Investigation on the structure and magnetic properties at low temperature for nanocrystalline γ′-Fe4N thin films

Nanocrystalline γ′-Fe4N thin films were synthesized on single crystal Si (1 0 0) substrate by facing target magnetron sputtering at a mixture of Ar/N2 gas discharge, and their structure, morphology, and magnetic properties at room temperature and low temperature were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and vibrating sample magnetometer (VSM), respectively. The results showed that the films were γ′-Fe4N with an average grain size of 55 nm, which had a highly preferred orientation along (1 1 1). The ratio of remanent magnetization Mr over saturated magnetization Ms at room temperature for γ′-Fe4N was 0.528, which indicated that the films had a single easy magnetized direction. In the temperature range of 80–350 K, the saturation magnetization Ms and coercive force Hc increased, whereas the ratio of Mr/Ms decreased with decreasing temperature. The relation between the coercive force Hc and temperature T observed a T1/2 law, and the coercivity at T = 0 K of 356.87 Oe and blocking temperature of 667 K were acquired, respectively. However, the relation between Ms and T did not observe the Bloch T3/2 law, which meant that the interaction among spin waves should be considered. Through fitting Ms–T equation, the saturation magnetization at T = 0 K of 202 emu/g, Bloch constant of 1.0 × 10−4 K−3/2, and exchange interaction constant of 1.537 × 10−21 were obtained, respectively.