Light-induced magnetization changes in aggregated and isolated cobalt ferrite nanoparticles

Abstract

The light-induced magnetization changes in cobalt ferrite nanoparticles are reinvestigated to probe the mechanism of photomagnetic behavior and to uncover the essential criteria required to observe the effect. Irradiation with white light results in pronounced demagnetization as evidenced by a decrease in the coercivity (ΔHc ∼ 3 kOe at 10 K) and a drop in the high field magnetization at 70 kOe. Wavelength dependent studies show the optical excitation driving the effect is broad in nature. Power and temperature (T) dependent measurements reveal strikingly different photomagnetic behaviors for the high field magnetization and coercive fields with energy scales of 25 K and 200 K, respectively. Importantly, the magnitude of the light-induced change in the magnetization is found to be specific to the synthesis protocol, with aggregated nanoparticles showing a larger effect than isolated particles. Mössbauer spectroscopy provides additional evidence of the differences between the aggregated and isolated nanoparticle samples. For T ≲ 25 K, the photo-response arises from magnetic disorder generated by an elevated electronic temperature in the surface layer of the particles, thereby leading to a decrease in magnetic volume. For 25 K ≲ T ≲ 200 K, the electronic and phononic reservoirs are more intimately coupled, so the photo-induced magnetic response follows the temperature dependence of the magneto-crystalline anisotropy. A similar response in manganese ferrite suggests that the mechanism is general.