20th Magnetic Materials Center Seminar
June 26, 2006, 16:00
7th floor seminar room, Sengen
Magnetic properties and microstructure for FePt-SiO2 granular films
T. O. Seki
For achieving higher areal density in the perpendicular recording system, high signal to media noise ratio (SNRm) must be maintained without degrading the thermal stability of nanosized particles. Since the size of magnetically isolated particles is expected to become 7 nm for the area density of 1 Tb/inch2, L10 ordered FePt receives intense research interest for its very high magnetocrystalline anisotropy. The size and size distribution of the particles is also known to have a crucial effect on recording properties. The magnetic thin film with the narrow size distribution is expected to reduce the thermal decay problem. Although many investigations have reported the fabrication of FePt nanoparticles with narrow size distributions by chemical method, employment of the sputtering process is more desirable for commercial applications. However, there is no report of successful processing of FePt nanoparticle arrays with narrow size distributions with the sputtering method. In this study, I attempt to reduce the particle size distribution of FePt thin films that were sputter codeposited with SiO2 on MgO substrates. The particles are dispersed in SiO2 matrix. Last my talk, I showed the microstructure and magnetic properties for FePt-SiO2 granular films with total film thickness of 10nm. Achieved minimum particle size was 15 nm. This particle size and size distribution are not yet stable for application of media. In this presentation, I report my recent trial to improve above mentioned points.

TEM study on grain boundary structure of NdFeB sintered magnets with and without Cu addtion
W. Li
A conventional, ideal, anisotropic magnet consists of well aligned grains of hard magnetic phase, and the hard magnetic phase should be completely separated by a nonmagnetic intergranular phase. But for a complete surrounding of the hard magnetic grains the volume fraction of the liquid phase during sintering should exceed 22%, which is not fulfilled in Nd-Fe-B magnets. In Nd-Fe-B magnets, the hard magnetic Nd2Fe14B phase are not completely separated by non magnetic phase. Therefore, the magnetic interactions such as dipolar interaction and exchange coupling between misaligned grains reduce the coercivity of the magnets. Substituent and dopant elements influence the microstructure and thus the coercivity of the magnets. It is well-known that the Cu addition can increase the coercivity of Nd-Fe-B magnets. In my presentation I will report our preliminary result in the study on the microstructure of Nd-Fe-B magnets with and without Cu addition. I will also report the result in the magnetic properties of specimens by melt-spinning and SPS methods.

Phase separation in bulk forming metallic glasses
K. Hono
Phase separation in supercooled liquid state has been reported in many bulk forming metallic glasses, but its thermodynamical justification has not been made. In this talk, we will overview previous investigations on experimental reports of phase separtions in bulk forming metallic glasses. We will then show our experimental results that ruled out the preceeding phase separations to the onsets of crystallization. Finally, we will show our recent thermodynamical considerations on the miscibility gap islands that are predicted from the systems which are composed of the elements with all negative pair interactions but have large difference in their values. We will demonstrate that phase separation is very unlikely to occur in bulk forming metallic glasses even if we consider the presence of the miscibility gap islands.