Josephson Effects in a Bose-Einstein Condensate of Magnons

A phenomenological theory is developed, that accounts for the collective dynamics of a Bose-Einstein condensate of magnons. In terms of such description we discuss the nature of spontaneous macroscopic interference between magnon clouds, highlighting the close relation between such effects and the well known Josephson effects. Using those ideas we present a detailed calculation of the Josephson oscillations between two magnon clouds, spatially separated in a magnonic Josephson junction

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

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

Time-dependent strain-tuning topological magnon phase transition

Collinear magnets in honeycomb lattices under the action of time-dependent strains are investigated. Given the limits of high-frequency periodically varying deformations, we derive an effective Floquet theory for spin system that results in the emergence of a spin chirality. We find that the coupling between magnons and spin chirality depends on the details of the strain such as the spatial dependence and applied direction. Magnonic fluctuations about the ferromagnetic state are determined, and it is found that spatially homogeneous strains drive the magnon system into topologically protected phases. In particular, we show that certain uniform strain fields play the role of an out-of-plane next-neighbor Dzyaloshinskii-Moriya interaction. Furthermore, we explore the application of nonuniform strains, which lead to a confinement of magnon states that for uniaxial strains, propagates along the direction that preserves translational symmetry. Our work demonstrates a direct and novel way in which to manipulate the magnon spectrum based on time-dependent strain engineering that is relevant for exploring topological transitions in quantum magnonics

Green's function formalism for spin transport in metal-insulator-metal heterostructures

We develop a Green's function formalism for spin transport through heterostructures that contain metallic leads and insulating ferromagnets. While this formalism in principle allows for the inclusion of various magnonic interactions, we focus on Gilbert damping. As an application, we consider ballistic spin transport by exchange magnons in a metal-insulator-metal heterostructure with and without disorder. For the former case, we show that the interplay between disorder and Gilbert damping leads to spin current fluctuations. For the case without disorder, we obtain the dependence of the transmitted spin current on the thickness of the ferromagnet. Moreover, we show that the results of the Green's function formalism agree in the clean and continuum limit with those obtained from the linearized stochastic Landau-Lifshitz-Gilbert equation. The developed Green's function formalism is a natural starting point for numerical studies of magnon transport in heterostructures that contain normal metals and magnetic insulators.Comment: 13 pages, 8 figure