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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)

   null (Ed.)

   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 by 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. 

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2. The spin polarization of palladium on magneto-electric Cr ₂ O ₃

   https://doi.org/10.1088/1361-648X/acc710  

   Komesu, Takashi; Echtenkamp, Will; Binek, Christian; Dowben, Peter A. (April 2023, Journal of Physics: Condensed Matter)

   Abstract While induced spin polarization of a palladium (Pd) overlayer on antiferromagnetic and magneto-electric Cr₂O₃(0001) is possible because of the boundary polarization at the Cr₂O₃(0001), in the single domain state, the Pd thin film appears to be ferromagnetic on its own, likely as a result of strain. In the conduction band, we find the experimental evidence of ferromagnetic spin polarized in Pd thin films on a Cr₂O₃(0001) single crystal, especially in the thin limit, Pd thickness of around 1–4 nm. Indeed there is significant spin polarization in 10 Å thick Pd films on Cr₂O₃(0001) at 310 K, i.e. above the Néel temperature of bulk Cr₂O₃. While Cr₂O₃(0001) has surface moments that tend to align along the surface normal, for Pd on Cr₂O₃, the spin polarization contains an in-plane component. Strain in the Pd adlayer on Cr₂O₃(0001) appears correlated to the spin polarization measured in spin polarized inverse photoemission spectroscopy. Further evidence for magnetization of Pd on Cr₂O₃is provided by measurement of the exchange bias fields in Cr₂O₃/Pd(buffer)/[Co/Pd]_nexchange bias systems. The magnitude of the exchange bias field is, over a wide temperature range, virtually unaffected by the Pd thickness variation between 1 and 2 nm. 

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3. Excitation and detection of coherent sub-terahertz magnons in ferromagnetic and antiferromagnetic heterostructures

   https://doi.org/10.1038/s41524-022-00851-2  

   Zhuang, Shihao; Hu, Jia-Mian (August 2022, npj Computational Materials)

   Abstract Excitation of coherent high-frequency magnons (quanta of spin waves) is critical to the development of high-speed magnonic devices. Here we computationally demonstrate the excitation of coherent sub-terahertz (THz) magnons in ferromagnetic (FM) and antiferromagnetic (AFM) thin films by a photoinduced picosecond acoustic pulse. Analytical calculations are also performed to reveal the magnon excitation mechanism. Through spin pumping and spin-charge conversion, these magnons can inject sub-THz charge current into an adjacent heavy-metal film which in turn emits electromagnetic (EM) waves. Using a dynamical phase-field model that considers the coupled dynamics of acoustic waves, spin waves, and EM waves, we show that the emitted EM wave retains the spectral information of all the sub-THz magnon modes and has a sufficiently large amplitude for near-field detection. These predictions indicate that the excitation and detection of sub-THz magnons can be realized in rationally designed FM or AFM thin-film heterostructures via ultrafast optical-pump THz-emission-probe spectroscopy. 

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4. 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)

   null (Ed.)

   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 electrical 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. 

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5. Nonvolatile Electric‐Field Control of Ferromagnetic Resonance and Spin Pumping in Pt/YIG at Room Temperature

   https://doi.org/10.1002/aelm.201800663  

   Yu, Rui; He, Kang; Liu, Qi; Gan, Xucheng; Miao, Bingfeng; Sun, Liang; Du, Jun; Cai, Hongling; Wu, Xiaoshan; Wu, Mingzhong; et al (February 2019, Advanced Electronic Materials)

   Abstract Electric‐field‐controlled magnetism is of importance in realizing energy efficient, dense and fast information storage and processing. Strain‐mediated converse magneto‐electric (ME) coupling between ferromagnetic and ferroelectric heterostructure shows promise for realizing electric‐controlled magnetism at room temperature and is attracting a number of recent investigations. However, such ME‐effect studies have mainly focus on magnetic metals. In this work, high quality yttrium iron garnet (Y₃Fe₅O₁₂(YIG)) films are deposited directly onto (100)‐oriented single‐crystal Pb (Mg_1/3Nb_2/3)_0.7Ti_0.3O₃(PMN‐PT) substrates by means of magnetron sputtering. The electric‐field‐induced polarization switching and lattice strain in the PMN‐PT substrate results in two distinct magnetization states in the YIG film that are nonvolatile and electrically reversible. Because of the direct contact between the YIG and the PMN‐PT substrate, an efficient ME coupling and an almost 90° rotation of the easy axis of the YIG film can be realized. Furthermore, the electric‐field‐controlled hysteresis loop‐like ferromagnetic resonance field shifts and spin pumping signals are observed in Pt/YIG/PMN‐PT heterostructures. Thus, the obstacle is overcome via growing high‐quality YIG thin films directly onto PMN‐PT substrates and an efficient manipulation of magnetism and pure spin current transport by electric field is thereby realized. These findings are instructive for future low‐power magnetic insulator‐based spintronic devices. 

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