Magnetic and structural properties of NdFeB thin film prepared by step annealing

The crystallization of the amorphous phase into the tetragonal Nd2Fe14B (Φ) phase and the corresponding changes in magnetic properties have been examined by step annealing experiment using a 2 μm thick NdFeB film sample. Microstructural and magnetic analysis indicate that the film was magnetically soft as deposited with the coercivity Hci⊥ <16 kA m−1 and the remnant magnetization 4πMr⊥ <0.02 T. Annealing at a temperature of 550 °C, a coercivity value around 784 kA m−1 was developed and diffraction analysis showed evidence of Φ phase 002l peaks being aligned perpendicular to the film plane. At an optimum annealing temperature of 575 °C, the remnant magnetization of this anisotropic thin film is around 0.60 T with intrinsic coercivity of ∼1340 kA m−1. Annealing the film sample at 200 °C≤Tann≤750 °C showed variations in magnetic properties that were mostly due to the change in the perpendicular anisotropy. Based on 4πMs⊥ values plotted against Tann, a dip in 4πMs⊥ values was observed as Tann increased in the soft-to-hard magnetic characteristics transition region and rose as the hard crystalline phase started to form. The results show that the magnetic properties of the NdFeB film were slightly influenced by the presence of NdO, film surface roughening and the small increase in crystal size as a consequence of repeated heat treatment. At Tann ∼300 °C, the nominal saturation magnetization indicated a certain degree of weak perpendicular magnetic anisotropy in the film sample considered to be essential in the enhancement of coercivity in crystallized films.