Nd-Ce-Fe-B纳米复合薄膜的磁性及交换耦合作用

采用磁控溅射技术制备了具有永磁特征的Nd-Ce-Fe-B多层纳米复合薄膜,并对其进行了退火处理.通过改变退火温度,研究其对薄膜磁性能和晶体结构的影响.结果表明,随着退火温度的提高薄膜磁性能逐渐增大,但当温度达到695℃以上时,薄膜的磁性能急剧下降.当退火温度为675℃时,薄膜的矫顽力Hci=10.1 kOe(1Oe=79.5775 A/m),垂直于薄膜表面方向的剩余磁化强度4πM_(r⊥)=5.91 kG(1 G=10~3/(4π)A/m).薄膜的X射线衍射结果表明,磁性薄膜具有较好的c轴取向.通过对薄膜磁化反转过程的研究,发现随着外加磁场的增大,M_(rev)的极小值向M_(irr)减小的方向移动,这与畴壁弯曲模型类似,表明在薄膜中存在较强烈的局部钉扎作用,而剩余磁化强度曲线表明这种钉扎作用在薄膜矫顽力机制中并不占支配作用.此外,薄膜的Henkel曲线结果表明在薄膜中存在较强的交换耦合作用,在经过685℃退火的薄膜中磁相互作用更加显著.

Magnetic properties and exchange coupling of Nd-Ce-Fe-B nanocomposite films

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.