Microstructure Control And Magnetic Properties Of L1₀-FePt/Fe Exchange Coupled Nano-composite Thin Films

Nanocomposite magnets have attracted considerable attention since 1991. However, how to obtain optimal two-phase microstructure is an open problem for the development of nanocomposite magnets. Since controlling the microstructure of the nanocomposite magnets on nanometer scale becomes a key technology to achieve excellent magnetic properties, in this work, a granular FePt film, which could be fabricated by tuning sputtering pressure, is employed. And then, soft magnetic layer is deposited on the L10-FePt island structure in order to obtain optimal two-phase nanostructure.A series of FePt thin films and FePt/Fe multilayers were prepared by DC magnetron sputtering and post annealing under high vacuum. The crystal structure, morphology and composition were characterized by X-ray diffraction technique, transmission electron microscopy and scanning electron microscopy with energy dispersive spectroscopy respectively. Meanwhile the magnetic properties were measured by vibrating sample magnetometer at room temperature. The main results are as follows:(1) The sputtering pressure and film thickness have great influences on surface morphology and grain size of as-deposited FePt thin film. With increasing the sputtering pressure, the grain size decreases, at the same time the continuous film changes into granular film. As the film thickness increases, the grain size becomes larger and the film morphology changes from granular film into continuous one. By controlling the sputtering pressure, FePt granular films with island structure have been successfully prepared.(2) Sputtering pressure and film thickness have great effects on magnetic properties of annealed thin films. FePt phases in as-deposited thin films are disordered soft magnetic phase with FCC crystal structure. After annealing at 600? under high vacuum, disordered soft magnetic phases turn into ordered hard magnetic phases with FCT crystal structure. Annealing time has a great influence on the magnetic properties of L10-FePt film. The coercivities display a trend of increasing with increasing annealing time.(3) The Fe layers with different thicknesses were deposited on L10-FePt granular film to obtain soft and hard magnetic phase layer-by-layer structure. The energy product increases with Fe layer thickness and reaches a peak, and then decrease rapidly. It could be obtain good magnetic properties, when the FePt thickness is 20 nm and Fe thickness is 2 nm or 4 nm.(4) L10-FePt/Fe multilayer films were studied in this work. For [L10-FePt(20nm)/Fe(2nm)]n and [L10-FePt(20nm)/Fe(4nm)]n multilayer films, with the increasing the number of layers n, exchange-coupling interaction and energy product (BH)max increase.(5) Annealing temperature and time have also much effect on the magnetic properties of FePt/Fe multilayers. After annealed at 600? for 10 min, the energy product of [Llo-FePt(20nm)/Fe(4nm)]5 achieved 172.29 kJ/m3. Meanwhile, Mr/Msat value achieves 0.7258, which is much higher than 0.5 of expected value for isotropic magnet. It means that there are strong remanence enhancement and strong exchange coupling between soft and hard phase in this film.