Effect of Dy-doping on the structural and magnetic properties of Co-Zn ferrite nanocrystals for magnetocaloric applications

Magnetic nanoparticles for magnetocaloric applications should combine small coercivity, low demagnetization temperature, and high pyromagnetic coefficients while keeping the magnetization as high as possible. The strong dependence of the magnetic properties of cobalt-zinc mixed ferrite with specific dopant species enables this material to be considered a promising candidate for magnetocaloric applications. On this basis, pure and Dy-doped Co 0.7 Zn 0.3 Fe 2 O 4 cobalt-zinc ferrite nanocrystals have been synthesized by conventional and modified (i.e., flow rate controlled addition of reactants) coprecipitation routes. The modified approach allows the control of ferrite crystal growth at the nanoscale and hence tuning of the corresponding magnetic properties. The magnetic properties of the produced nanocrystals were determined as a function of their structure, nominal dopant concentration, and crystal size. X-ray diffraction, transmission electron microscopy, and Raman spectroscopy analyses suggested both the actual incorporation of the dopants into the host ferrite lattice and the promoting effect on crystal size of the flow rate at which the reactants are contacted. The average crystallite size varied from 13 nm (no control of flow rate) to 28 nm when the ferrite was synthesized at 1 ml/min. Doping caused the maximum magnetization of the ferrite to decrease; this parameter dropped from 60 emu/g (nondoped ferrite) to 55 emu/g when the ferrite was doped with 0.01 at.% of Dy. The maximum magnetization of the Dy ( y = 0.01 ) Co-Zn ferrite went up to 62 emu/g when the synthesis was carried out under flow-controlled conditions. The presence of 0.01 at.% Dy in the ferrite caused the demagnetization temperature to decrease from 350 ° C (nondoped ferrite) to 320 ° C . The demagnetization temperature was further decreased down to 308 ° C when the ferrite powders were synthesized under flow rate controlled conditions.