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After the first observation of the ultrafast demagnetization process, a huge experimental and theoretical effort was devoted to reveal the microscopic mechanism governing the ultrafast optically induced loss of magnetic order in ferromagnetic materials. Almost all experimental fs-demagnetization studies so far employed fs light pulses of 1.55 eV to trigger the magnetization-dynamics. Hence, the role of the photon energy of the exciting light pulse has been largely neglected so far. Therefore, we have implemented an all-optical time-resolved MOKE setup with variable pump photon energy in the range of 1.55 to 3.10 eV. As prototypical system, we investigated the ultrafast demagnetization dynamics of thin Ni films on insulating and conducting substrates. The characteristic parameters of the demagnetization process, i.e., the demagnetization time for various excitation photon energies are compared with simulations applying a temperature-based model. These simulations consider the density of states of Nickel and interface effects to describe the non-equilibrium dynamics of the spin-carrying excited electrons. For metallic substrates, we further discuss the magnitude and timescale of the optically injected spin carriers across the magnetic/non-magnetic interface.References: M. Hofherr, et al., Physical Review B 96, 100403 (2017) U. Bierbrauer, et al., J. Phys.: Condens. Matter 29, 244002 (2017) B.Y. Mueller and B. Rethfeld, Phys. Rev. B 90, 144420 (2014)

MMM 2020

Photon energy dependent fs-demagnetization dynamics of thin Nickel films

The 65th Annual Conference on Magnetism and Magnetic Materials (MMM 2020) was held in a virtual form. The COVID-19 pandemic, with its associated uncertainty about international travel and the ability to gather in large groups, meant that the Steering Committee decided to cancel the face-to-face meeting that had been planned in Palm Beach, Florida November 2-6, 2020.

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technical paper

Voltage-controlled spintronic devices utilizing the spin degree of freedom are desirable for future applications, and may allow energy-efficient information processing. A pure spin current can be created by thermal excitations in magnetic systems via the spin Seebeck effect (SSE). However, controlling such spin currents, by purely electrical means, has been a fundamental challenge. Here, we investigate voltage control of the SSE in an antiferromagnetic insulator. We demonstrate that the SSE response generated in this material can be effectively controlled by applying a bias voltage, owing to the sensitivity of the SSE to the orientation of the magnetic sublattices as well as the existence of magnetoelectric couplings in the present system. Our experimental results on the voltage-controlled switching of magnetic sublattices are explained using a model based on the magnetoelectric effect. Acknowledgments: all work at Argonne was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division. The use of facilities at the Center for Nanoscale Materials, an Office of Science user facility, was supported by the US Department of Energy, Basic Energy Sciences under Contract No. DE-AC02-06CH11357.

Voltage control of magnon spin currents in an antiferromagnetic insulator

Ferrimagnetic thulium iron garnet (Tm3Fe5O12 or TIG) grown on GGG substrate is known to have perpendicular magnetic anisotropy and magnetic compensation [1,2]. We investigated magnetization configuration of bismuth and gallium substituted Tm3Fe5O12 films by anomalous Hall effect (AHE) induced in a proximately coupled Pt film, and by x-ray magnetic circular dichroism (XMCD) between 10-200 K. In the TIG/Pt heterostructure, anomalous Hall hysteresis changes sign first at 135 K and then at around 60 K. Besides, above 135K, three hysteresis loops are formed under small (less than 1T) field sweep and in a wide temperature range (135-200K). XMCD measurement confirms 135 K as the compensation temperature, and shows that there is no magnetic moment flip around 60 K. Hence the sign change of AHE at 60 K is attributed to competition between spin Hall effect and magnetic proximity effect contributions. XMCD result also demonstrates that above compensation point, both Fe and Tm sublattice moment flips three times under field sweep at a fixed temperature, while their magnitude remains unchanged. This behavior is not likely from canted spin phase in ferrimagnet [3] since spin canting typically requires a field much larger than 1 T. The reason for this field-induced sublattice moment reorientation is unclear yet.References: [1] C. Tang et al., Phys. Rev. B 94, 140403 (2016). [2] Q. Shao et al., Phys. Rev. B 99, 104401 (2019). [3] J. Bernasconi and D. Kuse, Phys. Rev. B 3, 811 (1971).

Magnetization configuration of Bi and Ga substituted Tm3Fe5O12 probed by proximate anomalous Hall effect and x-ray magnetic circular dichroism

Magneto-dielectric materials have been recently attracting great attention owing to their significant advantages for miniaturization of antennas.1,2 Precise measurement of permeability and permittivity over a wide frequency range is crucial to develop and evaluate advanced magneto-dielectric materials.3,4 The Nicolson-Ross-Weir (NRW) method is widely used to extract electromagnetic constitutive parameters from measured scattering parameters.5 One of the drawbacks of this method is requiring direct measurement of the actual sample position in the measurement fixture, which leads to uncertainty in the measured permeability and permittivity. In this work, we improve the standard NRW method by removing the need for prior knowledge of sample position. For any isotropic material, the sample position in the measurement fixture can be precisely determined by matching the phasor angles of the reflection coefficients at the sample surfaces. The phasor matching algorithm is verified by two kinds of magneto-dielectric materials, ferrite ceramics and ferrite/polymer composites, measured in two sets of test fixtures of different geometries. The uncertainty is independent of the actual sample position along the measurement cell as shown in Fig. 1. The measured permeability and permittivity spectra of the ferrite sample are plotted in Fig. 2, showing quite small and stable standard deviations at lower frequencies. The experimental results indicate that the improved NRW method reduces the errors in calculated permeability and permittivity by one order of magnitude compared to those obtained by the conventional NRW method. Therefore, the proposed position-independent NRW method is a precise, efficient and statistically robust technique to characterize the electromagnetic properties of magneto-dielectric materials over a wide frequency range.References: 1R. C. Hansen and M. Burke, Microw. Opt. Technol. Lett., Vol. 26, no. 2, p. 75 (2000). 2K. Buell, H. Mosallaei, and K. Sarabandi, IEEE Trans. Microw. Theory Tech., Vol. 54, p. 135 (2006). 3Q. Li, Y. Chen, C. Yu et al., J. Am. Ceram. Soc., Vol. 105, (2020). 4Q. Li, Y. Chen, and V. G. Harris, J. Appl. Phys., Vol. 125, p. 185107 (2019). 5J. R. Baker-Jarvis, Nat. Inst. Stands. Tech. Tech. Note 1341, (1990).

A Position-Independent Approach to Accurate Measurement of Broadband Electromagnetic Constitutive Parameters of Magneto-Dielectric Materials

Magnetic characterization is an undeniable scheme for the identification of magnetic and mechanical properties of materials [1] [2]. More pertinent is the unavoidable urge for localized magnetic monitoring schemes in line with the current growing industrial expectations [3] [4]. Herein, the adequacy and consistency of the magnetic needle probe (MNP) method over the conventional search coil method were concealed by the instrumentation prerequisite coupled to very weak levels of the sensor signals and the limited theoretical investigation on the method, generally considered quite blurry. The first hindrance taken care of by technological upgrades, studies on the basis of the electromagnetic theory from eddy-current distribution in magnetic materials has advanced the interpretation of the MNP method for local magnetic measurements [5] [6]. The objectives of this paper are twofold; it presents an electromagnetic modelling of the MNP method in a rectangular bar from a micro-magnetic perspective and exploits the model to derive the magnetic anhysteretic behaviour of the ferromagnetic materials. In the first objective, the advance theoretical background of the MNP method is modelled for anti-parallel domain structure using Finite Difference (FD) approach and compared to the analytical approach in [5]. Then, the model is used to simulate the reversible stochastic motion of domain walls for a hysteresis free magnetic material. The dependence of the anhysteretic model on the domain width (d) and the ratio of domain width to the thickness (d/c) of a Fe-3%Si sheet tells of the sensitivity of the magnetic microstructure on the magnetic signature. Noticeable hysteresis losses were however observed at smaller domain width translating to a state of high magneto-crystallographic anisotropy where closure domain effects add the resultant magnetization in the magnetic material. A complete account of the domain constituency is envisaged for future studies.References: [1] Y. A. Tene, P. Tsafack, B. Ducharne and al, IEEE Trans. Mag., vol. 55, no. 7, 2019. [2] B. Ducharne, B. Gupta and al, IEEE Trans. on Mag., vol. 54, no. 11, 6202606, Nov. 2018. [3] A. Pulnikov, V. Permiakov and al, J. Magn. Magn. Mater., vol. 47, no. 9, p. 254–255, 2003. [4] K. S. H. Nguedjang, D. Y. A. Tene and al, J. Magn. Magn. Mater., pp. 1-10, 505, 166767, March 2020. [5] T. Yamaguchi, K. Senda and al, J. Physique IV, vol. 8, p. 717–20, 1998. [6] K. Senda, M. Ishida and al, Electr. Eng. Japan, vol. 126, p. 942–9, 1999.

Electromagnetic Modelling of the Magnetic Needle Probe Method from Parallel Domain Structure by FD Approximation

A technique to measure the full magnetic torque vector (three orthogonal torque components) on micro- and nano-scale magnetic materials is introduced. Measurements of a single axis or component of torque are most common and are very useful for hysteresis measurements of thin film structures in particular, where high shape anisotropy yields a near-proportionality of in-plane magnetic moment and the magnetic torque along the perpendicular in-plane axis. More generally, any component of cross-product torque involves a linear combination of perpendicular moment and magnetic susceptibility, yielding the rotational magnetic anisotropy or stiffness of the spin texture in response to torque. The method is demonstrated using a modified, single-paddle silicon-on-insulator resonant torque sensor [1]. The torsion mechanical susceptibilities to x- and z-torques are maximized by clamping the sensor at a single point. Figure 1 shows a completed device supporting a permalloy / cobalt oxide bilayer disk, deposited on a silicon paddle which is supported by a single beam. Mechanically-resonant AC torques are driven by an RF field containing a frequency component for each fundamental torsional mode of the device, and the resulting displacements read out through optical position-sensitive detection. A typical hysteresis measurement obtained as an in-plane DC bias field is swept is shown in Figure 2. The measurements are compared with micromagnetic simulations of the mechanical torque to verify the interpretation of the signals. As an application example, simultaneous observations of hysteresis in the net magnetization along with the field-dependent in-plane anisotropy is highly beneficial for studies of exchange bias.References: [1] Z. Diao et al., J. Vac. Sci. Technol. B., Vol. 31, p.051805 (2013)

Mechanical Torque Vector Measurements for Micromagnets

Among several advantageous, additive manufacturing facilitates rapid prototyping and the realization of objects that cannot be made with traditional manufacturing methods. Recently, additive manufacturing of magnetic structures is being investigated including the inkjet [1], Fused Filament Modelling (FFM) [2], and UV Assisted Direct Write methods [3]. Soft or hard magnetic particles are included in polymer binder to realize objects interesting for magnetic shielding, magnetic flux guiding, or for their permanent magnetic properties. Magnetic characterization of 3D printed samples with a Vibrating Sample Magnetometer (VSM) however is not straightforward, particularly when larger samples are characterized that have a non-zero infill percentage. Determination of the saturation magnetization is difficult because of the lack of calibration standards for such inhomogeneous samples. Therefore, we modeled the sensitivity of a biaxial VSM for different 3D printed samples produced by FFM. The magnetic potential of a distributed coil with finite dimensions was approximated from the first 3 Legendre polynomial terms and used to determine the point dipole sensitivity of a MicroSense VSM employing a Mallinson pickup coil set using an approach similar as described in [4]. The obtained point sensitivity function was used to calculate the sample signal of various 3D printed samples with different infill orientations and infill percentages using Mathematica. The calculations show that the measured signal is strongly affected by the infill percentage and orientation. For 2D shaped samples (1x1x0.05 cm3) with low infill percentages, the signal differs by up to a factor 2 depending on the sample orientation. The calculated signal of cubed samples (1x1x1 cm3) can vary as much as 30% with infill percentage. A Mathematica spreadsheet was developed that allows one to calculate the signal produced by any specific 3D printed sample in a biaxial VSM that employs a Mallinson coil set configuration. This work was supported in part by NSF through an DMR-MRI Grant under Award 1726970.References: [1] M.P. Paranthaman, C.S. Shafer, M. Elliot, JOM 68 (2017) 1978. [2] C. Huber, C. Abert, F. Bruckner, et al., App. Phys. Lett. 109 (2016) 162401. [3] Alan Shen, Xiaoguang Peng, Callum P. Bailey et al., Mater. and Des.183 (2019) 108133. [4] Co-Cr layers for perpendicular magnetic recording, J.P.C. Bernard, C.P.G. Schrauwen, PhD. Thesis University of Twente, ISBN 90-9003454-4.

Effect of Field Orientation and Infill Percentage on the VSM calibration factor of FFM 3D printed samples

Modern inductive magnetometry is very widely used for bulk samples and thin films alike. SQUID-based, superconducting second-order gradiometer magnetometry has become the de facto standard, worldwide. While a decade ago, it was dominated by primarily extraction-style systems, where the sample is moved at relatively long distances (~ base of the gradiometer) and at low frequencies (~ 1 Hz); nowadays, higher frequency (many 10ns of Hz), shorter translation length systems, rapidly gain popularity. The benefits of faster measurements must be paid by a compromise of capability elsewhere: increased sample geometry- and position-dependent systematic errors are two just two examples. Here, the influence of the dominating radial sample offset and radial projection systematic uncertainties is analysed [1] and a new data processing algorithm is advised, which allows for the determination of the radial sample offset, minimizing its impact. The normalized point spread functions (PSF) of the MPMS3 and its predecessor, the MPMS system [2], are shown on Fig. 1. The PSF of the MPMS3 is sharper, larger in amplitude. [3] The change of shape and magnitude of the PSF of the MPMS3 as a function of radial sample offset is also shown on Fig. 1. There is a pronounced amplitude increment and a narrowing of the z-axis carrier, with radial offset. [4] The normalized response as a function of oscillation amplitude is shown on Fig. 2. With our algorithm, it is possible to fit it and extract the radial offset, by means of generalized non-linear least squares regression, with an uncertainty of about 10 %, as illustrated using Monte-Carlo simulated data, on Fig. 2. [5] It is hoped that this algorithm will be utilized by the magnetic community and will help reduce the spread of data in very many experimental situations.References: [1] Based on a direct quadrature integration approach for the computation of the coupled flux. [2] Quantum Design, San Diego, CA, USA. [3] The MPMS3 gradiometer is more sensitive to radial offsets or spread of the moment distribution. [4] This results in a very substantial increase in the second harmonic pickup, > 200 % in the worse case scenario: millimetric radial errors result in systematic errors of 10ns of percent. [5] This leads to a relative uncertainty of about 2-3 % in a general experimental scenario – a rather substantial and welcome error containment.

An Algorithm for Radial Offset Systematic Error Correction in Second-Order Gradiometer-Based Vibrating Sample Magnetometers

The lifetime of magnetic excitations in metals is governed by scattering rates between magnons and electrons. Recent investigations [1, 2] document how magnetization dynamics vary with composition in magnetic metal alloys due to changes in their electronic band structures; e.g. Co0.25Fe0.75 thin-films display low magnetic damping (α ~ 10-4). In my talk, I will present experimental measurements of magnetic damping, ultrafast magnetization dynamics, and thermal transport properties of CoFe magnetic alloys, varying with alloy composition. To explore the magnetization dynamics, I use a time-resolved MOKE set-up to observe ultrafast demagnetization in a few hundred femtoseconds followed by precessional dynamics. Through TDTR measurements, I check if lower electron-magnon scattering rates lead to improved thermal transport. My investigation sheds light on how magnon-electron scattering rates govern dynamics on femtosecond (ultrafast demagnetization), nanosecond (magnetic precession and damping), and microsecond time-scales (thermal transport of energy). *This work was supported by the U.S. Army Research Laboratory and the U.S. Army Research Office under grant # W911NF-18-1-0364.References: [1] Qin, H. J., et al. Nature communications 6 (2015): 6126. [2] Schoen, Martin AW, et al. Nature Physics 12.9 (2016): 839.

Effects of CoFe Alloy Composition on Magnetization Dynamics and Magnetic Thermal Transport

Engineering of magnetic materials for developing better spintronic applications relies on the control of two key parameters: the spin polarization and the Gilbert damping responsible for the spin angular momentum dissipation. Both of them are expected to affect the ultrafast magnetization dynamics occurring on the femtosecond time scale. Here, we use engineered Co2MnAlxSi1-x Heusler compounds grown by molecular beam epitaxy to adjust the degree of spin polarization at the Fermi energy P and investigate how it correlates with the damping. We first demonstrate experimentally that substituting Si by Al in Co2MnAlxSi1-x Heusler compounds allows us to get a tunable spin polarization at EF from ~60% in Co2MnAl to 100% in Co2MnSi, indicating the transition from metallic to half metallic behaviors. Second, a strong correlation between the spin polarization and the Gilbert magnetic damping is established in these films. The damping decreases when increasing the spin polarization from 1.1 10-3 for Co2MnAl (with 63% spin polarization) to an ultra-low value of 4.10-4 for the half-metallic ferromagnet Co2MnSi [1]. This allows us investigating the relation between these two parameters and the ultrafast demagnetization time characterizing the loss of magnetization occurring after femtosecond laser pulse excitation. The demagnetization time is observed to be inversely proportional to 1-P and as a consequence to the magnetic damping, which can be attributed to the similarity of the spin angular momentum dissipation processes responsible for these two effects. Altogether, our high quality Heusler compounds allow controlling the band structure and therefore the channel for spin angular momentum dissipation [2].References: [1] C. Guillemard et al, Phys. Rev. Appl.. 11, 064009 (2019) [2], W. Zhang, C. Guillemard, et al, Advanced Materials, 1908357 (2020)

Engineering Co2MnAlxSi1-x Heusler compounds as a model system to correlate spin polarization, intrinsic Gilbert damping and ultrafast demagnetization

In the previous two decades, important technological advancements have expanded the range of temporal resolution in transmission electron microscopes (TEM). Commercial direct-counting and single-electron detectors have revealed dynamics in the ms-timescale. Laser-actuated photoemission microscopes [1, 2] combined with beam scanning, spatially-parsed large area detectors [3], and sparse-sensing algorithms [4], can now unlock phenomena at the ms to sub-ps timescales. Further optimization of the photoemission stage [5] and beam bunching technologies could potentially extend the temporal resolution into the deep fs-regime. Following our earlier concept paper [6], we now present the modifications to a pair of commercial instruments – one Schottky (200 keV) [7] and one thermionic (300 keV) [8] that can confer temporal information spanning the ns and ps range with MHz to GHz repetition rates, in the stroboscopic mode without an excitation laser. The key enabling technology is a pair of broadband phase-matched modulating and demodulating RF pulsers. We have demonstrated 11 ps and 30 ps on the 200 keV and 300 keV microscopes respectively. The placement of the pulsers, mounted immediately below the gun, allows for the preservation of all optical configurations otherwise available to the unmodified instrument, and therefore makes these instruments dual-mode, both stroboscopic time-resolved (strobe) mode and conventional continuous waveform (CW) mode. To show that the modifications preserved imaging and diffraction functionalities of the instrument, we obtained Au images and diffraction patterns using both a continuous and a strobed beam. We will also show the first proof-of-principle demonstration of these new instruments through images of RF waves moving though an interdigitated MEMS device.References: [1] VA Lobastov, R Srinivasan, AH. Zewail, Proc. Natl. Acad. Sci. 102 (2005) 7069−7073. [3] T LaGrange, et. al., Appl. Phys. Lett. 89 (2006) 044105 [3] T LaGrange, BW Reed, DJ. Masiel, MRS Bulletin 40 (2015) 22-28 [4] A Stevens, et al., Advanced Structural and Chemical Imaging 1 (2015). [5] DA Plemmons and DJ Flannigan, Chemical Physics Letters 683 (2017) 186–192 [6] J Qiu, et. al., Ultramicroscopy 161 (2016) 130. [7] C Jing, et al., Ultramicroscopy, 207 (2019) 112829. [8] JW Lau, et. al., Review of Scientific Instruments, 91 (2020) 021301. https://doi.org/10.1063/1.5131758 [*] We are grateful for the invaluable assistance from JEOL USA. This work was supported by DOE BES SBIR program Grant no. DE-SC0013121 and NIST award SB1341-16-CN-0035. During this work, K. Schliep was supported by the Research Associateship Program of the National Research Council and M. Katz was partially supported by the Professional Research Experience Program at the University of Maryland. Work at BNL was supported by the US DOE-BES, MSED, under Contract No. DESC0012704.

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