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Yttrium Iron Garnet (YIG) is the benchmark material for ferromagnetic resonance; it is up to today the only magnetic material that is being integrated in on-shell electronic devices for its radiofrequency properties that combine high resonance quality factor (10&lt;sup&gt;4&lt;/sup&gt;) and frequency tunability. It is the goal of YIG magnonics to utilize this potential for a large variety of applications ranging from beyond complementary metal-oxide semiconductor (CMOS) computation to radiofrequency front-end and back-end analogue signal processing. YIG devices have however a severe drawback that is that they should include thermal stabilization. This is a necessity since YIG has a relatively low Curie temperature (∼550K) which results in a high sensitivity of its magnetization to temperature (∼4 Gauss/Kelvin). Here, we present a novel approach to solve this long standing issue by engineering the magnetic anisotropy in Bi substituted YIG. We evidence for those films that a vanishing effective magnetization yields high thermal stability over very wide temperature range: from 260K to 400K, where the frequency thermal drift is 50 times smaller than that of YIG. These types of properties can be leveraged for new applications where the temperature dependence of YIG is detrimental, while keeping the extremely low damping and insulating character of YIG&lt;sup&gt;[1]&lt;/sup&gt;. For instance, this was key for the excitation of coherent spin-waves using SOT&lt;sup&gt;[2]&lt;/sup&gt;. We anticipate that fabricating a material whose FMR frequency is temperature independent open new opportunities for the field of spintronics.&lt;br/&gt;&lt;br/&gt;In his seminal article&lt;sup&gt;[3]&lt;/sup&gt; of 1948 Charles Kittel established the Ferromagnetic resonance (FMR) conditions of magnetic samples with arbitrary shapes stressing the importance of correcting the effective magnetic field that sets the Larmor frequency by taking into account the demagnetization field and the effect of uniaxial anisotropy. In today notations the FMR frequency can hence be written for a thin film in the in-plane magnetization configuration as: f=γ*(H&lt;sub&gt;a&lt;/sub&gt;*(H&lt;sub&gt;a&lt;/sub&gt;+(M&lt;sub&gt;S&lt;/sub&gt;-H&lt;sub&gt;u&lt;/sub&gt;)))&lt;sup&gt;1/2&lt;/sup&gt;. Where γ is the gyromagnetic factor, H&lt;sub&gt;a&lt;/sub&gt; is the applied magnetic field, M&lt;sub&gt;S&lt;/sub&gt; is the saturation magnetization and H&lt;sub&gt;u&lt;/sub&gt; is the out-of-plane magnetization and the effective magnetization is defined as M&lt;sub&gt;eff &lt;/sub&gt;= M&lt;sub&gt;S&lt;/sub&gt; - H&lt;sub&gt;u&lt;/sub&gt;. It arises from this equation that the only temperature dependent terms are M&lt;sub&gt;S&lt;/sub&gt;(T) and H&lt;sub&gt;u&lt;/sub&gt;(T). For most ferromagnetic materials H&lt;sub&gt;u&lt;/sub&gt; depends on M&lt;sub&gt;S&lt;/sub&gt; with power law dependence defined by Callen and Callen theory&lt;sup&gt;[4]&lt;/sup&gt;. Nonetheless, by engineering the magnetic anisotropy in Bi-YIG films, we were able to balance the saturation magnetization with the uniaxial anisotropy between 260K and 400K. These measurements were done on thin Bi&lt;sub&gt;1&lt;/sub&gt;Y&lt;sub&gt;2&lt;/sub&gt;Fe&lt;sub&gt;5&lt;/sub&gt;O&lt;sub&gt;12&lt;/sub&gt; films of 21nm grown on (111)-oriented sGGG substrates with Pulsed Laser Deposition. We varied the substrate growth temperature within a range of 100°C and probed the films effective magnetization using FMR: we evidenced a magnetization state transition from in-plane to out-of-plane as the substrate growth temperature decreases from 500°C to 400°C. To ensure the nominal Bi content as well as the magneto-elastic anisotropy were maintained over all the growth temperature range, X-Ray Diffraction measurements were done and evidenced a perfect structural match between thin Bi&lt;sub&gt;1&lt;/sub&gt;Y&lt;sub&gt;2&lt;/sub&gt;IG films. Magneto-Optical Kerr Effect (MOKE) measurements were performed complementary to visualize not only the magnetic domains but also the magnetic anisotropy transition. With room temperature FMR measurements, we obtained the optimal substrate growth temperature range to be 420°C to 430°C: temperatures for which the effective magnetization M&lt;sub&gt;eff &lt;/sub&gt;≈ 0G. Interestingly, for thin films closest to the compensation, FMR measurements performed at lower temperatures evidenced that effective magnetization stay constant in a range of 140K: suggesting Hu(T)=Ms(T) contradictory with Callen and Callen theory&lt;sup&gt;[4]&lt;/sup&gt;. Finally we measured the frequency thermal drift was about 1mT between 260K and 400K for films close to the compensation, FMR frequency is temperature independent.

**References:**

[1] L. Soumah, N. Beaulieu, L. Qassym, Nature Communication 9, 3355 (2018)&lt;br/&gt;[2] M. Evelt, L. Soumah, A.B. Rinkevich, Physical Review Applied 10, 041002 (2018)&lt;br/&gt;[3] C. Kittel, Physical Review 73, 155 (1948)&lt;br/&gt;[4] H. B. Callen and E. Callen, Journal of Physics and Chemistry of Solids 27, 1271 (1966).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/e5e1e60b717ac1a92c59f4a6cb88abd2.jpg)
Fig. 1 Polar Magneto-Optical Kerr Effect (MOKE) measurement for a Bi-YIG thin film close to the compensation evidences the presence of magnetic domains.

![](https://assets.underline.io/uploads/markdown_image/1/image/7befc984861421507db377f3d886061b.jpg)
Fig. 2 Variation of the effective magnetization Meff(T) for Bi&lt;sub&gt;1&lt;/sub&gt;YIG thin films within a range of 140K probed by Ferromagnetic Resonance (FMR) measurement.

INTERMAG 2021

 Fine tuning of the Magnetic Anisotropy results in Temperature Independent Ferromagnetic Resonance Frequency for Bi-YIG thin films grown by Pulsed Laser Deposition

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This Conference provides an outstanding opportunity for worldwide participants to meet their colleagues and collaborators and discuss developments in all areas of magnetism research.

Yttrium Iron Garnet (YIG) is the benchmark material for ferromagnetic resonance; it is up to today the only magnetic material that is being integrated in on-shell electronic devices for its radiofrequency properties that combine high resonance quality factor (104) and frequency tunability. It is the goal of YIG magnonics to utilize this potential for a large variety of applications ranging from beyond complementary metal-oxide semiconductor (CMOS) computation to radiofrequency front-end and back-end analogue signal processing. YIG devices have however a severe drawback that is that they should include thermal stabilization. This is a necessity since YIG has a relatively low Curie temperature (∼550K) which results in a high sensitivity of its magnetization to temperature (∼4 Gauss/Kelvin). Here, we present a novel approach to solve this long standing issue by engineering the magnetic anisotropy in Bi substituted YIG. We evidence for those films that a vanishing effective magnetization yields high thermal stability over very wide temperature range: from 260K to 400K, where the frequency thermal drift is 50 times smaller than that of YIG. These types of properties can be leveraged for new applications where the temperature dependence of YIG is detrimental, while keeping the extremely low damping and insulating character of YIG[1]. For instance, this was key for the excitation of coherent spin-waves using SOT[2]. We anticipate that fabricating a material whose FMR frequency is temperature independent open new opportunities for the field of spintronics. In his seminal article[3] of 1948 Charles Kittel established the Ferromagnetic resonance (FMR) conditions of magnetic samples with arbitrary shapes stressing the importance of correcting the effective magnetic field that sets the Larmor frequency by taking into account the demagnetization field and the effect of uniaxial anisotropy. In today notations the FMR frequency can hence be written for a thin film in the in-plane magnetization configuration as: f=γ*(Ha*(Ha+(MS-Hu)))1/2. Where γ is the gyromagnetic factor, Ha is the applied magnetic field, MS is the saturation magnetization and Hu is the out-of-plane magnetization and the effective magnetization is defined as Meff = MS - Hu. It arises from this equation that the only temperature dependent terms are MS(T) and Hu(T). For most ferromagnetic materials Hu depends on MS with power law dependence defined by Callen and Callen theory[4]. Nonetheless, by engineering the magnetic anisotropy in Bi-YIG films, we were able to balance the saturation magnetization with the uniaxial anisotropy between 260K and 400K. These measurements were done on thin Bi1Y2Fe5O12 films of 21nm grown on (111)-oriented sGGG substrates with Pulsed Laser Deposition. We varied the substrate growth temperature within a range of 100°C and probed the films effective magnetization using FMR: we evidenced a magnetization state transition from in-plane to out-of-plane as the substrate growth temperature decreases from 500°C to 400°C. To ensure the nominal Bi content as well as the magneto-elastic anisotropy were maintained over all the growth temperature range, X-Ray Diffraction measurements were done and evidenced a perfect structural match between thin Bi1Y2IG films. Magneto-Optical Kerr Effect (MOKE) measurements were performed complementary to visualize not only the magnetic domains but also the magnetic anisotropy transition. With room temperature FMR measurements, we obtained the optimal substrate growth temperature range to be 420°C to 430°C: temperatures for which the effective magnetization Meff ≈ 0G. Interestingly, for thin films closest to the compensation, FMR measurements performed at lower temperatures evidenced that effective magnetization stay constant in a range of 140K: suggesting Hu(T)=Ms(T) contradictory with Callen and Callen theory[4]. Finally we measured the frequency thermal drift was about 1mT between 260K and 400K for films close to the compensation, FMR frequency is temperature independent.

**References:**

[1] L. Soumah, N. Beaulieu, L. Qassym, Nature Communication 9, 3355 (2018) [2] M. Evelt, L. Soumah, A.B. Rinkevich, Physical Review Applied 10, 041002 (2018) [3] C. Kittel, Physical Review 73, 155 (1948) [4] H. B. Callen and E. Callen, Journal of Physics and Chemistry of Solids 27, 1271 (1966).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/e5e1e60b717ac1a92c59f4a6cb88abd2.jpg)
Fig. 1 Polar Magneto-Optical Kerr Effect (MOKE) measurement for a Bi-YIG thin film close to the compensation evidences the presence of magnetic domains.

![](https://assets.underline.io/uploads/markdown_image/1/image/7befc984861421507db377f3d886061b.jpg)
Fig. 2 Variation of the effective magnetization Meff(T) for Bi1YIG thin films within a range of 140K probed by Ferromagnetic Resonance (FMR) measurement.

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Introduction Rogowski coils (RC) are used for the measurement of large alternating current, particularly as a short circuit detector in electric substations. Another application is in electricity meters. A regular, air-core RC cannot be saturated and therefore its dynamic range is limited only by noise voltage and supply voltage of the accompanying integrator or ADC [1]. An RC used for precise current measurement should have a consistent cross-sectional area and turn-to-turn spacing, limiting the winding arrangement to a single layer. The sensitivity of such coils is poor, causing a low signal-to-noise ratio [2]. Our proposed design adds a low-permeability ferromagnetic core to improve the sensitivity and thus reduce the required number of turns. Ferromagnetic core has been already tested for different types of RC, e.g. self-integrating RCs which are used for measurement of short current pulses. In this case increasing the core permeability decreases the low frequency limit of self-integrating operation [3]. Contrary to that, our target application is precise current sensing at the mains frequency and its harmonics. Such a sensor should be able to work with high accuracy (typical requirement is 0.5% or 0.2%) in the nominal range, as well as above this range with reduced accuracy with partly saturated core as a fault current detector [4]. Because the coil does not require a load resistor on its winding, there is no concern about overheating during overload conditions.However, there are some open questions, how will the ferromagnetic core affect the frequency characteristics of the RC, its ability to suppress external currents and resistance against the position deviations of the measured (primary) conductor. Several different materials, either in the form of a finished toroidal core or a 3-D printer filament filled with magnetic particles were tested, both theoretically by a FEM simulation and practically by measurement. FEM Simulation We have made a series of 3D FEM simulations to evaluate the effect of the core permeability on rejection of external currents and position of the measured conductor. In both cases the perfectly homogenous coil would be absolutely immune against both these influences. Practical coil always has uneven winding density, in particular between the first and last turn [5]. Also the cross-sectional area of individual turns varies slightly. We simulated the role of these imperfections in crosstalk from an external conductor as a function of its position and core permeability. In general, the error is diminishing with increasing permeability due to demagnetization effect. However, for practical permeability of 20 the error reduction is only very small (from 0.23 % to 0.15 %). The permeability effect on the resistance against the position deviations of the measured (primary) conductor is even lower and for permeability of 20 it is negligible. The effect of nonuniformity of the core permeability along its circumference was found to be insignificant as well. Samples used for measurement The first batch of test specimens (labeled A, B, C) are toroidal powder cores with an outer diameter of 79 mm and rated relative permeability of 26 (Kool Mju and Sendust materials) or 14 (Sendust). The winding is electrostatically shielded by a copper adhesive tape, care was taken to avoid the creation of a short-circuited turn. We also tested 3D printed cores from two types of plastic filament filled with magnetic particles and for the full paper we plan to test resin-based cores of various sizes. Measurement procedure Two important sensor characteristics were measured: linearity and frequency response. Test current was provided by a Techron 7548 power amplifier with an additional step-down transformer. Reference current measurement was done by a LEM Ultrastab current sensor (IT60S for frequency response, IT1000S for linearity). Two different measurement approaches were tested: either integrating the induced voltage with a Lakeshore 480 fluxmeter in AC mode, or measuring the voltage directly with a HF2LI lock-in amplifier. Both methods give similar results in amplitude, but the phase can only be measured by a lock-in amplifier. A primary conductor with a loop length of 10 meters was passed once through the measured coil to minimize the influence of the return current. Fig. 1 shows the frequency dependence: there is no observable sensitivity drop due to frequency dependence of the permeability. The sensitivity increase at high frequencies is due to the resonance with parasitic coil capacitance and its amplitude could be reduced by a parallel damping resistor [6] at the expense of heat being generated in the associated circuitry. Fig. 2 shows the measured linearity error at 93 Hz, which is in a good agreement with the catalogue values of permeability vs. flux density in the core. A nominal flux line length of 0.196 m was used to convert flux density to primary current. Conclusion We have demonstrated that adding a ferromagnetic core to a RC of the classical dI/dt type is beneficial for its sensitivity, which would allow the design of very precise RCs for lower currents. The RC linearity is decreased, but remains acceptable for most applications. The amplitude error caused by self-resonance can be reduced by standard methods such as a damping resistor, or by using a different material with high core loss. Acknowledgement This study was supported by the Grant Agency of the Czech Republic within the Nanofluxgate project (GACR GA20-27150S).

**References:**

[1] S. Tumanski, Meas. Sci. Technol. 18, p. R31, 2007 [2] J. D. Ramboz, IEEE Trans. Instrum. Meas. 45, p. 515, 1996. [3] J. D. Zhu, Q. G. Zhang, J. B. Jia et al., Plasma Sci. Tech. 8, p. 457l, 2006. [4] L. A. Kojovic, M. T. Bishop, and D. Sharma, IEEE Trans. Ind. Appl. 49, pp. 8, 2013. [5] L. Ferkovic, D. Ilic, and R. Malaric, IEEE Trans. Instr. Meas. 58, p. 122, 2009. [6] M. H. Samimi, A. Mahari, M. A. Farahnakian et al., IEEE Sens. J. 15, p. 651 (2015).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/b316cd355ace0c3db4ef22e26ec8fa2a.jpg)
Fig. 1 Frequency response

![](https://assets.underline.io/uploads/markdown_image/1/image/fd59bddadd4847af0a850bbd099da4d6.jpg)
Fig. 2 Linearity

 Rogowski Coil with Ferromagnetic Core

A growing interest in electric vehicles challenges soft magnetic materials to improve efficiency and power density. Metal amorphous nanocomposites (MANCs) have lower coercivities and eddy current losses, allowing for greater efficiency and higher switching frequency. The later allows for high motor speeds and improved power density. However, commercially available and laboratory demonstrated MANCs have shortcomings. Fe-based MANCs, such as FINEMET have excellent magnetic properties, but mechanical properties limit their application in high speed electric motors (HSMs). Fe-Co based MANCs, such as HITPERM, have high saturation inductions and good mechanical properties, but elevated magnetostrictive losses. Co-based MANCs and more recent Fe-Ni based MANCs have low losses and good mechanical properties, but lower saturation inductions (1-1.2 T), that limit power density in HSMs. Recently, much work has explored improving saturation induction in MANCs by increasing the content of magnetic elements, which comes at the cost of glass-forming ability (GFA). These efforts where generally limited to trial and error testing. Additionally, all work to date has focused on Fe-based alloys. In this work, a method of using Thermocalc simulation to locate near-eutectic compositions has been applied to Fe-Ni based alloys. Minima in liquidus temperature and solidification range were found for a ternary composition range and used to identify compositions that retain good GFA as the percentage of magnetic elements is increased. The (Fe70 Ni30)x(B-Si-Nb)100-x alloy system for x=82% and x=85% was explored by Thermocalc simulations. This is an increase in magnetic element content compared to previously developed x=80% alloys. 3 compositions in the x=82% system, and 1 alloy in the x=85% system where identified and successfully cast as amorphous ribbon. The amorphous nature of the ribbon was confirmed by a bend test and XRD. Magnetic testing was performed by PPMS to measure saturation induction and Curie temperature of the amorphous material. The Curie temperatures increased to 407-438 °C for the x=82% alloys, and 462 °C for the x=85% alloy. Saturation induction increased to 1.28-1.36 T for the x=82% alloy and 1.48 T for the x=85% alloy, which is a significant improvement over the ~370 °C Curie temperature and 1.2 T saturation of previous Fe-Ni alloy. Crystallization behavior was studied by XRD for the x=85% alloy, showing that crystallization process follows a 2-step process of Amorphous → BCC+FCC+Amorphous → BCC+FCC+Fe3B+Fe23B6. Post annealing magnetic properties were studied by strip testing, and saturation induction as high as 1.48 T was seen after optimal annealing. TEM was used to study structure of as cast and annealed material. 2 alloys with x=82% compositions were identified with an optimal crystalline size of 10-20 nm, while the x=85% alloy had this structure in the as cast state. This opens the possibility of using the alloy in the as cast state, without requiring annealing. In conclusion, alloys with good GFA and improved magnetic properties were identified by Thermocalc simulation.

**References:**

 N. Aronhime, V. DeGeorge, V. Keylin, JOM. 69 (2017) 2164–2170 N. Aronhime, E. Zoghlin, V. Keylin, Scr. Mater. 142 (2018) 133–137 J.M. Silveyra, A. Leary, V. DeGeorge, J. Appl. Phys. 115 (2014) 17A319 J.M. Silveyra, P. Xu, V. Keylin, J. Electron. Mater. 45 (2015) 219–225 A. Urata, H. Matsumoto, S. Sato, J. Appl. Phys. 105 (2009) 2007–2010 C. Wang, A. He, A. Wang, J. Pang, Intermetallics. 84 (2017) 142–147 A. Wang, C. Zhao, A. He, J. Alloys Compd. 656 (2016) 729–734 Y. Krimer, N. Aronhime, P. Ohodnicki, J. Alloys Compd. 814 (2020) 1-9

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/1be95150355009fb148c6d8dd5009eca.jpg)

![](https://assets.underline.io/uploads/markdown_image/1/image/46a90665e7662989fbfe624a29693a8d.png)

 Development of High Saturation Induction Fe-Ni Based Metal Amorphous Nanocomposite by Optimization of Glass Forming Ability

The D03 Fe3Sn phase shows higher magnetization and smaller magnetocrystalline anisotropy compared to Fe3Si based on the comprehensive first principles calculations. For this reason, it is important to develop nanocrystalline soft alloys which have D03 Fe3Sn phase as primary crystallite. A new alloy design method, motivated by the work of Villars, was used to choose the alloy composition. Element Co was found to stabilize the D03 Fe3Sn phase. In this work, a new alloy system (Fe1-x-yCoxSny)87Nb3B9Cu1 was studied to achieve D03 Fe3Sn phase with the help of the non-equilibrium processing method. Differential scanning calorimetry was used to determine the primary crystallization temperatures of the amorphous ribbons. The amorphous ribbons were annealed in tube furnace at 723K, 773K, and 823K for 3600 seconds after encapsulating them in a fused quartz ampoule. X-Ray Diffraction (XRD) has been used to analyze structural properties of as-spun and heat-treated ribbons of all alloy compositions. The single broad peak of Fe60Co20Sn7Nb3B9Cu1 as-spun ribbons’ XRD diffractograms indicates an amorphous phase due to their lack of long-range order. The rest as-spun ribbons’ XRD diffractograms shows partial crystalline during melt-spinning process due to the low glass-forming ability of these series of alloys. XRD diffractogram of the heat-treated ribbons are comprised of a B2 phase, which has a BCC structure. The crystallite sizes for each heat-treated sample were estimated by Scherrer broadening to have values between 7 and 10 nm. The magnetic properties of as-spun and heat-treated ribbons were studied by vibrating sample magnetometry. Saturation magnetization and coercivity were determined from room temperature magnetic hysteresis loops. The highest magnetization value achieved was 161.7 Am2/kg for the Fe75Co4Sn8Nb3B9Cu1 ribbon annealed at 723K.

**References:**

P. Villars, “Three-dimensional structural stability diagrams for 648 binary AB3 and 389 binary A3B5 intermetallic compounds: III,” J. Less-Common Met., vol. 102, no. 2, pp. 199–211, 1984 P. Villars, “A three-dimensional structural stability diagram for 998 binary AB intermetallic compounds,” J. Less-Common Met., vol. 92, pp. 215–238, 1983 Alex Zunger, “Systematization of the stable crystal structure of all AB-type binary compounds: A pseudopotential orbital-radii approach,” Phys. Rev. B, vol. 22, no. 12, pp. 5839–5872, 1980 S.S. Batsanov, “Dielectric Methods of Studying the Chemical Bond and the Concept of Electronegativity,” vol. 51, no. 7, 1982.

**Images:**

 Development of an (Fe, Sn)-based Nanocrystalline Soft Magnetic Alloy

INTRODUCTION Fe-based amorphous ribbons play an important role in high efficiency energy conversion devices and have been widely used in the field of electric power, power electronics and renewable energy as transformers [1], inductors [2] and reactors [3]. Small improvements in core losses can have large energy savings [4]. The demands of high efficiency and energy saving pose a challenge to further reduce the core losses of amorphous ribbons. To date, numerous studies have been investigated on materials development [5], subsequent annealing and structure design of magnetic cores [6] to further reduce the core losses of Fe-based amorphous cores and components. However, few reports are found on the influence of process parameters on core losses of Fe-based amorphous ribbon produced by planar flow casting (PFC) process. The objectives of this work are to study the influence of the thermal conductivity of cooling wheel substrate on cooling rate of Fe-based amorphous ribbon, to investigate the effect of cooling rate on core losses of Fe-Si-B amorphous ribbons. MATERIALS AND METHODS A batch of copper beryllium (Cu-2Be) cylindrical rings with different thermal conductivity was manufactured. A cooling wheel system is composed of a stainless steel wheel core and the cylindrical ring. Amorphous alloy ribbons with nominal composition of Fe80Si9B11 (at. %) were produced by using the Cu-2Be as the substrate of cooling wheel, the ribbon width is 50±0.5 mm and the ribbon thickness is 25±0.5 mm. The core losses were measured using IWATSU SY-8232 B-H analyzer. All the measured samples are toroidal cores with inner diameter of 30 mm, outer diameter of 35 mm and height of 50 mm subjected to heat treatment at 365 °C for 60 minutes with applied longitudinal field of 400 A/m. A 2D numerical model was established to calculate heat and mass transfer during PFC process by using ANSYS FLUENT software, the parameters used in the numerical calculation were the same as that of practical casting of Fe-Si-B alloy ribbons. RESULTS AND DISCUSSIONS The PFC process was simulated and the thermal conductivity of 175.3 W/m×K and 206.5 W/m×K for the Cu-2Be rings as a substrate of cooling wheel were used. The main process parameters employed for the simulation are as the follows: the applied pressure at the bottom of nozzle is 45 kPa, the cooling wheel outer diameter is 300 mm, the thickness of the Cu-2Be ring is 10 mm, the linear velocity of cooling wheel surface is 20 m/s, the nozzle slit width is 0.3 mm, the nozzle wheel gap distance is 0.15 mm, the temperature of molten alloy 1420 oC, the initial temperature of Cu-2Be substrate and circumferential air is 32 oC, the glass formation temperature of Fe-Si-B alloy is assumed to be 750 oC. Fig. 1 shows the simulation results of upstream meniscus, downstream meniscus and growth of liquid/solid interface at the 130th revolution for substrate thermal conductivity of 175.3 W/m×K and 206.5 W/m×K, respectively. In Fig. 1, upstream meniscus and down meniscus are plotted by the line of volume fraction of melt=1 and the liquid/solid interface is plotted by the isothermal line of glass formation temperature Tg=750 oC. The growth of liquid/solid interface is from P3 to P4 for this case. The solidification time is t=L/U and the average cooling rate is h=(Tg-Ts)/ t, where L is the puddle length from P3 to P4, U is the linear velocity of cooling wheel surface and Ts is the surface temperature of cooling wheel beneath the puddle (not shown). Based on the data extracted from the liquid/solid interface of the puddle and the surface temperature of cooling wheel beneath the puddle, the average cooling rates are 1.26e-6 K/s and 1.44e-6 K/s for k=175.3 W/m×K and 206.5W/m×K, respectively. The cooling rate increases by 14.3% due to the increase of substrate thermal conductivity. To investigate the cooling rate on core losses of Fe-Si-B alloy during PFC process, Cu-2Be rings with thermal conductivity of 175.3 W/m×K and 206.5 W/m×K were shrink fitted onto the wheel core as the substrate of cooling wheel. Fig. 2 shows the core losses at 50 Hz as a function of induction for Fe-Si-B ribbons produced by cooling wheel with different thermal conductivity. The increase of thermal conductivity of cooling wheel from 175.3 W/m×K to 206.5 W/m×K results in the decrease of core losses from 0.171 W/kg to 0.133 W/kg, with the core losses reduction up to 22.1%. CONCLUSIONS The influence of thermal conductivity of cooling wheel substrate on cooling rate during PFC process was simulated by numerical method and the results show that the increase of thermal conductivity of cooling wheel substrate from 175.3 W/m×K to 206.5 W/m×K leads to the increase of cooling rate from 1.26e-6 K/s to 1.44e-6 K/s in PFC process. The effect of cooling rate on structure and soft magnetic properties of Fe-Si-B alloy ribbons was investigated. The results show that the increase of thermal conductivity of cooling wheel substrate from 175.3 W/m×K to 206.5 W/m×K during PFC process results in the decrease of core losses from 0.171 W/kg to 0.133 W/kg, with the core losses reduction up to 22.1%.

**References:**

[1] D. Azuma, R. Hasegawa, IEEE Trans. Magn., 44, 4104-4106 (2008) [2] R. Hasegawa, J. Non-Cryst. Solids, 387, 405-412 (2001) [3] N. Kurita, K. Onda, K. Nakanoue, IEEE Trans. Power Electr., 29, 3657-3668 (2014) [4] O. Gutfleisch, M. A. Willard, E. Brück, Adv. Mater., 23, 821-842 (2011) [5] C. Dong, A. Inoue, X. H. Wang, J. Non-Cryst. Solids, 500, 173-180 (2018) [6] D. Azuma, N. Ito, M. Ohta, J. Magn. Magn. Mater., 501, 166373 (2020)

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/eb7f8d32dca65c6294b9b09eda7cc5f5.jpg)
Fig. 1 The simulation results of upstream meniscus, downstream meniscus and growth of liquid/solid interface in quasi-static process for substrate thermal conductivity of 175.3 W/m×K and 206.5 W/m×K.

![](https://assets.underline.io/uploads/markdown_image/1/image/544d845cdb3c2ee849ef1324426ec190.jpg)
Fig. 2 Core losses at 50 Hz as a function of induction for Fe-Si-B ribbons produced by cooling wheel with different thermal conductivity.

 Reducing the Core Losses of Fe-Si-B Amorphous Ribbons by High Cooling Rate Planar Flow Casting

Crystallization from an amorphous precursor has been historically exploited to synthesize metastable soft magnetic nanocomposite systems comprised of transition metal rich ferromagnetic nanocrystals embedded within an intergranular amorphous phase [1,2]. Subsequent thermal processing steps are designed to produce a highly refined microstructure with grain sizes in the nm-range to enable effective exchange averaging and highly reversible magnetization processes as described by the random anisotropy model [3,4]. Successful realization of superior soft magnetic properties results from ubiquitous nucleation and self-limiting growth of nanocrystals due to expulsion of glass formers to the intergranular amorphous matrix, thereby requiring significant amounts of metalloids and early transition metals which reduce the effective saturation magnetization. Recent work has discovered the balance between nucleation and growth can alternatively be optimized through highly controlled thermal processing steps involving extremely rapid heating and subsequent cooling rates on the order of 102-103K/s referred to as “flash annealing”, such that a high density of nucleation sites can be achieved with limited nanocrystal growth even for alloy chemistries which do not produce nm-scale microstructures for conventional annealing [5]. High heating and cooling rates generated during laser radiation have offered great advantages for rapid thermal processing and surface treatment of ferromagnetic amorphous ribbons [6–8]. In this work, we report on pulsed-laser processing of a series of Fe-based and Co-based amorphous ribbons with emphasis being placed on phase transformations within the laser-irradiated regions as well as impacts on surrounding regions due to the detailed thermal effects associated with the laser pulse. Amorphous ribbons with nominal compositions Fe73.5Si13.5Nb3B9Cu1 and Co77.2Fe1.4Mn1.4Nb4B14Si2 were used for laser experiments. Laser annealing has been carried out using a Q-smart pulsed Nd-YAG (wavelength of 1064 nm) and laser beam diameter around 0.6 mm. The laser power was varied from 50-150 mJ and the time delay associated with each laser power has been calculated using the FWHM of the laser output pulse. Microstructure and crystalline phases after laser annealing were characterized using both XRD and SEM. Temperature profiles achieved during laser annealing have been estimated using FEA heat transient analysis. Fig. 1 shows XRD patterns of as-cast and laser annealed Fe- and Co-based ribbons at different laser power. As can be seen, as-cast and samples annealed at 60 mJ are fully amorphous, whereas samples treated at higher laser power (i.e., at 100mJ and above) present a mixture of boride and α-Fe phases with an average grain size of the latter being on the order of 8-10 nm estimated from the FWHM of the corresponding crystalline peak. In conventional annealing methods, the formation of boride phases are typically observed only after reaching secondary crystallization temperatures in these alloy systems. Because the borides are already observed in laser annealed samples with significant volume fraction for the lowest pulse power levels at which crystalline peaks can be detected, a need exists to revisit the crystallization mechanism in light of the detailed thermal histories experienced across and surrounding the irradiated region and the Gaussian distribution of the laser beam across the irradiated surface. Laser energy density decreases exponentially in the radial direction and has a maximum at the center of the laser spot leading to greater temperature values at the center and a temperature gradient from the center towards the edges. In addition, laser beam intensity also produces a stress distribution which can be associated with its Gaussian profile resulting in an inhomogeneous stress across the beam diameter, tensile at the center and compressive at the edges. Such temporally and spatially dependent temperature and stress profiles can alter free volume, nucleation rate, and diffusion kinetics leading to rapid crystallization events and alternative pathways towards the crystalline phase. Temperature as a function of time of a simulated laser-irradiated spot at the top surface and below the top surface of an amorphous ribbon is shown in Fig. 2. The temporal temperature profile suggests temperatures above 2000K at the top surface which is beyond the melting point and most likely results in a local melting at the top surface layer with crystallization possible upon cooling. Temperatures estimated approximately 2 microns below the surface (within the skin penetration depth of Fe at the optical wavelength of Nd-YAG) are on the order of 1000K and likely not to reach melting temperatures, consistent with the surface deformation observed as illustrated in the SEM images of Fig. 2c. These temperatures significantly decrease within the distance from the center towards the edge of the laser spots suggesting diffusion of heat from the centers outwards and therefore different crystallization kinetics are expected.

**References:**

[1] M. E. McHenry, M. A. Willard, and D. E. Laughlin, Progress in Materials Science, vol. 44, no. 4, pp. 291–433, Oct. 1999 [2] A. Talaat, D. W. Greve, M. V. Suraj, and P. R. Ohodnicki, Journal of Alloys and Compounds, vol. 854, p. 156480, Feb. 2021 [3] G. Herzer, Acta Materialia, vol. 61, no. 3, pp. 718–734, Feb. 2013, doi: 10.1016/j.actamat.2012.10.040. [4] Y. Yoshizawa, S. Oguma, and K. Yamauchi, Journal of Applied Physics, vol. 64, no. 10, pp. 6044–6046, Nov. 1988 [5] K. Suzuki et al AIP Advances, vol. 9, no. 3, p. 035311, Mar. 2019, doi: 10.1063/1.5079778. [6] S. Katakam et al., Scripta Materialia, vol. 66, no. 8, pp. 538–541, Apr. 2012 [7] C. Smith et al., Materials Letters, vol. 122, pp. 155–158, May 2014 [8] L. Lanotte and V. Iannotti, Journal of Applied Physics, vol. 78, no. 5, pp. 3531–3533, Sep. 1995

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/b76b25d19ac56909b73662b71acd2ab3.jpg)
Fig. 1 XRD patterns of as-cast and laser-annealed Fe-based (a), and Co-based ribbons (b)

![](https://assets.underline.io/uploads/markdown_image/1/image/428c178d2a796bb0b353dfa63449a262.jpg)
Fig. 2. FEA simulations of temperature as a function of time at a laser power of 100mJ for (a) top surface and (b) below top surface of an amorphous ribbon. Red (0.6 mm), green (0.4 mm), and blue (0.2 mm) lines represent the distance from the center towards the edge of the laser spot. (c) SEM image of the ribbon surface layer showing evidence for local surface melting at the center of the laser irradiated region.

 Laser Processing of Soft Magnetic Metal Amorphous Nanocomposites

Fe-Co-Ni magnetic thin films with continuously varying compositions have been fabricated by the magnetron co-sputtering method. The films were characterized by high-throughput techniques. The results reveal the correlation between a range of compositions and the electrical properties of the films fabricated at various process parameters. Introduction: Soft magnetic materials are essential elements of electro-magnetic energy transformation technologies and they are widely used in various distribution, conversion, generation devices, such as transformers, motors, converters, generators, actuators, sensors etc. However, it usually takes long time for a new material before its acceptance for commercial application. Therefore, it is very crucial to speed up the discovery of new materials and incorporate them into various systems. High-throughput methods are efficient research methods [1-3] to explore this immense search space to identify new or optimized materials. Fe-Co based alloys received great attention due to high saturation magnetization. And what makes them even more fascinating is that their magnetic and electric properties can be tuned by controlling the alloy composition and microstructure. Among these compositions, Fe-Co-Ni films revealed significant induced magnetic anisotropy, low coercivity and high anisotropy of magnetoresistivity, what makes them a good candidate for technical applications. The resistivity is an important parameter to determine power loss in many soft magnetic applications. In this work we studied the effect of process parameters on the structure and resistivity of compositionally graded Fe-Co-Ni films to find process conditions at which optimum properties are obtained. Experimental details: Fe-Co-Ni films with continuously varying alloy compositions were fabricated by the magnetron sputtering method. Two experimental procedures were designed: a) Fe-Co-Ni films were deposited on SiO2/Si substrates by co-sputtering of permalloy and Co target materials using DC sources at different substrate temperatures (Ts): in particular, ambient temperature, 300 °C and 500 °C; b) the film was deposited at ambient temperature and annealed for 2 hours in vacuum at 500 °C. The base pressure in the chamber was controlled at 4 x 10 - 4 Pa. Depositions were performed in argon gas environment while the pressure was kept approximately at 0.3 Pa. For compositional, structural and electrical properties high-throughput Electron Probe Micro-Analysis (EPMA), X-ray Diffraction, four-point probe measurements were performed to determine chemical compositions, structure and resistivities of the films. Results and discussion: Fe-Co-Ni films with continuously varying alloy compositions were fabricated by magnetron sputtering method. The results of high-throughput Electron Probe Micro-Analysis measurements across Fe-Co-Ni films (Fig.1) revealed that atomic composition of Ni was highest, while Co was lowest in the film fabricated at Ts = 300 °C. The ratios of Co and Ni atomic compositions in deposited graded films were calculated as well. We found that Fe-Co-Ni film fabricated at Ts = 300 °C shows the lowest ratio as compared to other films. XRD measurements and analysis of multiple selected areas on Fe-Co-Ni film fabricated at ambient substrate temperature exhibited amorphous structure. As the substrate temperature increased diffraction peaks appeared, showing a clear change in structure. The diffraction patterns showed that only one FCC phase appeared. The peak intensity increased with temperature and the grain sizes became larger. The increase of peak intensity and average grain size indicated that increasing the substrate temperature from room temperature to 500 °C enhances the crystallinity of Fe-Co-Ni films. As for the annealed samples the intensity of the peak was low, and peak shifted towards lower angles indicating crystal lattice expansion. In addition, oxide peaks have been observed. Fig. 2 shows the measured resistivities as a function of composition of various process parameters. Resistivities of the graded film deposited at Ts =300 °C exhibited higher values than resistivities of the films deposited at ambient, 500 °C, and at ambient temperature followed by annealing at 500 °C. These results indicate that Ni plays important role in tuning resistivities of the films. Conclusion: Fe-Co-Ni films with continuously varying compositions have been fabricated by magnetron sputtering and characterized by high-throughput experimentation methods. The results revealed the correlation between continuously varying alloy compositions and selected properties of the films fabricated at various process parameters. Acknowledgement: This work is supported by the AME Programmatic Fund by the Agency for Science, Technology and Research, Singapore under Grant No. A1898b0043

**References:**

[1] Sean W. Fackler, Vasileios Alexandrakis, Dennis König, A. Gilad Kusne, Tieren Gao, Matthew J. Kramer, Drew Stasak, Kenny Lopez, Brad Zayac, Apurva Mehta, Alfred Ludwig and Ichiro Takeuchi, Science and Technology of Advanced Materials 2017, Vol.18, No.1, 231–238 [2] H. Koinuma, I. Takeuchi: Nat. Mater. 3 (2004) 429 –438. [3] Alfred Ludwig, npj Computational Materials (2019) 5:70

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/a7c2b3186ab8ed446ef672dcbe850a63.jpg)
Fig. 1 Elemental compositions of the graded Fe-Co-Ni films

![](https://assets.underline.io/uploads/markdown_image/1/image/dcd0207cd6dc3ac30e5af139256a3be1.jpg)
Fig. 2 Resistivities of Fe-Co-Ni films with continuously varying alloy compositions fabricated at different process parameters

 Effect of process parameters on the properties of compositionally graded FeCoNi films: A High Throughput Strategy

Recently, pearlitic wires, which experienced large drawing strains awoke the interest of material scientists due to their astonishing high strength of up to 7 GPa [1]. Because of the large amount of deformation applied during wire drawing, the grain size refines into the nanocrystalline regime and microstructural sizes of 10 nm and below are reported. The present study focusses on the capabilities to magnetically soften pearlitic steel by this “top-down”, conventional metal-forming process, as these length scales are already below the ferromagnetic exchange length of pure Fe. To do so, volume saturation magnetization and temperature dependent coercivities of wires experiencing high drawing strains (ε &gt; 5) were investigated. The results are compared to the ones of the initial wire, having more than twenty times the diameter and consequently a much coarser microstructure. To investigate the magnetic softening upon wire drawing, SQUID magnetometry and VSM measurements were performed. For the accurate determination of the volume saturation magnetization, the precise determination of the volume of the small samples is necessary. A confocal laser scanning microscope was used – the results allow the 3D reconstruction of the surfaces of the samples, for an example see inlay in Figure 1. From the surface reconstruction the volume can be calculated. It was found that the volume saturation magnetization is higher for the very thin wires in comparison to the initial state. While the thick, non-deformed wire shows a saturation magnetization very close to the one of pure pearlite, the saturation values for the thin wires are close to the one of pure body centred cubic (bcc) Fe (Figure 1). The slightly higher values, even in comparison to pure Fe, can be explained by C being supersaturated in the Fe matrix. The C necessary for supersaturation originates from the dissolution of cementite (Fe3C) during the wire drawing process [1]. Regarding the potential of wire drawing for magnetic softening, a decrease in coercivity was indeed found. The measured coercivities of the thin wires, determined with VSM, are smaller (24µm: 520 A/m, 36µm: 610 A/m) compared to the thick wire (1510 A/m). It is evident from magnetic measurements that there is a small but no substantial further refinement in grain size upon drawing the wire from 36 µm to 24 µm, although this constitutes a large degree of further deformation. Restoration mechanisms, as they are also known from other techniques involving severe deformation, seem to keep the grain size almost constant. For the thin wires, the grain size is calculated from the room temperature coercivity measurements. The grain size is found to be below the ferromagnetic exchange length of pure Fe, thus the random anisotropy model is applied. Using magnetic constants of pure Fe yields a grain size of 11 nm. This is in excellent agreement with atom probe tomography studies on the same material, where a subgrain size of 10 nm and below was found [2]. To further confirm the finding of dissolving cementite, the coercivity was determined as a function of temperature. Figure 2 includes calculated coercivities based on temperature dependent magnetocrystalline anisotropies for pure bcc Fe from [3–5]. Comparison reveals a very good match of measured and literature data; thus it can be safely assumed that the cementite has largely dissolved. Residual stresses, which might prevail from the drawing process would lead to an increased coercivity. Thus, a further softening of the drawn wires can be expected after thermal treatment, as it was the case for severe plastically deformed material, presented in [6]. One of the thin wires was annealed at 150°C for 30 min, which resulted in a decrease in coercivity by about 76 A/m or 12%. This proves the temperature stability of the microstructure at that temperature, as an increase in grain size would result in an increase in coercivity. The decrease in coercivity can be explained by reduced magneto-elastic effects, as residual stresses after wire drawing increase the effective anisotropy. In summary, pearlitic material was drawn to very high strains, resulting in very thin wires with nanocrystalline microstructure. Results of magnetic measurements indicate grain sizes close to 10 nm, the dissolution of cementite and a supersaturation of bcc Fe with C. It was possible to magnetically soften the coarse grained pearlitic steel from ~1500A/m to ~500A/m by this wire drawing process. However, the almost constant coercivity and grain size for very high drawing strains demonstrates that the magnetic softening seems to be maxed out. This project received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (Grant No. 757333).

**References:**

[1] Y.J. Li, P. Choi, C. Borchers, et al., Atom probe tomography characterization of heavily cold drawn pearlitic steel wire, Ultramicroscopy. 111 (2011) 628–632. [2] Y. Li, D. Raabe, M. Herbig, et al., Segregation Stabilizes Nanocrystalline Bulk Steel with Near Theoretical Strength, Physical Review Letters. 113 (2014). [3] H.-P. Klein, E. Kneller, Variation of Magnetocrystalline Anisotropy of Iron with Field and Temperature, Phys. Rev. 144 (1966) 372–374. [4] B. Westerstrand, P. Nordblad, L. Nordborg, The Magnetocrystalline Anisotropy Constants of Iron and Iron-silicon Alloys, Phys. Scr. 11 (1975) 383–386. [5] I.M. Puzei, V.V. Sadchikov, Dependence of the magnetic-anisotropy energy in iron on the magnetic field, Sov. Phys. JETP. 70 (1990) 137–139. [6] M. Stückler, L. Weissitsch, S. Wurster, et al., Magnetic dilution by severe plastic deformation, AIP Advances. 10 (2020) 015210.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/6e4821f573e1bd315f9ff4211724fec7.jpg)
Figure 1: Saturation magnetization for all investigated wires, measured at applied fields ≥ 2 T, taking into account the individual sample volume determined by confocal laser scanning microscopy. The values of pure bcc Fe and pearlite are included. As an inlay, the 3D-reconstruction of the wire surface of a small section of one of the 36µm-wires is shown.

![](https://assets.underline.io/uploads/markdown_image/1/image/885834416e94cd889410edd26013c0ba.jpg)
Figure 2: Measured and corrected temperature-dependent coercivity of one of the 36µm-wire as a function of temperature. Results are compared with coercivities calculated according to literature [3–5] on temperature dependent magneto-crystalline anisotropy. The coercivity of the annealed 36µm-wire is also shown.

 Soft magnetic properties of severely drawn pearlitic-ferritic wires with nanocrystalline microstructure

Organophosphates (OP) have been used to kill insects in many areas such as homes and workplaces, especially in agricultural pest control [1]. In many people, the remains of OP can lead to many health problems to range from paralysis to death. For this reason, it is very important to detect OPs, which is the most important way of passage to the human organism, as a sensitive, fast, and cheap way. Nowadays, conventional OP detection is carried out by techniques such as mass spectroscopy, gas chromatography, and high-performance liquid chromatography. Although these methods give accurate and sensitive results, they require time-consuming processes such as sample preparation, solid-phase extraction, and comparison of sample peaks with references. Moreover, the cost of these methods is quite high. Therefore, the development of an alternative method as sensitive, fast, practical and cheap for detection of OPs was carried out in this study. The system used in the detection of OPs is a magnetoelastic sensor (MES). Magnetoelastic sensors are preferred because of their low cost, disposable application, wireless and easier to use. Magnetoelastic materials begin to vibrate (shorten and elongate) when exposed to a time-varying (AC) magnetic field. The vibration frequency of such samples varies depending on the geometry, elastic coefficient and mass (density) of the sample [2]. In magnetoelastic sensors, the amount of mass usually deposited on the sensor surface causes a shift in the resonance frequency and the shift in the magnetoelastic measurement system is measured. In this way, the mass accumulated on the sensor surface can be measured. It is important that the sensor surface is functionalized according to the type of molecule being detected. The magnetoelastic sensor will be more precise and superior if it measures a very low mass change. Therefore, enlarging the sensor surface area makes a significant contribution to the sensitivity of the sensor. In this context, electrospun nanofibers were coated on the sensor surface in order to increase the surface area of magnetoelastic sensors for the first time within this study. Within the scope of this project, 2826MB (Fe40Ni38Mo4B18) amorphous ferromagnetic ribbons with 35 mm long and 5 mm in width were used as MES. Then, the nanofibers were functionalized with glutaraldehyde (GA) in order to covalently bind acetylcholinesterase (AChE) enzyme to the surface. The resonance frequency of the functionalized amorphous strip was measured to be around 59 000 Hz. OP in the 10% EtOH solution was dripped with a micropipette on the surface of the functionalized MES then allowed drying. It was observed that the EtOH solution evaporate and the remaining OP covalently binds the acetylcholinesterase (AChE) enzyme. When 50 ppm OP in 10% EtOH solution were dropped, a decrease of approximately 22 Hz was observed in the resonance frequency at 500 A/m DC bias magnetic field. Figure 1 shows the measured resonance frequency change when different amounts of OP were dropped onto the MES. As can be seen, MES shows a linear change according to the amount of OP measured in the range of 0-150 ppm in 10% EtOH solution. To determine whether the frequency shift comes only from the OP, only 10% EtOH solution was dripped onto the functionalized MES surface in different amounts and it was found that no shift in the frequency had occurred. Acknowledgment This work was supported by TUBITAK with 119F144 project number.

**References:**

[1] M. Eddleston, M.R. Phillips, BMJ. 328, 42 (2004). [2] C.A. Grimes, P.G. Stoyanov, D. Kouzoudis, K.G. Ong, Rev. Sci. Instrum., 70, 4711 (1999).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/c1ac49f02f0ff5add6db01a39b73119c.jpg)
Fig.1. Variation of resonance frequency as a function of diazinon organophosphates amount.

 Detection of Diazinon Organophosphates Using Magnetoelastic Sensor

Owing to the augmentation of wireless communication, the development of electromagnetic interference (EMI) shielding structures is an open area for research. The devices working at microwave frequencies are hazardous to human health and can also result in the malfunctioning of various sensitive equipments. Various techniques are reported by the researchers for the development of an efficient EMI shielding structure, some of them are composite materials, nanocomposites, metamaterials, and frequency selective surfaces (FSSs) [1]-[3]. FSSs are an array of periodic slots or apertures having excellent electromagnetic (EM) wave filtering properties. Major challenges across the research community are to create a miniaturized, angular stable, wideband, polarization-insensitive, and flexible EMI shielding structure. Multiband structures are often used by researchers to get a wide bandwidth. Whereas, miniaturization is attained either by techniques like 3-D, convoluted geometries, and lumped capacitances. All these methods of bandwidth improvement and miniaturization result in complex design and therefore fabrication becomes tedious. Almost all reported EMI shielding structures uses pure dielectric substrates like FR4 and Rogers. However, bandwidth and miniaturization can be improved by increasing the overall inductance of the shielding structure. The introduction of lumped inductors can be a solution but it not only increases the fabrication complexities but also may result in unwanted parasitic capacitance. Also introducing active or reactive components limits the conformal applications of shielding structures. One possible solution is to use magnetic substrates, which will increase the overall inductance of the shielding structure. Therefore, in this work electromagnetic properties of two magnetic substrates (Ferrite and magnetic silicon rubber) are measured and investigated to improve the bandwidth and miniaturization of an EMI shielding structure. Further, a polarization-insensitive, angular stable FSS is designed and the equivalent circuit model (ECM) is developed to have a physical insight into the structure. Moreover, a competent deep neural network (DNN) approach is employed for fast and accurate optimization of the shielding structure. At last, a prototype is developed and measured for validation of the proposed technique. The schematic of the proposed shielding structure is depicted in Figure 1(a). It comprises an array of periodic unit cells, a magnetic ferrite substrate, and a silicon rubber sheet. Figure 1(b) demonstrates an FSS unit cell with corresponding geometrical design variables. Here, ro is the radius of the outer circle and ri corresponds to the radius of the inner circle. d is the length of the patch, w is the width, and periodicity of the unit cell is represented by p. The electromagnetic properties of magnetic substrates are measured using a waveguide-based microwave measurement technique. Figure 1(c) depicts the measured relative permittivity (εr) and permeability (μr) of both the substrates. The real part of μr varies between 2.10 and 1.49 and the imaginary part varies between 0.34 and 0.0025 for magnetic silicon rubber when the frequency changes from 8.2 to 12.4 GHz. The real part of εr is 5.50 and the imaginary part varies between 0.20 and 0.005. On the other hand, the ferrite substrate real part of μr changes varies between 1.17 and 1.13 and the imaginary part varies between 0.04 and 0.001. Moving a step further, ECM of the FSS backed substrates is developed and the overall impedance is calculated. A huge database is created using the ECM and a deep neural network is trained using the Levenberg-Marquardt algorithm. The entire dataset is divided into three sections randomly for training, validation, and testing in the proportion of 70%, 15%, and 15%. Once the training, testing, and validation of the network is done, the shielding structure is optimized for resonant frequency and bandwidth. A prototype is then fabricated from the optimized parameters using the printed circuit board (PCB) technique. Figure 2(a) shows the image of the fabricated prototype. Meanwhile, the effect of the magnetic properties of the substrates on the transmission performance of the shielding structure is investigated. When the permittivity of the substrates dominates, a bandwidth of 2.75 GHz is obtained with resonant frequency at 10.42 GHz as shown in Figure 2(b). The bandwidth of surfaces with quite high impedance is directly proportional to the square root of overall inductance. To increase the bandwidth and to achieve miniaturization substrates with dominant magnetic properties are used. The overall inductance of the structure is increased by using substrates with dominant μr. Figure 2(b) depicts when substrates with dominant permeability are used in place of permittivity dominant substrates resonance shifts to lower value as the overall inductance of the structure increased. Also, the bandwidth obtained from such magnetic substrate increased to 3.37 GHz from 2.75 GHz. Therefore, it can be concluded that using magnetic substrates bandwidth enhancement and miniaturization of the shielding structure can be achieved.

**References:**

[1] M. Sonehara, T. Sato, M. Takasaki, H. Konishi, K. Yamasawa and Y. Miura, "Preparation and Characterization of Nanofiber Nonwoven Textile for Electromagnetic Wave Shielding," IEEE Transactions on Magnetics, vol. 44, no. 11, pp. 3107-3110, Nov. 2008. [2] J. Lee, B. M. Jung, S. B. Lee, S. Lee and K. H. Kim, "FeCoNi-Coated Glass Fabric/Polycarbonate Composite Sheets for Electromagnetic Absorption and Shielding," IEEE Transactions on Magnetics, vol. 53, no. 11, pp. 1-4, Nov. 2017. [3] R. Panwar, S. Puthucheri, D. Singh and V. Agarwala, "Design of Ferrite–Graphene-Based Thin Broadband Radar Wave Absorber for Stealth Application," IEEE Transactions on Magnetics, vol. 51, no. 11, pp. 1-4, Nov. 2015.

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/cfe4cd73e62b6f4782155911aa08359e.jpg)
 
Figure 1. Proposed shielding structure (a) Schematic and FSS unit cell, and (b) Fabricated prototype

![](https://assets.underline.io/uploads/markdown_image/1/image/2627666b73e7db1a89ce3751c93f1ac6.jpg)
Figure 2. EMI shielding characteristics (a) Frequency-dependent electromagnetic characteristics of silicon rubber and ferrite substrates, and (b) Transmission performance for different configurations of FSS and substrates.

 Magnetic Substrate Coupled Broadband and Miniaturized Electromagnetic Interference Shielding Structure Using Deep Neural Network

Microwave absorbers are advanced electromagnetic structures that can competently deplete the incident electromagnetic (EM) wave, and widely utilized for stealth and electromagnetic interference mitigation applications. There have been enormous strides in the broadening of absorption bandwidth (BW) using thin and lightweight functional materials. Apart from functional materials, the shape and loading of the absorber are equally responsible for attenuating the EM wave. There are various potential structures capable of altering the equivalent EM properties of the material to achieve excellent absorption BW [1]-[2]. The simultaneous achievement of wide absorption bandwidth and lower coating thickness of the absorber layer is a very challenging task. In this work, an effort is made towards the design of thin and broadband triple-layer absorber by incorporating gradient air perforation into the absorbing sheets constituted of iron (Fe)-silicon carbide (SiC) composites synthesized by robust top-down fabrication technique. The gradient air perforation prompts viable absorption BW if the period and diameter of perforations are equivalent to the wavelength. Fe-SiC composites in distinct compositions (i.e., FS1, FS2, FS3, FS4, FS5, FS6, and FS7) have been utilized in the development of microwave absorber. The composites have been developed using a top-down fabrication technique accomplished via a high-energy planetary ball mill (Model: Insmart device, MBM-07, India). The milling process has been initiated by loading stainless steel containers with hard 20 mm and 10 mm diameter steel balls. The powder to ball weight ratio (PBR) of 1:10 was taken with the rotor speed to 400rpm. The EM properties of the composite were measured in the range of 8.2 to 12.4 GHz (i.e., X-band) using a waveguide-based microwave measurement technique. Further, Jaya’s-Grey Wolf based hybrid multiobjective optimization technique has been utilized for the modelling of a perforated multi-layered absorber using the measured EM parameters. The mathematical model of air perforation is integrated with the transmission line equations such that the developed structure appears like a gradient perforated triple-layer structure. The fitness function of maximum BW below -10dB with minimum thickness is organized in the hybrid optimization algorithm. An optimal sample has been fabricated using epoxy coating and drilling techniques based on the optimal design variables obtained using the algorithm. The schematic of 3D topology and fabricated porotype of perforated absorber constituted of Fe-SiC composites is shown in Figs. 1 (a) and (b), respectively. Fig. 2 shows the reflection coefficient curve (RC) of single-layer absorber at their optimal thicknesses. Among all Fe-SiC composite database, FS-7 composite achieves a minimum RC value of -24.7dB at 11.1 GHz at an optimal thickness of 1.7mm, covering an absorption BW of 3.1 GHz below -10dB threshold in X-band. Furthermore, FS-4 composite exhibits an absorption BW of 1.7GHz with a minimum RC value of -10.9 dB at 9.3 GHz at an optimal thickness of 1.7 mm. However, FS-3 composite achieves a minimum RC value of -9dB at 8.2 GHz demonstrating no absorption BW. In the proposed gradient structure, the hybrid optimization algorithm chooses composite FS-4 for layer 1 with an optimal thickness of 0.4 mm backed with a perfect electrical conductor. Consecutively, layers 2 and 3 (i.e., air absorber interface) are structured using FS-7 and FS-3 composites with an optimal thickness of 0.9 mm and 0.3 mm, respectively. The optimum air perforation diameter is 6 mm, 5mm, 4mm for FS-3, FS-7, and FS-4, respectively with a period of 20 mm. The magneto-dielectric properties of composite FS-3 are high as compared to FS-4 and FS-7 composites. Due to the larger diameter of air perforation, the effective magneto-dielectric properties are supervised to act as an impedance matching layer. A 1.6 mm multilayer structure (without hole) is compared with the proposed structure as shown in Fig. 2 (b). Due to gradient air perforation in the layered structure, the EM parameters are altered, which leads to the shift of resonance frequency. The obtained result reveals a better -10 dB absorption BW of 3.4 GHz with a minimum RC of -19.9 dB at 10.8 GHz. An exceptional microwave absorption is achieved in terms of peak RC and BW with a substantial reduction in the total coating thickness of 1.6 mm in comparison to a single-layer absorber. Moreover, due to perforation, the weight of the structure is also reduced. The gradient impedance alignment within the structure improves the other mechanisms of absorption, such as phase cancellation, internal reflection, and frequency dispersion within the structure which results in improved absorption BW.

**References:**

1. R. Panwar, S. Puthucheri, D. Singh and V. Agarwala, IEEE Transactions on Magnetics., vol. 51, no. 11, pp. 1-4 (2015). 2. L. Zhang, P. Zhou, H. Zhang, L. Lu, G. Zhang, H. Chen, H. Lu, J. Xie, and L. Deng, IEEE transactions on magnetics., vol. 50, no. 5, pp. 1-5 (2013).

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/a36d6e6475b0a7c1c80560119035c751.jpg)
Fig. 1. Schematic of perforated absorber (a) 3D topology and (b) fabricated porotype.

![](https://assets.underline.io/uploads/markdown_image/1/image/53df4f47bbf29bb3232a59d3911a61b0.jpg)
Fig. 2. Frequency-dependent RC curve of the absorber (a) single layer and (b) gradient perforated multilayered absorber in X-band.

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