Study of magnetic properties and microstructures of FePt island films
Date Issued
2010
Date
2010
Author(s)
Lin, Ger-Pin
Abstract
The development of recording media of continuous films is restricted by super-paramagnetic limit. Patterned media has been suggested as a potential solution for this physical limit. In this study, we describe a simple and cheap method to fabricate large-area patterned media.
The grain size of the magnetic films must be reduced to improve the signal-to-noise ratio. The exchange coupling effect between magnetic grains should be minimized in order to decrease transition noise. This discontinuous nano-size island magnetic film was suggested to reduce the exchange coupling effect between magnetic islands and increased the recording density. In order to increase the recording density, the size of magnetic islands must be reduced by modifying the experiment condition.
FePt films (15 nm) were fabricated with (Fe/Pt)n multilayer on amorphous glass substrates then post-annealed at 700 ℃ for 30 mins to obtain better perpendicular magnetic anisotropy and ordering degree. The chemical composition of FePt films is Fe59Pt41. The magnetic easy axis [001] of L10 FePt phase in the FePt films was perpendicular to the film plane. This nano-size island-shape FePt film with perpendicular magnetic anisotropy may be a good candidate for ultra-high density recording media.
The X-ray diffraction patterns of the annealed FePt films with heat treatment on amorphous glass substrates shown that the fct-FePt(001) peak and fct-FePt(002) peak appeared in 5-nm FePt film. However, the intensity of the fct-FePt(001) peak and fct-FePt(002) peak were weaker as the thickness of FePt film increasing to 30 nm, but the intensity of the fct-FePt (111) peak enhanced. As the thickness of FePt films was increased, the easy axis orientation of FePt films would turn from perpendicular to parallel to the film plane. However, the ordering degree of the FePt film would increase with FePt film thickness.
The particle size of 1-nm thick FePt film on glass that annealed at 700 ℃ for 10 minutes distributed between 2.5 to 5 nm, and the density of islands is 1.37×1013 islands/inch2. A discontinuous and well-separated nano-size island magnetic film can reduce the exchange coupling of the media and increase the recording density.
The driving force of FePt island formation was the surface energy difference between the substrate and FePt alloy. The nucleation site number of FePt islands increased with the surface energy difference between the substrate and FePt alloy. Therefore, the density of islands of 1-nm thick FePt film annealed at 700 ℃ for 10 minutes on carbon film (1.44×1013 islands/inch2) was slightly larger than that of 1-nm thick FePt film annealed at 700 ℃ for 10 minutes on glass. But, the fcc-FePt phase still existed in the 1-nm thick FePt film which annealed at 700 ℃ for 10 minutes on carbon film.
Due to the thick Ag under-layer, the grain growth and island cluster occurred easily in the FePt (1 nm~7.5 nm)/Ag (100 nm) bi-layer films, and the size distribution of metallic islands would be very broad. However, the Ag under layer could improve the ordering of FePt film. The lattice mismatch between Ag under layer and FePt film would provide additional strain energy and promote the occurrence of the fct-FePt (111) preferred orientation. The orientation of magnetic easy axis of FePt (1 nm~7.5 nm) films with Ag (100 nm) under-layer would turn from perpendicular to parallel to the film plane. Due to the Ag atom has larger diffusion coefficient during annealing process, the metallic islands had better roundness in FePt (1 nm)/Ag (1 nm ~3 nm) bi-layer films.
Investigation of the microstructures and magnetic properties of the ordered FePt films revealed that the 1 nm-FePt film annealed at 700 ℃ for 10 minutes had perpendicular magnetic anisotropy and formed well-separated FePt nano-size islands. It is suitable for ultra-high density magnetic recording media. But, the out-of-plane coercivity of the well-separated FePt nano-size islands was about 20 kOe, and it was too large for recording head to reverse its moments. In order to reduce the writing field of FePt nano-island film, we introduced Fe capping layer (0.5~5 nm) on the FePt nano-island films. The exchange coupling effect between Fe capping layer and FePt nano-island would decrease the writing field of the island films.
Subjects
FePt
magnetic recording media
Type
thesis
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