Glass substrate for magnetic disk and magnetic recording medium
With regard to a glass substrate 1 according to the present invention, a value of an amendment concentricity AC that has taken into consideration Sk and/or Ku calculated from a shape profile over the whole circumference of an inside hole, or the skewness is within a predetermined range. The glass substrate for a magnetic recording medium can stably read servo information including track information stored on a magnetic disk when the glass substrate is used for an HDD.
1. A glass substrate for a magnetic disk with an inside hole formed therein, wherein:
a circularity of the inside hole is 5.5 μm or less, and
a value of Sk calculated from a shape profile over a whole circumference of the inside hole is in a range of 0.55 to −0.55, and
Sk is given by
Sk
=
∑
i
=
1
n
R
i
3
n
σ
3
(
1
)
where n is the number of samples, and n=3600, R i is an ith difference between the shape profile and a least-square-approximated circle of the shape profile on a line drawn to the least-square-approximated circle of the shape profile from a center of a circle calculated by a least-square approximation of the shape profile over the whole circumference of the inside hole, and σ is a standard deviation.
2. The glass substrate for a magnetic disk as recited in claim 1 , wherein a value (Ku−3) obtained by subtracting 3 from Ku calculated from the shape profile over the whole circumference of the inside hole is in a range of 1.6 to −1.6, and
Ku is given by
Ku
=
∑
i
=
1
n
R
i
4
n
σ
4
(
3
)
where n is the number of samples, and n=3600, R i is an ith difference between the shape profile and a least-square-approximated circle of the shape profile on a line drawn to the least-square-approximated circle of the shape profile from a center of a circle calculated by a least-square approximation of the shape profile of the inside hole, and σ is a standard deviation.
3. The glass substrate for a magnetic disk as recited in claim 1 , wherein a circularity of the inside hole is 3 μm or less.
4. The glass substrate for a magnetic disk as recited in claim 1 , wherein the glass substrate is used for a magnetic disk having a storage capacity of at least 80 GBytes per one 2.5-inch disk.
5. The glass substrate for a magnetic disk as recited in claim 1 , wherein a value of AC calculated by using a value of Sk ID calculated from the shape profile over the whole circumference of the inside hole, a value of Sk OD calculated from a shape profile of an outer circumference of the glass substrate for a magnetic disk, and a value of a concentricity C is 9.6 μm or less,
AC is given by
A
C
=
C
(
1
+
Sk
OD
Sk
ID
)
(
4
)
Sk ID is given by
Sk
ID
=
∑
i
=
1
n
R
i
3
n
σ
3
(
5
)
where n is the number of samples, R i is an ith difference between the shape profile and a least-square-approximated circle of the shape profile on a line drawn to the least-square-approximated circle of the shape profile from a center of a circle calculated by a least-square approximation of the shape profile over the whole circumference of the inside hole, and σ is a standard deviation, and
Sk OD is given by
Sk
OD
=
∑
i
=
1
n
R
i
3
n
σ
3
(
6
)
where n is the number of samples, R i is an ith difference between the shape profile and a least-square-approximated circle of the shape profile on a line drawn to the least-square-approximated circle of the shape profile with the smallest value of the shape profile from a center of a circle calculated by a least-square approximation of the shape profile of the outer circumference, and σ is a standard deviation.
6. The glass substrate for a magnetic disk as recited in claim 1 , wherein an M-value calculated from a relationship between a radial position from a center of the glass substrate for a magnetic disk and a thickness at that radial position is within 20×10 6 mm 3 , and the M-value is given by
M
=
∑
i
=
0
n
r
i
2
Δ
t
i
(
7
)
Δ
t
i
=
t
i
-
t
0
(
8
)
where n is the number of samples, r i is a radius of a measurement point, t i is a thickness at the measurement point, and t 0 is a thickness at an initial measurement point (thickness of an inner circumferential edge).
7. A magnetic recording medium comprising:
the glass substrate for a magnetic disk as recited in claim 1 ; and
an underlayer, a magnetic layer, a protective layer, and a lubricating layer provided on a main surface of the glass substrate.