New Spintronic Speed Record

Spintronics is the science devoted to gaining greater control over digital information processing by exploiting electron spin along with electron charge in microcircuits. One drawback to implementing a scheme of magnetic-based memory cells for computers has been the relatively slower speed of spin transistors. Hans Schumacher of the Physikalisch-Technische Bundesanstalt, Braunscheig, Germany, has now devised the fastest-yet magnetic version of a random access memory (MRAM) cell, one that switches at a rate of 2 GHz, as good as or better than the fastest non-magnetic semiconductor memories.

The MRAM architecture is a sandwich, consisting of two magnetic layers, with a tunneling layer in between. When the magnetic layers are aligned (their spin orientation is the same) resistance in the cell is low; when they are counter-aligned resistance is high. These two conditions establish the binary 1 or 0 states. The speed of writing or reading data to and from the cells has, for MRAMs, been limited to cycle times of 100 MHz by magnetic excitations in the layers. This problem has now been overcome, according to Hans Schumacher (hans.w.schumacher@ptb.de), through a novel approach referred to as ballistic bit addressing.

In the case of the new MRAM architecture, the influence of magnetic excitations is eliminated through the use of very short (500 picosecond) current pulses for carrying out the write operation and that even a bit whose value will remain the same undergoes a complete 360-degree precession, whereas a change of status (say, from a 0 to a 1) will be achieved by pivoting the magnetic status of the cell through 180 degrees.

The 2-GHz switching speed (the rate at which writing can be accomplished) is faster than static RAM (or SRAM) memories, currently the fastest memories, can accomplish. Furthermore, the magnetic memories are non-volatile, which means that the status of the memory does not disappear if the computer is shut down. ( Schumacher, Applied Physics Letters, 25 July 2005; and Journal of Applied Physics, to appear 1 August 2005; general MRAM website at www.mram-info.com)