Representing data with the spin of an
electron transformed hard drive read heads, but could it do the same for logic
circuits too?Spintronics is one of the hottest topics in
science and tech, whether it concerns logic circuits based on the spin or
magnetic moment of an electron or qubits in quantum computers. With all that
excitement, it’s easy to forget that technology based on spintronics has been a
part of the computing experience for the better part of a decade. You know it
better as the read head of your hard drive.
In the mid- 1980s, two research teams,
headed respectively by Albert Fert and Peter Grünberg, independently discovered
the phenomenon of Giant Magneto Resistance (GMR), an effect found in thin film
materials composed of alternating ferromagnetic and non-magnetic layers.
has already been used in hard drive read heads for the best part of a decade
Fert and Grünberg observed a significant
difference in the resistance across such a material, depending on the alignment
of spins in adjacent layers. High resistance marks a region of antiparallel
alignment, and low resistance a region of parallel alignment – the difference
is significant enough to be used to read data from a hard drive. Their work won
Fert and Grünberg the 2007 Nobel Prize, led to an explosion in hard drive data
densities, and paved the way for a new way of thinking about storage, memory
At the heart of spintronics is the idea
that we can exploit not only the charge of an electron, but also its spin, or
magnetic moment. The instant we stop thinking about an electron as a moving
object (part of a flow of current), and consider at as an entity in itself, it
starts to become more interesting. However, getting from the lab to the PC
isn’t always an easy road.
‘The first spintronics device went into
production just eight or nine years after a new physical effect – giant magneto
resistance – was discovered,’ says Rolf Allenspach, head of IBM’s Physical of
Nanoscale systems research group in Zurich. However, the reason the technology
moved so astonishingly quickly from lab to production was because it was a
‘drop-in’ replacement, he explains. Nothing needed to be redesigned; the new
head just replaced the old one and did a better job.
Allenspach, head of IBM’s Physics of Nanoscales systems research group in
The road ahead for other spintronics
devices might not be as smooth, and certainly seems to be longer, but still
possible to navigate. One of the most active areas of research concerns
something called magnetic domain walls and spin-transfer torque, which is most
certainly not a ‘drop-in’ replacement for existing tech. the physics is at the
heart of both new types of memory, and room-temperature spintronics logic.
Both are based on the same basic idea. ‘A
conventional field effect transistor works because charge moves around, and we
can break that flow of electrons to produce the logic states of one and zero.
Spintronics, meanwhile, encodes the logic one or zero in the spin – either up
or down – of an electron. Instead of diverting flow, you flip the spin to
change the logic state,’ Allenspach explains. ‘In an ideal case, you wouldn’t
have any current flow, just the flow of spin. This is what reduces heat
You can flip the spin with magnetic field
(which is difficult to control inside devices), or with electric field (which
may be easier, especially at a small scale). This can be done at a very low
power density, which is a major reason why people are interested in this tech.
Significant work is being carried out on
developing these discoveries even further into the holy grail: universal
A reliable and non-volatile technology
capable of high-density data storage, fast read and writes times would consign
the standard hard drive, flash and all the familiar alphabetical RAM variants
to the history books.
Racetrack memory, which is the brainchild
of IBM researcher Stuart Parkin, is IBM’s stab at it. The firm first
demonstrated the technology in 2008, but at the end of last year it showed off
a version integrated with standard 200mm wafer CMOS technology.
memory, the brainchild of IBM researcher Stuart Parkin, stores data as
magnetised regions on nanowires that can be written, read and overwritten as
they whizz past fixed read and write heads
Racetrack stores data as magnetised regions
on nanowires that can be written, read and overwritten as they whizz past fixed
read and write heads. Since the bits themselves move along the nanowires, you
only need one transistor per racetrack, as opposed to one transistor per bit.
You can also stack the wires, creating data densities that not only equal but
surpass conventional hard drives. What’s more, the data is non-volatile: it
persists even when the power is switched off.
You may recall from GCSE physics that in a
ferromagnet, the spins of all the electrons are aligned. You can have regions
within a material where spins are aligned one way, right next to areas where
the spins are aligned the other way. The boundary between the two differently
aligned areas, the region in which the magnetic moment of the electrons
changes, is known as the magnetic domain wall. One direction of the wall is
logic state one, the other direction is logic state zero.
The spin of a given region can be
manipulated by a magnetic field, but this doesn’t necessarily move it along the
nanowire. Instead, you can move the domains with a pulse of spin-polarised current.
Generally, an electrical current isn’t polarised – it has an even mix of up and
down spin-states. However, if you load the current with more electrons spin-up,
for example, when in hits a magnetic domain wall, the spin – angular momentum –
of the electrons is transferred, switching the polarisation of the region on
either side of the domain wall.
This works because angular momentum is a
conserved quantity – you can’t get more than you already have, and you have to
keep what you have. So as the polarised electron in the pulse of current hits
the magnetised domain and switches from 0 to 1, another electron – one in the
magnetised domain – has to flip from 1 to 0.
This transfer of momentum can move a
magnetised bit along a wire at an impressive 150nm per nanosecond in ideal
conditions. This gives you a non-volatile form of high-density memory with
access times in nanoseconds; much faster than hard drives.
It isn’t perfect thought. Allenspach
acknowledges that the read-world performance is still some way below what
theory suggests should be possible. ‘We found we needed much longer current
pulses than we expected to move the domains,’ he says.
While we’ve had years of perfecting silicon
production, making perfect ferromagnets is still a new game. Tiny defects in
the crystalline structure were causing the magnetic domains to become stuck,
meaning that the current had to be applied for much longer to shift them along.
In a clear stretch of permalloy wire, though, the theory held, and pulses of
just a few nanoseconds were enough to move the domains along.
The real world gets in the way of
spintronics in logic too. This is too large a topic to cover in detail here,
but basically, this work can be divided into two main avenues, each of which
has promise, but also some very obvious difficulties that have yet to be
The first avenue works at room temperature
and is based on the same underlying physics as racetrack: ferromagnetism. This
would mean a total overhaul of computer design to get it into PCs.
The alternative approach, which is
compatible with current materials, only works below 2000 Kelvin.
That’s a bracing -730C in real money, so it isn’t much cop in your
average air-cooled desktop PC, and not really appropriate for the mass market.
garner the full potential of spintronics, further fundamental advances are
While these kinds still exist in memory and
logic research, and current technology works, the industry will be reluctant to
move from current standard technology that has proven reliability. The
implication is that, despite spintronics’ potential, for now, standard memory
is doing enough to stay in the race.
‘Whether or not any of this becomes
commercialised depends on the difficulties in the silicon industry,’ Allenspach
concludes. ‘It’s hard to predict if an ingenious engineer will find a new
solution for the problems facing existing technology, but we want to be ready
if the standard approach fails.’