| Summary: | 博士 === 國立成功大學 === 物理學系碩博士班 === 91 === Magnetic thin films of Fe3C nanograins embedded in an amorphous carbon matrix were synthesized by using a dc magnetron co-sputtering technique. The films containing pure Fe3C grains could be obtained only in proper sputtering conditions. Films containing only grains of Fe3C phase were subjected to post deposition annealing at temperatures of 250 to 600℃for 30 to 90 min, respectively. Auger electron spectroscopy (AES) was used to determine the atomic concentration and its fluctuations. The microstructure and phases of grains were determined by high-resolution transmission electron microscopy (HRTEM) and selected area diffraction (SAD) patterns. Very good crystallinity appeared at Ta>=250℃. From the HRTEM image, lattice spacings of 6.7 A - corresponding to Fe3C (001) planes - and 4.3 A - corresponding to Fe3C (100) planes - were observed. Higher annealing temperature and larger annealing time caused an increase of grain size. The largest grain (18 nm) was obtained at Ta = 550℃.
Both temperature and field dependent magnetizations, M(T) and M(H), were measured for samples of various carbon concentrations ( from 37 at. % to 85 at. %). M(T) were measured in both conditions of zero field cooling and a field cooling at H = 100 Oe. Experimental results of Xzfc(T) , obtained from MZFC(T), of zero field cooling, were theoretically fitted by using Wolhfarth''s model of non-interacting particles with log-normal distribution function of particle size. Only the films containing pure Fe3C grains are well fitted theoretically. Once the film was annealed, the grains were longer of single log-normal distribution but were of mixed log-normal and normal distributions. Blocking temperature, grain size, and dispersion of grain size distribution are obtained from fitting results. Saturation magnetization, and coercivity are obtained from the results of M(H) measurements. Films, as deposited, are superparamagnetic and show zero room temperature coercivity. The largest room temperature coercivity of 965 Oe is obtained for the sample of 72 at.% C made at the sputtering pressure of 4 mtorr and annealed at the temperature of 550℃ for 60 min.
The temperature-dependent resistance and the field-dependent magnetoresistance were measured for as-deposited films and films annealed at temperatures from 250 to 550℃for a period of 60 min. Results of temperature-dependent resistance show electrical tunneling conductance in as-deposited film and films annealed at Ta = 250 and 350℃only. The largest magnetoresistance ratio (MR) of 23 % at temperature Tm = 2 K was observed for Ta = 350℃. The variations of both the temperature dependence of resistance and the magnetoresistance with the annealing temperature are discussed.
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