Speed limit ahead

Tracking nuclear motion in single-molecule magnets using femtosecond X-ray absorption spectroscopy

The development of new data storage solutions is crucial for emerging digital technologies. Recently, all-optical magnetic switching has been achieved in dielectrics, proving to be faster than traditional methods. Despite this, single-molecule magnets (SMMs), which are an important class of magnetic materials due to their nanometre size, remain underexplored for ultrafast photomagnetic switching. Herein, we report femtosecond time-resolved K-edge X-ray absorption spectroscopy (TR-XAS) on a Mn(III)-based trinuclear SMM. Exploiting the elemental specificity of XAS, we directly track nuclear dynamics around the metal ions and show that the ultrafast dynamics upon excitation of a crystal-field transition are dominated by a magnetically active Jahn-Teller mode. Our results, supported by simulations, reveal minute bond length changes from 0.01 to 0.05 Å demonstrating the sensitivity of the method. These geometrical changes are discussed in terms of magneto-structural relationships and consequently our results illustrate the importance of TR-XAS for the emerging area of ultrafast molecular magnetism.

Femtosecond laser-induced spin dynamics in single-layer graphene/CoFeB thin films

Graphene/ferromagnet hybrid heterostructures are important building blocks of spintronics due to the unique ability of graphene to transport spin current over unprecedented distances and possible increase in its spin-orbit coupling due to proximity and hybridization. Here, we present magnetization dynamics over a femtosecond to nanosecond timescale by employing an all-optical time-resolved magneto-optical Kerr effect technique in single-layer graphene (SLG)/CoFeB thin films with varying CoFeB thickness and compared them with reference CoFeB thin films without an SLG underlayer. Gilbert damping variation with CoFeB thickness is modelled to extract spin-mixing conductance for the SLG/CoFeB interface and isolate the two-magnon scattering contribution from spin pumping. In SLG/CoFeB, we have established an inverse relationship between ultrafast demagnetization time (τ_m) and the Gilbert damping parameter (α) induced by interfacial spin accumulation and pure spin-current transport via a spin pumping mechanism. This systematic study of ultrafast demagnetization in SLG/CoFeB heterostructures and its connection with magnetic damping can help to design graphene-based ultrahigh-speed spintronic devices.

Switching of spins and entanglement in surface-supported antiferromagnetic chains

Previous experimental studies discovered universal growth of chains and nanowires of various chemical elements on a corrugated molecular network of Cu₃N on the Cu(110). Herein, performing combined ab initio and quantum Hamiltonian studies we demonstrate that such chains can be used for a fast spin switching and entanglement generation by locally applied magnetic pulses. As an example, we show that in antiferromagnetic Co chains a strong entanglement between ends of chains occurs during spin switching. A novel parity effect in spin dynamics is reported. Even-numbered chains are found to exhibit significantly faster spin switching than odd-numbered counterparts. Moreover, at certain parameters of the system the dimerization effect in the spin dynamics of the chains was found. Our studies give a clear evidence that tailoring spin dynamics and entanglement can be achieved by magnetic fields and by tuning exchange interactions in supported chains.

Laser-induced magnetization dynamics and reversal in ferrimagnetic alloys

This review discusses the recent studies of magnetization dynamics and the role of angular momentum in thin films of ferrimagnetic rare-earth-transition metal (RE-TM) alloys, e.g. GdFeCo, where both magnetization and angular momenta are temperature dependent. It has been experimentally demonstrated that the magnetization can be manipulated and even reversed by a single 40 fs laser pulse, without any applied magnetic field. This switching is found to follow a novel reversal pathway, that is shown however to depend crucially on the net angular momentum, reflecting the balance of the two opposite sublattices. In particular, optical excitation of ferrimagnetic GdFeCo on a time scale pertinent to the characteristic time of the exchange interaction between the RE and TM spins, i.e. on the time scale of tens of femtoseconds, pushes the spin dynamics into a yet unexplored regime, where the two exchange-coupled magnetic sublattices demonstrate substantially different dynamics. As a result, the reversal of spins appears to proceed via a novel transient state characterized by a ferromagnetic alignment of the Gd and Fe magnetic moments, despite their ground-state antiferromagnetic coupling.Thus, optical manipulation of magnetic order by femtosecond laser pulses has developed into an exciting and still expanding research field that keeps being fueled by a continuous stream of new and sometimes counterintuitive results. Considering the progress in the development of plasmonic antennas and compact ultrafast lasers, optical control of magnetic order may also potentially revolutionize data storage and information processing technologies.

Electron theory of fast and ultrafast dissipative magnetization dynamics

For metallic magnets we review the experimental and electron-theoretical investigations of fast magnetization dynamics (on a timescale of ns to 100 ps) and of laser-pulse-induced ultrafast dynamics (few hundred fs). It is argued that for both situations the dominant contributions to the dissipative part of the dynamics arise from the excitation of electron-hole pairs and from the subsequent relaxation of these pairs by spin-dependent scattering processes, which transfer angular momentum to the lattice. By effective field theories (generalized breathing and bubbling Fermi-surface models) it is shown that the Gilbert equation of motion, which is often used to describe the fast dissipative magnetization dynamics, must be extended in several aspects. The basic assumptions of the Elliott-Yafet theory, which is often used to describe the ultrafast spin relaxation after laser-pulse irradiation, are discussed very critically. However, it is shown that for Ni this theory probably yields a value for the spin-relaxation time T(1) in good agreement with the experimental value. A relation between the quantity α characterizing the damping of the fast dynamics in simple situations and the time T(1) is derived.

Subpicosecond magnetization reversal across ferrimagnetic compensation points

Subpicosecond magnetization reversal is experimentally demonstrated by ultrafast heating of a ferrimagnet across its compensation points, under an applied magnetic field. While the reversal is initiated by crossing the magnetization compensation temperature, the short reversal time is related to the angular momentum compensation, where the dynamics of the system is highly accelerated owing to the divergence of the gyromagnetic ratio. These results demonstrate the feasibility of subpicosecond magnetization reversal previously believed impossible.