Session T68: Magnetic Manipulation and Excitations in 2D Magnets

Efficient Control of 2D Magnets

The emergent two-dimensional (2D) layered magnets provide ideal platforms to enable the atomic-thin magneto-optical and magnetoelectric devices. Though many have envisioned that 2D magnets should allow efficient control of magnetism by a variety of external stimuli, true breakthroughs are still lacking, with limited proof-of-concept demonstrations reported thus far. There appear to be fundamental obstacles for efficient control, e.g., through electrical and optical means. In this talk I will analyze the challenges and present our theoretical and experimental progress on efficient electrical and optical control of 2D magnets. Specifically, the results show that the laser shinning of tens of uW/um² can effectively affect 2D magnets, and the voltage of a few volts can effectively change the magnetic anisotropy of 2D magnets. These efficient controls of 2D magnets potentially open up new avenues towards low-power spintronics and photonics.

    

Magnetic excitations in 2D van der Waals honeycomb ferromagnets

The recent discovery of 2D ferromagnets opens up a vast field of research in nanomaterials and spintronics. Probing the magnetic excitations and their interplay with other degrees of freedom (electronic, lattice, etc.) is crucial in order to understand the underlying interactions and potential applications to future spintronic devices. Furthermore, the nontrivial topology in their magnon bands could be used to realize dissipationless spin transport. In this talk, I will present the inelastic neutron scattering (INS) measurement of magnetic excitations in honeycomb ferromagnet insulators CrI3, CrGeTe3, and their isostructural compounds. Using INS, we identified topological gaps in the spin excitations in CrI3, then confirmed that the topological gap in CrI3 is mainly a result of the Dzyaloshinskii-Moriya (DM) interaction rather than the Kitaev interaction. We also investigated the strong spin-lattice coupling in CrGeTe3, and observed anharmonic non-linear magnon-phonon scattering in this compound. Such a process severely shortens magnon lifetime and renormalizes magnon energy. Our results provide crucial insights into the spin-driven phenomena in these 2D van der Waals ferromagnetic systems.   

Epitaxial growth of 2D van der Waals magnets and heterostructures with topological insulators

I will discuss our latest advances on the epitaxial growth of 2D van der Waals (vdW) magnets and their integration with topological insulators (TI). This work is motivated by the realization of topological phases such as the quantum anomalous Hall effect and highly efficient spin-orbit torque produced by TIs.

               Our initial studies of MnSe₂ growth on Bi₂Se₃ showed a tendency for the interdiffusion of Mn into the Bi₂Se₃. [1]. This ultimately led to the synthesis of MnBi₂Se₄ (MBS), a new magnetic TI [2]. Interestingly, the vdW phase is not the thermodynamically stable phase and bulk crystals do not exist, so the epitaxial stabilization of MBS creates the opportunity to explore the magnetic and topological properties of this material. We find that MBS is a layered antiferromagnet, similar to MnBi₂Te₄, but a difference is that the magnetic moments lie in the plane of the film. Angle-resolved photoemission experiments show the presence of a topological surface state with Dirac dispersion.

               For bilayers of 2D magnets and TIs, we have developed FGT films on Bi₂Te₃. We first optimized FGT by studying its growth on Ge(111) substrates, where we find that kinetic considerations play a major role [3]. Using cross-sectional scanning transmission electron microscopy and scanning tunneling microscopy, we optimize the FGT films to have atomically smooth surfaces and abrupt interfaces with the Ge(111). Subsequently, we have developed the growth of FGT on Bi₂Te₃ for the integration of 2D magnets with TIs. Interestingly, we observe room temperature ferromagnetism in FGT/Bi₂Te₃ heterostructures by varying the growth conditions.

    

Spin Manipulation in van der Waals Ferromagnets and Antiferromagnets

van der Waals materials have a number of properties that make them intriguing for spintronic devices.  They can be thinned to the two-dimensional limit while maintaining good magnetic properties, they can manifest a many different magnetic states and a wide range of resistivities, and they can be incorporated into heterostructures with pristine interfaces and large magnetoresistance.  Here, I will discuss several experiments designed to understand and enable control of the spin dynamics in these materials.  

Our research group has developed techniques to integrate insulating vdW ferromagnets with layers of heavy metals or topological insulators.  With the heavy metals we demonstrate efficient transmission of spin currents to generate spin-orbit torque on the vdW magnet.  With the topological insulators we show that an insulating magnet can induce an anomalous Hall effect whose amplitude can be tuned by gate voltage with a strong peak near the Dirac point.  This is the signature expected due to Berry curvature associated with an exchange gap in the topological surface states.  

We are also investigating the spin-dynamics in the layered antiferromagnet CrSBr, which has biaxial magnetic anisotropy with an easy axis in-plane.  Depending on the angle and magnitude of applied magnetic field, this anisotropy allows for a variety of states (antiparallel spin sublattices, spin-flop, and spin-flip), each with distinct behavior for the pair of antiferromagnetic resonances.  We have characterized these modes using resonance measurements are exploring whether they can be addressed individually and excited by spin-orbit torque.    

Anomalous thermal Hall effect in insulating van der Waals magnets

Thermal transport is also an important technique for studying low-energy excitations of quantum materials. Specifically, there has been considerable interest in using thermal transport measurements on insulating solids to probe their charge-neutral quasi-particles, i.e., magnons and phonons. In this talk, we will present our recent thermal transport studies on insulating van der Waals (vdW) magnets, focusing on ferromagnetic VI3. We show that this material exhibits an anomalous thermal Hall effect with large thermal Hall signal over a wide temperature region. The thermal Hall signal persists in the absence of an external magnetic field and flips sign upon the switching of the magnetization. The observed thermal Hall effect is of dual nature, dominated by topological magnons hosted by the ferromagnetic honeycomb lattice at higher temperatures while being driven by magnon polarons via magnon-phonon coupling at lower temperatures. If time permits, we will also present some new thermal transport studies on other 2D vdW magnets.