Magnetic excitations could provide information transfer without heat
loss
Magnons could transport information much more easily than electrical conductors

Date:
March 3, 2022
Source:
Technical University of Munich (TUM)
Summary:
Just as electrons flow through an electrical conductor, magnetic
excitations can travel through certain materials. Such excitations,
known in physics as 'magnons' in analogy to the electron,
could transport information much more easily than electrical
conductors. An international research team has now made an important
discovery on the road to such components, which could be highly
energy-efficient and considerably smaller.



FULL STORY ==========================================================================
Just as electrons flow through an electrical conductor, magnetic
excitations can travel through certain materials. Such excitations,
known in physics as "magnons" in analogy to the electron, could transport information much more easily than electrical conductors. An international research team has now made an important discovery on the road to such components, which could be highly energy-efficient and considerably
smaller.


==========================================================================
At present the transport and control of electrical charges forms the
basis for most electronic components. A major disadvantage of this
technology is that the flow of electric currents generates heat due to
the electrical resistance.

Considering the gargantuan number of electronic components in use
worldwide, the loss of energy is immense.

An energy-efficient alternative may be the use of spin waves to transport
and process information, because they do not produce nearly as much
waste heat.

Such components could also be much more compact. Scientists around the
world are thus looking for materials in which magnetic spin waves can
be used to transport information.

An international research consortium with significant participation of
the Technical University of Munich (TUM) has now taken an important step forward in this search. Their observations of spin waves on circular
paths in certain magnetic materials could also represent a breakthrough
for quantum technologies that use waves to transport information.

Propagation of magnetic waves in materials When you throw a stone
into water, you bring the water molecules out of their equilibrium
position. They start to oscillate, and a circular wave spreads out.

In a very similar way, the magnetic moments in some materials can be made
to oscillate. In this process, the magnetic moment performs a gyroscopic
motion with respect to its rest position. The precession of one moment
affects the vibration of its neighbor, and so the wave propagates.



==========================================================================
For applications utilizing these magnetic waves, controlling properties
such as wavelength or direction is important. In conventional ferromagnets
-- in which the magnetic moments all point in the same direction --
magnetic waves generally propagate in a straight line.

The propagation of such waves is quite different in a new class of
magnetic materials, which, like a box of uncooked spaghetti, consist
of a tight arrangement of magnetic vortex tubes. This magnetic order
was discovered nearly fifteen years ago by a team led by Christian
Pfleiderer and Peter Bo"ni at the Technical University of Munich using
neutron experiments.

Because of their non-trivial topological properties and in recognition
of the theoretical-mathematical developments of the British nuclear
physicist Tony Skyrme, these vortex tubes are known as skyrmions.

Propagation of magnetic waves on a circular path Since neutrons carry
a magnetic moment, they are particularly well suited for the study of
magnetic materials. Like a compass needle, they respond sensitively to
magnetic fields. Neutron scattering proved to be the only technique
capable of detecting spin waves on circular orbits since it provides
the requisite resolution over very large length and time scales.



========================================================================== Using polarized neutron scattering, Tobias Weber and his team from the
Institut Laue Langevin (ILL) in Grenoble, France have now proven that
the propagation of magnetic waves perpendicular to such skyrmions does
not occur in a straight line, but rather on a circular path.

The reason for this is that the direction of neighboring magnetic moments,
and thus the direction of the axis about which the precessional motion
occurs, changes continuously perpendicular to the magnetic vortex
tube. Analogously, when the precessional motion propagates from one
magnetic moment to the next, the direction of propagation also changes continuously. The radius and the direction of the circular path of the propagation direction of the spin waves depends on the strength and the direction of the magnetic moments' tilt.

Quantization of circular orbits "But there is even more to it," says
Markus Garst of the Karlsruhe Institute of Technology (KIT), who had
developed the theoretical description of spin waves in skyrmions and their coupling to neutrons some time ago. "There is a close analogy between the circular propagation of spin waves perpendicular to a skyrmion lattice
and the motion of an electron perpendicular to a magnetic field caused
by the Lorentz force." At very low temperatures, when the circular
orbits are closed, their energy is quantized. Predicted almost a hundred
years ago by Russian physicist Lev Landau, well-known for electrons this phenomenon is called Landau quantization.

In analogy, the influence of the vortex-like character of the skyrmions
on the spin waves can be elegantly interpreted as a fictitious magnetic
field. In other words, the very complicated interplay of the spin waves
with the skyrmion structure is actually very simple and can be described
just like the motion of electrons transverse to a real magnetic field.

Moreover, the propagation of spin waves perpendicular to skyrmions
also displays a quantization of the circular orbits. The characteristic
energy of the spin wave is thus also quantized, which opens the door to completely new applications. In addition, the circular orbit carries
a subtle twist, somewhat similar to a so-called Mo"bius strip. It is topologically non-trivial: The twist can only be removed by cutting and reconnecting the strip. All of this leads to a particularly stable spin
wave motion.

Successful international cooperation "The experimental determination
of spin waves in skyrmion lattices required both a combination of
world-leading neutron spectrometers and a massive advancement of the
software to interpret the data," explains TUM physicist Peter Bo"ni.

The research team employed instruments of the Institut Laue-Langevin
in France, the spallation source SINQ at the Swiss Paul Scherrer
Institute, the UK's ISIS neutron and muon source, and the Research
Neutron Source Heim Maier-Leibnitz (FRM II) at the Technical University
of Munich. Further work on theory and data analysis was carried out
at the U.S. Los Alamos National Laboratory and the Karlsruhe Institute
of Technology.

Marc Janoschek, who now works at the Paul Scherrer Institute, emphasizes:
"It is simply great to see that, after countless experiments at
world-leading spectrometers and the clarification of major experimental
and theoretical challenges during my time at Los Alamos, the microscopic detection of Landau quantization at the world's unique beamline RESEDA
at TUM's FRM II in Garching closes a circle that began almost fifteen
years ago with my first measurements at the Heinz Maier-Leibnitz Zentrum
in Garching." However, the motion of spin waves on circular orbits, which
are quantized to boot, is a breakthrough not only from the perspective
of fundamental research.

Christian Pfleiderer, managing director of the newly founded
Center for QuantumEngineering at TUM, emphasizes: "The
spontaneous motion of spin waves on circular orbits, whose
radius and direction arise from the vortex-like structure of
skyrmions, opens up a new perspective for realizing functional
devices for information processing in quantum technologies,
such as simple couplers between qubits in quantum computers." ========================================================================== Story Source: Materials provided by
Technical_University_of_Munich_(TUM). Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. T. Weber, D. M. Fobes, J. Waizner, P. Steffens, G. S. Tucker,
M. Bo"hm,
L. Beddrich, C. Franz, H. Gabold, R. Bewley, D. Voneshen,
M. Skoulatos, R. Georgii, G. Ehlers, A. Bauer, C. Pfleiderer,
P. Bo"ni, M. Janoschek, M. Garst. Topological magnon band structure
of emergent Landau levels in a skyrmion lattice. Science, 2022;
375 (6584): 1025 DOI: 10.1126/ science.abe4441 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/03/220303162042.htm

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