Study of ultra-strong magnetic properties of exchange-biased Nd4Fe76Cu0.5Nb1B18.5 nanocomposite alloys

Abstract

An amorphous alloy of composition Nd4Fe76Cu0.5Nb1B18.5 prepared by melt spinning technique at wheel speed of 25 m/s in an argon atmosphere has been studied to observe their ultra-hard magnetic properties. In the as-prepared condition sample was in fully amorphous state as has been revealed by X-ray diffraction. Crystallization kinetics was studied by Differential Scanning Calorimetry. When the sample has been annealed above the crystallization temperature it has been devitrified and transformed into a composite consisting of two suitably dispersed ferromagnetic and mutually exchange-coupled phases, one of which is hard phase in order to provide a high coercive field while the other is soft-magnetic phase providing a high squareness ratio. At room temperature hysteresis loop measurements showed the highest value of coercivity Hc of around 3.30 kOe, which has been obtained for the sample annealed at 933K for 10 min. A large value of maximum energy product (BH)max and remanent ratio Mr/Ms of around 9.87 MGOe and 0.765 have been found respectively at this annealing temperature. The demagnetization branch of hysteresis loop is convex when the soft and hard phases are exchange coupled in the optimum condition of annealing. Otherwise, the hysteresis loops are constricted. At 5K the values of Hc and (BH)max are found as 6.02 kOe and 13.2 MGOe respectively for the sample annealed at 933K for 10 min. At temperatures below 150K, the shape of the hysteresis loops has been changed that is related to the spin reorientation in the hard magnetic phase. Otherwise, the hysteresis loops are constricted.