Acta Metall Sin  2018, Vol. 54 Issue (3): 457-462    DOI: 10.11900/0412.1961.2017.00211
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Magnetic Viscosity of Anisotropic Rare Earth Permanent Films
Yachao SUN, Minggang ZHU(), Rui HAN, Xiaoning SHI, Nengjun YU, Liwei SONG, Wei LI
Division of Functional Material, Central Iron & Steel Research Institute, Beijing 100081, China;
Abstract

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 $Hc⊥$ 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.

 Fig.1  Hysteresis loops with the magnetic field applied perpendicular $(⊥)$ and parallel (//) to the plane of the thin films at room temperature (H—coercivity, J—magnetic polarization) Fig.2  XRD spectra of the NdFeB, CeFeB and NdFeB/CeFeB thin films Fig.3  Temperature dependence of coercivites Hc for the NdFeB, CeFeB and NdFeB/CeFeB thin films (Inset shows the locally enlarged curve) Fig.4  Time t (a) and ln(t+t0) (b) dependence of magnetization M/M0 for the NdFeB/CeFeB thin film at applied magnetic fields (H0=Hc, t0—time constant) Fig.5  Temperature dependence of the magnetization viscosity coefficient S for the magnetic thin films Fig.6  Temperature dependence of the initial magnetic decay rates dM/dt from 0 s to 10 s for the magnetic thin films Fig.7  Applied magnetic field dependence of S at room temperature for the magnetic thin films