Two-magnon bound state causes ultrafast thermally induced magnetisation switching.

There has been much interest recently in the discovery of thermally induced magnetisation switching using femtosecond laser excitation, where a ferrimagnetic system can be switched deterministically without an applied magnetic field. Experimental results suggest that the reversal occurs due to intrinsic material properties, but so far the microscopic mechanism responsible for reversal has not been identified. Using computational and analytic methods we show that the switching is caused by the excitation of two-magnon bound states, the properties of which are dependent on material factors. This discovery allows us to accurately predict the onset of switching and the identification of this mechanism will allow new classes of materials to be identified or designed for memory devices in the THz regime

Magnetisation switching of FePt nanoparticle recording medium by femtosecond laser pulses

Manipulation of magnetisation with ultrashort laser pulses is promising for information storage device applications. The dynamics of the magnetisation response depends on the energy transfer from the photons to the spins during the initial laser excitation. A material of special interest for magnetic storage are FePt nanoparticles, for which switching of the magnetisation with optical angular momentum was demonstrated recently. The mechanism remained unclear. Here we investigate experimentally and theoretically the all-optical switching of FePt nanoparticles. We show that the magnetisation switching is a stochastic process. We develop a complete multiscale model which allows us to optimize the number of laser shots needed to switch the magnetisation of high anisotropy FePt nanoparticles in our experiments. We conclude that only angular momentum induced optically by the inverse Faraday effect will provide switching with one single femtosecond laser pulse.EC under Contract No. 281043, FemtoSpin. The work at Greifswald University was supported by the German research foundation (DFG), projects MU MU 1780/8-1, MU 1780/10-1. Research at Göttingen University was supported via SFB 1073, Projects A2 and B1. Research at Uppsala University was supported by the Swedish Research Council (VR), the Röntgen-Ångström Cluster, the Knut and Alice Wallenberg Foundation (Contract No. 2015.0060), and Swedish National Infrastructure for Computing (SNIC). Research at Kiel University was supported by the DFG, projects MC 9/9-2, MC 9/10-2. P.N. acknowledges support from EU Horizon 2020 Framework Programme for Research and Innovation (2014-2020) under Grant Agreement No. 686056, NOVAMAG. The work in Konstanz was supported via the Center for Applied Photonics

Disparate ultrafast dynamics of itinerant and localized magnetic moments in gadolinium metal

The Heisenberg-Dirac intra-atomic exchange coupling is responsible for the formation of the atomic spin moment and thus the strongest interaction in magnetism. Therefore, it is generally assumed that intra-atomic exchange leads to a quasi-instantaneous aligning process in the magnetic moment dynamics of spins in separate, on-site atomic orbitals. Following ultrashort optical excitation of gadolinium metal, we concurrently record in photoemission the 4f magnetic linear dichroism and 5d exchange splitting. Their dynamics differ by one order of magnitude, with decay constants of 14 versus 0.8 ps, respectively. Spin dynamics simulations based on an orbital-resolved Heisenberg Hamiltonian combined with first-principles calculations explain the particular dynamics of 5d and 4f spin moments well, and corroborate that the 5d exchange splitting traces closely the 5d spin-moment dynamics. Thus gadolinium shows disparate dynamics of the localized 4f and the itinerant 5d spin moments, demonstrating a breakdown of their intra-atomic exchange alignment on a picosecond timescale

Element‐Specific Magnetization Damping in Ferrimagnetic DyCo 5 Alloys Revealed by Ultrafast X‐ray Measurements

The dynamic response of magnetically amp; 8208;ordered materials to an ultrashort external stimulus depends on microscopic parameters such as magnetic moment, exchange and spin amp; 8208;orbit interactions. Whereas it is well established that, in multi amp; 8208;component magnetic alloys and compounds, the speed of demagnetization and spin switching processes has an element amp; 8208;specific character, the magnetization damping has been assumed to be a universal parameter for all constituent magnetic elements irrespective of their different spin amp; 8208;orbit couplings and electronic structure. Here, we provide experimental and theoretical evidence for an element amp; 8208;specific magnetic damping parameter by investigating the ultrafast magnetization response of a high amp; 8208;anisotropy ferrimagnetic DyCo5 alloy to femtosecond laser excitation. Employing femtosecond laser pump X amp; 8208;ray magnetic circular dichroism XMCD probe measurements combined with atomistic spin dynamics ASD simulations using ab amp; 8208;initio calculated parameters we reveal a strikingly different demagnetization and remagnetization dynamics of the Dy and Co magnetic moments upon photo amp; 8208;excitation. These observations, fully corroborated by the ASD simulations, are linked to the element amp; 8208;specific spin amp; 8208;orbit coupling strengths of Dy and Co, which are incorporated in the phenomenological magnetization damping parameters. Our findings can be used as a recipe for tuning the speed and magnitude of laser amp; 8208;driven magnetic processes and consequently allowing to control various dynamic functionalities in multi amp; 8208;component magnetic material