Session N22: Spin-Dependent Phenomena in Semiconductors: van der Waals Magnetic Dynamics

Spin dynamics and exchange interactions from a van der Waals antiferromagnet

van der Waals (vdW) magnets provide unique opportunities to explore magnetic phenomena and spin dynamics down to the atomically-thin limit. Additionally, when paired with other vdW materials into 2D heterostructures, they can also enable new phenomena and functionalities arising from interfacial exchange interactions.

 

In this two-part talk, we will first present our recent progress in identifying the gigahertz antiferromagnetic resonances within the vdW antiferromagnet CrSBr with a significant triaxial magnetic anisotropy with the easy axis oriented within the vdW plane. [1]. This anisotropy modifies the form of the antiferromagnetic resonances and allows the excitation of different types of modes depending on the external field direction. We measure hybridized Left-handed and Right-handed chiral resonance modes when the magnetic field is applied parallel to the in-plane easy axis, in addition to the optical and acoustic modes when the field is applied perpendicular to the easy axis. We also achieve hybridization of the optical and acoustic modes through a rotation of the magnetic field away from the high symmetry axis.In addition, we will describe direct electrical detection of the antiferromagnetic resonance dynamics in few-layer CrSBr devices via their effect on tunnel magnetoresistance, and the effects of externally-applied spin-orbit torque on the mode damping. 

 

In the second part, we will show that exchange bias from CrSBr acting on the vdW ferromagnet Fe3GeTe2 induces a spatially non-uniform spin configuration through the thickness of the Fe3GeTe2 that is not readily achievable with conventional magnetic materials. We show that CrSBr exerts an in-plane exchange bias on the FGT, with a direction parallel to the in-plane anisotropy axis of the CrSBr.  This in-plane exchange bias provides  sufficient symmetry breaking needed to enable field-free spin-orbit-torque switching in Pt/Fe3GeTe2/CrSBr heterostructures [2]. Detailed temperature and thickness studies shows that a minimum thickness of the CrSBr layer of about 10 nm is required for a non-zero exchange bias at 30 K.   

Emergence of exchange bias in van der Waals magnetic alloy CrxPt1−xTe

Van der Waals magnetic alloys are able to use alloy concentration to introduce desirable properties for applications. CrxPt1₋xTe2 is a recently developed magnetic alloy noted for its stability in ambient conditions. We study the effects of the alternating Cr concentration in CrxPt1−xTe2 (x ≥ 0.4) by measuring and comparing the hysteresis of x = 0.35 and x = 0.45. We observe the emergence of an exchange bias effect in Cr0.45Pt0.55Te2, without typical exchange bias sources such as an adjacent antiferromagnetic. We perform Monte Carlo simulations utilizing parameters from first-principles calculations that recreate the exchange bias in hysteresis loops of Cr0.45Pt0.55Te2. From our simulations, we infer the source of our exchange bias to be magnetic moments locked into free energy minima that resist macroscopic symmetric magnetization reversal. This work presents multiple ways to introduce desirable magnetic properties to van der Waals magnets.   

Optimizing the Activation of Acoustic and Optical Magnons in Synthetic Antiferromagnets and Ferrimagnets through Field-like and Damping-like Torques

Synthetic magnetic materials, constructed via thin-film deposition, serve as a robust platform for exploring antiferromagnetic and ferrimagnetic behaviors. In such systems, both acoustic and optical magnon modes can be excited at GHz frequencies using ferromagnetic resonance-based (FMR) techniques. Typically, by employing a system of coupled Landau-Lifschitz-Gilbert (LLG) equations, one can predict magnon frequencies and interactions across a myriad of structures. Despite the general agreement between spectroscopy data and the eigenmodes predicted by the LLG model, a particular mode (or modes) may remain quiescent in the spectroscopy data across a range of external field strengths—an observation that has yet to be adequately explained. We develop a theoretical framework that models the coupling strength of a polarized driving field (as well as polarized damping-like torques) to a magnon mode over a range of external field strengths. We show that our model of coupling strength aligns with previously reported spectroscopy data for various synthetic antiferromagnetic and ferrimagnetic systems. Our work provides a predictive guide for the ability of a spectroscopic method to drive a particular magnon branch.   

Optical helicity control of magnetic dynamics in TMD/2D Magnet heterostructure

2D van der Waals magnets provide versatile opportunities for controlling magnetization switching and studying the dynamics. With 2D magnet in proximity with a transition metal chalcogenide (TMD), many novel phenomena will happen such as magnetic proximity effect, valley Zeeman splitting and valley polarization in TMDs. On the other hand, when the 2D magnet comes down to few-layer limit, its magnetization will be more easily influenced by spin transferred from TMD. Here, we investigate spin and magnetization dynamics in TMD/2D magnet heterostructures, specifically WS₂/2L-Fe₃GeTe₂ (FGT)/Bi₂Te₃ heterostructure, using an ultrafast optical pump-probe technique. Different wavelengths are used to pump resonantly in TMD layer and probe magnetization in 2D magnet layer separately.  By performing photoluminescence (PL) measurements prior to time-resolved measurements, we observed much reduced PL intensity in monolayer WS₂ when interfaced with FGT, indicating strong charge transfer. The strong charge transfer and spin-valley locking nature in TMDs provides an opportunity to inject specific spin from WS₂ into FGT and modulate magnetic properties of the FGT by pumping with different optical helicity. We successfully observed difference in ultrafast magnetization dynamics with left and right circular polarized pump, which indicates possibly the combination of a thermal demagnetization effect and a much faster spin injection process. Time-resolved measurements and control experiments are still ongoing to be able to better analyze and explain the observed dynamics. We also perform measurements on heterostructures of TMD/magnetic semiconductors.   

Ultrafast Widefield Imaging of van der Waals Antiferromagnet

Two-dimensional (2D) antiferromagnetic materials have recently emerged as promising physical platforms for realizing exotic phenomena ranging from topological magnon transport to spin-driven nano-oscillators. However due to lack of suitable techniques, the spatially resolved dynamics of the few- layer limit has remained largely unexplored. Here we report the development of a novel ultrafast optical technique that allows us to fully capture the spatio-temporal evolution of a driven antiferromagnetic system, enabling further detection of unconventional spin excitations in these platforms.   

Magnons in 2D magnetic materials and the role of spin-orbit coupling in the topological properties

Magnetism in 2D materials is accompanied by novel phenomena related to the properties of spin-waves (the so-called magnons). Magnons exhibit topological properties in the family of the layered chromium trihalides like in the case of the paradigmatic CrI₃. In this work we present an ab initio approach to calculate magnons within the framework of many-body perturbation theory and solving the Bethe-Salpeter Equation. The approach is implemented with full spinorial wave functions, accounting for the spin-orbit interaction. The resulting magnon dispersions do not rely on any assumptions of the microscopic magnetic interactions. We characterize the magnon dispersion for the family of monolayers chromium trihalides and we compare the results with the spin wave dispersion obtained from the Heisenberg model. We find clear differences between ab initio results and the spin wave model. Moreover, we focus on the bandgap at the K point of the dispersion. We rationalize the topological bandgap in terms of the spin-orbit coupling originated by the halide atoms.   

Emergence of 100% Spin Polarization in Non-Collinear Magnets

Recently, large tunneling magnetoresistance (TMR) effects have been predicted and demonstrated in antiferromagnetic tunnel junctions (AFMTJs) with non-collinear antiferromagnetic (AFM) electrodes, that is promising for future information technology applications. These results indicated a very large transport spin polarization provided by AFM metals, which is unexpected due to the non-collinearity of their magnetic moments. In this work, we elucidate the origin of this giant spin polarization using simple tight-binding models and calculations based on density functional theory (DFT). Using tight-binding Hamiltonians to describe an infinite 1D chain of non-collinear magnetic moments and a 2D Kagome lattice in a non-collinear AFM phase, we show that fully spin-polarized conduction channels appear in these systems and reveal their origin. Using DFT calculations, we further demonstrate the presence of 100% spin-polarized states in non-collinear AFM antiperovskites XNMn3 (X = Ga, Sn, …) leading to giant TMR values in AFMTJs based on XNMn3 electrodes. Our results demonstrate great potential of non-collinear AFM metals for spintronics.   

Influence of heteroatom doping on hyperfine physics in graphene nanostructures

Recent developments in the field of on-surface synthesis (OSS) strategies have led to a surge in the fabrication of atomically perfect triangular flakes of nanographenes with increasing size. Additionally, the insertion of heteroatoms (nitrogen, boron, deuterium), or functional groups has been found to alter the electronic and magnetic properties of these nanographenes. We explore the effect of such heteroatom insertions on the physics of hyperfine interaction (HFI) in these molecular magnets. Using all-electron density functional theory (DFT) calculations such as ORCA, we report the hyperfine tensors for molecules such as the nitrogen-doped triangulenes ([n]-aza triangulenes) as well as boron-doped graphene nanoflakes. Our results indicate appreciable modifications in the magnitudes of hyperfine splitting for the $^{13}$C and $^{1}$H atoms in the presence of the dopants. We also discuss some implications of reported HFI for electron-spin decoherence and spin-dephasing times for application in spin-qubit-based operations.   

Theory of Hyperfine Interactions in Graphene-Based Nanostructures: σ - π Correlations

Open-shell nanographene molecules offer unique possibilities in applications such as spintronics and quantum computing due to their novel physical properties related to magnetism and unconventional topological phases of matter. We focus on the electron-nuclear interactions characterized by the hyperfine couplings in these molecular magnets. Using theoretical tools based on all-electron density functional theory (DFT), tight-binding (TB) models with single π-orbitals within the Mean-Field Hubbard (MFH) description, and perturbative techniques, we demonstrate that the physical origin of these hyperfine splittings relates to exchange interactions between π and σ orbitals. Our results establish the well-known McConnell's equation for the proton hyperfine splittings and provide an intuitive way of understanding the electronic structure of the molecule.