We have carried out a systematic study on the effect of high-temperature annealing on the structural and magnetic properties of ultra-small crystalline cobalt ferrite nanoparticles prepared by the hydrothermal method. The structural and the magnetic characterizations of nanoparticles were investigated by XRD, TEM, FTIR, and VSM. The results of Rietveld refinement revealed that the nanoparticles have a cubic single-phase spinel structure. High-temperature annealing was found to increase the lattice parameter and average crystallite size of the cobalt ferrite nanoparticles. The TEM measurements showed that the nanoparticles were monodisperse and spherical in shape. The FTIR results confirmed the single-phase nature of the prepared nanoparticles. The magnetic measurements showed that the nanoparticles were ferromagnetic over a wide temperature range. We have observed that the high-temperature annealing increased both the saturation magnetization and magnetocrystalline anisotropy and decreased the coercivity. We demonstrated that magnetic field induced superparamagnetism can be induced by applying stronger magnetic fields that were able to shift the blocking temperature (T_B) down to below room temperature, resulting in a superparamagnetic behavior above T_B.

我们对高温退火对通过水热法制备的超小结晶钴铁氧体纳米粒子的结构和磁性的影响进行了系统的研究。通过XRD，TEM，FTIR和VSM研究了纳米粒子的结构和磁性。Rietveld精炼的结果表明，纳米颗粒具有立方单相尖晶石结构。发现高温退火增加了钴铁氧体纳米颗粒的晶格参数和平均微晶尺寸。TEM测量表明纳米颗粒是单分散的并且是球形的。FTIR结果证实了所制备纳米颗粒的单相性质。磁性测量表明，纳米颗粒在很宽的温度范围内都是铁磁性的。我们已经观察到高温退火同时增加了饱和磁化强度和磁晶各向异性，并降低了矫顽力。我们证明了磁场感应的超顺磁性可以通过施加更强的磁场来感应，该磁场能够改变阻断温度（T _B）降至室温以下，从而导致在T _B之上出现超顺磁行为。