Associate Professor, Faculty of Engineering, Tohoku-Gakuin University
Nano-structure and magnetic properties for FePt ordered alloy films with high coercivity
The magnetization processes for assemblies of ferromagnetic nano-particles with a large uniaxial magnetocrystalline anisotropy is of great scientific and technological interest, since they can be used in future magnetic devices, such as high-density perpendicular magnetic recording media, artificial patterned media and nano-scale high performance magnets. Recently, a lot of work have been made to process FePt films and particles by conventional thin film preparation techniques and a chemical synthesis technique, because the L10 ordered FePt phase possesses a large uniaxial magnetocrystalline anisotropy. However, high coercivity together with highly aligned crystal orientation has been realized in only a few studies. We have found that huge coercivities exceeding 70 kOe at room temperature and 100 kOe at 4.5 K can be obtained in defect-free perfectly aligned single domain FePt particles (Fig.1). The difference between the magnetization processes for single-domain particles and multi-domain particles is demonstrated clearly by the initial magnetization curves (Fig. 2). Nominal film thicknesses are 3 nm (a), 5 nm (b), 8 nm (c), 10nm (d), 12 nm (e), 15 nm (f), 18 nm (g), 20 nm (h), 25 nm (i), 30 nm (j) and 40nm (k). The films with single domain particles are very difficult to be magnetized (Fig. 2 (a) and (b)). Hence, the magnetization progresses only by magnetization rotation in the particles. However, with increasing film thickness, the volume fraction of multi-domain particles increases and consequently, the fraction of the magnetization that is magnetized at a low magnetic field, which corresponds to the magnetic domain wall displacement, increases. These results provide valuable information not only for theoretical analysis of magnetization processes in nano-meter scale magnets but also for future possible magnetic applications.
