Effect of Post Annealing on the Microstructure and Magnetic Properties of NdFeB/α-Fe/NdFeB Thin Films

A series of nanocomposite thin films, composed of Nd2Fe14B and α-Fe, has been prepared by DC-magnetron sputtering combined ion beam sputtering onto Si (100) substrates. The effects of post annealing on the microstructure and magnetic properties of [NdFeB/α-Fe/NdFeB]-type thin films have been investigated. The X-ray diffraction (XRD) study showed that annealing of the films for 30 min at temperatures 550, 600, 650, 700 ℃ resulted in the appearance of diffraction peaks, characteristic for Nd2Fe14B tetragonal structure, α-Fe and Nd2O3 phases. The investigation using the Vibrating Sample Magnetometer (VSM) with a maximum applied field of 2 T indicated that with the increase of the annealing temperature, the magnetic properties of the multilayer films were improved and reached peak value at 650 ℃ (Hci=41.72 kA·m-1, Mr/Ms=0.4, (BH)max=30.35 kJ·m-3), after which the magnetic properties were decreased greatly. Along with the increase of the thickness of α-Fe layer from Tα-Fe>16 nm, the coercivity Hci, saturation magnetization Ms, and remanence ratio Mr/Ms all declined. As the Atomic Force Microscope (AFM) indicated, after being annealed at 650 ℃ for 30 min, the sample was showed fine surface morphology with grain size 60 nm≤dα-Fe≤80 nm and 100 nm≤dNdFeB≤150 nm.