Photoexcited spin-glass state in (Mg, Fe){Mg,Fe,Ti}O4 spinel ferrite films

Abstract

Spin-glass states above room temperature have been found in (Mg, Fe){Mg,Fe,Ti}2O4 spinel ferrite thin films formed on Al2O3 (0001) and SrTiO3 (111) substrates. The films show a long-time relaxation of the magnetization in zero-field-cooled operation and a frequency dependence of the cusp temperature in ac susceptibility measurement, both of which are typical characteristics of a spin glass. This high-temperature spin-glass state has been achieved by the fine tuning of spin states through the control of composition, random oxygen deficiencies, and the stress induced by the film/substrate lattice mismatch, all of which give rise to effects that enhance the exchange interaction of spins in the ferrite films. We have demonstrated the change of magnetic state by means of light irradiation from spin glass to a ferrimagnet over a wide range of temperatures below 290 K. The direct photoexcitation of spins with photon energy in the visible-light region (1.7-3.2 eV), which corresponds to the spin excitation energy of crystal field for Fe ions located in both tetrahedral and octahedral sites, is effective for realizing photoinduced magnetization. © 2000 American Institute of Physics.