Theoretical Prediction of a Rotating Magnon Wave Packet in Ferromagnets

Abstract

We theoretically show that the magnon wave packet has a rotational motion in two ways: a self-rotation and a motion along the boundary of the sample (edge current). They are similar to the cyclotron motion of electrons, but unlike electrons the magnons have no charge and the rotation is not due to the Lorentz force. These rotational motions are caused by the Berry phase in momentum space from the magnon band structure. Furthermore, the rotational motion of the magnon gives an additional correction term to the magnon Hall effect. We also discuss the Berry curvature effect in the classical limit of long-wavelength magnetostatic spin waves having macroscopic coherence length.

Thermal Hall effect of spins in a paramagnet

“…Coupling of the energy density H i to the pseudogravitational potential ψ i is an effective way to derive the thermal response function. [6][7][8]17 In brief, the total Hamiltonian including the gravitational coupling H = …”

“…14 Stimulated by their observations, we go beyond the existing magnon description of the thermal Hall effect [4][5][6][7][8][9][10] and formulate the phenomenon using the spin language entirely. It is then applied to discuss Hall effects of spin both in the paramgnetic as well as the ferromagnetic regime.…”

“…Magnons -quantized small fluctuations of an ordered magnet -can in principle exhibit similar Hall transport driven by thermal gradient, as first predicted theoretically by Katsura, Nagaosa, and Lee 4 and confirmed experimentally in an insulating pyrochlore magnet Lu 2 V 2 O 7 by the Tokura group 5 . Formulation of the magnon Hall effect was perfected by Murakami and collaborators in a series of papers [6][7][8] after correcting for the missing, magnetization current term in the original derivation of Ref. 4.…”

Topological thermal Hall effect in frustrated kagome antiferromagnets

“…However, a temperature gradient can induce a heat current and the DMI-induced Berry curvature acts as an effective magnetic field in momentum space. This leads to a thermal version of the Hall effect characterized by a temperature dependent thermal Hall conductivity [1,3]. Thermal Hall effect is now an emerging active research area for probing the topological nature of magnetic spin excitations in quantum magnets.…”

“…Topological phases of matter are an active research field in condensed matter physics, mostly dominated by electronic systems. Quite recently the concepts of topological matter have been extended to nonelectronic bosonic systems such as quantized spin waves (magnons) [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] and quantized lattice vibrations (phonons) [18][19][20][21][22][23]. In the former, spin-orbit coupling manifests in the form of Dzyaloshinsky-Moriya interaction [24,25] and it leads to topological spin excitations and chiral edge modes in collinear ferromagnets [5,6].…”