Reduction of intergranular exchange coupling and grain size for high Ku CoPt-based granular media: Metal-oxide buffer layer and multiple oxide boundary materials

Abstract

Investigation of magnetic properties and microstructure of granular media with various multiple oxides as the grain boundary material is reported. Saturation magnetization (Ms), uniaxial magnetocrystalline anisotropy (Ku), and magnetic grain diameter (GD) of the granular media show linear correlation with volume weighted average for melting point (Tm) of each oxides (Tmave). Ku of magnetic grains (Kugrain) shows a trade-off relation with GD that it is a big challenge to satisfy both high Kugrain and small GD by only controlling Tmave. To obtain a granular medium with appropriate Kugrain, GD, and low degree of intergranular exchange coupling, the combination of Tmave control of grain boundary material by mixing oxides and employment of a buffer layer are required. Here the degree of intergranular exchange coupling is estimated from the slope of M-H loop at around coercivity (α). By applying this technique, a typical granular medium with Kugrain of 1.0×107 erg/cm3, GD of 5.1 nm, and α of 1.2 is realized.

Effects of utilizing a granular nucleation layer on magnetic properties and microstructure of CoPt–B2O3 granular medium

“…The possible origin of H c at various d NL values in the granular medium is the existence of an inhomogeneous structure in the medium 13,[18][19][20] or the noncoherent magnetic reversal mode [21][22][23] or the change in the degree of thermal stability 24,25) or intergranular exchange coupling. [26][27][28][29] In accordance with the result in the previous section, each magnetic grain grows columnarly with c-plane sheet texture orientation such that the existence of an inhomogeneous structure in the medium and the noncoherent magnetic reversal mode are not the primary issues for the granular medium. To determine the degrees of thermal stability and intergranular exchange coupling of the granular medium, vK grain u =kT and the slope (α) at approximately H c are evaluated.…”