Atomistic modelling of metamagnetic transition in FeRh with four-spin exchange

The metamagnetic transformation of FeRh from antiferromagnetic (AFM) to ferromagnetic (FM) ordering makes it suitable for a wide scope of applications, ranging from magnetic recording media to antiferromagnetic spintronics and magnetic refrigeration. Exchange spring systems of FeRh coupled with a hard magnetic layer (FePt) are a promising approach for heat-assisted magnetic recording technologies that assure high density of stored information. It has been shown that different temperature scalings of the exchange interactions can lead to a first-order phase transition in FeRh systems (Barker, J., & Chantrell, R. W. (2015). Higher-order exchange interactions leading to metamagnetism in FeRh. Physical Review B, 92(9), 094402). This model assumes the presence of a higher-order exchange term in the form of four-spin exchange, that arises from four consecutive hops of electrons from one spin configuration to the spin-flipped one, the higher order four-spin interaction being mediated in FeRh by the Rh atoms. At small temperatures the four-spin exchange is responsible for the AFM ordering, while, at higher temperatures the FM ordering is given by the bilinear exchange since the four-spin term decreases more rapidly with temperature than the bilinear term. In this work, the first-order phase transition that appears in FeRh is systematically studied via the four-spin parametric model that was previously given in literature. A degeneracy in the ground-state of the four-spin exchange system is found. The effect of the parameters entering into the spin Hamiltonian was systematically analysed. The model has been implemented for FeRh and then developed in order to consider other materials with different crystal structures. As nanoscale applications of the FeRh systems are more practical due to the high cost of Rh, the finite size effects of FeRh grains and thin films are systematically investigated. As a further test of the model, ultrafast simulations have been performed. In accordance to the literature, it is found that, by laser-heating the FeRh system, the ferromagnetic ordering is generated in picosecond time-scales. Additionally the dynamical and equilibrium properties of FePt/FeRh bilayers have been systematically investigated, as this system is of particular interest for recording media applications

Model of Magnetic Damping and Anisotropy at Elevated Temperatures : Application to Granular FePt Films

An understanding of the damping mechanism in finite-size systems and its dependence on temperature is a critical step in the development of magnetic nanotechnologies. In this work, nanosized materials are modeled via atomistic spin dynamics, the damping parameter being extracted from ferromagnetic resonance (FMR) simulations applied for FePt systems, generally used for heat-assisted magnetic recording media (HAMR). We find that the damping increases rapidly close to TC and the effect is enhanced with decreasing system size, which is ascribed to scattering at the grain boundaries. Additionally, FMR methods provide the temperature dependence of both damping and the anisotropy, which are important for the development of HAMR. Semianalytical calculations show that, in the presence of a grain-size distribution, the FMR line width can decrease close to the Curie temperature due to a loss of inhomogeneous line broadening. Although FePt has been used in this study, the results presented in the current work are general and valid for any ferromagnetic material

All-optical control of spin in a 2D van der Waals magnet

Two-dimensional (2D) van der Waals magnets provide new opportunities for control of magnetism at the nanometre scale via mechanisms such as strain, voltage and the photovoltaic effect. Ultrafast laser pulses promise the fastest and most energy efficient means of manipulating electron spin and can be utilized for information storage. However, little is known about how laser pulses influence the spins in 2D magnets. Here we demonstrate laser-induced magnetic domain formation and all-optical switching in the recently discovered 2D van der Waals ferromagnet CrI(3). While the magnetism of bare CrI(3) layers can be manipulated with single laser pulses through thermal demagnetization processes, all-optical switching is achieved in nanostructures that combine ultrathin CrI(3) with a monolayer of WSe(2). The out-of-plane magnetization is switched with multiple femtosecond pulses of either circular or linear polarization, while single pulses result in less reproducible and partial switching. Our results imply that spin-dependent interfacial charge transfer between the WSe(2) and CrI(3) is the underpinning mechanism for the switching, paving the way towards ultrafast optical control of 2D van der Waals magnets for future photomagnetic recording and device technology

A route to minimally dissipative switching in magnets via THz phonon pumping

Advanced magnetic recording paradigms typically use large temperature changes to drive switching which is detrimental to device longevity, hence finding non-thermal routes is crucial for future applications. By employing atomistic spin-lattice dynamics simulations, we show efficient coherent magnetisation switching triggered by THz phonon excitation in insulating single species materials. The key ingredient is excitation near the $P$-point of the spectrum in conditions where spins typically cannot be excited and when manifold $k$ phonon modes are accessible at the same frequency. Our model predicts the necessary ingredients for low-dissipative switching and provides new insight into THz-excited spin dynamics.Comment: 8 pages including supplementary informatio

Spin-lattice dynamics model with angular momentum transfer for canonical and microcanonical ensembles

A unified model of molecular and atomistic spin dynamics is presented enabling simulations both in microcanonical and canonical ensembles without the necessity of additional phenomenological spin damping. Transfer of energy and angular momentum between the lattice and the spin systems is achieved by a phenomenological coupling term representing the spin-orbit interaction. The characteristic spectra of the spin and phonon systems are analyzed for different coupling strength and temperatures. The spin spectral density shows magnon modes together with the uncorrelated noise induced by the coupling to the lattice. The effective damping parameter is investigated showing an increase with both coupling strength and temperature. The model paves the way to understanding magnetic relaxation processes beyond the phenomenological approach of the Gilbert damping and the dynamics of the energy transfer between lattice and spins

Non-equilibrium heating path for the laser-induced nucleation of metastable skyrmion lattices

Understanding formation of metastable phases by rapid energy pumping and quenching has been intriguing scientists for a long time. This issue is crucial for technologically relevant systems such as magnetic skyrmions which are frequently metastable at zero field. Using Atomistic Spin Dynamics simulations, we show the possibility of creating metastable skyrmion lattices in cobalt-based trilayers by femtosecond laser heating. Similar to the formation of supercooled ice droplets in the gas phase, high temperature ultrafast excitation creates magnon drops and their fast relaxation leads to acquisition and quenching of the skyrmion topological protection. The interplay between different processes corresponds to a specific excitation window which can be additionally controlled by external fields. The results are contrasted with longer-scale heating leading to a phase transition to the stable states. Our results provide insight into the dynamics of the highly non-equilibrium pathway for spin excitations and pave additional routes for skyrmion-based information technologies

Non-equilibrium heating path for the laser-induced nucleation of metastable skyrmion lattices

Understanding formation of metastable phases by rapid energy pumping and quenching has been intriguing scientists for a long time. This issue is crucial for technologically relevant systems such as magnetic skyrmions which are frequently metastable at zero field. Using Atomistic Spin Dynamics simulations, we show the possibility of creating metastable skyrmion lattices in cobalt-based trilayers by femtosecond laser heating. Similar to the formation of supercooled ice droplets in the gas phase, high temperature ultrafast excitation creates magnon drops and their fast relaxation leads to acquisition and quenching of the skyrmion topological protection. The interplay between different processes corresponds to a specific excitation window which can be additionally controlled by external fields. The results are contrasted with longer-scale heating leading to a phase transition to the stable states. Our results provide insight into the dynamics of the highly non-equilibrium pathway for spin excitations and pave additional routes for skyrmion-based information technologies

Probing spin dynamics of ultra-thin van der Waals magnets via photon-magnon coupling

Layered van der Waals (vdW) magnets can maintain a magnetic order even down to the single-layer regime and hold promise for integrated spintronic devices. While the magnetic ground state of vdW magnets was extensively studied, key parameters of spin dynamics, like the Gilbert damping, crucial for designing ultra-fast spintronic devices, remains largely unexplored. Despite recent studies by optical excitation and detection, achieving spin wave control with microwaves is highly desirable, as modern integrated information technologies predominantly are operated with these. The intrinsically small numbers of spins, however, poses a major challenge to this. Here, we present a hybrid approach to detect spin dynamics mediated by photon-magnon coupling between high-Q superconducting resonators and ultra-thin flakes of Cr2Ge2Te6 (CGT) as thin as 11 nm. We test and benchmark our technique with 23 individual CGT flakes and extract an upper limit for the Gilbert damping parameter. These results are crucial in designing on-chip integrated circuits using vdW magnets and offer prospects for probing spin dynamics of monolayer vdW magnets