Structure, magnetic properties, and exchange coupling in thermally processed NdDyFeCoB/α-Fe nanoscale multilayer magnets

The structure and magnetic properties of multilayer magnets with a hard phase (HP1= Nd13.5 Dy1.5 Fe68 Co10 B7 or HP2= Nd14 Dy1 Fe68 Co10 B7) and soft phase Fe, prepared by sputtering and subsequent heat treatment, have been investigated. For SiTi (20 nm) [HP1 (2.0 nm) Fe (0.5 nm)] ×200Ti (10 nm) multilayer film, transmission electron microscopy results show that Fe disperses in amorphous NdDyFeCoB alloy. After annealing at 575 °C for 5 min, the hard Nd2 Fe14 B phase typically with grains of about 50 nm and some amount of α-Fe coexist in the film. The grain size of the hard and soft phases can be controlled by the proper thickness of the layer and the period. A coercivity of 7.7 kOe and a ratio Mr Ms of 0.74 are achieved in the multilayer SiTi (20 nm) [HP2 (2 nm) Fe (0.5 nm)] ×200Ti (10 nm). A good squareness of the hysteresis loop (measured even at 200 K) is observed, due to the effective exchange coupling between the magnetically soft and hard nanograins in the film. A pinning-type mechanism dominates the magnetization reversal process. In addition, the effect of the effective anisotropy, the grain size, and interfaces between the magnetically hard and soft phases on the exchange coupling is discussed.