Spin glass dynamics: Effects of field and finite size on microfabricated mesoscopic samples

Mesoscopic samples of Heisenberg spin glass (SG) with uniform sizes from 100 to 200 nm are prepared using a microfabrication method. A reduction in the peak temperature of zero-field-cooled (ZFC) magnetization in mesoscopic samples, compared with that of a film sample with the same thickness, was observed. The magnetization relaxation measurement indicates that the mesoscopic SG almost reaches the equilibrium state on a measurable time scale. The reduction in the peak temperature and the equilibration of the magnetization on an experimental time scale are interpreted as the size limitation of SG correlation length based on a droplet picture. The barrier exponents of ψ ∼ 0.65 for a logarithmic growth law and b ∼ 0.166 for a power growth law were estimated assuming that the droplet size is equal to the system size at the peak temperature of ZFC magnetization at ∼100 s. The field dependence of T _f, at which the ZFC and field-cooled magnetizations merge, was investigated in a mesoscopic sample and a film sample with the same thickness. The difference in T_f between both samples disappears in fields above 100 Oe, i.e., the size effect disappears. This can be explained on the basis of the field crossover length predicted in the droplet picture.