Magnetic materials for data storage
Hard disk drive (HDD) is one of the most widely used data storage device
for its economical merit and reliability. HDD used to be used only as data
storage devices for computers, but its use is explosively expanding to
home electronics such as HDD video recorder and portable music players.
Accordingly the demand for higher storage capacity or recording density
is increasing. Therefore, there is a need to increase the areal density
of magnetic recording futher. The areal density of the current HDD is in the level 200 Gbit/in2, and futher increase up to the level of 600 Gbit/in2 is expected within five years. The HDD system is composed of magnetic recording media, write head, and read head. In addition, small high energy density permanent magnet is used in a spindle motor to rotate disks.
As shown in Fig. 1, a recording medium is composed of magnetically isolated
ferromagnetic particles. As the recording density increases, the size of
the particles becomes smaller and smaller, so the ferromagnetic material
with high magnetocrystalline anisotropy is required to overcome the thermal
aggitation problems. The thermal stability of the particles shown by KuV/kBT must be larger than 60 for long term sorage of recorded data, where Ku is magntocrysalline anisotropy constant, V is volue of a particle, KB is Boltzman constant, and T is absolute temperature. This equation indicates that when the particle size becomes smaller than a few nm, the material with high Ku must be used. This is the reason why high Ku materials such as L10-FePt is considered promissing as future recording media. The currently
used material is Co-Cr-Pt alloys, in which Pt is added to enhance the anisotropy
of hcp-Co and Cr to obtain better intergranular magnetic isolation. The
density of currently used recording media is around 200 Gbit/in2. Till recently, the magnetic signals were recording within the plane of
the films and this method is called "longitudianl recording".
However, as the magnetic recording density increases, the size of bit has
become so tiny that the magnetization within a bit interfere with the magnetization
of another bit. To overcome this problem, the most up-to-date recording
method is "parpendicular recording" method, in which the direction of magnetization of bits are in the
normal direction to the films. In this case, the magnetizations of neighboring
bits do not interfere each other. The modern perpendicular recording method
was proposed by Professor Iwasaki at Tohoku Univerisity in 1976, and it
was finally commercialized after 30 years from its first invention.
For writing, magnetic field must be generated from induction current applied
to a soft magnetic core. The maximum head field is given by 0.6Bs according
to Nakamoto and Bertman (2002), so the magnetic field that can be generated
at the write head is 15 kOe using FeCo alloy whose saturation magnetic
fluxdensity (Bs) is 2.4 T. As mentioned above, high Ku material is required
for thermal stability in high density recording, while the coercivity of
the media must be smaller than 15 kOe. Read head used to be the soft magnet
pole like the write head, but much higher head sensitivity is requied in
the recent high density recording as the bit size is so tiny. To read weak
magnetic signals, giant magnetoresistance (GMR) heads are used in the current recording system. However, there is a strong
demand for improving the MR values for the GMR heads, as the bit size is
becoming increasingly smaller. Hence, many efforts are being done to increase
the GMR head sensitivity. Current confined path current perpendicular to plane (CCP-CPP) GMR head was developed to increase the sensitivity of the read head.
Link to Hitachi Global Storage Technologies
