46 research outputs found
The temperature dependence of quantum spin pumping generated using electron spin resonance with three-magnon splittings
On the basis of the Schwinger-Keldysh formalism, we have closely investigated
the temperature dependence of quantum spin pumping by electron spin resonance.
We have clarified that three-magnon splittings excite non-zero modes of magnons
and characterize the temperature dependence of quantum spin pumping. Our
theoretical result qualitatively agrees with the experiment by Czeschka et al.
that the mixing conductance is little influenced by temperature [F. D. Czeschka
et al., Phys. Rev. Lett., 107, 046601 (2011)].Comment: 16 pages, 5 figures, clear pictures are available at URL specified in
the documen
Temperature dependence of spin currents in one- and three-dimensional insulators
The temperature dependence of spin currents in insulators at the finite
temperature near zero Kelvin is theoretically studied. The spin currents are
carried by Jordan-Wigner fermions and magnons in one- and three- dimensional
insulators. The quasiparticle description of one-dimensional spin systems is
valid only in the finite temperature near zero Kelvin. These spin currents are
generated by the external magnetic field gradient along the quantization axis
and also by the two-particle interaction gradient. In one-dimensional
insulators, quantum fluctuations are strong and the spin current carried by
Jordan-Wigner fermions shows the stronger dependence on temperatures than the
one by magnons.Comment: 11 pages, 2 tables, accepted for publication in Int.J.Mod.Phys.
Magnon transport through microwave pumping
We present a microscopic theory of magnon transport in ferromagnetic
insulators (FIs). Using magnon injection through microwave pumping, we propose
a way to generate magnon dc currents and show how to enhance their amplitudes
in hybrid ferromagnetic insulating junctions. To this end focusing on a single
FI, we first revisit microwave pumping at finite (room) temperature from the
microscopic viewpoint of magnon injection. Next, we apply it to two kinds of
hybrid ferromagnetic insulating junctions. The first is the junction between a
quasi-equilibrium magnon condensate and magnons being pumped by microwave,
while the second is the junction between such pumped magnons and noncondensed
magnons. We show that quasi-equilibrium magnon condensates generate ac and dc
magnon currents, while noncondensed magnons produce essentially a dc magnon
current. The ferromagnetic resonance (FMR) drastically increases the density of
the pumped magnons and enhances such magnon currents. Lastly, using microwave
pumping in a single FI, we discuss the possibility that a magnon current
through an Aharonov-Casher phase flows persistently even at finite temperature.
We show that such a magnon current arises even at finite temperature in the
presence of magnon-magnon interactions. Due to FMR, its amplitude becomes much
larger than the condensed magnon current.Comment: 12 pages, 5 figures, accepted for publication in Phys. Rev.
Wiedemann-Franz Law for Magnon Transport
One of the main goals of spintronics is to improve transport of information
carriers and to achieve new functionalities with ultra-low dissipation. A most
promising strategy for this holy grail is to use pure magnon currents created
and transported in insulating magnets, in the complete absence of any
conducting metallic elements. Here we propose a realistic solution to this
fundamental challenge by analyzing magnon and heat transport in insulating
ferromagnetic junctions. We calculate all transport coefficients for magnon
transport and establish Onsager relations between them. We theoretically
discover that magnon transport in junctions has a universal behavior, i.e. is
independent of material parameters, and establish a magnon analog of the
celebrated Wiedemann-Franz law which governs charge transport at low
temperatures. We calculate the Seebeck and Peltier coefficients which are
crucial quantities for spin caloritronics and demonstrate that they assume
universal values in the low temperature limit. Finally, we show that our
predictions are within experimental reach with current device and measurement
technologies.Comment: 10 pages, 3 figures, updated into published version from PR
A short note on spin pumping theory with Landau-Lifshitz-Gilbert equation under quantum fluctuation; necessity for quantization of localized spin
We would like to point out the blind spots of the approach combining the spin
pumping theory proposed by Tserkovnyak et al. with the Landau-Lifshitz-Gilbert
equation; this method has been widely used for interpreting vast experimental
results. The essence of the spin pumping effect is the quantum fluctuation.
Thus, localized spin degrees of freedom should be quantized, i.e. be treated as
magnons not as classical variables. Consequently, the precessing ferromagnet
can be regarded as a magnon battery. This point of view will be useful for
further progress of spintronics.Comment: 10pages, 1 figure. This article is closely related to the work by K.
N.; arXiv:1201.194
Optomagnonic Barnett effect
Combining the technologies of quantum optics and magnonics, we find that the
circularly polarized laser can dynamically realize the quasiequilibrium magnon
Bose-Einstein condensates (BEC). The Zeeman coupling between the laser and
spins generates the optical Barnett field, and its direction is controllable by
switching the laser chirality. We show that the optical Barnett field develops
the total magnetization in insulating ferrimagnets with reversing the local
magnetization, which leads to the quasiequilibrium magnon BEC. This
laser-induced magnon BEC transition through optical Barnett effect, dubbed the
optomagnonic Barnett effect, provides an access to coherent magnons in the high
frequency regime of the order of terahertz. We also propose a realistic
experimental setup to observe the optomagnonic Barnett effect using current
device and measurement technologies as well as the laser chirping. The
optomagnonic Barnett effect is a key ingredient for the application to
ultrafast spin transport.Comment: 5+7 pages, 3 figures, 1 tabl
Tunable Magnonic Thermal Hall Effect in Skyrmion Crystal Phases of Ferrimagnets
We theoretically study the thermal Hall effect by magnons in skyrmion crystal
phases of ferrimagnets in the vicinity of the angular momentum compensation
point (CP). To this end, we start by deriving the equation of motion for
magnons in the background of an arbitrary equilibrium spin texture, which gives
rise to the fictitious electromagnetic field for magnons. As the net spin
density varies, the resultant equation of motion interpolates between the
relativistic Klein-Gordon equation at CP and the nonrelativistic
Schr{\"o}dinger-like equation away from it. In skyrmion crystal phases, the
right- and the left-circularly polarized magnons with respect to the order
parameter are shown to form the Landau levels separately within the uniform
skyrmion-density approximation. For an experimental proposal, we predict that
the magnonic thermal Hall conductivity changes its sign when the ferrimagnet is
tuned across CP, providing a way to control heat flux in spin-caloritronic
devices on the one hand and a feasible way to detect CP of ferrimagnets on the
other hand.Comment: 6 pages, 3 figures, 2 pages of supplemental materia
Josephson and Persistent Spin Currents in Bose-Einstein Condensates of Magnons
Using the Aharonov-Casher (A-C) phase, we present a microscopic theory of the
Josephson and persistent spin currents in quasi-equilibrium Bose-Einstein
condensates (BECs) of magnons in ferromagnetic insulators. Starting from a
microscopic spin model that we map onto a Gross-Pitaevskii Hamiltonian, we
derive a two-state model for the Josephson junction between the weakly coupled
magnon-BECs. We then show how to obtain the alternating-current (ac) Josephson
effect with magnons as well as macroscopic quantum self-trapping in a
magnon-BEC. We next propose how to control the direct-current (dc) Josephson
effect electrically using the A-C phase, which is the geometric phase acquired
by magnons moving in an electric field. Finally, we introduce a magnon-BEC ring
and show that persistent magnon-BEC currents flow due to the A-C phase.
Focusing on the feature that the persistent magnon-BEC current is a steady flow
of magnetic dipoles that produces an electric field, we propose a method to
directly measure it experimentally.Comment: 8 pages, 6 figures, updated into published versio