180 research outputs found
Magnon-polarons in cubic collinear Antiferromagnets
We present a theoretical study of excitations formed by hybridization between
magnons and phonons - magnon-polarons - in antiferromagnets. We first outline a
general approach to determining which magnon and phonon modes can and cannot
hybridize in a system thereby addressing the qualitative questions concerning
magnon-polaron formation. As a specific and experimentally relevant case, we
study Nickel Oxide quantitatively and find perfect agreement with the
qualitative analysis, thereby highlighting the strength of the former. We find
that there are two distinct features of antiferromagnetic magnon-polarons which
differ from the ferromagnetic ones. First, hybridization between magnons and
the longitudinal phonon modes is expected in many cubic antiferromagnetic
structures. Second, we find that the very existence of certain hybridizations
can be controlled via an external magnetic field, an effect which comes in
addition to the ability to move the magnon modes relative to the phonons modes.Comment: arXiv admin note: text overlap with arXiv:1808.0901
An all-electrical torque differential magnetometer operating under ambient conditions
An all-electrical torque differential magnetometry (also known as cantilever
magnetometry) setup employing piezoelectric quartz tuning forks is
demonstrated. The magnetometer can be operated under ambient conditions as well
as low temperatures and pressures. It extends the allowed specimen mass range
up to several 10 g without any significant reduction in the sensitivity.
Operation under ambient conditions and a simple all-electrical design of the
magnetometer should allow for an easy integration with other experimental
setups. The uniaxial magnetic anisotropy of a 25 m diameter iron wire,
measured under ambient conditions with a high signal to noise ratio, was found
to be in good agreement with its literature value. Further applications of the
technique are discussed.Comment: 9 two-column pages, 9 figure
Exchange-enhanced Ultrastrong Magnon-Magnon Coupling in a Compensated Ferrimagnet
The ultrastrong coupling of (quasi-)particles has gained considerable
attention due to its application potential and richness of the underlying
physics. Coupling phenomena arising due to electromagnetic interactions are
well explored. In magnetically ordered systems, the quantum-mechanical
exchange-interaction should furthermore enable a fundamentally different
coupling mechanism. Here, we report the observation of ultrastrong intralayer
exchange-enhanced magnon-magnon coupling in a compensated ferrimagnet. We
experimentally study the spin dynamics in a gadolinium iron garnet single
crystal using broadband ferromagnetic resonance. Close to the ferrimagnetic
compensation temperature, we observe ultrastrong coupling of clockwise and
anticlockwise magnon modes. The magnon-magnon coupling strength reaches more
than 30% of the mode frequency and can be tuned by varying the direction of the
external magnetic field. We theoretically explain the observed phenomenon in
terms of an exchange-enhanced mode-coupling mediated by a weak cubic
anisotropy
Electrical detectability of magnon-mediated spin current shot noise
A magnonic spin current crossing a ferromagnet-metal interface is accompanied
by spin current shot noise arising from the discrete quanta of spin carried by
magnons. In thin films, e.g., the spin of so-called squeezed magnons have been
shown to deviate from the common value , with corresponding changes in
the spin noise. In experiments, spin currents are typically converted to charge
currents via the inverse spin Hall effect. We here analyze the magnitude of the
spin current shot noise in the charge channel for a typical electrically
detected spin pumping experiment, and find that the voltage noise originating
from the spin current shot noise is much smaller than the inevitable
Johnson-Nyquist noise. Furthermore, we find that due to the local nature of the
spin-charge conversion, the ratio of spin current shot noise and
Johnson-Nyquist noise cannot be systematically enhanced by tuning the sample
geometry, in contrast to the linear increase in dc spin pumping voltage with
sample length. Instead, the ratio depends sensitively on material-specific
transport properties. Our analysis thus provides guidance for the experimental
detection of squeezed magnons through spin pumping shot noise.Comment: Revised manuscript title, added paragraph of interpretation of the
shot noise enhancement with temperature, added brief discussion of the low
temperature limit, references adde
Control of nonlocal magnon spin transport via magnon drift currents
Spin transport via magnon diffusion in magnetic insulators is important for a
broad range of spin-based phenomena and devices. However, the absence of the
magnon equivalent of an electric force is a bottleneck. In this work, we
demonstrate the controlled generation of magnon drift currents in yttrium iron
garnet/platinum heterostructures. By performing electrical injection and
detection of incoherent magnons, we find magnon drift currents that stem from
the interfacial Dzyaloshinskii-Moriya interaction. We can further control the
magnon drift by the orientation of the magnetic field. The drift current
changes the magnon propagation length by up to 6 % relative to diffusion.
We generalize the magnonic spin transport theory to include a finite drift
velocity resulting from any inversion asymmetric interaction, and obtain
results consistent with our experiments.Comment: 6 pages, 3 figure
Why do Particle Clouds Generate Electric Charges?
Grains in desert sandstorms spontaneously generate strong electrical charges;
likewise volcanic dust plumes produce spectacular lightning displays. Charged
particle clouds also cause devastating explosions in food, drug and coal
processing industries. Despite the wide-ranging importance of granular charging
in both nature and industry, even the simplest aspects of its causes remain
elusive, because it is difficult to understand how inert grains in contact with
little more than other inert grains can generate the large charges observed.
Here, we present a simple yet predictive explanation for the charging of
granular materials in collisional flows. We argue from very basic
considerations that charge transfer can be expected in collisions of identical
dielectric grains in the presence of an electric field, and we confirm the
model's predictions using discrete-element simulations and a tabletop granular
experiment
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