Coercivity in lean rare earth NdFeB and PrFeB nanocomposite hard magnetic materials

NdFeB and PrFeB nanocomposite hard magnetic materials of the exchange-spring type were prepared by splat-cooling. Hysteresis cycles reveal that magnetisation processes may be analysed in terms of two contributions arising from the two different phases which exist in these systems. The soft phase magnetisation reverses-first at a field of approximately 0.3 T which is found to be little temperature dependent. Reversal is determined by exchange coupling with the hard phase. A grain size of 20 nm is deduced. This value is in agreement with structural analysis. The hard-phase magnetisation reverses at a larger field value which is found to increase steadily with decreasing temperature (2.75 T in the PrFeB samples and 1.1 T in the NdFeB sample at 20 K). Reversal is found to be determined by processes which are reminiscent of those characterising reversal in usual sintered or melt-spun RFeB magnets. At room temperature, the activation volume deduced from magnetic after-effect measurements is 2200 nm(3). The contribution of inter-particle dipole interactions to magnetisation reversal is found to be small. (C) 1998 Elsevier Science S.A. All rights reserved.