210 research outputs found
Spintronics and spin currents: from magnetic memories to black holes
Physikalisches Kolloquium Technische Universität Kaiserslauter
Bulk and edge spin transport in topological magnon insulators
We investigate the spin transport properties of a topological magnon
insulator, a magnetic insulator characterized by topologically nontrivial bulk
magnon bands and protected magnon edge modes located in the bulk band gaps.
Employing the Landau-Lifshitz-Gilbert phenomenology, we calculate the spin
current driven through a normal metaltopological magnon insulatornormal
metal heterostructure by a spin accumulation imbalance between the metals, with
and without random lattice defects. We show that bulk and edge transport are
characterized by different length scales. This results in a characteristic
system size where the magnon transport crosses over from being bulk-dominated
for small systems to edge-dominated for larger systems. These findings are
generic and relevant for topological transport in systems of nonconserved
bosons.Comment: 5 pages, 5 figures, 3 pages Supplemental Material with 2 additional
figure
Electronic Pumping of Quasiequilibrium Bose-Einstein Condensed Magnons
We theoretically investigate spin transfer between a system of
quasiequilibrated Bose-Einstein condensed magnons in an insulator in direct
contact with a conductor. While charge transfer is prohibited across the
interface, spin transport arises from the exchange coupling between insulator
and conductor spins. In normal insulator phase, spin transport is governed
solely by the presence of thermal and spin-diffusive gradients; the presence of
Bose-Einstein condensation (BEC), meanwhile, gives rise to a
temperature-independent condensate spin current. Depending on the thermodynamic
bias of the system, spin may flow in either direction across the interface,
engendering the possibility of a dynamical phase transition of magnons. We
discuss experimental feasibility of observing a BEC steady state (fomented by a
spin Seebeck effect), which is contrasted to the more familiar spin-transfer
induced classical instabilities.Comment: 7 pages, 4 figure
Many-body theory of spin-current driven instabilities in magnetic insulators
We consider a magnetic insulator in contact with a normal metal. We derive a
self-consistent Keldysh effective action for the magnon gas that contains the
effects of magnon-magnon interactions and contact with the metal to lowest
order. Self-consistent expressions for the dispersion relation, temperature and
chemical potential for magnons are derived. Based on this effective action, we
study instabilities of the magnon gas that arise due to spin-current flowing
across the interface between the normal metal and the magnetic insulator. We
find that the stability phase diagram is modified by an interference between
magnon-magnon interactions and interfacial magnon-electron coupling. These
effects persist at low temperatures and for thin magnetic insulators.Comment: 10 pages and 5 figure
Long-Range Phonon Spin Transport in Ferromagnet-Nonmagnetic Insulator Heterostructures
We investigate phonon spin transport in an insulating
ferromagnet-nonmagnet-ferromagnet heterostructure. We show that the
magnetoelastic interaction between the spins and the phonons leads to nonlocal
spin transfer between the magnets. This transfer is mediated by a local phonon
spin current and accompanied by a phonon spin accumulation. The spin
conductance depends nontrivially on the system size, and decays over millimeter
length scales for realistic material parameters, far exceeding the decay
lengths of magnonic spin currents.Comment: 6 pages, 4 figure
Breaking of Coulomb blockade by macrospin-assisted tunneling
A magnet with precessing magnetization pumps a spin current into adjacent
leads. As a special case of this spin pumping, a precessing macrospin
(magnetization) can assist electrons in tunneling. In small systems, however,
the Coulomb blockade effect can block the transport of electrons. Here, we
investigate the competition between macrospin-assisted tunneling and Coulomb
blockade for the simplest system where both effects meet; namely, for a single
tunnel junction between a normal metal and a metallic ferromagnet with
precessing magnetization. By combining Fermi's golden rule with magnetization
dynamics and charging effects, we show that the macrospin-assisted tunneling
can soften or even break the Coulomb blockade. The details of these effects --
softening and breaking of Coulomb blockade -- depend on the macrospin dynamics.
This allows, for example, to measure the macrospin dynamics via a system's
current-voltage characteristics. It also allows to control a spin current
electrically. From a general perspective, our results provide a platform for
the interplay between spintronics and electronics on the mesoscopic scale. We
expect our work to provide a basis for the study of Coulomb blockade in more
complicated spintronic systems.Comment: 7 pages, 3 figure
Spin current cross-correlations as a probe of magnon coherence
Motivated by the important role of the normalized second order coherence
function, often called , in the field of quantum optics, we propose a
method to determine magnon coherence in solid-state devices. Namely, we show
that the cross-correlations of pure spin-currents injected by a ferromagnet
into two metal leads, normalized by their dc value, replicate the behavior of
when magnons are driven far from equilibrium. We consider two
scenarios: driving by ferromagnetic resonance, which leads to the coherent
occupation of a single mode, and driving by heating of the magnons, which leads
to an excess of incoherent magnons. We find an enhanced normalized
cross-correlation in the latter case, thereby demonstrating bunching of
nonequilibrium thermal magnons due to their bosonic statistics. Our results
contribute to the burgeoning field of quantum magnonics, which seeks to explore
and exploit the quantum nature of magnons
Dynamic phase diagram of dc-pumped magnon condensates
We study the effects of nonlinear dynamics and damping by phonons on a system
of interacting electronically pumped magnons in a ferromagnet. The nonlinear
effects are crucial for constructing the dynamic phase diagram, which describes
how "swasing" and Bose-Einstein condensation emerge out of the
quasiequilibrated thermal cloud of magnons. We analyze the system in the
presence of magnon damping and interactions, demonstrating the continuous onset
of stable condensates as well as hysteretic transitions.Comment: 9 pages, 6 figure
Magnon-Mediated Indirect Exciton Condensation through Antiferromagnetic Insulators
Electrons and holes residing on the opposing sides of an insulating barrier
and experiencing an attractive Coulomb interaction can spontaneously form a
coherent state known as an indirect exciton condensate. We study a trilayer
system where the barrier is an antiferromagnetic insulator. The electrons and
holes here additionally interact via interfacial coupling to the
antiferromagnetic magnons. We show that by employing magnetically uncompensated
interfaces, we can design the magnon-mediated interaction to be attractive or
repulsive by varying the thickness of the antiferromagnetic insulator by a
single atomic layer. We derive an analytical expression for the critical
temperature of the indirect exciton condensation. Within our model,
anisotropy is found to be crucial for achieving a finite , which increases
with the strength of the exchange interaction in the antiferromagnetic bulk.
For realistic material parameters, we estimate to be around 7 K, the same
order of magnitude as the current experimentally achievable exciton
condensation where the attraction is solely due to the Coulomb interaction. The
magnon-mediated interaction is expected to cooperate with the Coulomb
interaction for condensation of indirect excitons, thereby providing a means to
significantly increase the exciton condensation temperature range.Comment: 7+13 Pages, 2+1 figures. Added discussion of retardation effects.
Accepted for publication in Phys. Rev. Let
Magnon antibunching in a nanomagnet
We investigate the correlations of magnons inside a nanomagnet and identify a
regime of parameters where the magnons become antibunched, i.e., where there is
a large probability for occupation of the single-magnon state. This antibunched
state is very different from magnons at thermal equilibrium and
microwave-driven coherent magnons. We further obtain the steady state
analytically and describe the magnon dynamics numerically, and ascertain the
stability of such antibunched magnons over a large window of magnetic
anisotropy, damping and temperature. This means that the antibunched magnon
state is feasible in a wide class of low-damping magnetic nanoparticles. To
detect this quantum effect, we propose to transfer the quantum information of
magnons to photons by magnon-photon coupling and then measure the correlations
of photons to retrieve the magnon correlations. Our findings may provide a
promising platform to study quantum-classical transitions and for designing a
single magnon source.Comment: 6 pages, 3 figure
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