Active Magnetic Matter

SPICE Workshop on Spin textures: Magnetism meets Plasmonics, July 23rd - 25th 2024

Nina del Ser

Active matter describes many-body systems whose individual units move in a non-deterministic way. Flocks of birds, schools of fish and crowds of people are a few prominent examples. In this talk, I will propose how to realise active matter in magnets. As a general principle, driving a magnet with a weak oscillating magnetic field activates its Goldstone mode(s). In a ferrimagnet, this induces rotation of the antiferromagnetic $xy$-order in either a clockwise or an anticlockwise direction, depending on the sign of the ferromagnetic component. At a ferrimagnetic domain wall, two opposite directions of rotation meet, resulting in so-called "dynamical frustration". To resolve this, the domain wall acquires a translational velocity $\pm v$, with the direction of motion chosen by spontaneous symmetry breaking, which gives us our non-deterministically moving unit. When many moving domain walls are assembled, their behaviour is markedly different to that of the equilibrium system. In particular, we find a linear growth in time of the magnetic correlation length (much faster than in equilibrium), and a very high resilience (up to several orders in magnitude higher) of the magnetic correlation length to noise.