New method to track ultrafast change of magnetic state

Date:
August 25, 2020
Source:
Bielefeld University
Summary:
Physicists have developed a precise method to measure the ultrafast
change of a magnetic state in materials. The international research
team recorded emissions of terahertz radiation.



FULL STORY ==========================================================================
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.


========================================================================== 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 is one millionth
of one millionth of a second. This could allow the operation of magnetic memories at terahertz 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 emission," 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 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.' The remaining co-authors of this publication have dedicated it to the memory of
their colleague and a pioneer in the field of ultrafast magnetism,
Dr. Eric Beaurepaire from the University of Strasbourg. He was one of
the originators of this study, but passed away during its final stages.


========================================================================== Story Source: Materials provided by Bielefeld_University. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Wentao Zhang, Pablo Maldonado, Zuanming Jin, Tom S. Seifert, Jacek
Arabski, Guy Schmerber, Eric Beaurepaire, Mischa Bonn, Tobias
Kampfrath, Peter M. Oppeneer, Dmitry Turchinovich. Ultrafast
terahertz magnetometry.

Nature Communications, 2020; 11 (1) DOI: 10.1038/s41467-020-17935-6 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200825110635.htm

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