Size-dependent magnetocaloric effect in GdVO4 nanoparticles

We investigated the magnetization and electron spin resonance (ESR) in nanocrystals of antiferromagnet GdVO4 with the grain size ranging from 30 nm to 5 µm. Our nanoparticles with an average size reduction from 5 µm to 300 nm exhibited enhanced paramagnetic-like behavior, in which the weakly coupled spins can be well aligned along the direction of an external magnetic field, resulting in a giant entropy change of -ΔSM = 45 J/kg K at 3 K for ΔH = 7 T. Moreover, with a further reduction of grain size to ~ 30 nm, the saturation magnetization together with the negative entropy change -ΔSM, max decreased dramatically. The aggregation effect is believed to be responsible for the decline in -ΔSM, max, in which small nanoparticles agglomerating to 'large' grains made them behave like bulk material. Finding the critical point of positive size effect on magnetocaloric effect offered a new way to achieve large magnetic entropy change.