MnxZn1-xFe2-yRyO4 (R = Gd, Eu) Ferrite Nanocrystals for Magnetocaloric Applications

We investigated the dependence of magnetic properties with composition in nanocrystalline Mn-Zn ferrites. Ferrite crystals were synthesized by a modified coprecipitation route where nucleation and crystal growth conditions were controlled during the reaction of 0.011M Fe(III) and 0.055M[Zn(II) + Mn(II)] solutions under alkaline conditions. When doping, suitable amounts of Gd(III) or Eu(III) salts were added to solutions. XRD, FT-IR and Raman spectroscopy analyses verified the formation of the ferrite structure. An average crystallite size varying from 8 to 12nm with composition was estimated by using Scherrer´s equation. SQUID analyses suggested that ferrite nanocrystals were superparamagnetic at room temperature. The maximum magnetization values were dependent on the composition of the Mn-Znferrite and varied from 15 emu/g up to 49.8 emu/g for ´x´ 0.0 and 0.8, respectively. The variation in magnetization with temperature for the ferrites was also investigated. A Tc of 79.4degC and a pyromagnetic coefficient of 8.9 A/m-degC were determined for an aqueous-based fluid bearing Mn_0.7Zn_0.3 Fe₂O₄ nanocrystals. The variation in both the Tc and the pyromagnetic coefficient were also measured for various concentrations of Gd(III) and Eu(III) species in the Mn-Zn ferrites.