United States

Mn2RuxGa (MRG) is a half-metallic compensated ferrimagnet crystallising in a cubic inverse Heusler structure (XA) with a substrate-induced biaxial strain of about 1% leading to perpendicular magnetic anisotropy of the net moment. The Mn atoms occupy two different crystallographic sites, Wyckoff positions 4a and 4c. [1] From band structure calculations we find that the states originating from the Mn4c sites dominate the transport. Anomalous Hall effect (AHE) resistance is proportional to the z-component of the magnetisation of the Mn4c sublattice. [2] The AHE measurements show loops with high remanence and only one magnetic phase, with coercivity diverging close to compensation. SQUID magnetometry measures the net moment, i.e. the sum of Mn4a and Mn4c sublattice moments. SQUID-measured magnetisation, in the same geometry as AHE, exhibits a combination of an apparent soft magnetic response in addition to a component with the coercivity observed by AHE. We infer that the net magnetic moment is canted due to non-collinearity of the sublattice moments. Our DFT calculations suggest that the non-collinearity may be due to competing exchange on the Mn4c sites as bulk antisymmetric exchange averages to zero in the -43m point group [3]. The Mn4c sublattice and net magnetisations differ by an order of magnitude, so a small canting of the Mn4c moment can result in a large canting of the net moment. X-ray magnetic circular dichroism measurements were performed to map sublattice magnetisations in the z-x plane. Both sublattices can be distinguished by L-edge absorption due to charge-transfer separation of the Mn absorption edges at the two sites by ≈ 1 eV. [2] Analysis of the mz-mx moments shows that they are indeed non-collinear (Fig.1). From balancing of torques, anisotropy fields at the Mn4a and Mn4c sublattices were found to be easy-plane -1.8 T and easy-axis 3 T, respectively (Fig.2.).References: [1] H. Kurt, K. Rode, P. Stamenov, et al., “Cubic Mn2Ga thin films: crossing the spin gap with ruthenium”, Phys. Rev. Lett., 112, p. 27201, 2014. [2] D. Betto, N. Thiyagarajah, Y.-C. Lau, et al., “Site-specific magnetism of half-metallic Mn2RuxGa thin films determined by x-ray absorption spectroscopy”, Phys. Rev. B, 91, p. 094410, 2015. [3] O. Meshcheriakova, S. Chadov, A.K. Nayak, et al. &quot;Large Noncollinearity and Spin Reorientation in the Novel Mn2RhSn Heusler Magnet&quot;, Phys. Rev. Lett. 113, p. 087203, 2014.

MMM 2020

Investigation of non-collinearity in Mn2RuxGa compensated ferrimagnetic half-metallic thin 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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Mn2RuxGa (MRG) is a half-metallic compensated ferrimagnet crystallising in a cubic inverse Heusler structure (XA) with a substrate-induced biaxial strain of about 1% leading to perpendicular magnetic anisotropy of the net moment. The Mn atoms occupy two different crystallographic sites, Wyckoff positions 4a and 4c. [1] From band structure calculations we find that the states originating from the Mn4c sites dominate the transport. Anomalous Hall effect (AHE) resistance is proportional to the z-component of the magnetisation of the Mn4c sublattice. [2] The AHE measurements show loops with high remanence and only one magnetic phase, with coercivity diverging close to compensation. SQUID magnetometry measures the net moment, i.e. the sum of Mn4a and Mn4c sublattice moments. SQUID-measured magnetisation, in the same geometry as AHE, exhibits a combination of an apparent soft magnetic response in addition to a component with the coercivity observed by AHE. We infer that the net magnetic moment is canted due to non-collinearity of the sublattice moments. Our DFT calculations suggest that the non-collinearity may be due to competing exchange on the Mn4c sites as bulk antisymmetric exchange averages to zero in the -43m point group [3]. The Mn4c sublattice and net magnetisations differ by an order of magnitude, so a small canting of the Mn4c moment can result in a large canting of the net moment. X-ray magnetic circular dichroism measurements were performed to map sublattice magnetisations in the z-x plane. Both sublattices can be distinguished by L-edge absorption due to charge-transfer separation of the Mn absorption edges at the two sites by ≈ 1 eV. [2] Analysis of the mz-mx moments shows that they are indeed non-collinear (Fig.1). From balancing of torques, anisotropy fields at the Mn4a and Mn4c sublattices were found to be easy-plane -1.8 T and easy-axis 3 T, respectively (Fig.2.).References: [1] H. Kurt, K. Rode, P. Stamenov, et al., “Cubic Mn2Ga thin films: crossing the spin gap with ruthenium”, Phys. Rev. Lett., 112, p. 27201, 2014. [2] D. Betto, N. Thiyagarajah, Y.-C. Lau, et al., “Site-specific magnetism of half-metallic Mn2RuxGa thin films determined by x-ray absorption spectroscopy”, Phys. Rev. B, 91, p. 094410, 2015. [3] O. Meshcheriakova, S. Chadov, A.K. Nayak, et al. "Large Noncollinearity and Spin Reorientation in the Novel Mn2RhSn Heusler Magnet", Phys. Rev. Lett. 113, p. 087203, 2014.

technical paper

Ferromagnetic materials have played a significant role in magnetic memory devices. However, due to inherent limitations associated with stray field generation and vulnerability to external stimulation, antiferromagnetic materials are increasingly investigated as alternatives. Specifically, metallic tetragonal antiferromagnets with easy-plane magnetism are appealing because of the degenerate domains with 90° difference of Néel vector for binary information storage and the possibility of electrical switching. To understand the switching process, the magnetocrystalline anisotropy is essential, acting as an energy barrier between two degenerate states. In this study, we used density functional theory (DFT) to unravel the origin of the magnetocrystalline anisotropy in Fe2As, one of the metallic tetragonal antiferromagnets with easy-plane magnetism. The tetragonal structure allows two possible anisotropies out-of-plane and in-plane. Here, we calculate the spin-orbit interaction (SOI) in a DFT framework and the magnetic dipole-dipole interaction (MDD) based on the DFT ground state structure using a classical model. The computed out-of-plane |K1| anisotropy coefficient is 830 kJ/m3, composed to one-third of MDD and two-thirds of SOI, both of which exhibit the same two-fold symmetry. Conversely, the MDD contribution for in-plane anisotropy is almost imperceptible, leading to an in-plane |K22| anisotropy coefficient of 280 J/m3 with four-fold symmetry, due to SOI. The lowest frequency of a spin-wave is calculated using our anisotropy result, and it is predicted as 0.64 THz. Based on the frequency and anisotropy, the fast dynamics and high speed of switching are anticipated. However, low thermal stability might be observed due to the small in-plane anisotropy barrier. ** The Illinois Materials Research Science and Engineering Center is supported by the National Science Foundation MRSEC program under NSF Award Number DMR-1720633.

Ab initio Study of Magnetocrystalline Anisotropy in Antiferromagnetic Fe2As

The physics of the chiral magnet MnSi touches onto several fundamental questions in condensed-matter physics, from the stabilisation of exotic states like chiral skyrmions [1] to the interplay between localised and itinerant magnetism. Under mechanical pressure, or chemical pressure in the form of Fe doping in Mn1-xFexSi, a non-Fermi liquid behaviour sets-in which has been attributed to a chiral spin liquid and quantum fluctuations [2]. The origin and nature of these phenomena are still an open question, which we address by combining small angle neutron scattering and neutron spin echo spectroscopy [3-5]. Our results show that the destabilisation of the long range helical periodicity, close to the quantum critical points, is driven by the modification of the electronic state but with different microscopic mechanisms for mechanical and chemical pressure. Mechanical pressure enhances the itinerant electron character of MnSi leading to a softening of the magnetic moment. On the other hand, chemical doping increases the weight of achiral RKKY interactions. The resulting frustration explains both the destabilisation of long range helical periodicity and the enhancement of the precursor region with increasing doping.References: 1. S. Mühlbauer et al., Science 323, 915 (2009). 2. C. Pfleiderer, et al., Nature 414, 427 (2001) 3. L. J. Bannenberg et al., Phys. Rev B 100, 054447 (2019). 4. L. J. Bannenberg, et al., Phys. Rev. B 98, 184431 (2018). 5. C. Pappas, et al., in preparation

Helimagnetic correlations close to the Quantum Critical Points of MnSi under mechanical and chemical pressure. INVITED

The ferromagnetic Heusler compound Co2MnGa is a promising Weyl semimetal with the Fermi energy in the vicinity of the Weyl nodes. Bulk Co2MnGa recently attracted attention because of an exceptionally large anomalous Nernst effect [1,2] and exotic surface states [3]. Simultaneously, Co2MnGa thin films were successfully prepared and showed interesting transport properties [4]. Beside others, the non-trivial topology of the band structure of Weyl semimetals leads to unexpected magneto-thermoelectic transport phenomena which will be discussed in this talk. We report on our observation of a record large anomalous Nernst coefficient in Co2MnGa thin films - up to -3µV/K [5, 6], and discuss the procedure for the quantitative determination of the anomalous Nernst coefficient. Several samples with comparable saturated magnetization were studied, yielding significantly different anomalous Nernst responses. We conclude that the microscopic origin of the anomalous Nernst effect in Co2MnGa is complex and contains contributions from the intrinsic Berry phase and surface states. We further employ a series of Co2MnGa thin films to study the validity of the Mott relation by measuring all four transport coefficients - the longitudinal resistivity, anomalous Hall, Seebeck and anomalous Nernst coefficients [6]. We show that the measured anomalous Nernst signal can be connected to the remaining three magneto-thermal transport coefficients in the whole sample series. This is strong evidence that the Mott relation holds also in Weyl semimetals, where the intrinsic Berry curvature dominates the transport. In the last part of the talk we focus on another magneto-transport phenomena and its thermal counterpart – anisotropic magnetoresistance and anisotropic magneto-thermopower. We show that due to high epitaxial quality of the Co2MnGa films both non-crystalline and crystalline contributions to the anisotropic magnetoresistance can be measured. Interestingly, we do not observe the same crystalline contributions in the magneto-thermopower measurements [7].References: [1] S. N. Guin et al., NPG Asia Materials 11, 16 (2019) [2] A. Sakai et al., Nat. Phys. 13, 1119-1124 (2018) [3] I. Belopolski et al., Science 365, 1278-1281 (2019) [4] A. Markou et al., PRB 100, 054422 (2019) [5] H. Reichlova et al., APL 113, 212405 (2018) [6] G. H. Park, H.R. et al. PRB 101, 060406(R) (2020) [7] P. Ritzinger, H.R. et al., in preparation

Magneto-Thermal Transport in Weyl Semimetal Co2MnGa Thin Films INVITED

Magnetic navigation systems (MNSs) capable of generating various types of magnetic fields have been widely studied for the effective manipulation of biomedical magnetic robots [1-4]. Previous researchers proposed a geometrically and electrically efficient MNS comprised of a simple triad of electromagnetic coils (TEC) that can generate two-dimensional (2D) magnetic robot motions [3]. However, the TEC suffered from the restricted working area of the magnetic robot due to the nonlinearly distributed magnetic field throughout the system. In this research, we propose a method to precisely generate 2D magnetic torque and force of a magnetic robot regarding the robot's current position inside the TEC. Since the TEC is composed of three identical circular coils located at the vertices of an equilateral triangle, the magnetic field at a certain position inside the TEC can be expressed in terms of the three coils' currents and a position vector by using Jacobian and transformation matrices [4]. From this expression, we established several constraint equations to optimally manipulate the magnetic robot’s 2D motions at any position within the TEC. Fig. 1 shows several simulated magnetic fields of the TEC to propel a magnetic robot (red dots) in the direction of the x-axis. It shows that the TEC can generate a relatively uniform magnetic force within the nearby area of a magnetic robot, making it possible to move the magnetic robot in the desired direction (Fig. 2). We then constructed the TEC and a closed-loop visual tracking system to demonstrate that the TEC can manipulate a magnetic robot inside the TEC in a closed-loop manner. The results showed that the proposed method and system can improve the magnetic robot’s admissible working area in the TEC effectively and precisely.References: [1] M. Kummer, J. Abbott, and B. E. Kratochvil et al., IEEE Transactions on Robotics, Vol. 26, pp. 1006-1017, 2010. [2] Q. Cao, X. Han, B. Zhang, and L. Li et al., IEEE Transactions on Applied Superconductivity, Vol. 22, 2012. [3] H. Lee and S. Jeon, AIP Advances. Vol. 10, 2020. [4] G. Go, H. Choi, and S. Jeong et al., Sensors and Actuators, Vol. 205, P. 215-223, 2014.

Development of a Position-Based Precise Magnetic Torque and Force Generation Method Using a Triad of Electromagnetic Coils for Biomedical Magnetic Robot Applications

The structural and magnetic properties Fe1-xPtx nanoparticles are highly dependent on the composition x, which can be tailored to suit potential applications such as ultrahigh density storage media, biomedicine and catalysis.[1, 2] Most of the studies so far have been focused on equiatomic Fe-Pt with the highly anisotropic L10 structure. In bulk, FePt3 in the L12 structure is paramagnetic at room temperature and shows two coexisting antiferromagnetic transitions at TN1~160 K and TN2~120 K. [3] Recently, L10 FePt nanoparticles have been chemically synthesized through a halide mediated route without requiring any post annealing. [4, 5]. In this work, we studied the effect of elemental iodine as our halide intermediary in the synthesis of FePt nanoparticles using a co-reduction of Fe(acac)3 and (NH4)2PtCl2 with 1,2-hexadecanediol in trioctylamine and oleylamine. Our study showed that the elemental iodine facilitated primarily the formation of FePt3. When iodine was not used, the as-made nanoparticles had mostly the disordered fcc in structure (Figure 1). Annealing all of the as-made nanoparticles led to an increased particle size and transformation to the L12 ordered structure. The longer the annealing time, the greater the atomic ordering (Figure 1). Thermomagnetic data showed that the magnetic properties depend strongly on the degree of atomic ordering. The as-made particles with the disordered fcc structure are ferromagnetic with an ordering temperature of 380 K. The samples annealed at 700°C for 30 minutes showed a lower ordering temperature of ~250 K and another magnetic transition at 135 K possibly due to an antiferromagnetic structure. Room temperature hysteresis loops showed that the coercivity of our samples increased with annealing at 700°C up to 0.23 kOe. Future work is focused on finding the optimum conditions to obtain a highly ordered and monodispersed L12 FePt3 nanoparticles without the need for any additional heat treatment. This work was supported by DOE Grant FG02-90ER45413.References: [1] Sun, S., Adv. Mater., 18, 393 (2006) [2] Zhu, K., Ju, Y., Yang, Z., Acc. Chem. Res., 51, 404-413 (2018) [3] A.T. Heitsch, D.C. Lee, B.A. Korgel, J. Phys. Chem. C 114, 2512–2518 (2010) [4] W. Lei et al., Nano Lett., 18, 7839-7844 (2018) [5] W. Lei et al., Nanoscale, 9, 12855 (2017)

Structural and Magnetic Properties of Iodide-Mediated Chemically Synthesized FePt3 Nanoparticles

The magnetic properties of hexaferrite nanoparticles critically depend on the intrinsic anisotropy induced by its size as well as the morphology. This study demonstrates the controllable growth of barium hexaferrite (BaFe12O19) platelets using a salt-assisted sol-gel method. We employed salts like KBr, KCl, KI, and K2CO3 as a matrix to assist the formation of BaFe12O19 nanoplatelets from its precursor mixture gamma-Fe2O3 and BaCO3. Within the detection limit of the x-ray diffraction technique, pure barium hexaferrite phases are obtained through the salt-assisted sol-gel process. The reaction of pre-calcined precursors in the presence of potassium-based salts favors a template mechanism, resulting in platelet-shaped barium hexaferrite (fig 1). The salt matrix acts as a barrier, preventing possible particle coarsening at high-temperature treatments and promotes the evolution of the hard magnetic phase. The width and thickness of the plates decrease with the addition of high melting point potassium halide salts. The magnetization and coercivity at 300 K of the barium hexaferrite particles were also systematically analyzed as a function of the dimension of platelets. Barium hexaferrite platelets of 0.20 μm width and 0.07 μm thickness yielded coercivity of the order of 4.6 kOe and saturation magnetization, close to 71 emu/g. Plate-shaped hexaferrite particles could be stacked and aligned by applying an external magnetic field and pressure, which could result in high-performance bulk hexaferrite magnets.References: [1] W. Yongfei, L. Qiaoling, Z. Cunrui, and J. Hongxia, J. Alloys Compd. Vol.467, p.284 (2009). [2] A. Z. Eikeland, M. Stingaciu, A. H. Mamakhel, M. Saura-Múzquiz, and M. Christensen, Sci. Rep. Vol.8, p.7325 (2018).

Size-Dependent Magnetic Properties of Barium Hexaferrite Platelets Synthesized Using a Salt Assisted Sol-Gel Technique

GdMn2 is a member of a series of Laves phase containing a rare-earth element and a magnetic 3d-transition metal with very peculiar magnetic properties[1]. Doping with a non-magnetic element such as indium affects these properties[2,3]. GdMnIn is reported to crystalize in the hexagonal MgNi2-type structure presenting a spin-glass behavior with no magnetic order attributed to the triangular spin frustration of magnetic ions[3]. The observed absence of long-range interactions by magnetization measurements along with the almost impossible measurements with neutron diffraction due to the presence of Gd with very high neutron absorption cross section makes the investigation of local exchange interactions in this compound very difficult. In the present work, measurements of hyperfine interactions at In sites using 111In(111Cd) probe nuclei were carried out by perturbed angular correlations (PAC) technique to investigate the local magnetic exchange in GdMnIn compound. Results displayed in Fig. 1 for the behavior of the major component (Vzz) of the electric field gradient (efg) tensor and the distribution of electric quadrupole frequency (δ) show a strong instability of the electronic environment of In sites when temperature decreases indicating a Mn-In disorder. The weak magnetic hyperfine field at In sites also shown in Fig.1 with an anomalous temperature dependence below around 140 K along with the increase in the angle between Bhf and Vzz directions suggest a weak long-range exchange interaction.References: [1] K. Krop, Physica B 319, 9–16 (2002). [2] S.K. Dhar, C. Mitra, P. Manfrinetti, R. Palenzona, and A. Palenzona, J. Phase Equilibria 23, 79-82 (2002). [3] S. De Negri, D. Kaczorowski, A. Grytsiv, E. Alleno, M. Giovannini, R. Gorzelniak, P. Rogl, C. Godart, A. Saccone, and R. Ferro, J. Alloys Compounds 365, 58–67 (2004).

Local inspection of magnetic properties in GdMnIn by measuring hyperfine interactions

Herein, employing first-principles prediction and rigid-band analysis, we have demonstrated an extremely large uniaxial magnetic anisotropy (Ku) up to 3 MJ m−3 in rare-earth (RE) free magnet α''-phase Fe16N2 by replacing Fe with more valence electron elements, Co to Zn in the periodic table, which is more than 60% of Ku (4.9 MJ m−3) attained for RE-included permanent magnet Nd2Fe14B, while improving thermal stability. Such a supreme Ku can be attributed to the mutual mechanisms of the Jahn-Teller orbital splitting and excess electron-induced energy level changes in the rigid-band selection rule. Moreover, we reveal that the replacement by Ti in addition to the above replacements can improve the thermal stability further by an order of magnitude in terms of the formation enthalpy without much suppressing Ku. These results provide an instructive guideline for simultaneous improvements of the thermal stability and energy product in 3d-only permanent magnets without RE or heavy-metal elements. Keywords: Rare-earth free permanent magnets, First-principle calculations, Uniaxial magnetic anisotropy E-mail: ochirkhuyag.t@inu.ac.kr , *schong@ulsan.ac.kr , †odkhuu@inu.ac.kr This work is supported by Future Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (2016M3D1A1027835) and by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean government (MOTIE) (20192010106850, development of magnetic materials for IE4 class motor).

First-Principles Prediction of the Enhanced Uniaxial Magnetic Anisotropy and Thermal Stability in α''-Fe16N2

Due to remarkable success in many uses today such as motors in an electric vehicle and memory devices, there have been intensive research interests centered at synthesizing a rare earth (RE)-free permanent magnetic material that has a cheap cost for mass production and various methods to improve a uniaxial magnetic anisotropy (Ku), saturation magnetization (µ0Ms) as well as thermal stability of the previously known materials. RE-free L10 ordered FeNi alloys boast with affordable cost, large µ0Ms and also notable Ku owing to their tetragonal structure but has very low stability and degree of ordering. Herein, through first-principles electronic structure calculations, we have studied possible improvements of the structural stability and magnetic properties of L10-FeNi by an interstitial doping with 2p-electron elements (B, C, and N). It is found that the additions of 12.5 at.% B and N enhance Ku up to 0.9 and 1.8 MJ/m3, respectively, which are 35% and about 3 times larger than that (0.66 MJ/m3) of FeNi. More importantly, unlike the C and N dopants, the B greatly improves the stability and reaches Ku of 2.8 MJ/m3 at 25 at.%. These results demonstrate the feasibility of possible enhancements on the magnetic anisotropy and energy product of 3d-only metals for RE-free permanent magnet applications.

Improved Stability and Magnetic Anisotropy in L10 -FeNi by Interstitial Doping: A First-Principles Prediction

Glass-ceramic (GC) materials have formability advantages of glasses controlled crystallization of functional ceramic phases. Here, we obtain GCs containing the hard magnetic phases Ba- or Sr-hexaferrite from a borate glass. Twelve compositions were explored, varying B2O3 and Ba/Fe or Sr/Fe ratio. Selected compositions were heat treated from 30○C to 700-900○C. Resulting phases were identified using X-ray diffraction (XRD) as α-Fe2O3 plus SrB2O4 and SrFe12O19 or their Ba equivalents. Vibrating sample magnetometry (VSM) also identified Fe3O4. The best compositions for hexaferrite GCs contained 40-42 mol% B2O3 and Fe/(Sr,Ba) ratios of 0.5-1. Heat treatments for one Sr glass were performed at 625○C or 675○C to assess phase development at the peak crystallization temperatures determined by thermal analysis, but both treatments yielded only SrB2O4 by XRD; however, coercivity increased from the quenched glass (395 Oe) to 625○C (460 Oe) to 675○C treatment (520 Oe). Selected glasses were studied with in-situ crystallization, measuring magnetic properties during heating to 800○C in the VSM. Hysteresis measurements on initial heating showed predominantly paramagnetism. After cooling and reheating, wasp-waisted loops were observed with Hc > 650 Oe, dropping to 0 at 650○C, with changes occurring 290○C, 490○C (TC of SrFe12O19) and 550○C (TC of Fe3O4 suggesting oxidation despite Ar heating gas). First order reversal curve (FORC) measurements confirmed low and high coercivity phases. These results suggest that crystallization of magnetic phases is most significant on cooling, and that this glass might allow alignment of the anisotropic hexaferrite crystals while heating and cooling with an applied field.

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