Veit Koeninger

Surface acoustic waves on thin films of giant magnetostrictive alloys

Abstract Thin films of giant magnetostrictive (Tb,Dy)Fe 2 alloys were prepared and the magnetostriction and the velocity of a surface acoustic wave (SAW) were measured. This velocity depends on the mechanical constants of the material which can be changed under the influence of an applied magnetic field. The large magnetostrictions of the (Tb,Dy)Fe 2 materials can therefore be used for magnetically tunable SAW devices such as delay lines, signal converters and filters. Tb χ Dy 1− χ Fe 2 films of different concentrations were sputtered on piezoelectric LiNbO 3 substrates equipped with SAW-exciting interdigital electrodes. The influences of film composition, film thickness and magnetic field on SAW velocity and magnetostriction were investigated.

Pressure-composition-temperature relationships of the Sm2Fe17N system

The pressure-composition-temperature (P-C-T) relationships of the Sm 2 Fe 17 -N system were measured volumetrically at temperatures between 623 and 673 K. Clear pressure plateaux could be reproducibly measured between [N]/[Sm 2 Fe 17 ]=2.0 and 3.0, indicating the coexistence of Sm 2 Fe 17 N 2 and Sm 2 Fe 17 N 3 phases. As the N concentration [N]/[Sm 2 Fe 17 ] approaches from 0 to 2.0, the N chemical potentials and the lattice parameters increase sharply, indicating the induction and accumulation of internal stresses caused by the uptake of N. The inclinations of the measured plateaux suggest that the formation of Sm 2 Fe 17 N 3 from Sm 2 Fe 17 N 2 seems to proceed under highly constrained lattice conditions. The Lacher model was fitted to the measured P-C-T curves and a partial phase diagram of the Sm 2 Fe 17 -N system was constructed. The critical temperature below which Sm 2 Fe 17 N 3 exists was estimated as 793±30 K. The exposure of a stoichiometric Sm 2 Fe 17 sample to N 2 gas at 673 K over a period of 1 month exhibited no appreciable disproportionation. The partial molar enthalpy and entropy calculated for the formation of Sm 2 Fe 17 N 3 were -46.9±2.4 kJ (mol N 2 ) -1 and -25.4±1.3 J K -1 (mol N 2 ) -1 respectively.

Preparation and characterization of (Tb,Dy)Fe2 giant magnetostrictive thin films for surface acoustic wave devices

Abstract Thin films of giant magnetostrictive (Tb,Dy)Fe 2 alloys were prepared by three different deposition processes: ion beam sputtering, ion plating and flash evaporation. The flash evaporation process readily yields films with little deviation in composition from that of the source materials, while the ion-beam-sputtering process results in a composition deviation which depends on the substrate location. The ion-plating process yields a high rate of deposition but leads to a marked composition variation between source and film materials. The deposition conditions, mainly the background pressure, also have an important influence on the film characteristics.

Giant magnetostrictive materials: thin film formation and application to magnetic surface acoustic wave devices

Abstract This paper points out current problems in the research and development of giant magnetostrictive materials (GMMs), and the use of thin film GMMs instead of bulk is emphasized because of the ready and inexpensive production and high applicability. One of the most prominent applications using GMM films is the magnetic surface acoustic wave device with tunability under an external magnetic field. Thin films of the giant magnetostrictive Tb 0.3 Dy 0.7 Fe 2 alloy were prepared by vacuum flash evaporation. The film structure was able to be changed from amorphous to crystalline by controlling the substrate temperature and deposition rate. The crystalline films exhibited much higher magnetization and larger magnetostrictions than did the amorphous films. The contamination of the films in vacuum facilitated amorphization and markedly reduced the magnetostrictions of the films. X-ray diffraction and Auger electron spectroscopy analyses revealed that the ratio of vacuum to deposition rate is an appropriate indicator to assess the contamination effect of films.

Diffusion behavior of N atoms in Sm2Fe17

The rate of N diffusion in Sm 2 Fe 17 was measured using an Sm 2 Fe 17 N 2.4 sample with a particle size of 5 μm. At this N concentration, the two Sm 2 Fe 17 N 2 and Sm 2 Fe 17 N 3 phases were assumed to coexist. The measured diffusion coefficient yielded a value, D=2.7x10 -12 cm 2 s -1 at 623 K which is much higher than those measured for larger particle sizes of the sample. The marked change in the diffusivity can be attributed to the contribution of the pre-exponential factor D 0 to the diffusion coefficient. The sample size dependence of the N diffusivity suggests strong interactions of the diffusing N atoms with defects induced and accumulated in the nitrided sample. The rate of the N diffusion seems to change depending on the readiness of strain relief of the sample in the nitride formation, resulting in a different diffusion process depending on the N concentration and the particle size.