Physicists discover new magnetoelectric effect

Date:
September 14, 2020
Source:
Vienna University of Technology
Summary:
A special material was found, which shows a surprising new effect:
Its electrical properties can be controlled with a magnetic
field. This effect works completely differently than usual. It
can be controlled in a highly sensitive way.



FULL STORY ========================================================================== Electricity and magnetism are closely related: Power lines
generate a magnetic field, rotating magnets in a generator produce
electricity. However, the phenomenon is much more complicated: electrical
and magnetic properties of certain materials are also coupled with
each other. Electrical properties of some crystals can be influenced
by magnetic fields -- and vice versa. In this case one speaks of a "magnetoelectric effect." It plays an important technological role, for
example in certain types of sensors or in the search for new concepts
of data storage.


==========================================================================
A special material was investigated for which, at first glance, no magnetoelectric effect would be expected at all. But careful experiments
have now shown that the effect can be observed in this material, it only
works completely differently than usual. It can be controlled in a highly sensitive way: Even small changes in the direction of the magnetic field
can switch the electrical properties of the material to a completely
different state.

Symmetry controls the coupling "Whether the electrical and magnetic
properties of a crystal are coupled or not depends on the crystal's
internal symmetry," says Prof. Andrei Pimenov from the Institute of Solid
State Physics at TU Wien. "If the crystal has a high degree of symmetry,
for example, if one side of the crystal is exactly the mirror image of the other side, then for theoretical reasons there can be no magnetoelectric effect." This applies to the crystal, which has now been examined in
detail -- a so- called langasite made of lanthanum, gallium, silicon and oxygen, doped with holmium atoms. "The crystal structure is so symmetrical
that it should actually not allow any magnetoelectric effect. And in
the case of weak magnetic fields there is indeed no coupling whatsoever
with the electrical properties of the crystal," says Andrei Pimenov. "But
if we increase the strength of the magnetic field, something remarkable happens: The holmium atoms change their quantum state and gain a magnetic moment. This breaks the internal symmetry of the crystal." From a purely geometrical point of view, the crystal is still symmetrical, but the
magnetism of the atoms has to be taken into account as well, and this is
what breaks the symmetry. Therefore the electrical polarization of the
crystal can be changed with a magnetic field. "Polarization is when the positive and negative charges in the crystal are displaced a little bit,
with respect to each other," explains Pimenov. "This would be easy to
achieve with an electric field -- but due to the magnetoelectric effect,
this is also possible using a magnetic field." It's not the strength,
it's the direction The stronger the magnetic field, the stronger its
effect on electrical polarization. "The relationship between polarization
and magnetic field strength is approximately linear, which is nothing
unusual," says Andrei Pimenov. "What is remarkable, however, is that
the relationship between polarization and the direction of the magnetic
field is strongly non-linear. If you change the direction of the magnetic
field a little bit, the polarization can completely tip over. This is a
new form of the magnetoelectric effect, which was not known before." So
a small rotation may decide whether the magnetic field can change the electrical polarization of the crystal or not.

Possibility for new storage technologies "The magnetoelectric effect
will play an increasingly important role for various technological applications," says Andrei Pimenov. "In a next step, we will try to change magnetic properties with an electric field instead of changing electrical properties with a magnetic field. In principle, this should be possible
in exactly the same way." If this succeeds, it would be a promising
new way to store data in solids. "In magnetic memories such as computer
hard disks, magnetic fields are needed today," Pimenov explains. "They
are generated with magnetic coils, which requires a relatively large
amount of energy and time. If there were a direct way to switch the
magnetic properties of a solid-state memory with an electric field,
this would be a breakthrough."

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


========================================================================== Journal Reference:
1. Lukas Weymann, Lorenz Bergen, Thomas Kain, Anna Pimenov, Alexey
Shuvaev,
Evan Constable, David Szaller, Boris V. Mill, Artem M. Kuzmenko,
Vsevolod Yu. Ivanov, Nadezhda V. Kostyuchenko, Alexander I. Popov,
Anatoly K.

Zvezdin, Andrei Pimenov, Alexander A. Mukhin, Maxim
Mostovoy. Unusual magnetoelectric effect in paramagnetic
rare-earth langasite. npj Quantum Materials, 2020; 5 (1) DOI:
10.1038/s41535-020-00263-9 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/09/200914112159.htm

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