More Like this

1. Large magnetic anisotropy and magnetostriction in thin films of ${\mathrm{CoV}}_2{\mathrm{O}}_4$

   https://doi.org/10.1103/PhysRevMaterials.7.014408  

   Kim, Sangsoo; Thompson, Christie J.; Xin, Yan; Beekman, Christianne (January 2023, Physical Review Materials)

   Spinel cobalt vanadate CoV2O4 has been grown on (001) SrTiO3 substrates. Using torque magnetometry experiments, we find that the previously observed temperature-induced anisotropy change, where the easy axis changes from the out-of-plane [001] direction to a biaxial anisotropy with planar <100> easy axes, occurs in a gradual second-order structural phase transition. This paper characterizes this transition and the magnetic anisotropies in the (001), (100), and (-110) rotation planes, and explores their field dependence up to 30 T. Below 80 K, hysteretic features appear around the hard axes, i.e., the out-of-plane direction in (-110) and (010) rotations and the planar <110> directions in (001) rotations. This is due to a Zeeman energy that originates from the lag of the magnetization with respect to the applied magnetic field as the sample is rotated. The appearance of the hysteresis, which persists up to very high fields, shows that the anisotropy at low temperature is rather strong. Additionally, field-dependent distortions to the symmetry of the torque response in increasing applied fields shows that magnetostriction plays a large role in determining the direction and magnitude of the anisotropy. 

   more » « less
2. Large spin–orbit torque in bismuthate-based heterostructures

   https://doi.org/10.1038/s41928-023-01080-1  

   Edgeton, Anthony L; Harris, Isaac A; Campbell, Neil G; Chai, Yahong; Mazur, Marcel M; Gurung, Gautam; Huang, Xiaoxi; Susarla, Sandhya; Tsymbal, Evgeny Y; Ralph, Daniel C; et al (December 2023, Nature Electronics)

   The wider application of spintronic devices requires the development of new material platforms that can efficiently manipulate spin. Bismuthate-based superconductors are centrosymmetric systems that are generally thought to offer weak spin–orbit coupling. Here, we report a large spin–orbit torque driven by spin polarization generated in heterostructures based on the bismuthate BaPb1-xBixO3 (which is in a non-superconducting state). Using spin-torque ferromagnetic resonance and d.c. non-linear Hall measurements, we measure a spin–orbit torque efficiency of around 2.7 and demonstrate current driven magnetization switching at current densities of 4×10^5 A〖cm〗^(-2). We suggest that the unexpectedly large current-induced torques could be the result of an orbital Rashba effect associated with local inversion symmetry breaking in BaPb1-xBixO3. 

   more » « less
3. Electronic structure of Co $3d$ states in the Kitaev material candidate honeycomb cobaltate ${\mathrm{Na}}_3{\mathrm{Co}}_2{\mathrm{SbO}}_6$ probed with x-ray dichroism

   https://doi.org/10.1103/PhysRevB.107.214443  

   van Veenendaal, M.; Poldi, E. H.; Veiga, L. S.; Bencok, P.; Fabbris, G.; Tartaglia, R.; McChesney, J. L.; Freeland, J. W.; Hemley, R. J.; Zheng, H.; et al (June 2023, Physical Review B)

   none (Ed.)

   The recent prediction that honeycomb lattices of Co2+ (3d7) ions could host dominant Kitaev interactions provides an exciting direction for exploration of new routes to stabilizing Kitaev’s quantum spin liquid in real materials. Na3Co2SbO6 has been singled out as a potential material candidate provided that spin and orbital moments couple into a Jeff = 1/2 ground state, and that the relative strength of trigonal crystal field and spin-orbit coupling acting on Co ions can be tailored. Using x-ray linear dichroism (XLD) and x-ray magnetic circular dichroism (XMCD) experiments, alongside configuration interaction calculations, we confirm the counterintuitive positive sign of the trigonal crystal field acting on Co2+ ions and test the validity of the Jeff = 1/2 description of the electronic ground state. The results lend experimental support to recent theoretical predictions that a compression (elongation) of CoO6 octahedra along (perpendicular to) the trigonal axis would drive this cobaltate toward the Kitaev limit, assuming the Jeff = 1/2 character of the electronic ground state is preserved. 

   more » « less
4. Anomalous Landau Level Gaps Near Magnetic Transitions in Monolayer ${\mathrm{WSe}}_2$

   https://doi.org/10.1103/PhysRevX.14.031018  

   Foutty, Benjamin A; Calvera, Vladimir; Han, Zhaoyu; Kometter, Carlos R; Liu, Song; Watanabe, Kenji; Taniguchi, Takashi; Hone, James C; Kivelson, Steven A; Feldman, Benjamin E (August 2024, Physical Review X)

   First-order phase transitions produce abrupt changes to the character of both ground and excited electronic states. Here we conduct electronic compressibility measurements to map the spin phase diagram and Landau level (LL) energies of monolayer ${\mathrm{WSe}}_2$in a magnetic field. We resolve a sequence of first-order phase transitions between completely spin-polarized LLs and states with LLs of both spins. Unexpectedly, the LL gaps are roughly constant over a wide range of magnetic fields below the transitions, which we show reflects spin-polarized ground states with opposite spin excitations. These transitions also extend into compressible regimes, with a sawtooth boundary between full and partial spin polarization. We link these observations to the important influence of LL filling on the exchange energy beyond a smooth density-dependent contribution. Our results show that ${\mathrm{WSe}}_2$realizes a unique hierarchy of energy scales where such effects induce reentrant magnetic phase transitions tuned by density and magnetic field. Published by the American Physical Society2024 

   more » « less
5. Photon Energy-Dependent Optical Spin-Orbit Torque in Heavy Metal-Ferromagnet Bilayers

   https://doi.org/10.1002/adfm.202307753  

   Wang, Chuangtang; Xu, Yihao; Liu, Yongmin (October 2023, Advanced Functional Materials)

   Abstract The manipulation of magnetization through optically generated ultrafast spin currents is a fascinating area that needs a thorough understanding for its potential future applications. In this work, a comprehensive investigation of helicity‐driven optical spin‐orbit torque in heavy metal/ferromagnetic metal heterostructures is presented, specifically cobalt capped with gold or platinum, subject to laser pumping at different wavelengths. The results demonstrate up to tenfold enhancement in optical spin‐orbit torque quantum efficiency for gold compared to platinum of the same thickness when pumped with a visible laser. Additionally, the study provides the first experimental analysis of the photon energy dependence of optical spin‐orbit torque and derives the optical spin orientation spectra for both gold/cobalt and platinum/cobalt heterostructures. A key insight gained from the study is the impact of photon energy‐dependent spin transport in the system, which suggests the use of a high photon energy pump for efficient spin transport. These findings highlight the potential of spin current generation and manipulation in gold/ferromagnet heterostructures for a wide range of applications such as all‐optical magnetization switching, spin‐wave generation and control, and spintronic terahertz emission. 

   more » « less