Magnetic Entropy Change of Nanocomposites Composed of a Silver Matrix and Grains of Iron-Oxide Or -Nitride

Abstract

Magnetocaloric effect of nanocomposites composed of iron-oxide or iron-nitride grains dispersed in a silver matrix was studied by calculating magnetic entropy change ΔS induced by a change in applied magnetic field H. These nanocomposites were synthesized by the inert gas condensation technique and nitridation by heat treatment in an ammonia gas stream. Average sizes of the iron-containing grains were 10-35 nm. Magnetic phases in the materials were Fe3O4 or γ -Fe2O3 for the oxide-composites and γ-Fe4N or ε -Fe3N for the nitride-composites. Values of the ΔS were obtained by applying a thermodynamic Maxwell's relation, (∂S / ∂H)T = (∂M / ∂T)H, to data set of magnetization M measured at various temperatures T. They clearly indicated significant enhancement due to the nanostructure as predicted.

Synthesis of a new type of GdAl2nanocapsule with a large cryogenic magnetocaloric effect and novel coral-like aggregates self-assembled by nanocapsules

“…According to theoretical calculation, McMichael concluded that the nanoparticles may exhibit an effective magnetic moment that is greater than the magnetic moment of the constituent atoms, so the magnetic entropy of the nanoparticles may have a maximum enhancement of the magnetocaloric effect (MCE) [14]. Yamamoto et al [15,16] investigated the entropy change in Fe 2 O 3 -Ag nanocomposites, confirming the prediction of McMichael. Provenzano et al [17] reported an MCE of R 3 Ga 5−x Fe x O 12 nanocomposites (R = Gd, Dy and Ho), but the largest S M is −1.7 J kg −1 K −1 .…”

“…By grouping spins together in superparamagnetic GdAl 2 nanocapsules, the magnetic moments are more easily aligned than in the paramagnetic system for certain ranges of field at low temperature, and the entropy of the spins is more easily changed by the application of a field. According to the experimental results of Yamamoto et al [15,16], the isothermal magnetic entropy change around 250 K in 9% Fe of Fe 2 O 3 -Ag nanocomposite reached about 2.1 × 10 −3 J mol −1 (Fe) in a magnetic field varying from 0 to 70 kOe, which exceeded that of normal paramagnetic Fe 3+ in the same temperature and magnetic field range by about two orders of magnitude. However, the extensive measure of the MCE is reduced by a factor proportional to the ratio of the volumes (or the mass) of the inactive matrix [28].…”

Recent developments in magnetocaloric materials

“…In recent years, only a few reports about the MCE of nanocomposites have been published. Thus, Yamamoto et al [211] and [212] investigated Fe 2 O 3 -Ag nanocomposites containing 9 at% and 40 at% of iron oxide particulate ranging in size from 10 to 35 nm. In the low iron content material, iron oxide was in the form of γ -Fe 2 O 3 , while in the high iron content material, the formation of α-Fe 2 O 3 has been detected.…”

“…This enhancement has been attributed to the superparamagnetic behaviour of iron oxide nanoparticles. The same authors [212] examined the MCE of iron nitride nanoparticles, also embedded into a silver matrix. Two different iron nitrides (γ -Fe 4 N and ε-Fe 3 N) were observed in a 6 at% Fe-Ag nanocomposite.…”