Soft and Hard Magnetic Properties of Nanocrystalline Fe-M-B (M=Zr, Nd) Base Alloys Containing Intergranular Amorphous Phase

This paper reviews our recent results of the soft and hard magnetic properties of nanocrystalline Fe-rich Fe-M-B (M=Nd or Zr) base alloys containing an intergranular amorphous phase. Based on the previous results that the soft magnetic alloys in Fe-Si-B-Nb-Cu and Fe-Zr-B systems do not have zero magnetostriction (λ_s), the effect of the additional Al or Si on the λ_s and magnetic properties was examined for the nanocrystalline Fe-Zr-B-Al and Fe-Zr-B-Si alloys. The soft magnetic properties of high Bs above 1.5 T and high μ_e above 1.5x10^4 combined with zero λ_s were obtained for the Fe_Zr_7B_3Al_2 and Fe_Zr_7B_3Si_4 alloys annealed for 3.6 ks in the annealing temperature (Ta) range of 823 to 923 K. This is in contrast to the previous result that the good soft magnetic properties of the nanocrystalline Fe_Zr_7B_3 alloy are obtained in the narrow Ta range around 923 K. The remarkable extension of the Ta range seems to be attributed to the zero λ_s. The replacement of Zr by Nd in the Fe_M_7B_3 alloys was found to cause rather good hard magnetic properties of 1.3 T for Br. 260 kA/m for iHc and 146 kJ/m^3 for (BH)_ in a triplex nanostructure of bcc-Fe with a size of 20 nm, tetragonal Fe_Nd_2B with a size of 15 nm and intergranular amorphous phase with a thickness of 5 to 10 nm. The hard magnetic properties are obtained for the nanostructure containing 80 % soft magnetic phases and 20 % Fe_Nd_2B in volume. The notable result is presumably due to the effective action of the intergranular amorphous network phase which can act as a resistance against the nucleation of the reversion of magnetic domain walls leading to the increase in iHc as well as an exchange magnetic coupling medium between bcc-Fe and bcc-Fe or tetragonal Fe_Nd_2B phases leading to the high Br. This presumption is also supported from the result that no good hard magnetic properties are obtained for the over annealed sample without intergranular amorphous phase as well as for the use of the sample which does not consist of a mostly single amorphous phase in the as-quenched state. Thus, the soft and hard magnetic properties for the Fe-M-B alloys are obtained only in the optimal nanostructures. The fabrication of a new nanostructure is expected to cause the appearance of other function properties even at compositions where no useful properties have not been obtained