Manipulate the magnetic properties of AlNiCo films by tuning non-magnetic phase structures

Abstract

AlNiCo films have very promising applications in magnetic encoders. However, the films must have sufficient coercivity and remanence for practical applications. We prepared AlNiCo (50 nm)/X(t) (X: Ta or Ti, t = 0–9 nm) films using magnetron sputtering followed by vacuum annealing at 750 °C for 30 min. Compared with AlNiCo (50 nm) film (Hc = 79 Oe, Mr = 10130 Oe), the coercivity (Hc = 289 Oe) of AlNiCo (50 nm)/Ti (7 nm) is increased by 266 %, and the remanence is 10,005 Oe. Especially, the coercivity (Hc = 663 Oe) of AlNiCo(50 nm)/Ta (7 nm) is improved by 739 %, but the remanence is 3671 Oe, meeting the performance requirements of magnetic code disks. The microstructure analysis demonstrates that the diffusion of Ta and Ti atoms during annealing leads to the formation of diffusely distributed non-magnetic phases (Fe2Ta and Co2Ta) and well-crystallized non-magnetic phases (Ni3(Al,Ti)) in films. The diffusely distributed non-magnetic phases have stronger pinning effects on the domain walls, which we believe is an important reason for the enhancement of the coercivity. Additionally, the formation of Fe2Ta and Co2Ta phases leads to the loss of magnetic moments, but Ni3(Al,Ti) does not. Therefore, the Mr of AlNiCo/Ta film decreases, while the Mr of AlNiCo/Ti film does not change.