As
this illustration shows, the researchers were able to measure the magnetization
dynamics in the iron nanofilm caused by ultrafast electronic and acoustic
processes. Courtesy: Bielefeld University/W. Zhang
An
international team of physicists from Bielefeld University, Uppsala University,
the University of Strasbourg, University of Shanghai for Science and
Technology, Max Planck Institute for Polymer Research, ETH Zurich, and the Free
University Berlin have developed a precise method to measure the ultrafast
change of a magnetic state in materials. They do this by observing the emission
of terahertz radiation that necessarily accompanies such a magnetization
change. Their study, titled Ultrafast terahertz magnetometry, is being
published recently in Nature Communications.
Magnetic memories
are not just acquiring higher and higher capacity by shrinking the size of
magnetic bits, they are also getting faster. In principle, the magnetic bit can
be ‘flipped’—that is, it can change its state from ‘one’ to ‘zero’ or vice
versa—on an extremely fast timescale of shorter than one picosecond. One
picosecond (1 ps = 10-12 s) is one millionth of one millionth of a second. This
could allow the operation of magnetic memories at terahertz (1 THz = 1 x 1012
hertz) switching frequencies, corresponding to extremely high terabit per
second (Tbit/s) data rates.
The actual
challenge is to be able to detect such a magnetization change quickly and
sensitively enough,’ explains Dr. Dmitry Turchinovich, professor of physics at
Bielefeld University and the leader of this study. ‘The existing methods of
ultrafast magnetometry all suffer from certain significant drawbacks such as,
for example, operation only under ultrahigh vacuum conditions, the inability to
measure on encapsulated materials, and so on.
Our idea was
to use the basic principle of electrodynamics. This states that a change in the
magnetization of a material must result in the emission of electromagnetic
radiation containing the full information on this magnetization change. If the
magnetization in a material changes on a picosecond timescale, then the emitted
radiation will belong to the terahertz frequency range.
The
problem is, that this radiation, known as “magnetic dipole emissionâ€, is very
weak, and can be easily obscured by light emission of other origins.’ Wentao
Zhang, a PhD student in the lab of Professor Dmitry Turchinovich, and the first
author of the published paper says: ‘It took us time, but finally we succeeded
in isolating precisely this magnetic dipole terahertz emission that allowed us
to reliably reconstruct the ultrafast magnetization dynamics in our samples:
encapsulated iron nanofilms.’
In their
experiments, the researchers sent very short pulses of laser light onto the
iron nanofilms, causing them to demagnetize very quickly. At the same time,
they were collecting the terahertz light emitted during such a demagnetization
process. The analysis of this terahertz emission yielded the precise temporal
evolution of a magnetic state in the iron film.
‘Once our
analysis was finished, we realized that we actually saw far more than what we
had expected,’ continues Dmitry Turchinovich. ‘It has already been known for
some time that iron can demagnetize very quickly when illuminated by laser
light. But what we also saw was a reasonably small, but a very clear additional
signal in magnetization dynamics. This got us all very excited.
This
signal came from the demagnetization in iron—actually driven by the propagation
of a very fast pulse of sound through our sample. Where did this sound come from?
Very easy: when the iron film absorbed the laser light, it not only
demagnetized, it also became hot. As we know, most materials expand when they
get hot—and this expansion of the iron nanofilm launched a pulse of terahertz
ultrasound within our sample structure.
This sound
pulse was bouncing back and forth between the sample boundaries, internal and
external, like the echo between the walls of a big hall. And each time this
echo passed through the iron nanofilm, the pressure of sound moved the iron atoms
a little bit, and this further weakened the magnetism in the material.’ This
effect has never been observed before on such an ultrafast timescale.
‘We are
very happy that we could see this acoustically-driven ultrafast magnetization
signal so clearly, and that it was so relatively strong. It was amazing that
detecting it with THz radiation, which has a sub-mm wavelength, worked so well,
because the expansion in the iron film is only tens of femtometres (1 fm =
10-15 m) which is ten orders of magnitude smaller,’ says Dr. Peter M. Oppeneer,
a professor of physics at Uppsala University, who led the theoretical part of
this study.
Dr. Pablo
Maldonado, a colleague of Peter M. Oppeneer who performed the numerical
calculations that were crucial for explaining the observations in this work,
adds: ‘What I find extremely exciting is an almost perfect match between the
experimental data and our first-principles theoretical calculations. This
confirms that our experimental method of ultrafast terahertz magnetometry is
indeed very accurate and also sensitive enough, because we were able to
distinguish clearly between the ultrafast magnetic signals of different
origins: electronic and acoustic.’