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Multiferroics that show coupling between electric and magnetic properties are promising materials for devices that transform information from one state into another, such as spin excitations into charge, phonons or photons. Among them, BiFeO3 (BFO) is one of the very few compound displaying room temperature multiferroicity. BFO leads for example to spectacular THz electromagnetic wave generation and photostriction properties. The prospects for applications in this area such as optically triggered piezotransducers require a perfect knowledge of the elastic properties of materials. Combining Time Resolved Acoustic Measurements (TRAM) with density functional theory (DFT) calculations and volumic and surface mode propagations simulations, we have studied the elastic properties of BiFeO3. Using TRAM, we have measured wave propagation along 3 differents directions in the (110) plane of BFO. Then, we determined three modes speeds : one longitudinal mode, one fast and one slow transverse modes. With DFT, we calculated a set of elastic constants for BiFeO3. We used these elastic constants to simulate the propagation of volumic and surface modes. The comparison between the calculated and measured speeds enables to ajust the elastic constants. In conclusion, we have been able to fully determine the elastic constant of BiFeO3 that are essential for future integration of BFO in acoustic micro/nano-devices.

MMM 2020

Elastic properties of the multiferroic BiFeO3 by Time Resolved Acoustic Measurements

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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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The signature of magnetism without having a ferromagnet in the sample stack is both novel and fascinating from fundamental research point of view. It is possible to alter the electronic states of non-ferromagnetic materials (Cu, Mn, Sc, Pt) to overcome the Stoner criterion and make them ferromagnetic at room temperature1,2. In this context it is desired to quantify the magnetic moment at the interface of such non-FM/OSC. We have studied the Cu/C60 heterostructure and quantified the magnetic moment induced in Cu using X-ray magnetic circular dichroism (XMCD) sum rules. The multilayer of Cu/C60 were prepared on Si (100) substrate without breaking the vacuum using DC magnetron sputtering and thermal evaporation for Cu and C60, respectively. SQUID magnetometry on sample S1:(Si(100)/Ta(5nm)/C60(8nm)/Cu(1.8nm)/C60(4nm)/Ta(2nm)) and S2:(Si(100)/Ta(5nm)/C60(16nm)/[Cu(1.8nm)/C60(8nm)]x2/Ta(2nm)) exhibit a clear ferromagnetic hysteresis even in the absence of any ferromagnetic element in the sample stack. The induced magnetic moment in the samples is corroborated to the charge transfer and the molecular coupling between the metal (Cu) and carbon. The origin of the charge state of C60 to a reconstructed interface is due to (4 4) 7-atom vacancy holes in the surface3. The possible reason of this induced magnetic moment is hybridization between dcu and pC60 orbitals1. In order to evaluate the spinterfacial magnetic moment, XMCD has been performed on the samples. Figure1 shows the XAS and XMCD spectra of the samples. The XAS spectra do not exactly match with Cu. The reason behind such misalignment is the hybridization between Cu and C60. Due to the charge transfer at the reconstructed Cu/C60 interface the density of state of Cu has been modified and exhibits ~ 0.01 μB /atom magnetic moment4. Our understanding of the interface between the NM/OSC heterostructures may bring the new insights to the field of organic spintronics.References: 1. F. Al Ma'Mari et.al., Nature 524, 69 (2015). 2. F. Al Ma’Mari et al., PNAS, 1620216114 (2017). 3. W.W.Pai et.al., PRL 104, 036103 (2010). 4. P. Sharangi et al., (unpublished)

Quantifying induced moment in non-magnetic Cu – C60 interfaces via x-ray magnetic circular dichroism

Mn-Ga alloys show promise for spintronic applications based on their tunable saturation magnetization (Ms) and goldilocks region perpendicular magnetic anisotropy (Ku).1 Thin films with perpendicular magnetic anisotropy have potential as low-dimensional highly reliable spintronic devises.2-5 The task remains to optimize magnetic properties for applications as a function of composition and dimensionality. In this study we attempt to understand the impact of composition on magnetic properties of Mn-Ga systems in the D022 structure. Density Functional Theory (DFT) calculations are used to study the role of vacancies on Ms and Ku of Mn-Ga family of alloys. While the DFT calculations capture the general trends seen in experiments, they are unable to quantitatively predict Ms and Ku with sufficient accuracy. This is attributed to the inherent complexities in the thin film growth of Mn-Ga systems, where the variations in Ms and Ku show a strong dependence on the growth variables. Machine Learning (ML) is implemented as a low-cost alternative to DFT, where we train and validate data-driven models based on Ms and Ku data compiled from surveying experimental literature. The ML models enable rapid predictions of Ms and Ku for previously unexplored compositions. New compositions are identified with implications in emerging spintronics applications.References: 1) H. Kurt, K. Rode, M. Venkatesan, P. Stamenov, PHYSICAL REVIEW B 83, 020405(R) (2011) 2) S. Iwasaki and K. Ouchi, IEEE Trans. Magn. 14, 849 (1978) 3) P. F. Carcia, J. Appl. Phys. 63, 5066 (1988) 4) T. Seki, T. Shima, K. Takanashi, Y. Takanashi, E. 82, 2461 (2003) 5) S. N. Piramanayagam, J. Appl. Phys. 102, 011301 (2007).

Density Functional Theory and Machine Learning Guided Study of Mn-Ga Alloys with Targeted Magnetic Properties

Ionic modulations of magnetic properties, controlled by a low voltage, are attractive for future energy efficient magnetic devices. [1,2] We study the magneto-ionic control of thin iron films with uniaxial in-plane anisotropy, which is induced by oblique sputtering. The polycrystalline iron films are natively oxidized to yield iron oxide/iron films suitable for magneto-ionic manipulation by electrolytic gating in alkaline electrolyte. [3] Angle resolved magnetic measurements reveal a uniaxial anisotropy (Fig.1 ). In the pristine state (Fig. 1a,b, black curves), the remanence and coercivity along the hard axis are enhanced, due to the formation of a blocked magnetic state resulting from interactions of the charged Néel walls. [4] A voltage-controlled transformation of the FeOx layer into metallic Fe upon electrolytic gating leads to a drastic decrease in coercivity in all directions (Fig. 1a, blue curve). At the same time, the remanence remains unchanged for angles close to the easy axis but almost vanishes in the vicinity of the hard axis upon the reduction process (Fig. 1b, blue curve). Anhysteretic measurements along the hard axis revealed that the anisotropy increases upon voltage application. The unexpected finding of an inverse anisotropy-coercivity relationship is further studied by investigating the magnetic microstructure by in-situ Kerr microscopy. We find a significant increase in domain size by 60% upon reduction. We attribute the observed coarsening of the magnetic microstructure to the anisotropy-related increase in the domain wall energy. At the same time, the anisotropy increase causes a decrease of the Néel wall tails. Both effects will result in a weakening of the Néel wall interactions during voltage application, which can explain the observed magnetic de-blocking, and thus, the collapse of remanence and coercivity. [5]References: [1] C. Navarro-Senent, A. Quintana, E. Menéndez, E. Pellicer, and J. Sort, APL Materials 7, 030701 (2019). [2] A. Molinari, H. Hahn, and R. Kruk, Advanced Materials 31, 1806662 (2019). [3] K. Duschek, M. Uhlemann, H. Schlörb, K. Nielsch, and K. Leistner, Electrochemistry Communications 72, 153 (2016). [4] A. Hubert and R. Schäfer, Magnetic Domains (Springer Berlin Heidelberg, Berlin, Heidelberg, 1998). [5] J. Zehner, I. Soldatov, S. Schneider, R. Heller, N. B. Khojasteh, S. Schiemenz, S. Fähler, K. Nielsch, R. Schäfer, K. Leistner, submitted

Voltage control of Néel wall interaction in thin films with uniaxial in-plane anisotropy

Magnon spin Nernst effect (SNE) has recently been proposed as a bulk effect in antiferromagnetic insulators without considering diffusions and boundaries1,2. However, real experiments3 deal with situations far from the ideal bulk picture in that: 1) the system is finite and shares imperfect boundaries with the detecting leads; 2) both the electrons in the leads and the magnons are diffusive; 3) the thermal power rather than the temperature gradient is calibrated. To properly accommodate these complexities, we formulate a diffusive theory of the SNE of antiferromagnetic magnons under a realistic device geometry. By solving the coupled spin diffusion equations of magnons and electrons with a finite spin transmission across the boundaries, we find that the detectable signal can be much larger than that predicted by the bulk picture. The diffusive effects also lead to a different phase diagram of the SNE compared to the bulk theory without diffusion. Counterintuitively, we also find that the diffusive magnon SNE exhibits an even symmetry with respect to a perpendicular magnetic field.References: 1. R. Cheng, S. Okamoto and D. Xiao, Phys. Rev. Lett., Vol. 117, 217202 (2016) 2. V. A. Zyuzin and A. A. Kovalev, Phys. Rev. Lett., Vol. 117, 217203 (2016) 3. Y. Shiomi, R. Takashima and E. Saitoh, Phys. Rev. B, Vol. 96, 134425 (2017)

Diffusive spin Nernst effect of antiferromagnetic magnons

Topological spin textures, such as skyrmions, have attracted intense interest in recent years. Magnetic skyrmions have been studied almost exclusively in bulk ferromagnets (FM) or FM multilayers. Antiferromagnets (AF) have also been predicted to harbor topological spin textures such as AF skyrmions. Topological Hall effect has been a primary indicator of spin textures in magnetic materials. We observe unambiguous evidence of topological Hall effect in Pt/Cr2O3 bilayers grown on Al2O3(0001) and Al2O3(11-20), where Cr2O3 epitaxial film is an antiferromagnetic insulator[1]. Although Cr2O3 is insulating, the topological magnetic textures can be detected through a novel phenomenon: “spin-Hall topological Hall effect” (SH-THE), similar to what recently observed topological Hall effect in Pt/Tm3Fe5O12 bilayers[2]. The Pt/Cr2O3 bilayers exhibit clear topological Hall resistivity for Cr2O3 thicknesses below 6 nm near and above room temperature, which is above the Néel temperature of Cr2O3, revealing the key role of thermal fluctuations in the formation of spin textures[3]. The similarity of topological Hall signals in (0001) and (11-20)-oriented Cr2O3 films indicates that the emergence of spin textures is insensitive to crystalline orientation. This first observational evidence of topological Hall effect in HM/AFI bilayers significantly expands our materials base to include the large family of AF insulators for the exploration of AF-based skyrmion technology.References: [1]. Yang Cheng, Sisheng Yu, Menglin Zhu, Jinwoo Hwang and Fengyuan Yang, “Evidence of Topological Hall Effect in Pt/Antiferromagnetic-Insulator Bilayers,” Phys. Rev. Lett. 123, 237206. [2]. A. S. Ahmed, A. J. Lee, N. Bagués, B. A. McCullian, A. M. A. Thabt, A. Perrine, P.-K. Wu, J. R. Rowland, M. Randeria, P. C. Hammel, D. W. McComb, and F.. Y. Yang, “Spin-Hall topological Hall effect in highly tunable Pt/ferrimagnetic-insulator bilayers,” Nano Lett., 19, 5683 (2019). [3]. Wenbo Wang, Matthew W Daniels, Zhaoliang Liao, Yifan Zhao, Jun Wang, Gertjan Koster, Guus Rijnders, Cui-Zu Chang, Di Xiao and Weida Wu, "Spin chirality fluctuation in two-dimensional ferromagnets with perpendicular magnetic anisotropy" Nat. Mater. 2019, 18, 1054.

Evidence of Topological Hall Effect in Pt/Antiferromagnetic-Insulator Bilayers

Curvilinear magnetism is of great fundamental and practical interest whose rapid development is inspired by novel experimental technologies and wide potential applications [1]. A general approach for description of curvilinear ferromagnets [2] has been recently developed and used for thin wires and shells uncovering magnetochiral effects in statics and dynamics [1,3]. Besides intensive research of ferromagnetic materials, their antiferromagnetically ordered (AFM) counterparts are promising candidates for spintronics applications by their low sensitivity to external fields and ultra high eigenfrequencies [4]. Here, we present a general approach for description of AFM textures in curvilinear spin chains [5]. We show that the magnetic dipole-dipole interaction in these systems can be reduced to a hard-axis anisotropy along the chain. Lagrangian of the curvilinear AFM spin chain in continuum limit corresponds to the biaxial chiral helimagnet. Helix geometry shows existence of two equilibrium magnetic states depending on values of curvature and torsion: (i) homogeneous state in the local reference frame, it is typical for helices with the curvature larger than torsion; and (ii) periodic state is quasi-homogeneous in the laboratory reference frame. For specific case of the AFM flat chain there is the only ground state, with the order parameter being oriented perpendicular to the chain plane. We show that in curvilinear system transverse and longitudinal magnon modes in the AFM helix and ring are coupled due to geometry-induced Dzyaloshinskii–Moriya interaction.References: [1] R. Streubel, J. Lee, D. Makarov et al, J. Phys. D, 49, 363001, (2016); A. Fernández-Pacheco et al, Nat. Comm., Vol. 8, p. 15756 (2017). [2] Y. Gaididei, V. P. Kravchuk, D. D. Sheka, Phys. Rev. Lett. 112, 257203 (2014); D. D. Sheka, V. P. Kravchuk, Y. Gaididei, J. Phys. A, Vol. 48, p. 125202 (2015). [3] O. V. Pylypovskyi, D. D. Sheka, V. P. Kravchuk et al, Sci. Rep. Vol. 6, p. 23316 (2016); O. V. Pylypovskyi, D. Makarov, V. P. Kravchuk et al, Phys. Rev. Applied, Vol. 10, p. 064057 (2018) [4] V. Balz, A. Manchon, M. Tsoi et al, Rev. Mod. Phys., Vol. 90, p. 015005 (2018) [5] D. Y. Kononenko, O. V. Pylypovskyi, K. V. Yershov et al., arXiv:2005.05835 (2020)

Effects of Geometry on Curvilinear Spin Chains

The demonstration of efficient manipulation of the antiferromagnetic (AF) order parameter has highlighted its highly attractive properties for spintronics, including lack of magnetic stray fields, robustness against magnetic field perturbations and intrinsic dynamics in the THz regime [1, 2, 3]. Specific functionalities often depend sensitively on the magnetic domain structure [4]. In fully compensated AFs with no demagnetizing fields, magnetoelastic effects are expected to dominate [5,6]. We report direct imaging and analysis of AF domains in patterned epitaxial CuMnAs thin films. A pronounced effect of patterned edges on the domain orientation and domain wall width is demonstrated, which extends over several microns. A local alignment of the Néel vector with microtwin line defects is observed, which, combined with the edge effect, leads to the formation of lens-shaped domains oriented perpendicular to the patterned edges. We observe magneto-structural reconfigurations in which defect lines and surrounding AF domains grow together over the timescale of weeks at room temperature and over minutes 30K above room temperature.References: [1] P. Wadley, B. Howells, J. Zelezny et al., Science., Vol. 351, p. 587 (2016) [2] X.Z. Chen, R. Zarzuela, J. Zhang et al., Physics Review Letters., Vol. 120, p. 207204 (2018) [3] P. Nemec, M. Fiebig, T. Kampfrath and A.V. Kimel, Nature Physics., Vol. 14, p. 229 (2018) [4] A.L. Ross et al., Nano Letters, Vol. 20(1), p. 306 (2020) [5] O. Gomonay and V. Loktev, Journal of Magnetism and Magnetic Materials., Vol. 242(2), p. 1418 (2002) [6] O. Gomonay, V. Baltz, A. Brataas and Y. Tserkovnyak, Nature Physics., Vol. 14, p. 213 (2018)

Antiferromagnetic Domain Structure in CuMnAs Devices

Non-collinear antiferromagnets such as D019-Mn3Sn have attracted much attention owing to their large anomalous Hall effect (AHE) originating from non-vanishing Berry curvature [1-5]. We recently reported a growth of epitaxial D019-Mn3Sn thin films with various orientations by sputtering, where anisotropic AHE was observed [5]. To realize functional devices harnessing the unique features of this material system, it is necessary to clarify the correlation of the physical properties with various factors of thin film. Here, we study the correlation between AHE and crystal structure of Mn-Sn alloy film by changing post-annealing temperature (Ta) and Mn-Sn composition. We prepare two series of Mn3+xSn1-x thin films which are deposited on MgO(110) substrate by sputtering. The first series have a fixed x = +0.06 and Ta is changed from 300 to 600oC. The second series have various x (-0.35 ~ +0.32) with a fixed Ta = 500oC. Crystal structures are characterized by X-ray diffraction (XRD). Transport measurement is performed for microfabricated Hall devices. From XRD measurement, we find that dominant phase of Mn-Sn changes from Mn1.75Sn to D019-Mn3Sn as Ta increases (series 1), and changes from Mn3Sn2 to D019-Mn3Sn as x increases (series 2) in the studied range. Transport measurement for the series 1 reveals that appearance of AHE coincides with the formation of D019-Mn3Sn and the magnitude increases with Ta [Fig. 1]. Meanwhile, for the series 2, the largest AHE is obtained at around the stoichiometric composition [Fig. 2]. We here note that the two series are prepared at different times, leading to different magnitude of AHE. From a comparison with XRD results, we find that magnitude of AHE is well explained by multiple factors of crystalline structure including proportion of D019-Mn3Sn crystallite, crystalline orientation, and the order parameter. The obtained results provide an important insight to deepen the physics of non-collinear antiferromagnet and to realize new-functional devices. This work is partly supported by JSPS Kakenhi No. 19H05622 and No. 19J13405.References: [1] S. Nakastuji, N. Kiyohara and T. Higo, Nature, Vol. 527, p.212 (2015). [2] A. K. Nayak, J. E. Fischer and S. S. P. Parkin, Sci. Adv., Vol. 15, p.e1501870 (2016). [3] T. Higo, D. Qu and S. Nakatsuji, Appl. Phys. Lett., Vol. 113, p.202402 (2018). [4] T. Ikeda, M. Tsunoda and Y. Ando, Appl. Phys. Lett., Vol. 113, p.222405 (2018). [5] J. Yoon, Y. Takeuchi and H. Ohno, Appl. Phys. Express, Vol. 13, p.013001 (2020).

Anomalous Hall Effect in Mn-Sn Thin Films - Correlation with Crystal Structure

In this work, we report on THz emission from FeMnPt-based synthetic antiferromagnetic structures (SAF), originating from the anomalous Hall effect (AHE) [1][2]. The peak amplitude of the THz emission is almost doubled as compared to the respective single layer with the same equivalent thickness. In addition, we demonstrate by both simulation and experiment that the THz emission provides a powerful tool to probe the magnetization reversal process of individual ferromagnetic layers in the SAF structure – an important building block for all types of spintronic devices. By measuring the polarization of the THz wave, we are able to determine the magnetization directions of the individual magnetic layers which are not possible or difficult to obtain by using other magnetic characterization techniques. Fig. 1 shows the shematic of the FeMnPt SAF structure under an in-plane external field. A femtosecond laser pulse is used to excite the sample, thereby generating the THz emission. Fig. 2 shows the temperoal profile of the THz emmission from both the FeMnPt SAF (solide-line: 0 Oe; dashed-line: 200 Oe) and a single layer FeMnPt (dotted-line). At zero field, The AHE in both layers of the SAF contributes to the THz emission, leading to a larger THz amplitude, which is almost doubled that of the single layer. By correalting the simulated and experimental results, the direction of magnetizaion in each FM layer during field sweeping can be determined becasue the THz emission is propotional to the dedection of mangetization in each FM layer. Our results show that THz emission provides a powerful tool to understand the magnetic reversal of SAFs.References: [1] Q. Zhang, Z. Luo, H. Li, et al., Physical Review Applied, vol.12, p.054027, (2019) [2] Q. Zhang, Y. Yang, Z. Luo, et al., Physical Review Applied, vol.13, p.054016, (2020)

Terahertz Emission From an Exchange-Coupled Synthetic Antiferromagnet

Exotic spin torques resulting from reduced symmetry in quantum materials stand out as strong possibilities for advancing the future of MRAM and spin-torque based neuromorphic computing. Magnetic ordering can reduce the symmetry of a material in a manipulatable way, generating customizable spin torques, and is thus an attractive trait in spin source materials. Antiferromagnets are particularly attractive due to the robustness of their magnetic ordering, which can be prepared into a state and remain stable in that state in a wide range of device setups. Iron rhodium (FeRh) exhibits a transition between a low-temperature antiferromagnetic state and a high-temperature ferromagnetic state. For appropriately grown thin film FeRh, this transition can be found close to room temperature, making both of its magnetic states easily accessible. We report large exotic spin torques generated in an antiferromagnetic FeRh spin source layer and investigate the role of magnetic ordering on the size and geometry of generated spin torques. We also show strong temperature dependence of the spin torque efficiency in the antiferromagnetic phase of FeRh. This work was supported as part of Quantum Materials for Energy Efficient Neuromorphic Computing, an Energy Frontier Research Center funded by the U.S. DOE, Office of Science.

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