In the early 1980 s, the soft and hard magnetic nano-two-phase permanent magnet materials were developed and exchange coupling model was put forward. Moreover, the theoretical maximum magnetic energy product could reach 120 MGOe (1 Oe=79.5775 A/m). However a great many of experimental research results are always disappointing for theoretical calculation, but previous studies have shown that there exists also a strong exchange coupling in hard magnetic phase, which can improve the magnetic property of magnet.
In this paper, nanocomposite Ta(50 nm)/NdFeB(100 nm)/Ta(2 nm)/NdCeFeB(100 nm)/Ta(2 nm)/NdFeB(100 nm)/Ta(40 nm) multilayer films with Ta underlayers and coverlayers are fabricated on Si substrates by direct current sputtering. A 50 nm Ta underlayer and a 40 nm coverlayer are sputtered at room temperature to align the easy axis of the RE₂Fe₁₄B grains to the direction perpendicular to the film plane and to prevent the magnetic film from oxidizing, respectively. The 2 nm Ta spacer layer serves as suppressing the diffusion of elements between different magnetic layers. The NdFeB and NdCeFeB magnetic film are deposited at 630℃ and 610℃, respectively, and then they are followed by in situ rapid thermal annealing at 645-705℃ for 30 min. The microstructures and morphologies of the films are characterized by X-ray diffractometry with Cu K_α radiation, atomic force microscope, and magnetic force microscope. The magnetic properties of the films are measured with vibrating sample magnetometer.
The influences of annealing temperature on magnetic property and crystal structure of the film are investigated. The results show that the magnetic property of the film improves gradually with the increase of annealing temperature, but deteriorates sharply when the temperature reaches above 695℃. When the annealing temperature is 675℃, the coercivity H_ci of the film reaches 10.1 kOe and the remanence 4πM_r⊥ is 5.91 kG (1 G=10³/(4π) A/m), with a magnetic field applied to the direction perpendicular to the plane of the Nd-Ce-Fe-B thin film. The X-ray diffraction results show that the grains of the hard magnetic phase (2:14:1 phase) grow almost along the substrate normal (c-axis direction), of course, with a certain misorientation. Through the magnetization reversal process of the Nd-Ce-Fe-B thin film, it is found that the minimum value of M_rev moves in the direction of decreasing M_irr as the applied magnetic field increases, which is similar to the domain wall bowing model. This indicates that there is a strong local domain wall pinning in the film. Moreover, the remanence curve shows that the pinning type mechanism is indeed not dominant in the magnetization reversal process of the Nd-Ce-Fe-B thin film after annealing at 685℃. In addition, Henkel plots are also investigated in the films at different annealing temperatures. It is believed that nonzero δ_m is due to the interaction between particles in the magnet. It can be stated based on the measuring results that there exists a strong magnetic exchange coupling effect in the Nd-Ce-Fe-B thin film.