Photo-induced bias of the hysteresis loop in ferrites at room temperature with long-time memory

New photo-magnetic effects with an indefinitely long-time memory are found at room temperature in the epitaxial Mg0.75Mn0.21Co0.04Fe2O4 ferrite film. Illumination of the ferrimagnetic material with low-intensity (0.4 W cm−2) circularly polarized light with or without a static magnetic field in the Faraday effect geometry results in a number of nonlinear effects in both space and time. In a uniform crystal with cubic symmetry, the long-lived photo-induced magnetization (PIM) with a unidirectional anisotropy appears along the direction of the incoming light. The effects depend on a combination of magnetic field H and the helicity of circularly polarized light σ. Two combinations H+,σ+ and H−,σ− lead to a photo-induced unidirectional anisotropy with a shift of the hysteresis loop along an applied field and a change in loop parameters. The loop contracts by a factor of two, the shift of the mid-point Hsh increases by factor of five surpassing the coercivity Hc, the coercivity Hc1 and remanence Mr1 (for decreasing applied field) reverse the sign, increasing by 9 Oe and reducing by a factor of 4.5, respectively. The effects cannot be erased by a conventional demagnetization (using an AC current that is reduced to zero amplitude), but can be removed using an illumination with two other combinations (H+,σ− and H−,σ+) as well as by heating at temperatures higher than the Curie temperature. This long-lived room-temperature memory effect may arise from the formation of complex photo-induced defects including photo-induced magnetic polarons. The possible mechanisms responsible for the appearance of a room-temperature photo-induced unidirectional anisotropy with a long-lived memory are discussed. These new photo-magnetic effects may find an application in magneto-optical memory devices.