Condensed Matter Research Seminar: Dr Unai Atxitia, Freie Universität Berlin

Dr Unai Atxitia, of the Department of Physics, Freie Universität Berlin, Germany, will be presenting a seminar on his research. All are welcome to attend.

Antiferromagnetic spintronics is a rapidly developing field of magnetism [1]. The possible advantages of antiferromagnets (AFM) over conventionally used ferromagnetic (FM) devices include their lack of stray fields and low susceptibility to external fields, as well as the significantly faster spin dynamics in such systems. However, so far little is known about the interplay between thermal effects and AFM spin dynamics. Here, we focus on two examples, i) superparamagnetic limit of AFM nanostructures and ii) spin thermoelectric conversion by ultrafast AFM domain wall motion. For many applications, the size of magnetic structures will have to be scaled down to the nanometer regime, where, eventually, thermal excitations will reduce the stability of the magnetic state. Although the stability of the bits of information against thermal fluctuations is a crucial aspect of future technological applications, the reversal rates of AFM nanostructures remains to be unexplored. In single-domain FM nanoparticles this is known as the superparamagnetic limit, where the whole structure can be described as a single macroscopic magnetic dipole. Analogously, a single-domain AFM nanoparticle may be described by a macroscopic Néel vector, being the difference of the two sublattice magnetizations. The spontaneous switching of the Néel vector under thermal fluctuations constitutes the superparamagnetic limit in AFMs.

Here we demonstrate theoretically that the reversal time is significantly decreased in AFMs compared to FMs [2]. This indicates a reduced thermal stability in AFM devices, but may prove advantageous in thermally assisted processes such as electric current-induced switching [3]. Spin thermoelectric conversion is a fascinating phenomenon that has been actively studied in the last decade. To date, research has focused on spin currents generated by temperature differences — Spin Seebeck effect. Much less studied is the inverse effect — Spin-Peltier effect (SPE)— about temperature differences generated by spin currents. Only recently, it has been experimentally demonstrated in FMs that temperature differences of the order of mK can be achieved by passing spin currents through the junction of two magnetic domains [4]. Here, we present a novel strategy to achieve orders of magnitude higher SPE on the AFM counterpart. We propose to use the unique properties of moving AFM domain walls; atomic scale confinement and ultrahigh velocity, to demonstrate that ultrafast AFM domain wall motion produces significant local increase of the temperature. Notably, by combining theoretical tools from the so far independent fields of ultrafast spin dynamics and AFM spintronics, we provide detailed information of the energy redistribution channels and their corresponding time and length scales.

Our proposal could find important applications in sensing functionalities, particularly for the detection of the elusive AFM magnetic textures — undetectable by magnetic means — as its location could be inferred from the temperature map of the sample. Moreover, this phenomenon could be useful for local heat-transfer in a device in which heat may be desired to be extremely localised in space.

[1] P. Wadley et al. Science 351, 587 (2016) [2] U. Atxitia et al. arXiv:1808.07665 (2018) [3] M. Meinert Phys. Rev. Appl. 9, 064040 (2018) [4] K. Uchida et al., Nature 558, 95 (2018) [5] R. M. Otxoa, U. Atxitia et al. arXiv:1903.08034 (2019)

Host: Dr Satoshi Sasaki