Magnetic properties and magnetic domains of Nd–Fe–B thin films

Anisotropic Nd–Fe–B thin films are fabricated by direct current magnetron sputtering on Si substrates heated to temperatures over a wide range. Surface morphology and magnetic domains of the Nd–Fe–B thin films prepared at different sputtering temperatures (25–600 °C) are observed by a scanning probe microscopy. The magnetic domains exhibit a rich variety of textures, changing from striped via maze to cloudlike as the sputtering temperature is increased. Variations in magnetic domains with substrate temperature are discussed using phase components and magnetic anisotropies of the thin films. In addition, patterns of magnetic domains are analyzed using the “disorder functions,” a set of characterizations of complex patterns with labyrinthine structures. The disorder function δ(1) and the structure factor δk do not change appreciably until a substrate temperature of 350 °C, but increases significantly beyond 400 °C. The disorder in magnetic domains increases with increasing sputtering temperature. A simultaneous enhancement of the anisotropic c texture and the hard-magnetic properties of the thin films are observed. The significant change of the disorder function at Ts=400 °C appears to be a precursor to the hardening of the Nd–Fe–B film. The most disordered magnetic domains of the film with the substrate temperature of 600 °C correspond to the optimum magnetic properties, with the maximum energy product (BH)max of 22.4 MG Oe.