[tt] ballistic spin torque reversal in MRAM @GHz

[Eugen Leitl]

http://thefutureofthings.com/news/5401/new-speed-record-for-magnetic-memories.html

New Speed Record for Magnetic Memories 

Monday, September 22, 2008 - Anuradha Menon

An experiment performed at the Physikalisch-Technische Bundesanstalt (PTB) in
Germany has uncovered that a spin-torque switching of a nanomagnet is as fast
as what is permitted according to the fundamental laws of physics’ limit.
This method of switching, also named ballistic switching, could allow for
increased speeds in future non-volatile magnetic memories.

Dr. Hans Werner Schumacher from the PTB Working Group (Credit: University of
Hannover) 

Current high speed memories, such as Dynamic Static Random Access Memory
(DRAM) and Static Random Access Memory (SRAM), have one critical drawback-
they require constant power in order to retain memories. Using a magnetic
memory chip known as Magnetic Random Access Memory (MRAM), information can be
stored in the form of magnetization of a magnetic cell even when no power is
maintained.

The newest generation of MRAM employs the spin-torque effect for programming
the magnetic bits. By using a current pulse, the spin-torque can be
controlled and the memory state of the cell can be programmed. To obtain a
digital state ‘0’, a positive current is applied and the magnetization is set
to a particular direction. A negative current, on the other hand, is applied
to reverse the direction of magnetization, creating a digital state of ‘1’.
The application of spin torque in MRAM leads the way for a higher storage
density compared to DRAM and Flash.

In a conventional spin-torque system, the current pulse creates a rotation of
magnetization of the memory cell, which is also termed ‘precession.’ Usually,
several precessional turns are made during magnetization to ensure reliable
reversal. Therefore, spin torque MRAMs currently in production must operate
with rather long write pulses of about 10 nanoseconds. This limits the MRAM
clock speed.

Experiments carried out at PTB Braunschweig, led scientists to the discovery
that a single precessional turn is sufficient for spin torque magnetization
reversal. This type of ‘ballistic’ spin torque reversal relates to the
particularly quick physical time limit of spin torque magnetization reversal.
The achievement was made using specifically tailored current pulse parameters
in combination with a small magnetic bias field.

By using the ballistic spin torque reversal, potential MRAMs could be
programmed by current pulses quicker than 1 nanosecond and subsequently,
achieving write clock rates well above 1 GHz. Thus, high-density and
non-volatile memory executing at the clock rates of the fastest volatile
memories were enabled.

TFOT previously covered advanced MRAM technology developed by Freescale
Semiconductor in 2006. You can also check out our article onLaser Hard Drives
on the Horizon, about researchers who succeeded in flipping the value of a
magnetic memory bit without any external magnetic field interference. 

Additional information on the new MRAM research can be obtained on the PTB
Braunschweig website.