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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).

MMM 2020

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

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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poster

A- and B-site ordered quadruple perovskites with the chemical formula AA'3B2B'2O12 can form with a 1:3 ratio at the A site. A unique feature for this specially ordered perovskite is that three different atomic sites (A', B, and B' sites) can all accommodate magnetic transition metals. As a consequence, multiple magnetic interactions can occur at A', B and/or B' sites, giving rise to a series of intriguing physical phenomena. CaCu3Fe2Re2O12 is a good example, which shows the half-metallic electronic structure, large magnetization, and very high Curie temperature [1]. Here we have investigated a series of ferrimagnetic compounds ACu3Fe2Re2O12(A=Ca, Sr, Ba, Pb, Sc, Y, La) by using density functional theory and Monte Carlo simulations. We found that all compounds with A2+ ions exhibit high Curie temperatures (above 405 K), and the compounds with A3+substitution yield even higher Tc (above 502 K). By examining interatomic exchange parameters, we found that the antiferromagnetic exchange couplings between Re and Cu as well as Re and Fe are responsible for this very high Curie temperature. For the compounds with A3+ substitution, an electron doping at the band around the Fermi level dominated by the Re ions strengthen the Re-Cu, Re-Fe and Re-Re exchange interactions, which cause an increase in the critical temperatures. In summary, this work demonstrates a design strategy of enhancing spin ordering temperatures by replacing A-site non-magnetic ions.References: 1. Chen, Wei-tin, Masaichiro Mizumaki, Hayato Seki, Mark S. Senn, Takashi Saito, Daisuke Kan, J. Paul Attfield, and Yuichi Shimakawa. "A half-metallic A-and B-site-ordered quadruple perovskite oxide CaCu3Fe2Re2O12 with large magnetization and a high transition temperature." Nature Communications 5, no. 1 (2014): 1-7.

Electronic structure and magnetism of half-metallic quadruple perovskite ACu3Fe2Re2O12(A=Ca, Sr, Ba, Pb, Sc, Y, La) by first principles theory

Rare-earth orthochromites (RCrO3) with orthorhombically distorted perovskite (ABO3) structure exhibit a wealth of magnetic phenomena such as temperature-induced magnetization reversal (TMR), spin-reorientation (SR), spin-flipping (SF), and exchange-bias (EB). These occur as a result of magnetic interactions among the cations such as Cr3+–Cr3+, Cr3+–R3+ and R3+–R3+, where R is the rare earth element, such as Gd, Sm, Tm [1]. Rare-earth orthochromites also form a class of magnetoelectric multiferroics which exhibit the coexistence of ferroelectric and magnetic orders [1]. Single crystalline GdCrO3, exhibit a giant magnetic entropy change (ΔSm) at a reasonable magnetic field, making it a potential candidate for the application in magnetic refrigeration [2]. TMR and EB have been reported in several RCrO3 compounds due to the competition between the R3+ moment and the canted weak ferromagnetic (FM) component of Cr3+ ions [3]. In the present work, GdCrO4 samples were obtained using sol-gel technique [4]. X-ray diffraction and Raman techniques were used to identify the phase of the samples. Calcination of the samples at 600 °C led to GdCrO4 phase formation [5]. Further calcination of the samples at 1000 °C led to the decomposition of GdCrO4 into GdCrO3. Upon heat treatment, the Cr5+ oxidation state in the GdCrO4-zircon phase reduces to the relatively stable Cr3+ together with oxygen loss to stabilize the GdCrO3 perovskite-structure [5]. The role of thermal decomposition of GdCrO4 to GdCrO3 on crystal structure and magnetic transitions, studied using XRD and vibrating sample magnetometer (VSM), will be discussed. Magnetization measurements as a function of temperature (M-T) with different probing magnetic fields were carried out to locate the various magnetic transitions in the samples under different measurement protocols such as zero field cooled (ZFC), field cool cooling (FCC) and field cool warming (FCW). Anomalies in the M-T curves observed at T ≈ 10 K and T ≈ 170 K correspond to spin - flip and Néel transitions, respectively. This is in agreement with previously reported values [1]. The hysteresis loops measured across the transition temperatures validate the magnetic transitions as observed in the M-T curves.References: [1] S. Mahana, U. Manju and D. Topwal, J. Phys. D: Appl. Phys. 51 (2018) 305002. [2] L. Yin, J. Yang, X. Kan, W. Song, J. Dai and Y. Sun, J. Appl. Phys. 117 (2015)133901. [3] B.B. Dash and S. Ravi, Solid State Sciences 83 (2018) 192. [4] P. Mohanty, A.R.E. Prinsloo, B.P. Doyle, E. Carleschi, C.J. Sheppard, AIP Advances 8 (2018) 056424. [5] A.J. Dossantos-Garcia, E. Climent-Pascual, J.M. Gallardo-Amores, M.G. Rabie, Y. Doi, J. Romero de Paz, B. Beuneu, R. Saez-Puche, Journal of Solid State Chemistry 194 (2012) 119.

Thermal Decomposition of GdCrO4 to GdCrO3: Structure and Magnetism

We present the results of density functional calculations of the proposed low temperature charge quadrupolar ordering in Ba2MgReO6. Ba2MgReO6 is a spin-orbit-driven Mott insulator with a symmetry-lowering structural phase transition at 33 K and a canted antiferromagnetic ordering of the small (around 0.25 μB) Re magnetic moments at 18 K. The goal of our calculations is to test the existence of the quadrupolar order, and to understand its relationship to the structural and magnetic orders, which we achieve by separately isolating the ordering of the charge, spins, orbital moments, and structure. We find that the charge quadrupole ordering exists, and can be realized in the absence of magnetic ordering. In addition to shedding light on the detailed behavior of Ba2MgReO6, our work provides insight into the intriguing physics of 5d1 double perovskites in general, and illustrates a method for the analysis of magnetic ordering interactions combined with higher orders such as charge quadrupoles.

Hidden Multipolar Orders in 5d1 Ba2MgReO6

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

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Electronic structure and magnetism of half-metallic quadruple perovskite ACu3Fe2Re2O12(A=Ca, Sr, Ba, Pb, Sc, Y, La) by first principles theory

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GMW Best Poster Award 3 - Electroresistivity, magnetic and galvanomagnetic properties of Co-based Heusler alloys in the states of half-metallic ferromagnet and spin gapless semiconductor