Magnetization and coercivity of nanocrystalline gadolinium iron garnet

Gadolinium iron garnet (GdIG) nanoparticles with mean particle size of about 37 nm have been synthesized by citrate precursor gel formation followed by annealing at 800 °C for 2 hours. Magnetic behavior of clustered GdIG nanoparticles was studied in temperature range from 5 K to above Curie temperature. The sample shows a magnetization compensation temperature T _comp∼286.5 K and a Curie temperature T_C∼560 K. In comparison with the bulk saturation magnetization, the sample exhibits lower spontaneous magnetization in the temperature region from 5 K to T_comp whereas higher spontaneous magnetization is observed at higher temperatures up to near the Curie point. The magnetization curves show a differential susceptibility in high fields which increases sharply below 50 K. At very low temperatures, irreversibility was observed in the magnetization loops, enduring in the fields up to ∼12.5 kOe. The spontaneous magnetization, high-field susceptibility and low-temperature irreversible effect were discussed based on a model for the interacting particles consisting of ferrimagnetically aligned core spins and disordered spins in surface layer which become frozen at low temperatures. We proposed a mechanism for the enhancement of the spontaneous magnetization above T_comp in which the Gd and Fe spins in the surface layer are largely decoupled at high temperatures and the surface Fe spins realign to the magnetic moment of the core. The magnetic coercivity H _c at low temperatures is governed by the effective anisotropy whereas in the vicinity of the compensation point a peak in the coercive force shows up as a result of the so-called paraprocess with the maximum value of 1.2 kOe at T_comp and by further increasing temperature the coercivity decreases and eventually vanishes at about 500 K. The interparticle interactions were found to play an important role in the hysteresis behavior of the sample.