Magnetic disk substrate, magnetic disk, and magnetic disk device
It is an object to provide a magnetic disk substrate highly reliable to prevent the occurrence of crash failure even if a magnetic disk is rotated at high speed, and suitable for a hard disk that starts and stops by the load/unload method, and a magnetic disk using such a substrate. The representative structure of a magnetic disk substrate is a disk-shaped glass substrate 10 having a generally flat main surface 11 , an end face 12 , a chamfered face 13 formed between the main surface 11 and the end face 12 , and an offset portion, at the periphery of the main surface 11 , raised or lowered with respect to a flat surface, other than the periphery, of the main surface 11 , and characterized in that the magnitude of the offset portion is approximately uniform over the entire circumference of the glass substrate 10.
1. A magnetic disk substrate being a disk-shaped substrate and having a generally flat main surface, an end face, a chamfered face formed between the main surface and the end face, and an offset portion, at a periphery of the main surface, raised with respect to a flat surface, other than the periphery, of the main surface, wherein:
said offset portion is formed over the entire circumference of said substrate;
when a distance from a center of the substrate to the end face is 100%, an extreme portion where rising is maximum in the offset portion is located in a range of 92.0 to 97.0% from the center of the substrate;
in the main surface, a circularity of a circle formed by the extreme portion is 400 μm or less; and
the substrate is configured to be adapted to a magnetic disk of a load/unload type in which a magnetic head is loaded and unloaded with respect to the main surface of the magnetic disk through its outer periphery.
2. A magnetic disk substrate according to claim 1 , wherein:
said substrate has a circular hole in its center, and
in said main surface, a concentricity between a circle formed by said extreme portion and said circular hole is 1200 μm or less.
3. A magnetic disk substrate according to claim 1 , wherein:
said offset portion includes rising and/or lowering with respect to the flat surface, other than the periphery, of said main surface, said rising and/or lowering formed at the periphery of said main surface, and
a magnitude of offset in said offset portion is approximately uniform over the entire circumference of said substrate.
4. A magnetic disk substrate according to claim 3 , wherein, in said offset portion, a change in magnitude of said offset is 5 nm or less.
5. A magnetic disk substrate according to claim 1 , wherein said substrate is a substrate for use in a magnetic disk to be mounted in a magnetic disk device adapted to rotate the magnetic disk at a rotational speed of at least 5400 rpm or more.
6. A magnetic disk wherein at least a magnetic layer is formed on the substrate according to claim 1 .
7. A magnetic disk according to claim 6 , wherein the disk is configured to operate together with the magnetic head having a flying height of 4 nm or less.
8. A magnetic disk according to claim 6 , wherein a recording density is 200 GBit/inch 2 or more.
9. A magnetic disk device mounted with the magnetic disk according to claim 6 .