Magnetization reversal processes in sintered and melt-quenched NdFeB materials have been investigated by measurements of reversible magnetization. For isotropic and anisotropic sintered NdFeB and anisotropic hot pressed die upset NdFeB (MQIII) reversible magnetization is the result of bowing of pinned domain boundary walls. In contrast in isotropic melt quenched NdFeB (MQI) reversible magnetization appears to be the result of the rotation of domain magnetization vectors and not domain wall bowing. The differences in the reversible magnetization processes of the four materials investigated are determined.
Thin film CoPt/Co bilayers have been prepared as a model system to investigate the relationship between microstructure and exchange coupling in two-phase nanocomposite permanent magnets. The bilayers were prepared by magnetron sputter deposition of near-equiatomic CoPt with a thickness of 25 nm onto oxidized Si wafers. In the as-deposited state, CoPt had the A1 (fcc) structure and was magnetically soft. Before reinsertion into the sputtering chamber for the deposition of 2.8–16.7 nm thick Co layers, the CoPt films were annealed at 700 °C for 120 min to produce the magnetically hard, fully ordered L10 phase. The presence of exchange coupling in the bilayers was verified by magnetic hysteresis and recoil measurements and showed that only for Co thicknesses below 6.3 nm was this layer (in its as-deposited state) coupled through its full thickness to the CoPt layer. Annealing the bilayer samples at 300 and 550 °C for 20 min resulted in improvement of the interlayer magnetic coupling and produced clear differences in the magnetic reversal coherency and the recoil curves. However, for some samples, the improved coupling resulted in a decrease in coercivity, indicating that there is an optimum in the coupling strength for the attainment of high coercivity. Transmission electron microscopy studies of the bilayers in plan view showed that the increased interlayer coupling with annealing was a result of improved granular epitaxy of Co to CoPt.
A model has been developed to examine the effect that different distributions of pinning and nucleation fields have upon the reversible magnetization behavior. The model allows each grain to be in one of three states, saturated in the positive and negative directions or containing a domain wall. The nucleation and pinning field distributions determine the rate of change of the number of grains in the three states with field. The results indicate that reversible magnetization curves display a well-defined minimum where the domain wall pinning fields exceed the reverse domain nucleation fields. Where the domain wall nucleation fields exceed the domain wall pinning fields no minimum is seen. The exact shape of the reversible magnetization curve and the position of the minimum are indicative of the dynamics of the reversal process. This model is able to reproduce the form of reversible magnetization curves measured previously in a sintered isotropic NdFeB based magnet and a Sm2(Co, Fe, Cu, Zr)17 type magnet.
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