The diameter of an ion beam is only
20 nm and its aiming accuracy is 60 nm. "Single The
charge of electron enables semiconductors to process information,
and its spin allows us to realize magnetic information
storage devices.
Even though these properties are normally utilized separately
using magnetic and semiconductor materials, respectively,
spintronics uses both properties in the same material.
There are two approaches for designing advanced spintronic
devices. One is to use both degrees of freedom in metal-based
magnetic materials, and the other is in semiconductors.
Prof. Ohno has expanded the possibilities of utilizing
semiconductors with various structures and functions
by making III-V semiconductors ferromagnetic.
The ferromagnetic semiconductor that Prof. Ohno developed
was (In, Mn)As in which part of the indium atoms in InAs
is replaced with manganese, which is a magnetic element.
He said, "When I went to the IBM T. J. Watson Laboratory
in 1988, joining Leo Esaki's group, I thought of conducting
two kinds of research if I was given the chance.
One was research which seemed too difficult
to accomplish, and the other was research through which
I could publish papers." It was widely known then
that if the Mn concentration was too high, a MnAs phase
would nucleate before making InAs magnetic. However, he
and his coworkers were able to synthesize single-phase
(In, Mn)As by decreasing the crystal growth temperature
to 250 degree C.
Several years later, he discovered that single-phase (In,
Mn)As was ferromagnetic at low temperatures. Although he
had an idea of causing semiconductors to have magnetism,
he never expected that semiconductors would be ferromagnetic.
A ferromagnetic material transforms to a paramagnetic material
when it exceeds a transition temperature. He produced a
field effect transistor using (In, Mn)As and was able to
change the transition temperature of (In, Mn)As by applying
an electric field. He showed that the transition temperature
of (In, Mn)As increases as its carrier concentration increases
in an applied electric field . This occurs because the
Mn spins align in the same direction due to their interaction
with charge carriers in the semiconductor. He says, "A
semiconductor has the capability of changing the number
of electrons. Although it has been about 26 to 27 centuries
since human beings found magnetic materials, this was the
first time that we prepared magnetic materials of which
the properties could be altered after fabrication was complete." However,
unless the transition temperature drastically increases,
these materials cannot be commercialized. As well as increasing
the Mn concentration, he has been searching for other alloying
elements with a stronger interaction between charge carriers
and Mn spins because the interaction depends on the type
of semiconductors.
The next generation magnetic device, MRAM (Magnetic Random
Access Memory), where its memory bits are highly integrated,
will require a high magnetic field or a large electric
current to record data. The ferromagnetic semiconductor,
(In, Mn)As, meets this requirement when it is used in a
field effect transistor. When an electric field of about
1.5MV/cm is applied to (In, Mn)As, its coercive force decreases,
and the magnetic field required for magnetization reversal
falls to one fifth of that currently used. When data is
recorded by a low magnetic field while applying an external
electric field, and then the field is turned off, the coercive
force is recovered, which means that a higher magnetic
field will be required to rewrite data.
Prof. Ohno thinks the key to a scientific breakthrough
is curiosity.
"However, it takes more than curiosity. You have to
decide what you are going to do and do it quickly. You need
to design things in such a way that you can go through a
number of trials without spending too much time. Then, quantity
turns to quality at a certain point," said Prof. Ohno.
His research is fundamental, but he is always thinking about
its applications. He says, "You can publish your paper
no matter what your research is. There are lots of research
areas, which others have not studied yet. However, time and
resources are limited. The first thing you have to do is
to choose what you will do or will not do, and then figure
out how to deepen your research. After all, what you choose
to do first will show your taste.
(Interviewer: Kuniko Ishiguro, Cosmopia Inc.)
For more information,
http://www.nanonet.go.jp/english/mailmag/2005/047a.html
