Theory of relaxation time of stochastic nanomagnets

We theoretically investigate the switching dynamics of stochastic nanomagnets and highlight the mechanism describing their relaxation time. We reveal the distinct switching mechanisms in perpendicular and in-plane easy-axis nanomagnets, and report that the relaxation time in in-plane nanomagnets varies by a few orders of magnitude only by changing the effective perpendicular anisotropy field, even though it does not contribute to the thermal stability factor. We introduce the entropy of nanomagnets into Brown's theory [Phys. Rev. 130, 1677 (1963)], and reveal that the system with faster precession as well as larger damping and smaller magnetic moment shows shorter relaxation time, and explains the different time scale of relaxation times in perpendicularand in-plane easy-axis nanomagnets. We also show that the attempt frequency changes in both perpendicular and in-plane nanomagnets depending on the magnitude of anisotropy and show that there is a lower limit of relaxation time in perpendicular systems. This work gives physical insights of thermally activated dynamics of nanomagnets as well as its material/device design guidelines for achieving shorter relaxation times.

Automated summary

By theoretically investigating the switching dynamics of stochastic nanomagnets, we uncover distinct mechanisms and reveal the impact of various factors on the relaxation time. Our work introduces the concept of entropy into understanding the dynamics, and provides insights into the material/device design for achieving shorter relaxation times.