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.

Light-Switchable Exchange-Coupled Magnet

“…The dark-state coercivity of 12.17 ± 0.04 kOe decreases to 9.43 ± 0.09 kOe with light and is accompanied by a reduction in the remnant and high-field-magnetization values. This behavior with light is consistent with earlier reports − on cobalt ferrite nanoparticles and has been attributed to decoupling of surface spins on the nanoparticles resulting in a reduction of magnetic volume and changing interparticle interactions within the aggregates . Similar magnetization vs field plots with and without light for the core–shell-like 2 are presented in Figure .…”

“…Cobalt ferrite nanoparticles were shown by Giri et al , to undergo a light-activated decrease in coercivity. A more recent study revealed white light irradiation also decreases high-field magnetization . The light-induced demagnetization effects were attributed to decoupling of the surface spins, leading to a change in volume of the magnetically ordered domains.…”

“…The light-induced demagnetization effects were attributed to decoupling of the surface spins, leading to a change in volume of the magnetically ordered domains. The broken symmetry and reduced numbers of exchange pathways mean the surface spins decouple on a different energy scale than the bulk. − A key observation was larger effects are seen for aggregates of particles relative to isolated nanoparticles, indicating the light process also reduces interparticle coupling . If the same processes are applied to a composite material pairing a hard magnet with a soft magnet, a light-switchable exchange-coupled magnet should result, providing a mechanism to amplify the effect of the light.…”

“…Examples of cobalt-ferrite-based exchange-coupled magnets are known with core–shell heterostructures, for example, FePt@CoFe 2 O 4 , ZnFe 2 O 4 @CoFe 2 O 4 and MnFe 2 O 4 @CoFe 2 O 4 . In this study, manganese ferrite was chosen as the soft magnet to couple with cobalt ferrite, building on our earlier work . Good epitaxy is expected, as the two ferrite materials have a relatively small lattice mismatch of around 1.7% .…”

Particle Size Effects on the Order–Disorder Phase Transition in [(CH3)2NH2]Mg(HCOO)3

“…The reagent concentrations reported by Pato-Doldán et al , for the solvent layering synthesis of 1 provided the starting point for the particle syntheses reported here. Although surfactants are sometimes used to isolate discrete particles of the formate networks, they were not used in the present work to avoid their impact on solid-state properties, which at the mesoscale can be altered by surface modification − and with an eye toward further work incorporating the particles into heterostructures.…”