Computer
files that allow us to watch videos, store pictures, and edit all kinds
of media formats are nothing else but streams of “0” and “1” digital
data, that is, bits and bytes. Modern computing technology is based on
our ability to write, store, and retrieve digital information as
efficiently as possible. In a computer hard disk, this is achieved in
practice by writing information on a thin magnetic layer, where magnetic
domains pointing “up” represent a “1” and magnetic domains pointing
down represent a “0”.
The
size of these magnetic domains has now reached a few tens of
nanometers, allowing us to store a terabyte of data in the space of just
about 4 square centimeters. Miniaturization, however, has created
numerous problems that physicists and engineers worldwide struggle to
solve at the pace demanded by an ever-growing information technology
industry. The process of writing information on tiny magnetic bits one
by one, as fast as possible, and with little energy consumption,
represents one of the biggest hurdles in this field.
As
reported this week in Nature, a team of scientists from the Catalan
Institute of Nanotechnology, ICREA, and Universitat Autonoma de
Barcelona, Mihai Miron, Kevin Garello, and Pietro Gambardella, in
collaboration with Gilles Gaudin and colleagues working at SPINTEC in
Grenoble, France, have discovered a new method to write magnetic data
that fulfils all of these requirements.
Magnetic
writing is currently performed using magnetic fields produced by wires
and coils, a methodology suffering severe limitations in scalability and
energy efficiency. The new technique eliminates the need for cumbersome
magnetic fields and provides extremely simple and reversible writing of
memory elements by injecting an electric current parallel to the plane
of a magnetic bit. The key to this effect lies in engineering asymmetric
interfaces at the top and bottom of the magnetic layer, which induces
an electric field across the material, in this case a cobalt film less
than one nanometer thick sandwiched between platinum and aluminum oxide.
Due
to subtle relativistic effects, electrons traversing the Co layer
effectively see the material’s electric field as a magnetic field, which
in turn twists their magnetization. Depending on the intensity of the
current and the direction of the magnetization, one can induce an
effective magnetic field, intrinsic to the material that is strong
enough to reverse the magnetization. The research team showed that this
method works reliably at room temperature using current pulses that last
less than 10 ns in magnetic bits as small as 200 x 200 square
nanometers, while further miniaturization and faster switching appear
easily within reach. Although there is currently no theory describing
this effect, this work has many interesting applications for the
magnetic recording industry, and in particular for the realization of
magnetic random access memories, so-called MRAMs. By replacing standard
RAMs, which need to be refreshed every few milliseconds, non-volatile
MRAMs would allow instant power up of a computer and also save a
substantial amount of energy.
An
additional advantage of the discovery reported here is that
current-induced magnetic writing is more efficient in “hard” magnetic
layers than in “soft” ones. This is somehow counterintuitive, as soft
magnetic materials are by definition the easier to switch using external
magnetic fields, but very practical since hard magnets can be
miniaturized to nanometer dimensions without losing their magnetic
properties. This would allow the information storage density to be
increased without compromising the ability to write it. The results of
this work have also led to three patent applications dealing with the
fabrication of magnetic storage and logic devices.
Perpendicular switching of a single ferromagnetic layer induced by in-plane current injection