Interaction effects in granular media

Controlling the relaxation of magnetisation in magnetic nano-structures is key to optimising magnetic storage devices. Present day magnetic storage devices have what is known as a granular structure where the magnetic orientation of a section of grains (see the schematic) store the binary information (1’s and 0’s). At the nano-scale these grains can interact which affects how the magnetisation reacts to an external stimulus and therefore how the magnetisation is controlled.

Schematic of an optical pump probe simulation on granular media.
Schematic of an optical pump probe setup granular media. The grains (represented as individual magnetic moments) have a given configuration (giving an initial magnetisation M’) and upon laser excitation relax to a new magnetic state. This can be used to probe the relaxation time-scales of the material.

In collaboration with experimental partners at Seagate Technology, in the Netherlands, as well as with, theoretical collaborators in the UK, our recently published article in Physical Review B we have shown that the effects of the exchange interaction between grains has a strong effect on the relaxation processes and time-scale of the dynamics. Experimentally a sample series with different intergrain exchange was measured using a pump-probe technique (optical ferromagnetic resonance) and showed that the damping decreased significantly with increasing interaction strength, confirmed by both (semi)-analytic and computational models, providing new insights into technologically relevant magnetic materials.

Without funding this work would not have been possible so the authors are gratefully to; the Marie Curie Incoming BeIPD-COFUND fellowship program at the University of Liège; the Advanced Storage Technology Consortium; and the European Commission under contract number 281043 (FEMTOSPIN). Thanks to Jamie Verwey for the schematic diagram.

BeIPD COFUND