United States

Thermomagnetic materials are a new type of magnetic energy materials, which enable the conversion of low temperature waste heat to electricity by three routes: Thermomagnetic motors, oscillators, and generators. Taking our recent work on thermomagnetic generators with different magnetic field topologies (TMG, Fig. 1 and 2) [1] as a starting point, in this talk we also analyse the material requirements for a more energy and economic efficient conversion. We present experiments of our TMG using different thermomagnetic materials (La-Fe-Si-Co with different transition temperatures and Gd). We compare the suitability of these materials and from this derive a generalized approach to evaluate the suitability of different thermomagnetic materials, which is different compared to magnetocaloric materials. We show that the thermodynamic efficiency of the best thermomagnetic materials outperforms thermoelectric materials when harvesting low temperature waste heat. Furthermore, the low raw materials cost results in a price per watt more than one order of magnitude lower than today’s power plants and thermoelectrics, which makes them also economically competitive. As summary, we present a materials library, which allows selecting the best available thermomagnetic materials in two Ashby plots as figures of merit and gives guidelines for future development of this material class [2]. At the end of this talk, approaches towards micro systems for harvesting low temperature waste heat are described.References: [1] A. Waske et al. Nature Energy 4, 68 (2019) [2] D. Dzekan et al, arXiv: 2001.03375 (2020)

MMM 2020

Thermomagnetic harvesting of low temperature waste heat INVITED

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

Theoretical prediction and rare experimental observation of room temperature ferromagnetism in transition metal doped semiconductors/insulators such as ZnO, TiO2, SrTiO3 and BaTiO3 has been of key fundamental interest in understanding materials [1-3]. In this work, 5 mol% of Mn doped SrTiO3 was synthesized by chemical coprecipitation [4]. This synthesis technique has been adopted specifically to ensure the most homogeneous doping of Mn in SrTiO3 lattice thus largely preventing the formation of any other impurity phases. Analysis of high resolution XRD data collected for the sample sintered at 1150°C revealed that the phase was of high purity. Magnetization measurements were carried out on the same sample where the variation of Magnetization (M) with temperature under both zero field cooled (ZFC) and field cooled conditions (FC) were measured under an applied magnetic field (H) of 100 Oe. The trend in ZFC is quite contrary to the expected paramagnetic nature in this system. The increase in ZFC magnetization values when the temperature was increased beyond 150 K has to be taken into consideration [Fig. 1]. Also the dispersion in the ZFC and FC curves is quite significant near room temperature and there is a difference of an order in the ZFC and the FC magnetization values around 300 K unlike conventional paramagnetic systems. Therefore the temperature dependent magnetization data (>150 K) clearly exhibits the sign for an impending ferromagnetic behavior around RT. The M versus H behavior was studied in the -1.5 T to 1.5 T range at three different temperatures - 10 K, 150 K and 300 K. Saturated hysteresis loop observed at RT corroborates our observation of the bifurcation observed in ZFC/FC behavior around 300 K [Fig.2]. At 10 K, hysteresis loop turns to look more paramagnetic-like indicating the presence of only a weak ferromagnetic ordering with a strong paramagnetic matrix. A suitable mechanism based on defects induced by Mn doping in SrTiO3 will be discussed to explain this observation of RT Ferromagnetic behavior [5].References: K. Ueda, H. Tabata and T. Kawai, Appl. Phys. Lett. 79, 988 (2001) J.-Y. Kim, J.-H. Park, H.-J. Noh, Phys. Rev.Lett. 90, 017401 (2003) M. Valant, T. Kolodiazhnyi, I. Arčon, Adv. Funct. Mater. 22, 2114 (2012) Preethi Meher K.R.S, Christine B, Pierre-Eymeric Janolin and K.B.R. Varma, Journal of Solid state Chemistry, 192, 296 (2012) R. Ishikawa, Y. Shimbo, I. Sugiyama, Phys Rev B, 96, 024440 (2017)

Clinching experimental evidence of "Room Temperature Ferromagnetic Behavior" in 5 mol% Mn doped SrTiO3

Rare-earth-lean permanent magnetic materials play a key role in many technology sectors like spintronics, energy transport, aerospace, and biomedicine. Structure and composition are likely to control material properties such as magnetic moment and coercivity. Here, we study thin-film libraries of Sm-Co-Ti, fabricated on Si wafer by combinatorial DC magnetron sputtering, with a focus on magnetic and structural properties over wide ranges of composition and thickness. The samples had a thickness ranging from 370 to 450 Å and SmxCoyTi100-x-y alloy compositions with x = 10-25 at% and y = 65-86 at%, as estimated by MATLAB theoretical calculations and subsequently confirmed by Rutherford backscattering spectroscopy. Structural properties were investigated by GIXRD, confirming all samples to be amorphous. MOKE, VSM, and SQUID magnetometry were used to investigate the magnetic properties. In-plane magnetic hysteresis loops show a coercive field of 10-250 mT and a saturation magnetization of approximately 800 kA/m in the full thin libraries. RT-MOKE measurements confirmed that the addition of Ti to the SmCo alloy makes it likely to obtain greater coercivity (~180 mT) as compared to SmCo (~100 mT) [1] while reducing the necessity for a high Sm content. Fig 1 shows representative in-plane and out-of-plane hysteresis loop measured with VSM and In-plane coercivity measurement along the center stipe of the libraries. The Sm-Co gradient is along this stripe, and the Ti content is roughly constant (~7 at%).References: [1] Magnus, F., et al (2013). Tunable giant magnetic anisotropy in amorphous SmCo thin films. Applied Physics Letters, 102(16), 162 402.

Combinatorial study of Sm-Co-Ti amorphous hard magnet thin films

The rare-earth (RE) supply crisis around 2010/2011 led to a drastic increase in prices and revealed the dependency on critical RE elements [1] for the production of high-performance permanent magnets. Significant efforts have been made since in the search for new material systems which present magnetic properties similar or superior to those of benchmark Nd2Fe14B, but which contain less critical elements [2]. One of such alternatives are Nd- and Sm-based intermetallic compounds with a tetragonal ThMn12-type crystal structure, exhibiting intrinsic magnetic properties comparable to Nd2Fe14B [3-6]. In this study, we carried out advanced characterization of grain boundaries of polycrystalline bulk SmFe11Ti samples. Detailed microstructural analysis was carried out using HR-SEM coupled with TEM and APT. Additionally, for the determination of the magnetic properties room-temperature isothermal magnetization measurements were carried out. To get a better understanding of the demagnetization behavior of the samples, in-situ demagnetization measurements were done using MOKE microscopy. The EDX analysis results indicate that the chemical composition of the suction casted and annealed sample is slightly off-stoichiometric SmFe11.1Ti0.9. The APT measurements revealed a distinct composition of grain boundaries compared to the adjoining 1:12 matrix grains where a grain boundary composition with more than 75 at. % of iron was measured. Additionally, three different phases, namely, monoclinic Sm3(Fe,Ti)29, cubic SmFe2-, and Fe2Ti-Laves phases, all form preferably at the grain boundaries and at triple junctions. The MOKE measurements reveal a strong ferromagnetic coupling of the 1:12 grains across the grain boundaries. Based on these results, we assume that the grain boundaries, with more than 75 at% of iron, behave like ferromagnetic regions and therefore no magnetic isolation is observed at the grain boundaries. Financial support of the future pioneering program Development of Magnetic Material Technology for High-Efficiency Motors (MagHEM) commissioned by the New Energy and Industrial Technology Development Organization (NEDO) of Japan and the DFG – Project-ID 405553726 – TRR 270 are gratefully acknowledged.References: [1] O. Gutfleisch et al., Adv. Mat. 23 (2011) 821. [2] K.P. Skokov and O. Gutfleisch, Scr. Mat. 154 (2018) 289. [3] K.H.J. Buschow, Journal of Appl. Phys. 63 (1988) 3130. [4] D.B. De Mooij and K.H.J. Buschow, J. Less Common Met. 136 (1988) 207. [5] F. Maccari et al., Acta Mat. 180 (2019) 15-23. [6] D. Palanisamy et al., Phys. Rev. Mat. 4 (2020) 054404.

Grain boundary segregation, phase formation, and their influence on the coercivity of rapidly solidified SmFe11Ti hard magnetic alloys

Itinerant Contribution to the Anisotropy of Rare-Earth Transition-Metal Permanent Magnets

One of the most intriguing outcomes of symmetry breaking and spin-obit interactions in magnetic systems is the possibility to create non-collinear and chiral spin textures. The Dzyaloshinskii-Moriya interaction (DMI) results from strong spin-orbit coupling and broken inversion symmetry to generate magnetization rotations with fixed chirality. The discovery of magnetic skyrmions originating from strong DMI in metal thin films has led to an explosion of efforts to manipulate magnetic phases originating from interfaces. We have studied interface-induced magnetism in epitaxial 3d/5d iridate/manganite superlattices. In LaMnO3/SrIrO3 superlattices, we find a large additional topological Hall effect arising from the interaction of charge carriers with a noncoplanar chiral spin texture induced by interfacial DMI [1]. I will describe how the interfacial atomic layer stacking and symmetry enabled by the nonmagnetic A-sites determine the competition between collinear and chiral magnetic interactions originating from the oxide interface. Our findings [2] provide insight to the manipulation of chiral magnetism from atomic-scale control of exchange interactions at oxide interfaces.References: [1] P. Bruno et al., Phys. Rev. Lett. 93, 096806 (2004). [2] E. Skoropata et al., Sci. Adv., 6, eeaz3902 (2020).

Interfacial tuning of chiral magnetic interactions for large topological Hall effects in LaMnO3/SrIrO3 heterostructures INVITED

Spin Torque Oscillators (STOs) are promising solutions in a wide variety of next generation technologies from read-head sensors in high-density magnetic recording arrays [1] to neural oscillator units for neuromorphic computing [2-3]. There are several metrics that can be used to quantify the performance of an STO such as power, quality factor, frequency tunability, etc., most of which are dependent on the design of the STO device itself. Furthermore, determining the most important metric will be contingent on its desired application, meaning that it is crucial to understand how the STOs design parameters influence all aspects of its performance so that its design can be optimized to perform the desired function. Our previous studies have investigated the influence of STO size [4] and shape anisotropy [5] on precession frequency and linewidth. In this work, we expand on our previous research and study the influence of device size and shape on several STO performance metrics and use correlation coefficients to quantify relative magnitude of these effects. This was done by analyzing spin torque oscillations generated from 20 magnetic tunnel junctions with in-plane anisotropy and patterned into elliptical nano-pillars with a wide range of sizes and aspect ratios. For each device, we acquired 20 to 50 data sets at various bias fields and currents and obtained power spectral density (PSD) plots for each set, examples of which are shown in Fig. 1a. Each PSD plot was used to measure frequency and linewidth (as illustrated in Fig. 1b) as well as output power. From these measurements, we then calculated quality factor and power-to-linewidth ratio for each set. We also analyzed each STOs performance in terms of the bias fields and bias currents required to maximize output power and signal quality as well as the frequency tunability with both field and current. All of these performance metrics were compared between all 20 STOs tested.References: [1] H. Suto et al., “Real-Time Measurement of Temporal Response of a Spin-Torque Oscillator to Magnetic Pulses”, Appl. Phys. Express, vol. 4, p. 013003, Dec. 2010, doi: 10.1143/APEX.4.013003. [2] D. E. Nikonov et al., “Coupled-Oscillator Associative Memory Array Operation for Pattern Recognition”, IEEE J. Explor. Solid-State Comput. Devices Circuits, vol. 1, pp. 85–93, Nov. 2015, doi: 10.1109/JXCDC.2015.2504049. [3] M. R. Pufall et al., “Physical Implementatin of Coherently Coupled Oscillator Networks”, IEEE J. Explor. Solid-State Comput. Devices Circuits, vol. 1, pp. 76-84, Aug. 2015, doi: 10.1109/JXCDC.2015.2468070. [4] X. Chao, M. Jamali, and J.-P. Wang, “Scaling effect of spin-torque nano-oscillators”, AIP Advances, vol. 7, issue 5, p. 056624 Jan. 2017, doi: 10.1063/1.4974014. [5] X. Chao, M. Jamali, and J.-P. Wang, “Shape anisotropy effects on spin-torque oscillators”, AIP Advances, vol. 10, issue 4, p. 045101, Mar. 2020, doi: 10.1063/1.5130480.

Influence of size and shape on key performance metrics in spin-torque oscillators

INTRODUCTION - This paper proposes a new application of skin effect suppression technology [1]-[5] for long wiring on high-speed & low-delay I/O board.In previous research [6], a lower loss structure of electroplated conductor/magnetic multilayer was clarified by changing thickness of each layer with an electromagnetic field calculation theory for rectangular multilayer transmission line. A challenge in this paper is to clarify the structure with the lowest loss by the method of changing each layer’s thickness. Also an electroplated conductor/magnetic multilayer transmission line will be able to used as a band pass filter, utilizing characteristics of the magnetic material. VERIFICATION - Cu and NiFe were selected as metal conductor material and negative permeability magnetic material, respectively. Permeability and Q factor of NiFe were estimated by the L.L.G. equation. The Cu and NiFe films are alternately stacked to form the multilayer, as shown in Fig. 1. An electromagnetic field distribution, which considers the skin effect in a rectangular multilayer transmission line, is derived based on Maxwell's equation. The current density of each layer and the total loss can be obtained by the equations (1) and (2) in Fig. 1. The loss of transmission line whose total thickness is 12.67 µm was compared under the following conditions. 1) A 33 layered structure by a constant ratio, Cu tN = 0.51 µm and NiFe tF = 0.25 µm. 2) A 33 layered optimal design we proposed at 16 GHz. 3) A 17 layered optimal design we proposed at 16 GHz. RESULTS AND DISCUSSION - Fig. 2 shows the frequency characteristics of loss in each model. At 16 GHz, where NiFe has the best Q value, compared to conventional thickness by a constant ratio, it was estimated that the loss is 80% in optimal thickness of 33 layers. In the structure we proposed, the loss increases compared to the conventional one in the outside band of optimal frequency. It can be expected to obtain a steeper filter function utilizing the frequency characteristics of the magnetic material by designing a transmission line we proposed.References: [1] B. Rejaei and M. Vroubel, J. Appl. Phys., Vol. 96, p.6863 (2004) [2] Y. Zhuang, B. Rejaei and H. Schellevis, IEEE Electron Device Lett., Vol. 29, pp.319–321 (2008) [3] M. Yamaguchi, N. Sato and Y. Endo, Proc. 40th Eur. Microw. Conf., pp.1182-1185 (2010) [4] N. Sato, Y. Endo and M. Yamaguchi, J. Appl. Phys., Vol. 111, pp.07A501-1-07501-3 (2012) [5] M. Yamaguchi, T. Yanai and H. Nakayama, IEEE Trans. Magn., Vol. 54, pp.1-5 (2018) [6] Y. Aizawa, H. Nakayama, K. Kubomura, R. Nakamura, and H. Tanaka, AIP Advance, Vol. 10, p.015124 (2020)

Consideration on Lowering Loss design of Skin Effect Suppressed Multi-layer Transmission Line with Positive/Negative (Cu/NiFe) Permeability Materials for High Data-Rate and Low Delay-Time I/O Interface Board

Half-metallic ferromagnets (HMFs) and spin gapless semiconductors (SGSs) are promising materials for spintronics with almost 100% spin polarization of charge carriers. They possess a gap near the Fermi energy for the current carriers with spin down. However, for the opposite spin projection, these materials have a difference: the energy gap is absent in HFMs [1], while it is zero in SGSs. Parameters of the energy gap and, consequently, physical properties can vary quite strongly in different X2YZ Heusler alloys [2, 3, 4], where Y-elements are 3d-transition metals and Z-components are s- and p-elements. Thus, the aim of the work is to follow the changes in the electrical, magnetic and galvanomagnetic properties and to establish their interconnection and basic behavior patterns of the Co2YSi (Y = Ti, V, Cr, Mn, Fe, Co, Ni) and Co2MnZ Heusler alloys (Z = Si, Al, Ge, Sb) in the HMF and/or SGS states. The electrical resistivity was measured in a wide temperature range from 4.2 to 300 K, the field and temperature dependences of magnetic and galvanomagnetic properties being studied too. Significant changes in the values of the coefficients of the normal and anomalous Hall Effect, magnetization, residual resistivity, type, and concentration of current carriers and their mobility were found. At the same time, the correlation between the changes in these electronic and magnetic characteristics depending on the number of valence electrons are observed. The work was performed within the framework of the state assignment of the Ministry of Science and High Education of Russia (the themes “Spin”, No. AAAA-A18-118020290104-2-2 and “Quantum” No. AAAA-A18-118020190095-4) with partial support from the RFBR (project No. 18-02-00739 and 20-32-90065) and the Government of the Russian Federation (Decree No. 211, Contract No. 02.A03.21.0006).References: [1] M.I. Katsnelson, V.Yu. Irkhin, L. Chioncel et al., Rev. Mod. Phys., Vol. 80, p. 315 (2008). [2] V.V. Marchenkov, Yu.A. Perevozchikova, N.I. Kourov et al., J. Magn. Magn. Mat., Vol. 459, p. 211 (2018). [3] Yu.A. Perevozchikova, A.A. Semiannikova, A.N. Domozhirova, P.B. Terentyev, E.B. Marchenkova, E.I. Patrakov, M. Eisterer, P.S. Korenistov and V.V. Marchenkov, Low Temp. Phys., Vol. 45, p. 789 (2019). [4] Yu.A. Perevozchikova, A.A. Semiannikova, P.B. Terentyev, M. Eisterer, P.S. Korenistov and V.V. Marchenkov, Journal of Physics: Conf. Series, Vol. 1389, p. 012110 (2019).

Electroresistivity, magnetic and galvanomagnetic properties of Co-based Heusler alloys in the states of half-metallic ferromagnet and spin gapless semiconductor

Spin transfer electronics or spintronics offers the possibility of enhancement of regular electronic devices [1]. Because of that, the demand for materials that acquire high values of spin polarization is growing. From several groups of materials, half-metallic Co2-based Heusler alloys are well appropriate materials for spintronic devices due to their high Curie temperature, high magnetic moment, and low Gilbert dumping [2]. The majority of Heusler alloys are usually prepared in the form of ribbons, bulk, or thin films. However, due to technological progress, Heusler alloys face a new production challenge in the nano-scale regime. Several publications deal with the preparation of Heusler nanowires, but with the lack of the spin polarization measurements [3]. Magnetic nanowires prepared in the form of dense array offer the possibility for the development of next-level 3D magnetic “racetrack” memory. Here, the information is stored in the domain wall pattern and shift with spin-polarized current [4]. Therefore, it is essential to determine the spin polarization of the material, which is considered for possible spintronic applications. In the present work, Co2FeSn nanowires have been prepared using template-assisted electrochemical deposition. The nanowires have been grown in the pores of Anodic Alumna Oxide membranes. The spin polarization values have been determined using Point-Contact Andreev Reflection spectroscopy. We show that the spin polarization is preserved in the nanoscale, and by creating a contact with the fresh surface, it is possible to achieve high spin polarization values up to 100 %. This research was supported by Slovak Grant Agency VEGA 1/0053/19, Slovak Grant Agency grant number APVV-16-0079 and VVGS-2019-1231.References: [1] J. P. DeGrave, A. L. Schmitt, R.S. Selinsky, Nano Lett., 11, 4431-4437(2011). [2] M. Jourdan, J. Minár, J. Braun, Nat. Commun., 5, 3974 (2014). [3] L. Galdun, V. Vega, Z. Vargova, ACS Appl. Nano Mater., 1, 7066-7074 (2018). [4] S. S. P. Parkin, M. Hayashi, L.Thomas, Magnetic Domain-Wall Racetrack Memory. Science, 320, 190-194 (2008).

Co2FeSn Heusler Nanowires with High Spin Polarization

Interest in ferrimagnetic metals as active elements in spintronic devices is growing due to their intriguing magnetization effects close to compensation [1]. Mostly studied in alloys, the SOTs that arise from the negative exchange interaction attribute to interesting Hall measurements and can be manipulated through control of temperature [1, 2]. Here we demonstrate another unique feature of these systems: temperature-driven switching. We measured the anomalous Hall effect (AHE) in a van der Pauw geometry on a Ta(3nm)/Pt(3nm)/Co(.5nm)/Gd(1.8nm)/Pt(3nm) synthetic ferrimagnet stack, where the antiferromagnetic exchange coupling between the Co and Gd layers forms the ferrimagnet [3]. The observed compensation temperature is between 150 and 155 K as seen in the diverging coercivity of the sample when measuring AHE vs. perpendicular external field (Figure 1). At 115 K, the AHE displays an external field dependent hysteresis with a coercivity around 300, and 1200 Oe at 150 K (Figure 1). Because this is beneath the compensation temperature, the saturation voltage at 1500 Oe is more negative than at -1500 Oe, creating two anomalous Hall voltage states influenced by the net Gd moment. At 155 and 165 K the coercivities are 850 and 250 Oe respectively, yet the saturation voltages are reversed (Figure 1). This indicates the influence of the net Co moment above the compensation temperature, and the divergence of the coercivity between 150 and 155 K. By then applying a static external field, the AHE state can be switched as the temperature of the sample is lowered or raised above the compensation temperature. In Figure 2, a constant external magnetic field of 450 Oe is applied, and the AHE shows temperature dependent coercivity and hysteresis. This confirms that temperature can be used to write a magnetization state of a ferrimagnet, with a memory window between 145 and 156 K in the case of this sample.References: [1] X. Jiang, L. Gao, J. Z. Sun, and S. S. P. Parkin, Phys. Rev. Lett. 97, 217202 (2006). [2] R. Mishra, J. Yu, X. Qiu, Phys. Rev. Lett. 118, 167201 (2017). [3] P. Hansen, C. Clausen, G. Much, J. Appl. Phys. 66, 756 (1989). Funding acknowledgements: B. L. Zink Group: NSF DMR 1709646

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Thermomagnetic harvesting of low temperature waste heat INVITED

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