Hitachi kicks open Door 4 in the House of Spinning Platters

The post-perpendicular magnetic recording world in the land of spinning disks had three land masses; HAMR, BPM and Shingled Writing. Now there's a fourth; microwave magnetic recording (MMR).

Hitachi, with its GST hard disk drive (HDD) subsidiary, has been working with Japan's NEDO (New Energy and Industrial Technology Development Organisation) to see if microwaves can be used to extend the life of today's perpendicular magnetic recording (PMR) beyond the 800Gbit/in² - 1Tbit/in² limit where the constant magnetic field of a recorded bit becomes unstable due to susceptibility to temperature fluctuations and the influence of neighbouring bits.

A kind of consensus had been emerging that the overlapped tracks idea of Shingled Writes could extend the life of PMR to possibly 1.5Tbit/in² and maybe beyond. Heat-assisted magnetic recording (HAMR) comes in some time in the 2013-2015 period and uses localised heating to make it possible to change the magnetic field of a bit in a reformulated recording medium that is stable and holds its magnetic state at normal temperatures.

This could take us towards 3Tbit/in² at which time, possibly 2020, bit-patterned media (BPM) which surrounds smaller bit areas with insulating rings could take us to 4Tbit/in² and beyond.

Earlier this month Hitachi and NEDO said they had demonstrated that MMR could take existing PMR technology from 1Tbit/in² to 3Tbit/in² and thrown a spanner into the three-way works.

Microwaves

With MMR, microwaves are used to overcome the resistance of recording grains forming a bit to having their magnetic detection changed, and the technology needs a microwave generator adding to a disk drive read/write head. This is what the researchers did.

They devised a magnetic spin-torque oscillator which generates a high-frequency magnetic field and was mounted on a HDD read/write head. The HF magnetic field is applied to the recording grains under the read/write head and makes them susceptible to having data written by the write part of the head using its weak magnetic field. This is the first time that MMR has been demonstrated and showed that theoretical computer simulations were correct.

The researchers say: "Spin torque is a current flow in a laminated medium composed of magnetic/nonmagnetic/magnetic materials that cause torque which tends to change the direction of magnetisation by means of the spin flow of magnetic materials." A magnetic resonance effect is being used thus:

An electric current flow across a magnetic multilayer composed of a free magnetic layer and fixed magnetisation direction layers results in a polarized electronic spin flow from the fixed layer to the free layer. Under suitable conditions, the magnetisation of the free layer turns simultaneously due to the generated torque from the spin flow. This phenomenon is called spin torque oscillation, which is when the high frequency magnetic field for assisted recording is emitted from the free layer. High frequency oscillation in the 10 GHz range was confirmed using the developed spin torque oscillator.

The researchers showed a 1Tbit/in² spin-torque oscillator head in action and used computer simulation to demonstrate that it could be scaled to the 3Tbit/in² level.