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1. Electric-field control of spin dynamics during magnetic phase transitions

   https://doi.org/10.1126/sciadv.abd2613  

   Nan, Tianxiang ; Lee, Yeonbae ; Zhuang, Shihao ; Hu, Zhongqiang ; Clarkson, James D. ; Wang, Xinjun ; Ko, Changhyun ; Choe, HwanSung ; Chen, Zuhuang ; Budil, David ; et al ( October 2020 , Science Advances)

   Controlling magnetization dynamics is imperative for developing ultrafast spintronics and tunable microwave devices. However, the previous research has demonstrated limited electric-field modulation of the effective magnetic damping, a parameter that governs the magnetization dynamics. Here, we propose an approach to manipulate the damping by using the large damping enhancement induced by the two-magnon scattering and a nonlocal spin relaxation process in which spin currents are resonantly transported from antiferromagnetic domains to ferromagnetic matrix in a mixed-phased metallic alloy FeRh. This damping enhancement in FeRh is sensitive to its fraction of antiferromagnetic and ferromagnetic phases, which can be dynamically tuned bymore »electric fields through a strain-mediated magnetoelectric coupling. In a heterostructure of FeRh and piezoelectric PMN-PT, we demonstrated a more than 120% modulation of the effective damping by electric fields during the antiferromagnetic-to-ferromagnetic phase transition. Our results demonstrate an efficient approach to controlling the magnetization dynamics, thus enabling low-power tunable electronics.« less
2. Voltage controlled Néel vector rotation in zero magnetic field

   https://doi.org/10.1038/s41467-021-21872-3  

   Mahmood, Ather ; Echtenkamp, Will ; Street, Mike ; Wang, Jun-Lei ; Cao, Shi ; Komesu, Takashi ; Dowben, Peter A. ; Buragohain, Pratyush ; Lu, Haidong ; Gruverman, Alexei ; et al ( December 2021 , Nature Communications)

   Abstract Multi-functional thin films of boron (B) doped Cr 2 O 3 exhibit voltage-controlled and nonvolatile Néel vector reorientation in the absence of an applied magnetic field, H . Toggling of antiferromagnetic states is demonstrated in prototype device structures at CMOS compatible temperatures between 300 and 400 K. The boundary magnetization associated with the Néel vector orientation serves as state variable which is read via magnetoresistive detection in a Pt Hall bar adjacent to the B:Cr 2 O 3 film. Switching of the Hall voltage between zero and non-zero values implies Néel vector rotation by 90 degrees. Combined magnetometry, spin resolved inverse photoemission,more »electric transport and scanning probe microscopy measurements reveal B-dependent T N and resistivity enhancement, spin-canting, anisotropy reduction, dynamic polarization hysteresis and gate voltage dependent orientation of boundary magnetization. The combined effect enables H  = 0, voltage controlled, nonvolatile Néel vector rotation at high-temperature. Theoretical modeling estimates switching speeds of about 100 ps making B:Cr 2 O 3 a promising multifunctional single-phase material for energy efficient nonvolatile CMOS compatible memory applications.« less
3. Bulk-like dielectric and magnetic properties of sub 100 nm thick single crystal Cr2O3 films on an epitaxial oxide electrode

   https://doi.org/10.1038/s41598-020-71619-1  

   Vu, N. M. ; Luo, X. ; Novakov, S. ; Jin, W. ; Nordlander, J. ; Meisenheimer, P. B. ; Trassin, M. ; Zhao, L. ; Heron, J. T. ( December 2020 , Scientific Reports)

   Abstract The manipulation of antiferromagnetic order in magnetoelectric Cr 2 O 3 using electric field has been of great interest due to its potential in low-power electronics. The substantial leakage and low dielectric breakdown observed in twinned Cr 2 O 3 thin films, however, hinders its development in energy efficient spintronics. To compensate, large film thicknesses (250 nm or greater) have been employed at the expense of device scalability. Recently, epitaxial V 2 O 3 thin film electrodes have been used to eliminate twin boundaries and significantly reduce the leakage of 300 nm thick single crystal films. Here we report the electricalmore »endurance and magnetic properties of thin (less than 100 nm) single crystal Cr 2 O 3 films on epitaxial V 2 O 3 buffered Al 2 O 3 (0001) single crystal substrates. The growth of Cr 2 O 3 on isostructural V 2 O 3 thin film electrodes helps eliminate the existence of twin domains in Cr 2 O 3 films, therefore significantly reducing leakage current and increasing dielectric breakdown. 60 nm thick Cr 2 O 3 films show bulk-like resistivity (~ 10 12 Ω cm) with a breakdown voltage in the range of 150–300 MV/m. Exchange bias measurements of 30 nm thick Cr 2 O 3 display a blocking temperature of ~ 285 K while room temperature optical second harmonic generation measurements possess the symmetry consistent with bulk magnetic order.« less
4. Controlling spin current polarization through non-collinear antiferromagnetism

   https://doi.org/10.1038/s41467-020-17999-4  

   Nan, T. ; Quintela, C. X. ; Irwin, J. ; Gurung, G. ; Shao, D. F. ; Gibbons, J. ; Campbell, N. ; Song, K. ; Choi, S. -Y. ; Guo, L. ; et al ( December 2020 , Nature Communications)

   Abstract The interconversion of charge and spin currents via spin-Hall effect is essential for spintronics. Energy-efficient and deterministic switching of magnetization can be achieved when spin polarizations of these spin currents are collinear with the magnetization. However, symmetry conditions generally restrict spin polarizations to be orthogonal to both the charge and spin flows. Spin polarizations can deviate from such direction in nonmagnetic materials only when the crystalline symmetry is reduced. Here, we show control of the spin polarization direction by using a non-collinear antiferromagnet Mn 3 GaN, in which the triangular spin structure creates a low magnetic symmetry while maintainingmore »a high crystalline symmetry. We demonstrate that epitaxial Mn 3 GaN/permalloy heterostructures can generate unconventional spin-orbit torques at room temperature corresponding to out-of-plane and Dresselhaus-like spin polarizations which are forbidden in any sample with two-fold rotational symmetry. Our results demonstrate an approach based on spin-structure design for controlling spin-orbit torque, enabling high-efficient antiferromagnetic spintronics.« less
5. Insulator-to-conductor transition driven by the Rashba–Zeeman effect

   https://doi.org/10.1038/s41524-020-00441-0  

   Tao, Lingling ; Tsymbal, Evgeny Y. ( November 2020 , npj Computational Materials)

   The Rashba effect has recently attracted great attention owing to emerging physical properties associated with it. The interplay between the Rashba effect and the Zeeman effect, being produced by the exchange field, is expected to broaden the range of these properties and even result in novel phenomena. Here we predict an insulator-to-conductor transition driven by the Rashba–Zeeman effect. We first illustrate this effect using a general Hamiltonian model and show that the insulator-to-conductor transition can be triggered under certain Rashba and exchange-field strengths. Then, we exemplify this phenomenon by considering an Ag₂Te/Cr₂O₃heterostructure, where the electronic structure of the Ag₂Temore »monolayer is affected across the interface by the proximity effect of the Cr₂O₃antiferromagnetic layer with well-defined surface magnetization. Based on first-principles calculations, we predict that such a system can be driven into either insulating or conducting phase, depending on the surface magnetization orientation of the Cr₂O₃layer. Our results enrich the Rashba–Zeeman physics and provide useful guidelines for the realization of the insulator-to-conductor transition, which may be interesting for experimental verification.

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