Magnetic tape, its cleaning method, and optical servotrack forming/cleaning apparatus

Abstract

A magnetic tape which comprises a nonmagnetic support, a magnetic layer which is formed on one surface of the nonmagnetic support, and a backcoat layer which comprises a binder and nonmagnetic powder containing carbon black as a component and which is formed on the other surface of the nonmagnetic support, having pits for optical servo formed thereon, characterized in that the average of the reflectance on the flat portion of the backcoat layer is 8.5% or higher, and that the maximum rate of fluctuation of the reflectance on the flat portion, depending on a position of the magnetic tape: [Maximum of absolute value of (Reflectance−Average reflectance)]×100/(Average reflectance) is 10% or lower. This magnetic tape is high in the initial S/N of the servo signal, and also high in the S/N of the servo signal found after the magnetic tape is run twice.

1 . A magnetic tape comprising a nonmagnetic support, a magnetic layer which is formed on one surface of the nonmagnetic support, and a backcoat layer which comprises a binder and nonmagnetic powder containing carbon black as a component, which is formed on the other surface of the nonmagnetic support, and which has pits for optical servo formed thereon, wherein
 a content of the nonmagnetic powder in the backcoat layer is from 50 wt. % to 60 wt. % based on the total weight of the nonmagnetic powder and the binder in the backcoat layer;   an average surface roughness Ra of a flat portion of the backcoat layer, which is measured with an atomic force microscope, is 30 nm or less; and   an average of reflection on a flat portion of the backcoat layer is from 8.5% to 14.9%.   
     
     
         2 . The magnetic tape according to  claim 1 , wherein a maximum rate of fluctuation of the reflectance on the flat portion of the backcoat layer depending on a position of the magnetic tape, which is defined by the following equation, is from 2.8% to 10%:
   [Maximum of absolute value of (Reflectance−Average of reflectance)]×100/(Average of reflectance).
   
     
     
         3 . The magnetic tape according to  claim 1 , wherein a half width of fluctuation of the surface roughness Ra, depending on a site of the magnetic tape, is 5 nm or less.