Non-magnetic coated particles having a defined parameter amp;agr; 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:PPnon-magnetic particles selected from the group consisting of particles (1) and (2):PP(1) coated acicular or spindle-shaped &agr;-Fe.sub.2 O.sub.3 particles which are produced by adhering not less than 0.01 to 20 wt % (calculated as Al, SiO.sub.2 or Al and SiO.sub.2, and based on acicular or spindle- shaped &agr;-Fe.sub. 2 O.sub.3 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 &agr;-Fe.sub.2 O.sub.3 particles as a core material, and have a molecular weight-dependent parameter &agr; represented by the following formula of not less than 0.5:P PAs=K.sub.1 ·M. sup.&agr;PP wherein M represents the number- average molecular weight of a binder resin, As represents the saturation adsorption of said binder resin, and K.sub.1 represents a constant used for measuring As and dependent on the binder resin and a solvent,P P(2) BaO.4.5Fe. sub.2 O.sub.3 particles having an average particle diameter of not more than 0.1 &mgr;m; andPPa binder resin.