<s>
A	O
hard	B-Device
disk	I-Device
drive	I-Device
platter	I-Device
or	O
hard	B-Device
disk	I-Device
is	O
the	O
circular	O
magnetic	B-Architecture
disk	I-Architecture
on	O
which	O
digital	B-General_Concept
data	I-General_Concept
is	O
stored	O
in	O
a	O
hard	B-Device
disk	I-Device
drive	I-Device
.	O
</s>
<s>
The	O
rigid	O
nature	O
of	O
the	O
platters	O
is	O
what	O
gives	O
them	O
their	O
name	O
(	O
as	O
opposed	O
to	O
the	O
flexible	O
materials	O
which	O
are	O
used	O
to	O
make	O
floppy	B-Device
disks	I-Device
)	O
.	O
</s>
<s>
Hard	B-Device
drives	I-Device
typically	O
have	O
several	O
platters	O
which	O
are	O
mounted	O
on	O
the	O
same	O
spindle	O
.	O
</s>
<s>
A	O
typical	O
magnetic	O
region	O
on	O
a	O
hard-disk	B-Device
platter	I-Device
(	O
as	O
of	O
2006	O
)	O
is	O
about	O
200	O
–	O
250	O
nanometers	O
wide	O
(	O
in	O
the	O
radial	O
direction	O
of	O
the	O
platter	O
)	O
and	O
extends	O
about	O
25	O
–	O
30	O
nanometers	O
in	O
the	O
down-track	O
direction	O
(	O
the	O
circumferential	O
direction	O
on	O
the	O
platter	O
)	O
,	O
corresponding	O
to	O
about	O
100	O
billion	O
bits	O
per	O
square	O
inch	O
of	O
disk	O
area	O
(	O
15.5Gbit/cm2	O
)	O
.	O
</s>
<s>
The	O
material	O
of	O
the	O
main	O
magnetic	B-Architecture
medium	I-Architecture
layer	O
is	O
usually	O
a	O
cobalt-based	O
alloy	O
.	O
</s>
<s>
In	O
today	O
's	O
hard	B-Device
drives	I-Device
each	O
of	O
these	O
magnetic	O
regions	O
is	O
composed	O
of	O
a	O
few	O
hundred	O
magnetic	O
grains	O
,	O
which	O
are	O
the	O
base	O
material	O
that	O
gets	O
magnetized	O
.	O
</s>
<s>
One	O
reason	O
magnetic	O
grains	O
are	O
used	O
as	O
opposed	O
to	O
a	O
continuous	O
magnetic	B-Architecture
medium	I-Architecture
is	O
that	O
they	O
reduce	O
the	O
space	O
needed	O
for	O
a	O
magnetic	O
region	O
.	O
</s>
<s>
Thus	O
,	O
much	O
of	O
the	O
development	O
in	O
hard	B-Device
drives	I-Device
has	O
been	O
in	O
reduction	O
of	O
grain	O
size	O
.	O
</s>
<s>
As	O
of	O
2015	O
,	O
laptop	B-Device
hard	I-Device
drive	I-Device
platters	O
are	O
made	O
from	O
glass	O
while	O
aluminum	O
platters	O
are	O
often	O
found	O
in	O
desktop	O
computers	O
.	O
</s>
<s>
The	O
coating	O
has	O
a	O
complex	O
layered	O
structure	O
consisting	O
of	O
various	O
metallic	O
(	O
mostly	O
non-magnetic	O
)	O
alloys	O
as	O
underlayers	O
,	O
optimized	O
for	O
the	O
control	O
of	O
the	O
crystallographic	O
orientation	O
and	O
the	O
grain	O
size	O
of	O
the	O
actual	O
magnetic	B-Architecture
media	I-Architecture
layer	O
on	O
top	O
of	O
them	O
,	O
i.e.	O
</s>
<s>
In	O
the	O
hard-disk	B-Device
drive	I-Device
the	O
hard-drive	B-Device
heads	I-Device
fly	O
and	O
move	O
radially	O
over	O
the	O
surface	O
of	O
the	O
spinning	O
platters	O
to	O
read	O
or	O
write	O
the	O
data	O
.	O
</s>
<s>
Extreme	O
smoothness	O
,	O
durability	O
,	O
and	O
perfection	O
of	O
finish	O
are	O
required	O
properties	O
of	O
a	O
hard-disk	B-Device
platter	I-Device
.	O
</s>
<s>
In	O
1990	O
,	O
Toshiba	O
released	O
the	O
MK1122FC	O
,	O
the	O
first	O
hard	B-Device
drive	I-Device
to	O
use	O
a	O
glass	O
substrate	O
,	O
replacing	O
the	O
aluminium	O
alloys	O
used	O
in	O
earlier	O
hard	B-Device
drives	I-Device
.	O
</s>
<s>
It	O
was	O
originally	O
designed	O
for	O
laptops	B-Device
,	O
for	O
which	O
the	O
greater	O
shock	O
resistance	O
of	O
glass	O
substrates	O
are	O
more	O
suitable	O
.	O
</s>
<s>
Around	O
2000	O
,	O
other	O
hard	B-Device
drive	I-Device
manufacturers	O
started	O
transitioning	O
from	O
aluminum	O
to	O
glass	O
platters	O
because	O
glass	O
platters	O
have	O
several	O
advantages	O
over	O
aluminum	O
platters	O
.	O
</s>
<s>
In	O
2005	O
–	O
06	O
,	O
a	O
major	O
shift	O
in	O
technology	O
of	O
hard-disk	B-Device
drives	I-Device
and	O
of	O
magnetic	O
disks/media	O
began	O
.	O
</s>
<s>
Originally	O
,	O
in-plane	O
magnetized	O
materials	O
were	O
used	O
to	O
store	O
the	O
bits	O
but	O
has	O
now	O
been	O
replaced	O
by	O
perpendicular	B-Device
recording	I-Device
.	O
</s>
<s>
Orienting	O
the	O
magnetization	O
perpendicular	O
to	O
the	O
disk	O
surface	O
has	O
major	O
implications	O
for	O
the	O
disk	O
's	O
deposited	O
structure	O
and	O
the	O
choice	O
of	O
magnetic	O
materials	O
,	O
as	O
well	O
as	O
for	O
some	O
of	O
the	O
other	O
components	O
of	O
the	O
hard-disk	B-Device
drive	I-Device
(	O
such	O
as	O
the	O
head	O
and	O
the	O
electronic	O
channel	O
)	O
.	O
</s>
