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Antiferromagnets are promising materials for spintronic applications due to their unique features such as ultrafast spin dynamics and robustness against external magnetic fields. An efficient way to manipulate the antiferromagnetic (AFM) order is to use spin-orbit torque (SOT), which utilizes the conversion of electric current to spin through the spin-orbit interaction. Current-induced switching of the AFM order has been demonstrated experimentally in several antiferromagnets including Mn2Au. It has a broken sublattice inversion symmetry, which gives rise to opposite spin polarization on each sublattice under an electrical current. The staggered spin polarization results in a non-staggered field-like (FL) SOT, which is suitable for the switching of the AFM order. It is also possible to utilize the damping-like SOT originated from a spin current generated by a heavy metal (HM) layer adjacent to Mn2Au. Here by using first-principles calculations, we calculate the SOTs of a Mn2Au/HM bilayer, where HM is W or Pt. By expanding the SOT using vector spherical harmonics, layer-resolved angular dependence of SOT is obtained. For FL SOT, it has a large contribution from the Mn2Au and a small contribution from the HM. For DL SOT, it has most of the contribution from the HM. FL SOT is non-staggered on each sublattice and it dominates the behavior of spin dynamics. DL SOT is staggered on each sublattice so that its net effect on spin dynamics is negligible.

MMM 2020

Spin-oribit torques in Mn2Au/HM bilayers

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

Electrical writing and readout of antiferromagnetic insulators is a key prerequisite in view of using this class of materials in applications. Only recently, this has been demonstrated in insulating AFM/heavy metal bilayers by utilizing the transverse spin Hall magnetoresistance effect (SMR) as the readout. In the electrical measurement, a steplike signal resulting from a magnetic SMR contribution was reported in NiO1 and α-Fe2O32, while non-magnetic contributions due to artifacts in the heavy metal layer were found to mimic the signal expected from the magnetic switching.3,4 These results have generated a debate on the magnitude of the magnetic contributions, the reliability of the SMR readout mechanism and even on the possibility of switching domains in AFMs electrically. In our work5 we use a table-top approach to demonstrate stable and reversible current-induced switching of large-area antiferromagnetic domains in NiO/Pt by direct imaging in a Kerr microscope.6 Concurrent transport and magneto-optical imaging measurements allow us to correlate the AFM domain switching fraction and magneto-transport signal response. We establish a procedure to subtract the non-magnetic contributions and extract the magnetic SMR response from the transverse resistance signal. The disentanglement of magnetic and non-magnetic contributions clarifies the presence of a significant SMR response, associated with the switching of the antiferromagnetic domains. We thus highlight the possibility to deduce details of the antiferromagnetic switching from simple transport measurements.References: 1 L. Baldrati, O. Gomonay, A. Ross, Phys. Rev. Lett. 123, 177201 (2019). 2 Y. Cheng, S. Yu, M. Zhu, Phys. Rev. Lett. 124, 027202 (2020). 3 A. Churikova, D. Bono, B. Neltner, Appl. Phys. Lett. 116, 022410 (2020). 4 C.C. Chiang, S.Y. Huang, D. Qu, Phys. Rev. Lett. 123, 227203 (2019). 5 F. Schreiber, L. Baldrati, C. Schmitt, Appl. Phys. Lett. 117, 082401 (2020). 6 J. Xu, C. Zhou, M. Jia, Phys. Rev. B 100, 134413 (2019).

Concurrent magneto-optical imaging and magneto-transport readout of electrical switching of insulating antiferromagnetic thin films

Antiferromagnets have gained significant interest in recent years due to their potential application in antiferromagnetic spintronic devices and neuromorphic computing where the antiferromagnet is the active element. A current problem in the development of such devices is a lack of understanding of the magnetic properties of antiferromagnets. Atomistic modelling is a powerful tool in understanding these properties as it has the ability to model the materials in atomistic detail but on a scale comparable to realistic device sizes. Here, an atomistic model [1] is used to model the technologically relevant anti-ferromagnetic material Iridium Manganese to calculate the ground state spin structure and magnetic anisotropy. Previously, only the properties of fully ordered Iridium Manganese alloys or disordered IrMn3 have been investigated due to the complex material anisotropy of disordered crystal structures. Here, we have overcome this problem by modelling the anisotropy using the Néel pair anisotropy model which has the capability to accurately model any order or composition of IrMn [2]. Initially, the model was validated by comparing the results for ordered and disordered IrMn3, against previous experimental and theoretical calculations. The properties of never before theoretically studied compositions and orders of IrMn were calculated with particular focus on the technologically relevant compositions close to IrMn4 - IrMn5 used in realistic devices. It was found that all the technologically relevant alloys of disordered IrMn have the same ground state structure. A full composition study of the fully ordered and fully disordered states was undertaken, and it was found that in both cases the Néel temperature increases as the Mn content is increased, stabilizing the spin structure. These findings are an important step towards understanding the fundamental properties of antiferromagnets for applications.References: [1] R. F.L. Evans, et al, J Phys Cond. Matter, 26, (2014) [2] S. Jenkins, et al Physical Review B, 100, (2019)

Atomistic spin dynamics simulations of Iridium Manganese Alloys

Magnetization switching plays a central role in the field of magnetism-based devices. A concrete example is the case of magnetic textures which play the role of carriers of information. A precise control of their mobility becomes crucial for potential applications such as racetrack memories [1], sensors [2], and logic-devices [3]. Both ferromagnets (FM) and antiferromagnets (AFM) can host multiple types of these textures. While FM have been investigated in depth, only recently a mechanism to excite AFM was proposed based on the fact that there are certain materials in which their magnetic sublattices form inversion partners, being possible to generate a current-induced Néel spin-orbit (SO) field [4]. With this objective in mind, we have evaluated the dynamics of a one-dimensional Néel-like DW in an embedded FM layer of the metallic layered Mn2Au excited through the Néel SO field. To do this, we have combined atomistic spin dynamics simulations of the real crystalline structure together with an analytical model that combines the Lagrangian formalism with the collective coordinates approach. When theoretically formalizing the problem, complications arise to define an unique Néel order parameter due to the complexity of the system. To avoid this, we have introduced a pair of staggered AFM-coupled Néel vectors. Taking this into account, we have evaluated three consecutive correlated SO field-based regions: one of acceleration driven by a ramped stimulus, another of steady-steady at constant speed, and a friction-driven regime in the absence of external fields. Traces of the inertial dynamics of the magnetic texture and a great stability of the DW even at speeds very close to the maximum magnon group velocity of the medium have been observed. On the other hand, it has been possible to predict the distance traveled by the magnetic soliton in its friction-driven motion, which is of the order of 5-10 times greater than the DW width at rest for moderate fields of 10-60 mT. This opens up the possibility of exploiting the phenomenon of inertia to reduce the power consumption in those technological applications such as racetrack memories in which the position of the magnetic texture needs to be precisely determined.References: [1] S. S. Parkin, M. Hayashi, and L. Thomas, Science, Vol. 320, p. 190-194 (2008) [2] R. Mattheis, S. Glathe, M. Diegel, et al., Journal of Applied Physics, Vol. 111, p. 113920 (2012) [3] P. Wadley, B. Howells, J. Zelezný, et al., Science, Vol. 351, p. 587-590 (2016) [4] J. Zelezný, H. Gao, K. Výborný, et al., Physical Review Letters, Vol. 113, p. 157201 (2014)

Inertial Domain Wall Motion Driven by Staggered Spin-Orbit Fields in Layered Antiferromagnets

Current induced spin-orbit torque (SOT) has been extensively investigated as a promising mechanism to switch magnetization electrically, which potentially allows higher energy efficiency in information technologies [1]. Metallic antiferromagnets (AFM) have been demonstrated to be an efficient source of SOT [2]. However, the relationship between the magnetic structure and SOT of AFM has been frequently overlooked, which not only clouds the origin of SOT in AFM but also hampers the engineering efforts for application development. Here we demonstrate that the magnetic structure of AFM IrMn has a strong influence on its SOT generation [3-4]. We fabricate epitaxial IrMn thin films of three phases, which have distinct crystal and magnetic structures, as shown in Fig. 1(a) and 1(b). The magnetic structures of the three phases have been carefully studied using the neutron diffraction technique. Using spin-torque ferromagnetic resonance technique (ST-FMR) [Fig. 1(c)] and IrMn/permalloy (Py) bilayers, we show that the measured SOT efficiencies () of L10-IrMn, L12-IrMn3 and γ-IrMn3 are 0.61±0.01, 1.01±0.03 and 0.80±0.01, respectively, as displayed in Fig. 1(d). These values are substantially larger than that of the polycrystalline IrMn, which is only 0.083±0.002. By altering the direction of electric current in the crystal lattice of L10-IrMn, we observe a 4-fold rotation symmetry in both the SOT efficiency and the in-plane magnetic anisotropy, as shown in Fig. 1(e). Fig. 1(f) shows that 4-fold symmetry disappears when a thin Cu spacer is inserted between L10-IrMn and Py while the measured SOT efficiency decreases to 0.22±0.03. These results suggest a magnetic structure enhanced SOT efficiency in L10-IrMn, which consists of a large isotropic bulk contribution and a comparable anisotropic interfacial contribution [3]. Moreover, we observe an anomalous out-of-plane damping-like torque in both L10-IrMn and L12-IrMn3 [4], as shown by the red curves in both Fig. 1(g) and 1(h). The microscopic origin of this torque appears to be an out-of-plane spin polarization. This is consistent with the intrinsic effects of the a laterally broken magnetic mirror symmetry () that is present commonly in both L10-IrMn and L12-IrMn3. We attribute the absence of this anomalous SOT in γ-IrMn3 to the lack of magnetic asymmetry due to a disordered magnetic structure.References: [1] A. Manchon et al, Rev. Mod. Phys. 91, 035004 (2019). [2] W. Zhang et al, Phys. Rev. Lett. 113, 196602 (2014). [3] J. Zhou et al, Sci. Adv. 5, eaau6696 (2019). [4] J. Zhou et al, Phys. Rev. B 101, 184403 (2020).

Large spin-orbit torque efficiency and magnetic symmetry effect in the antiferromagnet IrMn INVITED

Magnetorheological elastomers are smart materials that can deform and change shape based on the application of a magnetic field. These magnetic elastomers perform best in soft robotics and biomedical applications when they have increased anisotropy, or different properties along different axes. To increase this anisotropy, one can utilize hard magnetic particulate, magnetic annealing, and fused deposition modeling (FDM), also known as 3D printing. Hard magnets can be oriented using an external alignment field, and strontium ferrite is specifically interesting because it costs less than other rare-earth magnets (or materials). FDM is usually a final step in creating anisotropy in a sample (1), and it allows magnetorheological elastomers components to be more accessible for other applications. Our thermoplastic magnetic elastomer composite is created using solvent-casting techniques with 15 vol% SrFe12O19 particulate. The material is then extruded into FDM filaments using a Filastuder. During the extrusion process, some filament is magnetically annealed in an axial applied field created by NdFeB magnets, while other filament extruded without magnetic annealing acts as the control. Mechanical stress vs. strain curves of the extruded filaments are acquired using an MTS tensile testing machine. Magnetic hysteresis loops are measured using a vibrating sample magnetometer (VSM). In order to determine the relative level of magnetic anisotropy within the samples due to these factors, hysteresis loops with the field applied parallel and perpendicular to the axis of extrusion are compared to one another. Magnetoactive testing is used to measure the mechanical deflection angle as a function of the strength of a transverse applied magnetic field. Understanding the mechanical, magnetic, and magnetoactive properties of this magnetorheological elastomer provides insight into how magnetic anisotropy can be controlled by a combination of hard magnetic particulate, magnetic annealing, and FDM for optimized performance.References: 1. Patton, et. al., “Manipulating magnetic anisotropy in fused filament fabricated parts via macroscopic shape, mesoscopic infill orientation, and infill percentage.” Additive Manufacturing, Vol. 27, pp. 482-488 (2019).

Thermoplastic Elastomers with Strontium Ferrite Particulate and Magnetic Annealing used for Fused Deposition Modeling

The design of the materials showing first-order magnetic transitions (FOMTs) with smallest possible thermomagnetic hysteresis stimulates considerable research interest because hysteresis can be detrimental for certain applications, such as solid-state caloric cooling [1]. The overwhelming majority of materials with a reasonably small hysteresis contain transition metals, and their behavior is closely associated with the itinerant-electron metamagnetism of the transition metals present in the system [2,3]. We observed anhysteretic FOMT in a lanthanide-based compound Pr2In with the classical RKKY-type magnetic interaction between 4f electrons of trivalent Pr. Although such behavior is phenomenologically similar to Eu2In, another lanthanide system showing hysteresis-free FOMT [4], the crystal structure and magnetic interactions in Eu2In are markedly different from Pr2In due to the presence of divalent Eu [4,5]. In the vicinity of ferromagnetic to paramagnetic transition temperature (TC ~57 K) Pr2In exhibits a large magnetic field-induced entropy change with the maximum value of -ΔSM ~ 23 J/kg K for ΔH = 50 kOe, which is comparable to a number of other potential magnetocaloric materials in the temperature range of 40-80 K, reported in the literature [6]. The temperature and magnetic field dependence of ΔSM can be expressed as ΔSM ~ Hn , where the local exponent n varies with both H and T, showing a clear maximum with n>>2 , a characteristic feature of FOMT [7]. The observation of anhysteretic FOMT and a large MCE in Pr2In with conventional RKKY-type magnetic exchange promises further advancements in developing novel functional caloric materials by manipulating electronic structure of 4f-based materials. Acknowledgement: This work was performed at Ames Laboratory and was supported by DMSE, BES, Office of Science of the U.S.DOE. Ames Laboratory is operated for U.S DOE by Iowa State University under Contract No. DE-AC02-07CH11358.References: 1. N. A. Zarkevich et al., Journal of Alloys and Compounds 802, 712-722 (2019). 2. F. X. Hu et al., Applied Physics Letters 78, 3675 (2001) 3. A. Fujita et al. , Physical Review B 67, 104416 (2003) 4. F. Guillou et al., Nature Communications 9, 2925 (2018). 5. A. Biswas et al., Physical Review B 101, 224402 (2020). 6. L. Li et al., Journal of Alloys and Compounds 823, 153810 (2020). 7. J. Law et al., Nature Communication 9, 2680 (2018).

Large magnetocaloric effect due to anhysteretic first-order transition in Pr2In

Discovery of the giant magnetocaloric effect (MCE) in interrmetallic Gd5(Si2Ge2) [1] led to an increase the interest in the materials with large MCE due to the possibility of their practical application as a working body [2] in magnetic cooling devices. Recently so-called shape-memory ferromagnets based on Heusler alloys Ni-Mn-Z (Z = Ga, In, Sn, Sb) began to be considered as the promising materials. For example, for Ni50Mn35Sn15 the value of △S = 14,2 J/(kgK) at 187 K in a magnetic field of 50 kOe [3]. The aim of this work was to study the effect of variation in chemical composition (the relative concentration Ni/Mn) on the MCE in Ni43+xMn46-xSn11 (x = 0; 1) Heusler alloys. Ingots were prepared by arc melting in an inert atmosphere and subsequently subjected to annealing at 1100 K for 24 h followed by furnace cooling. The elemental analysis and X-ray diffraction studies were performed. The magnetic and galvanomagnetic properties were measure at the Atomistitut, TU Wien using an MPMS XL7 (Quantum Design) SQUID magnetometer. The magnetic properties were measured in magnetic fields of up to 70 kOe in the temperatures range 150-330 K. As a result of this work it was established that the variation in the chemical composition does not lead to a significant change of the maximum value of the magnetocaloric effect (△S = 44 J/(kgK) and 40 J/(kgK) for Ni43Mn46Sn11 and Ni44Mn45Sn11, respectively), but shift its maximum (≈ 32 K). It was found that in the investigated alloys a change in the relative concentration Ni/Mn (and hence the e/a) does not significantly influence on the value of Curie temperature TC. The results of this work were obtained within the state assignment of the Ministry of Education and Science of the Russion Federation (theme “Spin” No. AAAA-A18-118020290104-2) and supported in part by RFBR grant (project No. 18-02-00739).References: [1] V.K. Pecharsky, K.A. Gschneider, Physical Review Letters, Vol. 78, p. 4494 (1997) [2] V.K. Pecharsky, K.A. Gschneider, Journal of Magnetism and Magnetic Materials, Vol. 200, p. 44 (1999) [3] T. Krenke, E. Duman, M. Acet, E.F. Wassermann, X. Moya, L. Mañosa, A. Planes, Nature Materials, Vol. 4, p. 450 (2005)

Influence of Ni/Mn Concentration on the Magnetocaloric Effect in Ni43+xMn46-xSn11 (x = 0; 1) Heusler Alloys

Magnetorheological elastomers (MREs) are magnetoactive smart materials that exhibit mechanical deformation in the presence of magnetic fields. The greatest MRE performance is seen from magnetic annealing, which creates high internal anisotropy. It is hypothesized that increased anisotropy from Fused Deposition Modeling (FDM, extrusion-based 3D-printing) could also increase MRE performance. Alignment of magnetic particulate within the material used in this process would further bolster the anisotropy and, thus likely, the magnetoactive response of the printed part. This work studies how different magnetic annealing setups can be applied to thermoplastic MRE extrusion geometries and how each setup effects resulting magnetoactive properties. Three different magnetic fields were used in the annealing process: no magnetic field (0M), stronger field (5M), and weaker field (3M). The magnetic annealing setup was composed of four stacks of NdFeB magnets arranged longitudinally around the extrusion path for an axial applied field. 0M had no magnets, 5M had 5 NdFeB magnets, and 3M had 3 NdFeB magnets each separated by a steel spacer acting as a saturated magnet. A map of each magnetic vector field was characterized in 3D using a gaussmeter mounted to a robotic arm. Despite the different number of rare-earth magnets, the non-zero fields had fairly minimal differences. Both non-zero magnetic annealing setups generated greater magnetoactive response in the final MRE than zero field (Fig 1). Between the two non-zero fields the stronger field gave the greatest response but was difficult to make samples with. This work demonstrates that both magnetic annealing setups can enhance performance over no magnetic field; while the preferred setup may depend on a compromise between enhanced performance with ease of extrusion.

Magnetic Annealing of Extruded Thermoplastic Magnetorheological Elastomers for 3D-Printing via FDM

In search of potential replacement of highly expensive rare-earth metal (Gd)and its alloys [1-2] for environment friendly magnetic refrigeration technology [3] near room temperature , the earth-abundant Mn-based alloys, particularly Mn5Ge3 (TC =297 K) [4], have got much attention with reasonably high relative cooling power (RCP). In this work, we undertake Mn5Ge3 alloy and optimize the synthesis, particularly excess Mn (to compensate evaporation loss) in arc melting method, followed by annealing conditions. The maximum magneto-caloric effect is observed for Mn5Ge3 alloy with ferromagnetic ordering at TC = 299(2) K and MS (at 5 K) =142 emu/g (by dc magnetization study) from arc melting synthesis with 5 % excess Mn and vacuum annealing at 800oC for 7 days. The magnetic ordering for optimized sample is further evaluated by detailed critical exponent study using modified Arrot plot (MAP) and scaling method gives second order phase transition at TC = 297.5(1) K with exponents β = 0.348(1) and γ=1.193(2), which lies in close proximity to 3D Heisenberg model. The magnetic isotherms are further employed to study the magneto-caloric effect. Using Maxwell equations, the change in magnetic entropy, -△Sm│max is calculated as 4.7 J/kgK at 5 T applied field; the field dependence of -△Sm|max follow power law behaviour, -△Sm│max ∝ Hn with exponent, n = 0.62(1), in well consistent with n = 0.638 for 3D Heisenberg model. Finally, the material parameter for practical utilization in magnetic refrigeration i.e. RCP = -△Sm│max x δTFWHM is obtained as 450 J/kg for a field change of 5 T, which is significantly larger than earlier reported 388 J/kg for Mn5Ge3 synthesized with annealing at 750oC for 4 days. Although obtained RCP value is nearly 70 % that for the commonly used rare-earth Gd, importantly with production cost maximum 32 % only [see Table I] that makes Mn5Ge3 alloy highly promising for magnetic refrigeration application near room temperatureReferences: 1. S. Y. Dankov, A. M. Tishin, V. K. Pecharsky, Phys. Rev. B 57, 3478 (1998) 2.S. Gorsse, B. Chevalier, and G. Orveillon, Applied Physics Letters 92, 122501 (2008) 3.C. Aprea, A. Greco, A. Maiorino, Journal of Physics: Conference Series 655, 012026 (2015) 4. T. Tolinski and K. Synoradzki, Intermetallic, vol. 47, pp. 1–5, (2014)

Optimization of magnetic refrigerant capacity of Mn5Ge3 alloy near room temperature

Magnetorheological elastomers (MREs) are a smart material with the ability to physically deform when in the presence of an applied magnetic field. Extruding MREs into filament allows for the use of FDM manufacturing. FDM has been shown to manipulate the mechanical and magnetic properties of polymer matrix composites, which in turn could fine-tune the magnetoactive responses of printed MREs. This project investigates how the magnetic and magnetoactive properties of composite materials made of thermoplastic polyurethane (TPU) and 15 vol% iron particulate are affected by the size of the particulate within the matrix. Two different sizes of iron particulate are tested: 100 mesh (<150 micron) and spherical carbonyl 1-3 micron. Varying particulate size is of interest for FDM 3D printing because of limitations due to small nozzle sizing. This means that larger particulate can cause more issues of clogging, though it is less expensive and more accessible than smaller particulate. After the composites were fabricated, the resulting magnetic properties were tested via a VSM. Hysteresis loops showed that the smaller particulate samples had a lower magnetic susceptibility than the larger particulate samples. That means the larger particulate will exhibit greater magnetization under the same applied field. The magnetoactive properties were tested by measuring the angle of deformation under various transverse applied magnetic field strengths (limit 0.4T). The magnetoactive properties of the filaments are shown visually in Figure 1 and quantitatively in Figure 2. In both, it can be clearly seen that the larger particulate samples respond with a greater angle of deformation at every tested applied field. In conclusion, larger particulate leads to greater magnetic susceptibility and magnetoactive deformation, but the smaller particulate does have a more desirable size and shape for FDM.

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Spin-oribit torques in Mn2Au/HM bilayers

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