Electronic structures of NANOMET-based nanocrystalline soft magnetic Alloys: Magnetic properties

Abstract

We developed new nanocrystalline Fe-Co-B-P-Cu soft magnetic alloys and studied their magnetic properties by calculating electronic structures using first-principles calculations based on density functional theory within the local spin density approximation and the Brillouin function. The supercell Fe-B-P-Cu was equilibrated at 3000 K for 12 ps using ab initio molecular dynamics (AIMD). Consequently, Fe, B, P, and Cu atoms are distributed randomly in an amorphous matrix formed by cooling it to 0 K. The ferromagnetic bcc α-(Fe₅₀Co₅₀) crystallites were formed in the amorphous matrix by annealing the mixture at 300 K and cooling to 0 K. This resulted in the formation of nanocrystalline (Fe, Co)-B-P-Cu soft magnetic alloys. The ferromagnetic bcc α-Fe(Co) crystallites, embedded in an amorphous matrix, were confirmed experimentally for the first time. The experiment involved quickly cooling and heating a melted mixture of Fe, Co, B, Fe₃P, and Cu to create both amorphous ribbons and crystallites. The Fe-Co-B-P-Cu alloy designed for the experiment has a saturation magnetization (μ₀M_S) of over 1.9 T (T) at 0 K and ranging from 1.87 to 1.92 T at 300 K. The magnetocrystalline anisotropy constant (K) decreased significantly to 142 J/m³ from 5.2 × 104 J/m³ with an increase in the number of embedded crystallites from one to five, indicating that the alloy became magnetically soft. At 300 K, the experimental μ₀M_S ranged from 1.79 to 1.89 T. Additionally, as the concentration of Co increased, the Curie temperature (T_C) also increased and reached about 800 °C at 3.5 at.%. The study revealed that the average grain size of the synthesized Fe-Co-B-P-Cu alloys was 29 nm. The experimental μ₀M_S was slightly lower than the calculated value for all designed Fe-Co-B-P-Cu alloys, indicating that the recrystallization process was not entirely controlled. The experimental results confirmed that the designed alloy system has achieved the target saturation magnetization of over 1.8 T. These results effectively demonstrate the efficiency of the design method for nanocrystalline soft magnetic materials. Consequently, the alloy design method outlined in this paper will offer guidance for developing other soft magnetic materials.