Patent ID: 7695833

Claim:
A magnetic disk having at least a magnetic layer, a protection layer, and a lubrication layer formed on a substrate in this order, wherein: when selection is made of at least three liquid samples including a liquid sample containing a nonpolar substance, a liquid sample containing a polar substance, and a liquid sample containing a hydrogen bonding substance, and having mutually different surface tensions, contact angles between the respective liquid samples and a surface of the magnetic disk are measured to thereby derive a correlation between cosines (Y) of the contact angles and surface tensions (X) of the liquid samples as a natural logarithmic function by a method of least squares, and a value of X that is calculated by substituting 1 for Y in said correlation is given as a true critical surface tension of said magnetic disk, the true critical surface tension is greater than 0 and no greater than 17 mN/m (milli Newton/meter), wherein the protection layer contains carbon, hydrogen and nitrogen, a content of the hydrogen is set to 3 atm % or more and less than 20 atm % relative to a total of the protection layer, a content of the nitrogen relative to the carbon is 4 to 12 atm %, the lubrication layer contains only a perfluoropolyether compound having weight average molecular weight of 5000 or more, and a hydrofluoroether film is formed on a surface of the lubrication layer.