Fig. 1 TEM micrographs of as-sputtered (Fe55Pt45)Al37-O granular films and those annealed for 1 h at 500, 650, and 750C.
Microstructures and magnetic properties of FePt thin films
Due to a strong thrust to develop a ultrahigh density recording media exceeding 100 Gbit/in2, granular films with high anisotropy materials became a center of current research interest in the magnetic recording community [1-3]. Such high-density recording requires a bit dimension of only a few nanometer diameters, and thus precise control of the media microstructure, especially grain size, grain size dispersion and grain isolation to break exchange coupling is necessary. In order to overcome the thermal instability of magnetic recording expected from such nanoscale ferromagnetic particles, it is said that the value of KuV/kT (KuV, stored magnetic energy; kT, thermal energy) of a magnetic particle must be higher than 50, where Ku is the magnetocrystalline anisotropy constant and V is the bit volume [4]. Thus, tetragonal intermetallic alloys with the L10 ordered structure such as FePt and CoPt, which have 20-40 times higher Ku than today's hexagonal Co-alloy based media, are regarded as the best candidates for the next generation high density recording media [5-12].Recently, Stavroyiannis et al. [12] and Yu et al [13] successfully prepared nanocomposite CoPt/Ag and CoPt/C thin films, respectively. Post-annealing these films led to a microstructure consisting of magnetically hard L10 CoPt nanoparticles embedded in the nonmagnetic matrix. High coercivities exceeding 10 kOe were reported for the films with the magnetic grain size of around 10 nm. More recently, Watanabe et al. [14,15] reported that high coercivity FePt granular thin film can be fabricated by annealing the FePt-Al-O films that were produced by sputtering FePt and Al targets under oxygen flow. Figure 1 shows TEM micrographs of (Fe55Pt45)63Al37-O granular films that were annealed for 1 h at 500, 650 and 750 C. As-sputtered film show extremely fine ( 2 nm) isolated granular microstructure embedded in amorphous oxide matrix phase. Since the isolated particles are disordered and their size is less than 2 nm, they show superparamagnetic behavior. By annealing at 500 C, some disordered metallic particles coalesce and form interconnected microstructure. Because of this, the film show softmagnetic behavior. Ordering to the L10 structure occurs only above 650 C annealing; the metallic grains are coarsened to larger than 5 nm. At 750 C, the ordering progresses to a higher degree, by which the coercivity increases. However, the significant grain coarsening and coalescence occur, which makes this film unsuitable as recording media. This observation suggests that it is not possible to produce L10 ordered FePt nanogranular materials suitable for recording media cannot be produced by post-annealing disordered granular FePt film. In order to prepare a suitable granular microstructure, it is necessary to decrease the temperature for ordering so that the grain coarsening can be suppressed. However, since ordering progress by the diffusion of atoms, suppression of coarsening during ordering process does not appear to be feasible. Choosing a matrix phase which suppress the coarsening of the metallic particles appears to be necessary.
Fig. 1 TEM micrographs of as-sputtered (Fe55Pt45)Al37-O granular films and those annealed for 1 h at 500, 650, and 750C.
Another recent observation is that the ordering may not occur when a particle size is below a critical size. Ping et al. [16] observed that the ordering of the granular FePt film occur at a higher temperature by 100 C than a continuous film. Takahashi et al. [17] reported that ordering can progress in-situ by sputter-deposit FePt films at a substrate temperature of 300 C. In this case, however, ordering progress only when the thickness of the film become lather than 10 nm [18]. This result also suggest that there may be a critical size for ordering. Mori et al. [19] reported that the ordering temperature decreases when particle size become less than 5 nm due to the depression of Debye temperature, and this size dependence of ordering may be explained in line with his proposal. In order to produce FePt nanogranular films that is suitable for recording media, it is necessary to find a way to make ordered nanoparticles suppressing grain size coarsening and coalescence.[4].
2. Studies on high coercive and high (BH)max thin film permanent magnet
Since L10 ordered FePt phase have a large magnetocrystalline anisotropy of 7 X 107 erg/cc, it is possible to achive very high coercivity (HA~120 kOe) by arrigning magnetically isolated sigle domain particles. Fig. 2 shows FePt particles epitaxially grown on a single crystalline MgO (001) at 700C which exhibit a huge coercivity of 40 kOe. Since the FePt particles are epitaxially grown, the c-axis of each particles is directed to normal to the film plane. Because of this, this films shows very high perpendicular anisotropy.
図１ 40 kOeの保磁力を持つ島状成長したFePt薄膜 [5]
Although magnetically isolated sigle domain particles show huge coercivity, the coercivity decreases drastically when particles are interconnected by the growth of the film. This is because there is no pinning force for domain wall motion.
Fig. 2 Microstructure and coercivity change of FePt films epitaxially grown
on MgO (001) substrate as funcitions of film thickness [5]
Relevant Publications