Nonmagnetic bottom layer for magnetic recording medium, a magnetic recording medium and non-magnetic particles

摘要

A magnetic recording medium comprises, in order:(A) a non-magnetic substrate;(B) a non-magnetic undercoat layer comprising a binder resin and non-magnetic particles selected from:(1) acicular or spindle-shaped α-Fe₂O₃ particles which are coated with from 0.01 to 20 wt% (calculated as Al, SiO₂ or Al and SiO₂ and based on the uncoated α-Fe₂O₃ particles) of an oxide or a hydroxide containing Al, Si or both Al and Si, which coated particles have a molecular-weight-dependent parameter α of not less than 0.5 wherein α is defined by formula (I):$$ {\text{As = K₁ . M}}^{\text{α}}\text{ (I)}$$ wherein M is the number-average molecular weight of a binder, As is the saturation adsorption of the binder, and K₁ is a constant dependent on the binder and the solvent used for measuring As; and(2) BaO·4.5Fe₂O₃ particles having an average particle diameter of not more than 0.1µm; and(C) a magnetic recording layer comprising magnetic particles and a binder resin.