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Spin-torque nano-oscillators (STNOs) are spintronic devices operating in frequencies ranging from less than 100 MHz up to a few GHz [1]. Vortex-based STNOs have a distinct configuration exhibiting a vortex magnetization distribution of the free layer in the equilibrium state. STNOs are promising candidates for future-generation implementations, such as rf generation [2], detection [3] or bio-inspired computing [4]. However, their use as magnetic field sensors remains understudied. Vortex-based devices provide a large linear range for magnetic field detection [5] and their nanometric size allows for improved spatial resolution. Such advantages motivate the study of vortex STNOs for sensing applications. One of the main limiting factors for magnetic sensors, notably vortex-based ones, is their intrinsic 1/f noise, which inhibits the device’s detection capabilities at low-frequencies. Here, we study the low-frequency noise of vortex-based STNOs. We determine the Hooge parameter α [6] - a figure of merit used to characterize sensor’s noise properties. In the uniform states, α is in the same order of magnitude as those measured in state-of-the-art TMR sensors with similar RA product, α = 10-11 - 10-10 µm2. In the vortex state, the measured noise level is around one order of magnitude greater than in the parallel state (Fig. 1), due to the increased probability of pinning of the vortex core due to material defects and/or inhomogeneities at the free layer. Furthermore, we examine the impact of the vortex dynamics in the low-frequency noise. Above a certain critical current, the spin transfer torques surpass the intrinsic damping of the free layer resulting in self-sustained oscillations of the vortex core [7]. We find that these dynamics strongly modify the characteristics of the STNOs and, in consequence, the achievable performances of magnetic field sensors relying on magnetic vortex dynamics. We present a detailed analysis of these characteristics and how they relate to the device’s noise properties. The work is supported by the French ANR project “SPINNET” ANR-18-CE24-0012.References: [1] S. Kiselev et al., Nature Vol. 425, 380 (2003) [2] S. Wittrock et al., Phys. Rev. B Vol. 99, 235135 (2019) [3] A. S. Jenkins et al., Nat. Nanotech., Vol. 11, 360 (2016) [4] M. Romera et al., Nature Vol. 563, 230–234 (2018); J. Torrejon et al., Nature, Vol. 547, 428-431 (2017) [5] D. Suess et al., Nature Electronics volume 1, pages 362–370 (2018) [6] F. Hooge and A. Hoppenbrouwers, Physica 45, 386-392 (1969) [7] A. Dussaux et al., Nat. Comm., Vol. 1, 8 (2010)

MMM 2020

Analysis of 1/f Low-frequency Noise in Vortex-based Spin-torque Nano-oscillators for Magnetic Sensing Applications

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In this work, we demonstrate that the TbVO4 compound can be used as magnetic refrigerant in efficient and ecofriendly magnetic cryocoolers due to its giant magnetocaloric effect (MCE) at low temperature regime. The application of a relatively low magnetic field of 2 T along the easy magnetization plane (ab) gives rise to a maximum entropy change of about 20 J/kg K at 4 K. More interestingly, under sufficiently high magnetic fields, the entropy change (-ΔS) remains approximately constant over a wide temperature range which is highly appreciated from a practical point of view. For example, in the magnetic field change of 7 T, -ΔS that reaches roughly 22 J/kg K remains practically unchanged over about 34 K, leading to an outstanding refrigerant capacity of 823 J/kg. On the other hand, the change in the crystallographic symmetry from the tetragonal to the orthorhombic structure occurring close to 33 K as confirmed by Raman scattering data (Figure 1), results in a strong magnetocrystalline anisotropy. Accordingly, giant thermal effects are obtained simply by spinning the TbVO4 single crystals between the hard (c axis) and easy (ab plane) orientations (Figure 1) in constant magnetic fields. The rotation of TbVO4 crystals between their easy and hard magnetization directions in a constant magnetic field of 2 T, enables to generate entropy changes of about 18 J/kg K around the liquid helium temperature. Such rotating MCEs would open the way for the implementation of TbVO4 in more efficient and compact magnetocaloric coolig systems with a simplified design that could be dedicated to the hydrogen and helium liquefaction.

Giant anisotropy of the magnetocaloric effect in the orthovanadate TbVO4 single crystals

Fe nanoparticles are grown in a TiN/Fe/TiN heterostructure using a pulsed laser deposition (PLD) system. TiN/Fe/TiN multilayers are made by altering the average interlayer distance between two Fe nanoparticles and varying Fe nanoparticle sizes by changing the number of laser pulses impinging on a commercial Fe target. Crystallographic studies carried out using x-ray diffraction (XRD) have indicated crystalline nature of the film with an FWHM of 0.13o. By controlling the nanoparticle size in the confined layers, a superparamagnetic to ferromagnetic phase transition is observed during zero field cooled (ZFC) temperature (T) dependent magnetization (M) measurements and thereby tuned magnetocaloric properties in a controlled manner. A maximum magnetization of ~3.6 × 105 A/m which is 2 order of magnitude higher than our previously reported Fe2Ta thin films is obtained at 0.075 T. Applying the Maxwell relation to the MT curves at various fields, we have calculated vs H the integration of which provides quantitative information about the isothermal entropy change. We have observed a positive MCE with maximum entropy change of ~ 600 J/K-m3 for the magnetic field changing from 0.002 - 0.075 T in TiN/Fe/TiN system. A coexistence of positive and negative ΔS is observed below and above blocking temperature respectively.References: 1. V. Franco, J. Blázquez, B. Ingale and A. Conde, Annual Review of Materials Research., 42, pp. 305-342 (2012). 2. C. W. Miller, D. D. Belyea and B. J. Kirby, Journal of Vacuum Science & Technology A., 32(4), p. 040802 (2014). 3. O. Tegus, E. Brück and K. H. J. Buschow, Nature, 415(6868), pp. 150-152 (2002).

Enhanced Magnetic and Magnetocaloric Properties of Fe Nanoparticles in TiN Thin Film Matrix

The effect of hydrostatic pressure on magnetic entropy change in Fe7Se8 single crystals is determined. The combined magnetic entropy change, due to both magnetic field and pressure application is analyzed experimentally and theoretically. The studied material is a ferrimagnetic metal with Néel temperature (TN) equal 450 K. The first-order spin reorientation transition (SRT) from easy c-axis to easy c-plane is observed in this material at the temperature Tr ≈ 125 K [1]. This effect was shown to originate from the specific dependence of the magnetocrystalline energy on temperature and can be induced by applied hydrostatic pressure and/or by a magnetic field. Under applied hydrostatic pressure, Tr is shifting to the lower temperatures, while the effect of the magnetic field depends on the direction to the c-axis. Magnetic measurements show the decrease of Fe7Se8 magnetization under applied hydrostatic pressure. It is a consequence of iron vacancies redistribution induced by pressure. It has been observed that the entropy change ΔSmag and refrigeration capacity RC, are in strong dependence on Tr and magnetization. Indirect methods, based on magnetization measurements, were used to calculate ΔSmag (Fig.1). It has been shown that the combined application of the magnetic field and hydrostatic pressure can significantly affect entropy change. The sign of the ΔSmag is determined on the direction of the magnetic field H. In conclusion, we have shown, that entropy change in magnetic materials such as Fe7Se8, subjected to simultaneous variation of magnetic field and pressure, reach large values in a wide range of temperatures in the vicinity of Tr. This study was partly financed by the National Centre for Research and Development, Research Project no. PBS2/A5/36/2013References: [1] I. Radelytskyi et al., J. Appl. Phys. 124, 143902 (2018).

Combined Pressure and Magnetic Field Induced Caloric Effects in Fe7Se8 Single Crystals

Scaling laws have been widely applied for the study of second-order phase transition (SOPT) magnetocaloric materials as their magnetic entropy change, ΔSM, collapses onto a single universal curve [1]. However, for non-single phase SOPT cases, distortions in the scaled curves prevent good collapse. These distortions get more evident when the ratio of the different phases and the proximity of their transitions increase [2]. Based on scaling laws, a procedure is proposed and successfully applied to deconvolute the magnetocaloric responses of biphasic Gd80Pd20 composite (formed by Gd and Gd7Pd3 with Curie temperatures separated by ~45 K) [3, 4]. The global ΔSM curves of the composite were firstly rescaled by the typical universal scaling procedure considering separately each phase (i=1, 2), i.e. temperature rescaling using one reference temperature, Tr,i, as θi = (T-Tpk,i)/(Tr,i -Tpk,i) and normalizing the ΔSM with respect to the corresponding peak values, ΔSMpk,i. In this case of highly overlapped phase transitions, the typical power law behavior for SOPT (ΔSMpk ∝ Hn and Tr ∝ Hζ) is not fulfilled. An iterative procedure is then followed to account for the phase overlap, leading to a good agreement between experimental and calculated phase fractions and reconstruction of the experimental ΔSM curves (Fig. 2). This procedure can be applied to extract the response of a pure phase even in cases where the synthesis procedure gives rise to impurity phases, providing a tool to gauge the potential applicability of a material without dedicating large efforts for optimizing its synthesis. Work supported by AEI/FEDER-UE (grants MAT-2016-77265-R and PID2019-105720RB-I00), US/JUNTA/FEDER-UE (grant US-1260179), Consejería de Economía, Conocimiento, Empresas y Universidad de la Junta de Andalucía (grant P18-RT-746), Army Research Laboratory under Cooperative Agreement Number W911NF-19-2-0212 and Polish National Science Centre (Project 2019/03/X/ST5/00066).References: [1] V. Franco et al., Int. J. Refrig., 33 465–473 (2010). [2] V. Franco et al., J. Magn. Magn. Mater., 321 1115–1120 (2009). [3] Á. Díaz-García et al., JOM, (2020). https://doi.org/10.1007/s11837-020-04251-z. [4] P. Gebara et al., J. Magn. Magn. Mater., 500 166175 (2020).

Use of the Scaling Laws of the Magnetocaloric Effect to Deconvolute Overlapping Phase Transitions in Biphasic Composites

It is over twenty years since the discovery of a giant magnetocaloric effect (GMCE) in Gd-Si-Ge [1], and in that time two low-cost GMCE materials systems have been pursued towards commercialization: Mn-Fe-P-Si [2] and La-Fe-Si [3]. This relatively small number of target room temperature magnetic refrigerants underscores the need for time- and cost-effective magnetocaloric materials discovery projects. The ideal material candidates for magnetocaloric applications must be non-toxic, inexpensive, and readily manufacturable [4]. High- throughput workflows, guided by machine learning and other statistical methods in material informatics, are essential to streamlining the process of screening new possible materials, making the experimental process as targeted and efficient as possible. This presentation describes two approaches to materials screening based on simulated data. The first approach is a modification of the screening method of Bocarsly et al. [5], wherein lattice deformation across the magnetic phase transition (magnetic deformation) was computed based on the difference between unit cell dimension lengths with and without magnetic interactions in models based on density function theory (DFT). We add a phenomenological parameter calculated from the electronic density of states available from the Materials Project [6]. The addition of this new parameter can enhance the prediction of magnetic field-induced entropy change, ΔSM, with nearly 60% better correlation than using magnetic deformation alone. The second approach is motivated by the relative scarcity of consistent magnetic entropy data for stoichiometric compounds, which limits the ability of researchers to cross-check predicted ΔSM with experiment. We describe a method of scraping the Materials Project database for DFT results on magnetically ordered compounds [7] using the Pymatgen open-source Python library [8]. By analyzing the electronic density of states, computed unit cell volume and other parameters obtained through the Materials Project database for over 20,000 materials, we compile a data subset which we use to propose directions for magnetocaloric material research.References: [1] V. Pecharsky et al. 1997 Phys. Rev. Let. 78 4494 [2] N. H. Dung et al. 2011 Adv. Energy Mater 1 1215-1219. [3] M. Katter et al. US Patent Application No. 2012/0043497 A1 (23 Feb. 2012) [4] N. A. Zarkevich et al. 2018 J. Phys. D: Appl. Phys. 51 024002 [5] J. Bocarsly, et al. 2017 Chem. Mat. 29 (4), 1613-1622 [6] A. Jain, S.P. Ong et al. 2013 APL Materials, 2013, 1(1), 011002 [7] S. P. Ong et al. 2015 Comp. Mat. Sci. 97 209–215 [8] S. P. Ong et al. 2013 Comp. Mat. Sci. 68 314–319

Applying Materials Informatics Insights to Magnetocaloric Materials Screening and Discovery

Near-equiatomic iron-rhodium (FeRh) is a fundamentally and technologically interesting material exhibiting giant inverse magnetocaloric effect (MCE), potentially suitable for applications at the diminutive scale such as micro-cooling and medical diagnostic imaging, which require sharp first-order magnetic phase transitions (FOMPTs). Reports on FeRh having a sharp FOMPT in its bulk form are common,1 however the transition broadens considerably in some non-bulk forms, particularly thin-films (thickness < 50nm) and fine particles.2 In this work, an abrupt, bulk-like FOMPT has been demonstrated for the first time in FeRh particles produced using a solvent-free, reactive mechanochemical co-reduction of FeCl2 and RhCl3 precursors. Characterization using structural and magnetic probes indicates that annealing at temperatures ranging from 600°C to 800°C and subsequent quenching enables tuning of composition, microstructure, and physical properties of the particles. Depending upon the heat treatment condition, the fraction of fcc paramagnetic FeRh and B2 FeRh (both ferromagnetic (FM) α’ and antiferromagnetic (AFM) α”) phases can be controlled. The M-T analysis indicates that the first-order magnetostructural transition temperature, Tt, can be tailored between 380K and 325K, and both the △M between the AFM and FM states and the thermal hysteresis can be manipulated as well (Fig. 1). A maximum magnetic entropy change of △Smag ~10 J/kgK at μH=2T was noted in FeRh particles annealed at 800°C. These results will be discussed in the context of the compositional phase diagram and strain accommodation mechanism. Implications of this study on emerging biomedical applications, mainly thermal magnetic resonance imaging and magnetic hyperthermia therapy, will be presented. Acknowledgement: Research at VCU supported by VCU College of Engineering start-up funds and NSF-MRI Grant (1726617). Research in Ames (AL) supported by the M.S.E.D., Office of Basic Energy Sciences of the US DOE. AL is operated for the US DOE by Iowa State University under Contract No. DE-AC02-07CH11358.References: °1L. H. Lewis et. al., J. Phys. D: Appl. Phys. vol. 49. 323002 (2016) 2R. Barua, "Pathways for tailoring the magnetostructural response of FeRh-based systems." Ph.D. Dissertation (2014).

Bulk-like First-order Phase Transition in Mechanochemically Fabricated FeRh Particles

The complex interplay between structural, electronic and magnetic degrees of freedom in Mn-based magnetic Heusler compounds gives rise to a great variety of functional properties suitable for sustainable technological applications (energy conversion devices, sensors, actuators, spintronic components…) [1]. The adaptive lattice allows, by changing composition, the development of different crystallographic and magnetic structures, thus offering the possibility to tune properties and effects. Great efforts have been devoted in the last decades to investigate these materials by combining various experimental techniques and first-principles calculations, nonetheless a comprehensive and coherent description, able to drive the full control of Heusler properties, it is far from being achieved. In this work, we combine bulk-sensitive magnetometry, neutron diffraction and 55Mn nuclear magnetic resonance experiments with first-principles calculations to decouple and clarify the different contributions that define the ferromagnetic structure of off-stoichiometric NiMn-based Heusler compounds. We selected, as test material, the Ni48Mn36In16-xSnx series [2], showing a stable cubic austenitic phase down to 5 K, with a Curie transition slightly above room temperature. The replacement of In with Sn leads to a decrease of the saturation magnetization whereas the Curie temperature is featured by a non-monotonic variation (Fig. 1). First-principles calculations suggested the intrinsic nature of the behaviours of the critical temperature, which turns out to be related to the electronic properties. On the contrary, neutron diffraction and NMR experiments revealed that the decrease of magnetization by increasing the Sn content is caused by the disordering of the Mn magnetic moments sited in the mixed sublattice. The progressive replacement of In by Sn leads to a decrease of ordering temperature of Mn in the mixed site (Fig. 2), due to the destabilization of the complex magnetic interactions.References: [1] T. Graf, C. Felser and S. S. Parkin, Prog. Solid. State Ch., 39, 1-50 (2011). [2] G. Cavazzini, F. Cugini, M. E. Gruner, et al., Scripta Mater., 170, 48–51 (2019).

Decoupling of Magnetic Sublattices in NiMn-based Heusler Compounds

In a recent paper [1], the authors have evidenced how the thermally activated demagnetization processes taking place in magnetite nanoparticles (NPs) play a major role on deteriorating the dc coercivity values measured in these samples at temperatures above ca. 150 K. The mechanisms for that deterioration are the increase with the temperature of both the fluctuation field and the activation volume, the latter reaching at room temperature values of up more than 40 times the average particle volume value. We aim here at the quantitative evaluation of the inter-particles interactions underlying that phenomenology and, particularly, at the identification of the role played by the nanoparticles morphology. For that purpose, we have measured magnetite NPs prepared via oxidative precipitation by controlling their size through the nature of the base and nitrate salt and the ethanol content in the media, and their morphology (either octahedral or spherical) by changing the final base concentration [2]. Particle size, shape and crystal structure were determined by transmission electron microscopy (TEM), high-resolution electron microscopy (HRTEM), and X-rays diffraction (XRD), respectively. The magnetic properties of were measured by means of a vibrating sample magnetometer, working in the temperature range from 5 K up to 290 K by applying a maximum field of 9 T. Figure 1 displays the temperature dependence of the reduced saturation remanence (Mr/MS) clearly showing that in both morphologies, but to a larger extent in the spherical NPs, the measured isotropic powder exhibits reduced remanences below 0.5, which suggests the occurrence of demagnetizing interactions. The actual occurrence of that type of interactions was confirmed from the measurement (at 5 K, 125 K and 200 K) of the field dependencies of the isothermal and demagnetization remanences of the samples and the evaluation of the associated Henkel plots. The differences identified in those plots between the spherical and octahedral NPs are discussed in terms of the particle aggregation into close-flux structures.References: [1] E. Navarro, Y. Luengo, S. Veintemillas, M. P. Morales, F. Javier Palomares, U. Urdiroz, F. Cebollada, and J. M. González; AIP Advances 9, 125143 (2019) [2] Y. Luengo, M. P. Morales, L. Gutiérrez, S. Veintemillas-Verdaguer, J. Mater. Chem. C, 4, 9482-9488 (2016)

Temperature Dependence of the Magnetic Interactions Taking Place in Monodisperse Magnetite Nanoparticles Having Different Morphologies

Magnetic tunnel junction (MTJ) is a promising candidate for the development of ultrasensitive magnetic field sensor (TMR sensor) capable to detect extremely weak magnetic fields (0.5) opening the possibilities for biomedical applications1. The performance of these type of sensors is limited by the high low-frequency noise (1/f) which appears due to the presence of magnetic domain fluctuations or by defects inside the insulator barrier2. In this work, we study the normalization and voltage bias effect on the detection limit in TMR sensors based in MTJs. The multilayer stack is Ta(5)/SyF1/MgO/SyF2/Ta(10), where the synthetic ferrimagnet structures are SyF1: PtMn(25)/CoFe(2.3)/Ru(0.83)/CoFe(1)/Ta(0.1)/CoFeB(1.5) and SyF2: CoFeB(1.5)/Ta(0.1)/CoFe(1)/Ru(2.6)/CoFe(2)/PtMn(16) (nanometers). Samples were processes using optical UV lithography to define the diameter (D) and number of MTJs circular pillars in serie (N) inside the sensor. The RA product was typically around 13-20kΩμm2 at 0 mT and the TMR 200 % at room temperature. In order to provide a good interpretation of the results we propose a theoretical model to extract the real output voltage noise from standard electronic circuits. This model was used on a ¼ Wheatstone bridge setup, where the resistive elements were modelled as fluctuating resistances. Our TMR sensors show a sensitivity of 4 %/mT (<0.1%/mT) at 0mT (P state at -100mT)3. The 1/f noise was quantified by the Hooge-like parameter, which allow to compare the 1/f noise of different types of TMR sensors. We observed that well known equation works only below a threshold voltage, due to the non-linearity of the I-V curve related maybe to different symmetry conduction bands. At low bias voltage, devices show higher values at 0 mT (3.10-8µm2) than in the P state (2–4.10-9µm2). An interesting feature for applications is the almost constant detectivity (100nT/Hz0.5 at 10Hz) as a function of the bias voltage. This is a result of the compromise between the output noise voltage and the sensitivity. The subsequent output power and detection limit normalization allow us to design new TMR sensors with optimal conditions.References: [1] S. Cardoso, D. C. Leitao, T. M. Dias, J. Valadeiro, M. D. Silva, A. Chicharo, V. Silverio, J. Gaspar, and P. P. Freitas, J. Phys. D: Appl. Phys. 50, 213001 (2017). [2] J. Scola, H. Polovy, C. Fermon, M. P. Lecoeur, G. Feng, K. Fahy, and J. M. D. Coey. Appl., Phys. Lett., vol. 90, p. 252501, 2007 [3] J. M. Almeida, P. Wisniowski, and P. P. Freitas IEEE Trans Mag, VOL. 44, NO. 11, (2008)

Detection limit normalization and study of TMR magnetic field sensors

Soft magnetic materials showing a high tunnel magnetoresistance (TMR) effect are needed for improving the sensitivity of magnetic tunnel junction (MTJ) based sensors. We focused the Sendust alloy (Fe85Al5.4Si9.6 wt.%, hereinafter, this is called “FeAlSi”), which has soft magnetic property in D03-ordered structure. FeAlSi electrode is also expected to show a high TMR ratio by Δ1 electron coherent tunneling through MgO barriers as well as Fe electrodes with similar crystal structure to FeAlSi [1]. In previous work [2], we firstly succeeded to fabricate epitaxial D03-FeAlSi films with thickness of 30 nm on MgO substrates. Additionally, we observed good soft magnetic properties comparable to other free layer materials such as NiFe [3]. The purpose of this study is fabrication of MTJs using FeAlSi free layer and investigation of their TMR properties. All films were deposited on Si/SiO2 substrates using magnetron sputtering. The stacking structure for the characterization of magnetic and structural properties was Si/SiO2/Ta(5)/CoFeB(5)/MgO(20)/FeAlSi(30)/Ta(5)(in nm). TMR properties were investigated in the MTJs with stacking structure of Si/SiO2/Ta(5)/CoFeB(5)/MgO(20)/FeAlSi(30)/MgO(2)/CoFeB(3)/Ru(0.85)/CoFe(5)/IrMn(10)/Ta(5)/Ru(10). Crystal structure, magnetic properties, and TMR properties were measured by X-ray diffraction (XRD), vibrating sample magnetometer (VSM), and 4-probe DC measurement. Fig. 1 shows the XRD patterns for FeAlSi films prepared by optimal condition. A fundamental peak of FeAlSi (004) and superlattice peak of FeAlSi (002) from B2-ordered structure were clearly observed. The XRD result indicates that the FeAlSi film grown on SiO2 substrates with an MgO underlayer has (001)-orientation and contains B2-ordered structure. Fig. 2 shows magnetoresistance curve for MTJs with FeAlSi bottom electrode. We firstly observed TMR effect of 18.1% at RT using FeAlSi electrode with relatively small switching field. Although the further improvement of TMR effect is needed, the prepared FeAlSi thin film with soft magnetic property is a promising candidate for the free layer of MTJ based sensors with high sensitivity.References: [1] W. H. Butler et al., Phys. Rev. B, Vol. 63, p.054416 (2001). [2] S. Akamatsu et al., AIP Advances 10, 0 15302 (2020). [3] K. Fujiwara et al., Jpn. J. Appl. Phys., Vol. 52, p.04CM07 (2013).

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Giant anisotropy of the magnetocaloric effect in the orthovanadate TbVO4 single crystals

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