Transient ferromagnetic-like state mediating ultrafast reversal of antiferromagnetically coupled spins

Abstract

Ferromagnetic or antiferromagnetic spin ordering is governed by the exchange interaction, the strongest force in magnetism. Understanding spin dynamics in magnetic materials is an issue of crucial importance for progress in information processing and recording technology. Usually the dynamics are studied by observing the collective response of exchange-coupled spins, that is, spin resonances, after an external perturbation by a pulse of magnetic field, current or light. The periods of the corresponding resonances range from one nanosecond for ferromagnets down to one picosecond for antiferromagnets. However, virtually nothing is known about the behaviour of spins in a magnetic material after being excited on a timescale faster than that corresponding to the exchange interaction (10-100 fs), that is, in a non-adiabatic way. Here we use the element-specific technique X-ray magnetic circular dichroism to study spin reversal in GdFeCo that is optically excited on a timescale pertinent to the characteristic time of the exchange interaction between Gd and Fe spins. We unexpectedly find that the ultrafast spin reversal in this material, where spins are coupled antiferromagnetically, occurs by way of a transient ferromagnetic-like state. Following the optical excitation, the net magnetizations of the Gd and Fe sublattices rapidly collapse, switch their direction and rebuild their net magnetic moments at substantially different timescales; the net magnetic moment of the Gd sublattice is found to reverse within 1.5 picoseconds, which is substantially slower than the Fe reversal time of 300 femtoseconds. Consequently, a transient state characterized by a temporary parallel alignment of the net Gd and Fe moments emerges, despite their ground-state antiferromagnetic coupling. These surprising observations, supported by atomistic simulations, provide a concept for the possibility of manipulating magnetic order on the timescale of the exchange interaction.

Perpendicular exchange bias in ferrimagnetic spin valves

“…The absolute value of the EB can be tuned through the thickness of the interlayer spacer and even reversed with relatively low magnetic fields of several hundred Oersteds. This flexibility in controlling a robust PEB at room temperature may be of outmost importance for applications in modern ultrafast storage media 37 .…”

Ultrafast magnetization enhancement in metallic multilayers driven by superdiffusive spin current

“…The physics becomes even more intriguing for heterogeneous magnetic systems, such as magnetic compounds 14 , alloys 15,16 and exchange-coupled layered structures. Several sophisticated experiments have recently employed ultrashort soft X-ray pulses from synchrotron 2,15 or laser-driven high-harmonic (HHG) light sources 3,16,17 to disentangle the various processes using element-specific demagnetization techniques. This has been achieved by monitoring the absorption or reflection of light tuned to inner-shell absorption edges of magnetic materials.…”

Ultrafast changes in lattice symmetry probed by coherent phonons

“…T he ultrafast control of a material's properties via nonequilibrium states is a new method to manipulate and control the structural 1 , magnetic 2 and electrical 3 properties of materials that may not be accessible under equilibrium conditions. The timescale on which one phase is lost and a new phase emerges is of fundamental interest for understanding how these transitions occur as well as being important for potential applications.…”

“…These results indicate new ways in which ultrafast structural transitions can be measured through optical probes. 2 . At room temperature, VO 2 is a monoclinic M 1 -phase insulator with P2 1/c space group, consisting of four vanadium ions per unit cell.…”