  An annular magnetic memory that uses a spin-polarized current to switch the
magnetization direction or helicity of a magnetic region is proposed. The
device has magnetic materials in the shape of a ring (1 to 5 nm in thickness,
20 to 250 nm in mean radius and 8 to 100 nm in width), comprising a reference
magnetic layer with a fixed magnetic helicity and a free magnetic layer with a
changeable magnetic helicity. These are separated by a thin non-magnetic layer.
Information is written using a current flowing perpendicular to the layers,
inducing a spin-transfer torque that alters the magnetic state of the free
layer. The resistance, which depends on the magnetic state of the device, is
used to read out the stored information. This device offers several important
advantages compared to conventional spin-transfer magnetic random access memory
(MRAM) devices. First, the ring geometry offers stable magnetization states,
which are, nonetheless, easily altered with short current pulses. Second, the
ring geometry naturally solves a major challenge of spin-transfer devices:
writing requires relatively high currents and a low impedance circuit, whereas
readout demands a larger impedance and magnetoresistance. The annular device
accommodates these conflicting requirements by performing reading and writing
operations at separate read and write contacts placed at different locations on
the ring.
