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Magnetism in strained LaCoO3 (LCO) thin films has been an intriguing scientific subject for the last decade. While the bulk LCO is non-magnetic, strained LCO thin films exhibit robust ferromagnetism with a phase transition temperature (Tc) of 80 K.[1] Since this system is a representative example where epitaxial strain induces non-bulk magnetic properties, it has drawn lots of attentions in the field of magnetic oxide thin films. Despite extensive research over the past decade[2-6], the consensus on the origin of ferromagnetism has not yet to be achieved. A main challenge of this system is a complex microstructure inside the film; the existence of planar defects (seen as dark stripes in high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) images) and their association with oxygen vacancies are important issues. Through the systematic STEM-based investigations, we verified that the planar defects always exist in epitaxial LCOs which undergo a ferroelastic response and the pristine planar defects do not involve oxygen vacancies. Furthermore, we have recently found that the compressed units sandwiched by the dark stripes are responsible for ferromagnetism in epitaxial LCOs. In this presentation, we will show that the ferroelastic nature of ferromagnetic structural units can account for many unusual magnetic properties of this system.References: [1] D. Fuchs et al. Phys. Rev. B 75, 144402 (2007) [2] W. Choi et al. Nano Lett. 12, 4966 (2012) [3] N. Biskup et al. Phys. Rev. Lett. 112, 087202 (2014) [4] L. Qiao et al. Nano Lett. 15, 4677 (2015) [5] J. Fujioka et al. Phys. Rev. Lett. 111, 027206 (2013) [6] G. Sterbinsky et al. Phys. Rev. Lett. 120, 197201 (2018)

MMM 2020

Ferroelastic Nature and Ferromagnetism in Epitaxial LaCoO3

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

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.

Spin-oribit torques in Mn2Au/HM bilayers

We report magnetic -field -induced rotation of the antiferromagnetic Néel vector in epitaxial CuMnAs thin films. Firstly, using soft x-ray magnetic linear dichroism spectroscopy and magnetometry, we demonstrate spin-flop switching and continuous spin reorientation in films with uniaxial and biaxial magnetic anisotropies, respectively, for applied magnetic fields of the order of 2 T. The remnant antiferromagnetic domain configurations are determined using x-ray photoemission electron microscopy. Next, we show that the Néel vector reorientations are manifested in the longitudinal and transverse anisotropic magnetoresistance. Dependency of the electrical resistance on the orientation of the Néel vector with respect to both the electrical current direction and the crystal symmetry are identified, including a weak 4th fourth-order term which is evident at high magnetic fields. The results provide characterization of key parameters including the anisotropic magnetoresistance coefficients, magnetocrystalline anisotropy, and spin-flop field in epitaxial films of tetragonal CuMnAs, a candidate material for antiferromagnetic spintronics.References: 1. M. Wang, C. Andrews, S. Reimers, et.al. Phys. Rev. B 101, 094429 (2020). 2. P. Wadley, B. Howells, J. Zelezny, et.al. Science 351, 587 (2016).

Spin flop and crystalline anisotropic magnetoresistance in CuMnAs

Antiferromagnets have considerable potential as spintronic materials [1]. Notably, they manifest resonant modes at frequencies higher than can be observed in conventional ferromagnetic materials. Synthetic Antiferromagnets (SAFs), engineered structures of antiferromagnetic (AF) RKKY exchange coupled ferromagnetic(FM)/non-magnetic(NM)/ferromagnetic trilayers, mimic the properties of single phase antiferromagnets whilst possessing greater compatibility with current device technologies such as those used for STT-MRAM production [2,3]. In addition to the conventional acoustic (Kittel) mode, SAFs also possess a higher order resonant (optic) mode [3]. The structure of a SAF is shown in Fig.1a. We report the dynamic properties of SAFs with structure Ta(2 nm)/CoFeB(5 nm)/Ru(tRu)/CoFeB(5 nm)/Pt(4 nm) where tRu is the thickness of the Ru layer, acting as the NM spacer layer. The samples were fabricated using magnetron sputtering and characterized by X-ray Reflectivity (XRR) to measure layer thickness, vector Vibrating Sample Magnetometry (VSM) to verify AF interlayer coupling and a Vector Network Analyser – Ferromagnetic Resonance (VNA-FMR) setup to explore dynamic properties. Fig.1b shows the resonant dynamics of a single CoFeB layer and a CoFeB SAF. The data demonstrate the highest zero field frequencies observed in SAFs to date [4] with resonances of 18 GHz and 21 GHz at the 1st and 2nd instances of AF RKKY coupling respectively. A three part analytical model can reproduce the major features of the measured spectra.References: [1] T. Jungwirth, X. Marti, P. Wadley et al., Nat. Nanotechnol., Vol. 11, p. 231–241 (2016) [2] R. Hao, L. Wang and T. Min, IEEE Trans. Magn., Vol. 55, p. 1-6 (2019) [3] S. Li, C. Wang, X. Chu et al., Adv. Funct. Mater., Vol. 26, p.3738-3744 (2016) [4] H. Waring, N.A.B. Johansson, I. Vera-Marun et al., Phys. Rev. Appl., Vol. 13, p. 034035 (2020)

Zero Field Optic Mode Beyond 20 GHz in a Synthetic Antiferromagnet

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

Since the first experimental observation of electrical switching of antiferromagnet[1], the experiments on manipulation of antiferromagnetic order are exploded. However, the theoretical models are still very simplified (sigma model, macrospin approximation). This talk will discuss the recent developments of modeling antiferromagnets and ferromagnets considering a full numerical micromagnetic model. We benchmarked the approach with analytical theories for THz oscillator[2], domain wall and skyrmion dynamics[3], while predicting and describing some features for THz detectors, memories[4] and high speed DW motions [5]. Within this approach, it is possible to take into account three contributions for the exchange interactions (one homogeneous and two inhomogeneous) that are important to identify the two time scales of the antiferromagnetic dynamics. The first, at the THz range, that is related to relaxation processes from a misalignment of the antiferromagnetic state. The second one is related to the relaxation of domains and it is of the same order of the one of the ferromagnetic order. In the last part of the talk, It will discussed the origin of memristive behaviour [4] and the effect of grain sizes in the antiferomagnetic switching experiments.References: 1 P. Wadley, et al, Science, 351, 6273 (2016). 2 V. Puliafito, at al, Phys. Rev. B 99, 024405 (2019). 3 L. Sanchez-Tejerina, et al, Phys. Rev. B, 101, 014433, 2020. A. Salimath, et al Phys. Rev. B, 101, 024429, 2020. 4 S. Chen, G. Finocchio, et al, "Antiferromagnetic PtMn memory devices controlled by electric current" Nature Elettronics, 3, 92-98, 2020. 5 Heng-An Zhou, et al, " Compensated magnetic insulators for extremely fast spin-orbitronics", https://arxiv.org/abs/1912.01775.

Micromagnetic modeling of antiferromagnets INVITED

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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Ferroelastic Nature and Ferromagnetism in Epitaxial LaCoO3

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