Magnetic properties of amorphous (Fe1-xCox)77.5Nd4B18.5 alloys before and after crystallization

Abstract

The magnetic properties of amorphous (Fe1-xCox)77.5Nd4B18.5 alloys before and after crystallization were studied. The Fe atom moment mu Fe is found to increase with increasing x from 2.0 mu B for x=0 to a saturation value of 2.3 mu B for x=0.7 and then to decrease, when the Co atom moment mu Co is assumed to be constant for all x. The variation in Fe moment is in good agreement with the Co concentration dependence of the average hyperfine field. The Curie temperature TC is found first to increase and then to decrease, attaining a maximum value of 895 K at about x=0.7. This variation in TC with x is calculated using a phenomenological model described by Kouvel (1969), the exchange interaction temperatures TFe-Fe, TFe-Co and TCo-Co are calculated to be 584 K, 1010 K and 825 K, respectively. With increasing Co concentration x, the crystallization temperature is approximately constant for x<or=0.7 and then increases slightly with increasing x. The crystalline phases in crystallized samples were identified by X-ray diffraction. A high room-temperature coercive field Hc and a high-energy product (BH)max for the optimally annealed samples are obtained.