Magnetization reversal and nonexponential relaxation via instabilities of internal spin waves in nanomagnets

¹  Physics Department, Lehman College, City University of New York - 250 Bedford Park Boulevard West, Bronx, New York 10468-1589, USA
²  Groupe d'Etude de la Matière Condensée, CNRS UMR8634, Université de Versailles St. Quentin 45 av. des Etats-Unis, 78035 Versailles, France, EU

Corresponding author: dmitry.garanin@lehman.cuny.edu

Received: 1 October 2007
Accepted: 8 February 2008

Abstract

A magnetic particle with atomic spins ordered in an unstable direction is an example of a false vacuum that decays via excitation of internal spin waves. Coupled evolution of the particle's magnetization (or the vacuum state) and spin waves, considered in the time-dependent vacuum frame, leads to a peculiar relaxation that is very fast at the beginning but slows down to a nonexponential long tail at the end. The two main scenarios are linear and exponential spin-wave instabilities. For the former, the longitudinal and transverse relaxation rates have been obtained analytically. Numerical simulations show that the particle's magnetization strongly decreases in the middle of reversal and then recovers.