Do you remember the first computer you ever used? Or the TV you once watched
cartoons on? it's easy to take modern technology for granted, but if you
compare it to the room-size computers of the 1960's, you'll notice how quickly
things have changed! Or rather, how much they've shrunk! As technology advances,
the amount of data we need to store and manipulate is increasing, and the size of
devices is decreasing so much that conventional techniques of data storage
won't be good enough for long. Most modern technology is electronic: it uses
a property of electrons called charge to encode information.
One example is computer memory, often called RAM, which is where computers keep
information they're working with for several seconds or minutes. RAM contains
billions of tiny plates, called capacitors, that hold electrons. To store
information, computers send a current of electrons into the capacitors; some of
these electrons collect on the plates. If the number of electrons on a capacitor
is above a threshold, the computer assigns it a value of one; otherwise, it's a
zero. Unique patterns of zeros and ones encode information like letters, numbers
and images. As chips get smaller, so do capacitors, and we are faced with the
problem of charge leakage--electrons constantly slip away from the plates.
When the electron count falls below the threshold, the stored data is lost. To
avoid this, computers have to recharge their capacitors several times per
second, which requires more and more power as chips shrink.
Fortunately, an emerging technology called "spintronics" could solve this
problem. Spintronics uses spin, another property of electrons, to encode data. The
spin of an electron is either "up" or "down." It is helpful to think of "spin up"
electrons as little magnets with their north poles pointing up, and "spin down"
electrons with downwards pointing north poles. Data is stored based on the spin
states of electrons that are already present. This means that there's no
problem of "leakage" with spintronics- once information is stored, it stays there
until we want to change it! The spin -states of electrons change if we
actively influence them by exposing them to magnetic fields. To take advantage of
this, scientists developed magneto resistive RAM, which replaces capacitors
with tunneling magnetoresistance structures. TMR structures are made up of
an "insulator" sandwiched between two "ferromagnetic" materials. An insulator is
a material with very high resistivity- this means that electrons are very
unlikely to move through it.
A "ferromagnet" is a material in which the spins
tend to align along the same direction. The lower ferromagnetic layer is "fixed"--
its electrons are forced to have the same spin as each other by an adjacent
strong magnet. The upper ferromagnetic layer is "free"-- its electrons change their
spins in response to external magnetic fields, or through a phenomenon called
spin-transfer torque, which is where electrons change their spins to match
the spins of nearby flowing electrons. When the spins of the ferromagnetic
layers are parallel, the structure stores a 1. Opposite spins represent a 0. To read
the data and determine whether it's a 0 or a 1, the computer sends a tiny current
through the TMR structure. If the spins of the top and bottom layers are
parallel, electrons can "tunnel" through the insulator. "Tunneling" is a process
that is similar to a basketball moving through a solid wall- it's normally very
unlikely, but parallel spins make it possible.
If the spins are opposite, almost no electrons tunnel through. By seeing how
much current reaches the other side , a computer can tell whether the structure is
holding a 1 or 0. To write data, we send larger currents up or down the structure.
Sending electrons up the structure, from the fixed layer to the free layer,
switches the value from a 0 to 1. Because of spin transferred torque, the flowing
electrons adopt the fixed layer's spin as they move through it. The electrons in
the free layer then flipped to match the spins of the flowing electrons.
Since the free layer responds to changes in external magnetic fields--which are
generated by both computers and living organisms--TMR structures have numerous
applications beyond computers. When a neuron in your brain fires, your
heartbeats, or a white blood cell attacks a virus, tiny magnetic fields are created.
A spintronic sensor chip could detect the smallest deviations from your "normal"
magnetic fields detecting anything from bacterial infections to brain tumors.
Because magnetic fields are not altered by the structures they move through, it
could even do this from outside the body! And because electron spin changes our
near-instant spintronics technology could do so quickly . Ultimately,
spintronics technology holds promise in countless areas, from computer memory to
medicine and beyond!