Group Seminar 2018/19

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)