Author: ehilp

New Developments on Chromium Trihalides 2D Ferromagnets

David Soriano

The discovery of 2D ferromagnets in 2017 has opened new ways to explore novel physical phenomena at the nanoscale. In the first part of my talk, I will briefly discuss the recent advances in chromium trihalides, from fundamentals to applications.[1] In the second part, I will focus on our recent work regarding the role of Coulomb interactions in the intralayer exchange, the electrical tunability of interlayer magnetism in bilayer CrI3,[2] and the exchange proximity effects in van der Waals heterostructures containing chromium trihalides.[3,4] [1] Magnetic Two-Dimensional Chromium Trihalides: A Theoretical Perspective. D. Soriano, M. I. Katsnelson, and J. Fernández-Rossier. Submitted to Nano Letters.

[2] Magnetic polaron and antiferromagnetic-ferromagnetic transition in doped bilayer CrI3. D. Soriano, and M. I. Katsnelson. Phys. Rev. B 101, 041402(R) (2020)

[3] Van der Waals Spin Valves. C. Cardoso, N. A. García-Martínez, and J. Fernández-Rossier. Phys. Rev. Lett. 121, 067701 (2018)

[4] Exchange-bias controlled correlations in magnetically encapsulated twisted van der Waals dichalcogenides. D. Soriano, and J. L. Lado. arXiv:2006.09953

van der Waals layered magnetic semiconductors

Young Hee Lee

 

The ferromagnetic state in van der Waals two-dimensional (2D) materials has been reported recently in the monolayer limit. Intrinsic CrI3 and CrGeTe3 semiconductors reveal ferromagnetism but the Tc is still low below 60K. In contrast, monolayer VSe2 is ferromagnetic metal with Tc above room temperature but incapable of controlling its carrier density. Moreover, the long-range ferromagnetic order in doped diluted chalcogenide semiconductors has not been demonstrated at room temperature. The key research target is to realize the long-range order ferromagnetism, Tc over room temperature, and semiconductor with gate tunability. Here, Ferromagnetic order is manifested using magnetic force microscopy up to 360K, while retaining high on/off current ratio of ~105 at 0.1% V-doping concentration. The V-substitution to W siteskeep a V-V separation distance of 5 nm without V-V aggregation, scrutinized by high-resolution scanning transmission-electron-microscopy. More importantly, the ferromagnetic order is clearly modulated by applying a back gate. We also observe a ferromagnetic hysteresis loop together with oscillatory behavior at room temperature in diluted V-doped WSe2, while maintaining the semiconducting characteristic of WSe2 with a high on/off current ratio of five orders of magnitude. Our findings open new opportunities for using two-dimensional transition metal dichalcogenides for future spintronics.

On-line SPICE-SPIN+X Seminars

On-line Seminar: 15 July 2020 - 15:00 (CET)

Crystal time-reversal symmetry breaking and spin splitting in collinear antiferromagnets

Libor Šmejkal, JGU, Mainz

Relativistic bandstructure of solids generates functionalities of modern quantum, topological and spintronics materials. Common collinear antiferromagnets exhibit Kramers spin degenerate bands and for many decades were believed to be excluded from spin splitting physics and spontaneous Hall effects. Our recent prediction of crystal time-reversal symmetry breaking by anisotropic magnetization densities (see picture) due to the collinear antiferromagnetism combined with nonmagnetic atoms changes this perspective. Unlike the conventional relativistic spin-orbit interaction induced spin splitting, our crystal antiferromagnetic spin splitting is of exchange origin, can reach giant eV values, and can preserve spin quantum number.
In this talk, we will discuss the basic properties of this new type of antiferromagnetic spin splitting, its local magnetic symmetry origin and symmetry criteria for its emergence and we will catalogue broad class of material candidates. Furthermore, we will show that this antiferromagnetic spin splitting can generate a crystal Hall effect controllable via rearrangement of nonmagnetic atoms. Finally, we will present an experimental discovery of crystal Hall effect in ruthenium dioxide antiferromagnet.

 

 

Šmejkal, L., Mokrousov, Y., Yan, B. & MacDonald, A. H. Topological antiferromagnetic spintronics. Nat. Phys. 14, 242 (2018).
Šmejkal, L., Železný, J., Sinova, J. & Jungwirth, T. Electric Control of Dirac Quasiparticles by Spin-Orbit Torque in an Antiferromagnet. Phys. Rev. Lett. 118, 106402 (2017), arXiv (2016)
Šmejkal, L., González-Hernández, R., Jungwirth, T. & Sinova, J. Crystal time-reversal symmetry breaking and spontaneous Hall effect in collinear antiferromagnets. Sci. Adv. 6, eaaz8809 (2020), arXiv (2019)
Feng, Z.*, Zhou, X.*, Šmejkal, L.*, González-Hernández, R., Sinova, J., Jungwirth, T., Liu, Z. et al. Observation of the Crystal Hall Effect in a Collinear Antiferromagnet. arXiv (2020).

 

On-line SPICE-SPIN+X Seminars

On-line Seminar: 17 June 2020 - 15:00 (CET)

Coherent Sub-Terahertz Spin Pumping from an Insulating Antiferromagnet

Enrique del Barco, University of Central Florida

Emerging phenomena, such as the spin-Hall effect (SHE), spin pumping, and spin-transfer torque (STT), allow for interconversion between charge and spin currents and the generation of magnetization dynamics that could potentially lead to faster, denser, and more energy efficient, non-volatile memory and logic devices. Present STT-based devices rely on ferromagnetic (FM) materials as their active constituents. However, the flexibility offered by the intrinsic net magnetization and anisotropy for detecting and manipulating the magnetic state of ferromagnets also translates into limitations in terms of density (neighboring elements can couple through stray fields), speed (frequencies are limited to the GHz range), and frequency tunability (external magnetic fields needed). A new direction in the field of spintronics is to employ antiferromagnetic (AF) materials. In contrast to ferromagnets, where magnetic anisotropy dominates spin dynamics, in antiferromagnets spin dynamics are governed by the interatomic exchange interaction energies, which are orders of magnitude larger than the magnetic anisotropy energy, leading to the potential for ultrafast information processing and communication in the THz frequency range, with broadband frequency tunability without the need of external magnetic fields.

PDF file of the talk available here

I will present evidence of sub-terahertz coherent spin pumping at the interface of a uniaxial insulating antiferromagnet MnF2 and platinum thin films, measured by the ISHE voltage signal arising from spin-charge conversion in the platinum layer. The ISHE signal depends on the chirality of the dynamical modes of the antiferromagnet, which is selectively excited and modulated by the handedness of the circularly polarized sub-THz irradiation (see figure). Contrary to the case of ferromagnets, antiferromagnetic spin pumping exhibits a sign dependence on the chirality of dynamical modes, allowing for the unambiguous distinction between coherent spin pumping and the thermally-driven, chirality-independent spin Seebeck effect. Our results open the door to the controlled generation of coherent pure spin currents with antiferromagnets at unprecedented high frequencies.

This work has been primarily supported by the Air Force Office of Scientific Research under Grant FA9550-19-1-0307.

References

  • Priyanka Vaidya, Sophie A. Morley, Johan van Tol, Yan Liu, Ran Cheng, Arne Brataas, David Lederman, and Enrique del Barco, Subterahertz spin pumping from an insulating antiferromagnet, Science 368, 160-165 (2020)