利用直流磁控溅射技术在Si基底上制备了NdFeB、CeFeB和NdFeB/CeFeB薄膜。XRD和磁滞回线结果表明,所制备的薄膜样品均具有良好的c轴取向,其中NdFeB单层薄膜垂直于薄膜表面方向的室温矫顽力\begin{array}{c}{H}_{c\perp }\end{array}达到1377.4 kA/m。研究了3种不同薄膜样品的磁黏滞系数S随温度(5~300 K) 的变化趋势,发现在低温(5 K)条件下,3种薄膜的S值非常接近,且都小于1,这是由于低温时薄膜晶格内的热起伏已经不足以使畴壁跃过位垒达到能量更小的位置。通过研究温度和外场对3种薄膜磁黏滞性的影响,发现NdFeB/CeFeB薄膜的磁黏滞系数更接近于CeFeB单层薄膜,且远小于NdFeB薄膜,说明通过双硬磁复合能够有效降低薄膜磁化强度对时间的依赖性,提高其时间稳定性。

Rare earth permanent thin films are useful for magnetic microdevices such as micromotors, since its excellent magnetic properties are able to raise the performance of the devices. In order to judge the reliability of permanent magnet materials, it is quite theoretical and practical to study the time dependence behavior of magnetization, that is, magnetic viscosity or magnetic after-effect. In this work, NdFeB, CeFeB and NdFeB/CeFeB films were fabricated on the Si substrates by direct current (DC) magnetron sputtering. A Ta underlayer of 50 nm and a coverlayer of 40 nm were sputtered at room temperature to align the easy axis of the RE₂Fe₁₄B grains perpendicular to the film plane and to prevent oxidation of the magnetic films, respectively. NdFeB and CeFeB magnetic films were deposited at 903 and 883 K, respectively, and submitted to an in-situ rapid thermal annealing at 948 K for 30 min. The microstructure and magnetic properties of the films were characterized by XRD and physical property measurement system (PPMS). The results indicate that the films show excellent perpendicular anisotropy. A coercivity \begin{array}{c}{H}_{c\perp }\end{array} of 1377.4 kA/m is obtained for NdFeB monolayer film at room temperature. The magnetic viscosity coefficient (S) of the films was studied over a range of temperatures (5~300 K). It is found that the values of S for all films are less than 1, and are quite similar at low temperature (5 K). Both weakened thermal agitation and strengthened anisotropy energy barriers are supposed to decrease transition frequency (f) and prolong relaxation time (τ) at low temperature, which lead to S decreasing. The magnetic viscosity of NdFeB/CeFeB thin film is as similar as that of the CeFeB monolayer thin film, and both are much smaller than that of the NdFeB film. It is shown that the dual-hard magnetic layer structure can effectively reduce the viscosity coefficient and improve the time stability of the NdFeB/CeFeB thin film. Furthermore, the temperature dependence of the initial decay rates (dM/dt) from 0 s to 10 s was discussed. The initial magnetic decay of the film demonstrates a similar temperature behavior as the magnetic viscosity coefficient S.