Nano-granular FePt-Ag Composite Films- Its Magnetic and Magnetotransport Properties
Zhengang Zhang, Kyongha Kang, Naotoshi Omoto and Takao Suzuki
Information Storage Materials Laboratory, Toyota Technological Institute, JP
Keywords: FePt-Ag, nano-granular, GMR, recording media
Ordered FePt films have a very high magnetic anisotropy . By annealing FePt/Ag multilayer precursor films, isolated FePt nano particles have been obtained with high perpendicular magnetic anisotropy, which opens a window for ultra-high- density magnetic recording . To detect the film structure, we measured its magnetotransport properties at room temperature. Giant magnetoresistance effect (GMR) was found in this system, as shown in Fig.1. Up to our knowledge, this is the first time to show GMR effect in such a big anisotropy system. With annealing the phase of FePt changes from fcc to fct resulting in a very high uni-axial magnetic anisotropy in each FePt grain. Structural analysis reveals that the average FePt grain size is around 25 nm. Under the saturation magnetic field, all grain moments approach to the external magnetic field direction, at which state the film resistance is minimum; under the coercivity field, the FePt grain moments tends to be random in space, leading to the maximum state of resistance. The GMR ratio was found to depend on the film pre-annealing layer structure and annealing conditions, as shown in Fig. 2. With thicker FePt layer thickness and thinner Ag thickness, the FePt layer is not easy to be broken by the diffusion of Ag along the FePt grain boundaries, which brings large exchange coupling between FePt grains killing GMR effect. With higher annealing temperature and longer annealing time, FePt grains grow bigger, which leads to the decrease in grain surface area to volume ratio also diminishing GMR effect . For high-density magnetic recording, nano-sized magnetic grains should be well decoupled magnetically. This requirement on film structure is also applicable to get high GMR effect. Therefore, measuring GMR effect of magnetic recording media provides a new way to detect the media microstructure.
* Research was supported by the Academic Frontier Center of the Japanese Ministry of Education, Culture, Sports, Science and Technology.
Topic area: Materials Nanostructures and properties
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