Domain wall motion in antiferromagnets triggered by thermally induced magnonic spin currents is studied theoretically. It is shown by numerical calculations based on a classical spin model that the wall moves towards the hotter regions, as in ferromagnets. However, for larger driving forces the so called Walker breakdown which usually speeds down the wall is missing. This is due to the fact that the wall is not tilted during its motion. For the same reason antiferromagnetic walls have no inertia and, hence, no acceleration phase leading to higher effective mobility.The interest in antiferromagnetic and ferrimagnetic materials has increased recently for several reasons. One is the more complex spin structures which lead to additional spin wave modes with higher frequencies and, consequently, faster spin dynamics than in ferromagnets (FMs). Possible applications are in the field of ultrafast spin dynamics [1,2]. Also, ferrimagnets and antiferromagnets (AFMs) have attracted a lot of attention as low-damping, insulating magnets in the emerging field of spincaloritronics [3][4][5] which is on the combined transport of spin and heat. Finally, antiferromagnets are also discussed as future material for antiferromagnetic spintronics, since it has been shown that despite their lack of a macroscopic magnetization their magnetic state can be controlled via spin torque switching and can be read out via their magnetoresistive properties [6]. Spintronic phenomena call for exploitation in devices with magnetic storage functionalities, where a magnetic nanostructure has to be controlled efficiently and fast. The information can be stored in magnetic domains, in isolated magnetic nanoparticles, or even in domain walls (DWs) [7]. For the latter case synthetic AFMs have been shown to pave a new road towards higher DW mobility [8].For a ferromagnetic system, in Ref.[9] the existence of thermally driven domain-wall motion in temperature gradients was demonstrated by computer simulations based on different approaches, an atomistic spin model as well as a micromagnetic model based on the Landau-Lifshitz-Bloch (LLB) equation of motion. A thermodynamic explanation for this kind of DW motion rests on the minimization of the free energy of the DW (or the maximization of entropy). For a DW at finite temperature, the free energy is ΔFðTÞ ¼ ΔU − TΔS, where ΔU is the internal energy and ΔS the entropy of the DW. It is a monotonically decreasing function of temperature [9][10][11]. This rather general argument explains a DW motion towards the hotter parts of the sample where the free energy is lower [11][12][13] and it can be expected to hold for other magnetic textures as well. Furthermore, it has been shown by Schlickeiser et al.that the DW motion is caused by a so-called entropic torque. The exchange stiffness is weaker for higher temperature and therefore, an effective torque on the DW is created driving it towards the hotter region [11].A more microscopic explanation for DW motion in temperature gradients rests on the continuous stream of ...

Control of magnetic domain wall motion holds promise for efficient manipulation and transfer of magnetically stored information. Thermal magnon currents, generated by temperature gradients, can be used to move magnetic textures, from domain walls, to magnetic vortices and skyrmions. In the last years, theoretical studies have centered in ferro-and antiferromagnetic spin structures, where domain walls always move towards the hotter end of the thermal gradient. Here we perform numerical studies using atomistic spin dynamics simulations and complementary analytical calculations to derive an equation of motion for the domain wall velocity. We demonstrate that in ferrimagnets, domain wall motion under thermal magnon currents shows a much richer dynamics. Below the Walker breakdown, we find that the temperature gradient always pulls the domain wall towards the hot end by minimizating its free energy, in agreement with the observations for ferro-and antiferromagnets in the same regime. Above Walker breakdown, the ferrimagnetic domain wall can show the opposite, counterintuitive behavior of moving towards the cold end. We show that in this case, the motion to the hotter or the colder ends is driven by angular momentum transfer and therefore strongly related to the angular momentum compensation temperature, a unique property of ferrimagnets where the intrinsic angular momentum of the ferrimagnet is zero while the sublattice angular momentum remains finite. In particular, we find that below the compensation temperature the wall moves towards the cold end, whereas above it, towards the hot end. Moreover, we find that for ferrimagnets, there is a torque compensation temperature at which the domain wall dynamics shows similar characteristics to antiferromagnets, that is, quasi-inertia-free motion and the absence of Walker breakdown. This finding opens the door for fast control of magnetic domains as given by the antiferromagnetic character while conserving the advantage of ferromagnets in terms of measuring and control by conventional means such as magnetic fields. arXiv:1911.05393v1 [cond-mat.mtrl-sci]

It is well established that it is possible to switch certain antiferromagnets electrically, yet the interplay of Néel-spin-orbit torques and thermal activation is only poorly understood. Combining ab initio calculations and atomistic spin dynamics simulations we develop a multiscale model to study the current-induced switching in Mn 2 Au. We compute from first principles the strength and direction of the electrically induced magnetic moments, caused by the Rashba-Edelstein effect, and take these into account in atomistic spin dynamics simulations. Our simulations reveal the switching paths as well as the timescales for switching. The size of the induced moments, however, turns out to be insufficient to lead to fully deterministic switching. Instead, we find that a certain degree of thermal activation is required to help overcome the relevant energy barrier.

Semiconductors with native ferromagnetism barely exist and defined nanostructures are almost unknown. This lack impedes the exploration of a new class of materials characterized by a direct combination of effects on the electronic system caused by quantum confinement effects with magnetism. A good example is EuO for which currently no reliable routes for nanoparticle synthesis can be established. Bottom‐up approaches applicable to other oxides fail because of the labile oxidation state +II. Instead of targeting a direct synthesis, the two steps—“structure control” and “chemical transformation”—are separated. The generation of a transitional, hybrid nanophase is followed by its conversion into EuO under full conservation of all morphological features. Hierarchical EuO materials are now accessible in the shape of oriented nanodisks stacked to tubular particles. Magnetically, the coupling of either vortex or onion states has been found. An unexpected temperature dependence is governed by thermally activated transitions between these states.

We explore the possibility of ultrafast, coherent all-optical magnetization switching in antiferromagnets by studying the action of the inverse Faraday effect in CrPt, an easy-plane antiferromagnet. Using a combination of density-functional theory and atomistic spin dynamics simulations, we show how a circularly polarized laser pulse can switch the order parameter of the antiferromagnet within a few hundred femtoseconds. This nonthermal switching takes place on an elliptical path, driven by the staggered magnetic moments induced by the inverse Faraday effect and leading to reliable switching between two perpendicular magnetic states.