Thin Nd-Fe-B films have been prepared by pulsed laser deposition on sapphire substrates. A Ta buffer layer has been used to promote epitaxial growth. The deposition temperature has been varied between 400 \ifmmode^\circ\else\textdegree\fi{}C and 700 \ifmmode^\circ\else\textdegree\fi{}C and proved to be an important parameter for both, the phase formation and the control of microstructure and coercivity. For low temperatures the Nd-Fe-B films grow in small grains with irregular shape, and the generally observed $c$-axis texture is not fully developed. The coercivity reaches values of around 1.6 T. For medium preparation temperatures of 600--650 \ifmmode^\circ\else\textdegree\fi{}C the Nd-Fe-B films grow in small rectangular grains which are mostly isolated from each other. Maximum coercivities of 1.9 T together with in-plane to out-of-plane remanence ratios as low as 0.05 have been achieved. These films possess a nearly perfect magnetic texture with the easy magnetization axis perpendicular to the film plane. For higher deposition temperatures the microstructure coarsens substantially and also oxidation of Nd occurs. Additionally, the coercivity drops drastically. Optimally prepared films have been investigated by magnetic force microscopy. The perfect growth of the ${\mathrm{Nd}}_{2}{\mathrm{Fe}}_{14}\mathrm{B}$ phase and the special granular microstructure allow us to study the domain structure of isolated, single crystalline ${\mathrm{Nd}}_{2}{\mathrm{Fe}}_{14}\mathrm{B}$ grains on a submicrometer scale. The observed domain patterns show stripe domains, which are typical for films with large perpendicular magnetic anisotropy, and resemble the specific shape of the magnetic grains. The domain width varies from 100 to 300 nm depending on the grain height and can, on a first approximation, be described by domain theory of extended films.

Growth, microstructure, and magnetic properties of highly textured and highly coercive Nd-Fe-B films

Microstructure and permanent magnet properties of a perpendicular anisotropic NdFeB/Ta multilayered thin film prepared by magnetron sputtering

The magnetic properties of Ni thin films, in the range 20–500 nm, at the crystalline-nanocrystalline interface are reported. The effect of thickness, substrate and substrate temperature has been studied. For the films deposited at ambient temperatures on borosilicate glass substrates, the crystallite size, coercive field and magnetization energy density first increase and achieve a maximum at a critical value of thickness and decrease thereafter. At a thickness of 50 nm, the films deposited at ambient temperature onto borosilicate glass, MgO and silicon do not exhibit long-range order but are magnetic as is evident from the non-zero coercive field and magnetization energy. Phase contrast microscopy revealed that the grain sizes increase from a value of 30–50 nm at ambient temperature to 120–150 nm at 503 K and remain approximately constant in this range up to 593 K. The existence of grain boundary walls of width 30–50 nm is demonstrated using phase contrast images. The grain boundary area also stagnates at higher substrate temperature. There is pronounced shape anisotropy as evidenced by the increased aspect ratio of the grains as a function of substrate temperature. Nickel thin films of 50 nm show the absence of long-range crystalline order at ambient temperature growth conditions and a preferred [111] orientation at higher substrate temperatures. Thin films are found to be thermally relaxed at elevated deposition temperature and having large compressive strain at ambient temperature. This transition from nanocrystalline to crystalline order causes a peak in the coercive field in the region of transition as a function of thickness and substrate temperature. The saturation magnetization on the other hand increases with increase in substrate temperature.

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Effect of microstructural evolution on magnetic properties of Ni thin films

Anisotropic Pr-Fe-B thin films with perpendicular texture have been prepared by magnetron sputtering and subsequent heat treatment. After crystallization at 873 K, the films deposited at 773 K show a perpendicular anisotropic texture, due to the existence of a weak anisotropy in the as-deposited films. The deposition rate has been proved to be an important parameter for the control of microstructure, morphology, and coercivity. The coercivity mechanism of the anisotropic Pr-Fe-B films has been studied by analyzing the temperature dependence of coercivity from 5 to 300 K, based on the micromagnetic model. Nucleation of reversed domains, taking place preferentially at the grain surface where the magnetic anisotropy is reduced and the local demagnetization field is the highest, is determined to be the leading mechanism in controlling the magnetization reversal processes of the anisotropic Pr-Fe-B films. Though the grains in the films are strongly magnetically coupled, the magnetization reversal processes in the Pr-Fe-B thin films are not realized by uniform rotations of the magnetic moments.

Coercivity mechanism of anisotropic Pr2Fe14B thin films with perpendicular texture

We show that the generally accepted demagnetizing field corrections are not applicable to the broad class of modern granular magnetic materials, where magnetization reversal occurs via discrete switching of the moments of individual grains, and thus the self-demagnetizing field of a given grain is proportional to the spontaneous magnetization Ms, not to the sample magnetization M. This leads to the fact that the self-demagnetizing field must be considered as a contribution to the coercive field and that the slope of the magnetization variation is higher than the classical demagnetizing field slope. This description of demagnetization processes is confirmed by numerical modeling.

Revisiting magnetization processes in granular hard magnetic materials

Relatively good hard magnetic properties obtained from Nd2Fe14B films prepared by the rf sputtering technique were investigated in terms of the microstructural development Although as-deposited films on Mo substrates deposited at substrate temperatures (Ts) of 365 °C were amorphous with the dispersion of nanocrystalline NdO particles, columnar grains of Nd2Fe14B phase with the c-axis perpendicular to the film plane developed after annealing at 650 °C at an optimized heating rate Nd2Fe14B grain size was about 400 nm in average and NdO particles of about 10 nm were dispersed within the grains and along the grain boundaries These films exhibited good perpendicular hard magnetic properties of iHc=1356 kA/m and (BH)MAX∼216 kJ/m3

T. Okuda

Papers

Structure and magnetic properties of high coercive NdFeB films with a perpendicular anisotropy