Structural and magnetic characterization of Al microalloying nanocrystalline FeSiBNbCu alloys

The magnetic properties of nanocrystalline Fe77Si10B9Cu1Nb3-xAlx (x = 0 and 1) alloys have been investigated and their structural and electromagnetic parameters have been quantitatively studied by X-ray diffraction, transmission electron microscopy and Mossbauer spectra under one-step and two-step annealing processes. The nanocrystalline structure consists of single alpha-Fe(Si) phase embedded in a residual amorphous phase. Both saturation magnetic flux density (B-s) and permeability (mu) of the nanocrystalline alloys are increased by substituting 1 at% Al for Nb and one-step annealing, from B-s= 1.41 T to 1.47 T and from mu = 18,000 to 23,000 at 1 kHz, respectively. The two-step annealing has little effect on the B-s, coercivity (H-c) and grain size of the nanocrystalline alloys, but greatly improves the mu of the Al-doped alloy, reaching up to 28,000 at 1 kHz. The improved mu can be attributed to the increased magnetic moment and exchange stiffness constant, homogeneous chemical structure and reduced magnetostriction. The Al-doped nanocrystalline alloy with high B-s, high mu, low H-c and good frequency stability are good candidates for magnetic shielding pieces of wireless charging.