Non-magnetic coated particles having a defined parameter alpha of not less than 0.5

摘要

The disclosure describes a non-magnetic undercoating layer for a magnetic recording medium, coated on the surface of a non-magnetic substrate comprising: non-magnetic particles selected from the group consisting of particles (1) and (2): (1) coated acicular or spindle-shaped alpha -Fe2O3 particles which are produced by adhering not less than 0.01 to 20 wt % (calculated as Al, SiO2 or Al and SiO2, and based on acicular or spindle-shaped alpha -Fe2O3 particles as a core material) of an oxide or a hydroxide containing Al, Si or both Al and Si to the surfaces of acicular or spindle-shaped alpha -Fe2O3 particles as a core material, and have a molecular weight-dependent parameter alpha represented by the following formula of not less than 0.5:As=K1xM alpha wherein M represents the number-average molecular weight of a binder resin, As represents the saturation adsorption of said binder resin, and K1 represents a constant used for measuring As and dependent on the binder resin and a solvent, (2) BaO.4.5Fe2O3 particles having an average particle diameter of not more than 0.1 mu m; and a binder resin.