Magnetocaloric effect of ferromagnetic particles

The entropy change accompanying the removal of an applied magnetic field (i.e. the magnetocaloric effect) is calculated for a system of magnetic spins independent of each other and also clustered together into independently acting magnetic particles. Mean-field-theory calculations are made for interacting, single magnetic spins and also for interacting, magnetic particles. In both cases, there are found temperature and field regimes where the entropy changes are larger when the spins are coupled together into nanometer-sized magnetic particles. Entropy change data is shown for a new Gd₃Ga_5-xFe _xO₁₂ (x⩽2.5) magnetic nanocomposite confirming the calculations and possessing magnetocaloric effects larger than the low temperature paramagnetic refrigerant, gadolinium gallium garnet