Spin Nernst Effect of Magnons in Collinear Antiferromagnets

Abstract

In a collinear antiferromagnet with easy-axis anisotropy, symmetry guarantees that the spin wave modes are doubly degenerate. The two modes carry opposite spin angular momentum and exhibit opposite chirality. Using a honeycomb antiferromagnet in the presence of the Dzyaloshinskii-Moriya interaction, we show that a longitudinal temperature gradient can drive the two modes to opposite transverse directions, realizing a spin Nernst effect of magnons with vanishing thermal Hall current. We find that magnons around the Γ-point and the K-point contribute oppositely to the transverse spin transport, and their competition leads to a sign change of the spin Nernst coefficient at finite temperature. Possible material candidates are discussed.PACS numbers: 75.30. Ds, 75.50.Ee, 75.76.+j Recent years have seen a surge of interest in utilizing magnons for information encoding and processing [1][2][3][4][5]. Being an elementary excitation in magnetically ordered media, a magnon carries not only energy but also spin angular momentum [6]. The latter is of intrinsic interest in spintronics, since it would allow the transfer of spin information without Joule heating. Such a realization has led to the emerging field of magnon spintronics [7], in which magnons are expected to play similar roles as spin-1 2 electrons. However, there is one caveat: while the electron spin forms an internal degree of freedom and is free to rotate, the magnon spin in a ferromagnet (FM) is fixed by its chirality, which can only be right-handed with respect to the magnetization.By contrast, it is well established that in a collinear antiferromagnet (AF) with easy-axis anisotropy, symmetry admits two degenerate magnon modes with opposite chirality [8], and hence opposite spin [9,10]. These two modes can be selectively excited and detected via both electrical [11][12][13] and optical [14][15][16] means, which enables an internal space to encode binary information similar to the electron spin. It is therefore possible to explore the magnonic counterparts of phenomena usually associated with the electron spin. For example, a spin field-effect transistor of magnons using collinear AF has been recently proposed [17], in which a rotation in the magnon spin space can be realized by a gate-tunable Dzyaloshinskii-Moriya interaction (DMI).Drawing the above analogy, we theoretically demonstrate in this Letter a magnon spin Nernst effect (SNE) in a collinear AF, which is similar to the electron spin Hall effect [18]. The magnon SNE is intimately related to the magnon Hall effect [19][20][21][22][23][24]; it can be viewed as two opposite copies of the magnon Hall effect for each spin species, i.e., magnons with opposite spins flow in opposite transverse directions driven by an applied temperature gradient (Fig. 1). We show that the SNE is realizable on a honeycomb lattice by including the second nearest-neighbor DMI. The SNE coefficient is calculated through a semiclassical theory of magnon dynamics, supplemented by general symmetry analyses. Finally, we propose MnP...

Antiferromagnetic spintronics

“…This effect does not rely on the presence of time-reversal symmetry breaking, unlike the thermal Hall effect (thermal analog of the Hall effect). Several recent papers (Cheng, Okamoto, and Xiao, 2016;Zyuzin and Kovalev, 2016) demonstrated the spin Nernst effect in models of honeycomb lattice magnets with Dzyaloshinski-Moriya interactions.…”

Magnon Spin Nernst Effect in Antiferromagnets

“…Recently we became aware of a paper [49] which discusses SNE in antiferromagnets. The theory presented in the present paper appears to be more general, see results (5), (6), (8), and (9).…”

Magnonic topological insulators in antiferromagnets

“…1 (b). Spins of length S on each bipartite sublattice, denoted by A and B, satisfy 60,61,66 S A = −S B = Se z in the ground state. The magnet is described by the following spin Hamiltonian, 42,43 …”

“…[13], we developed it further into the magnonic 'quantum' Hall effect and appropriately defining the thermal conductance for bosons, we found that the magnon Hall conductances in such topological FMs obey a Wiedemann-Franz 50 (WF) law for magnon transport 4,47 . In this paper, motivated by the recent experimental 51 demonstration of thermal generation of spin currents in AFs using the spin Seebeck effect [52][53][54][55][56][57][58][59] and by the report [60][61][62] of the magnonic spin Nernst effect in AFs, we develop Ref. [46] further into the AF regime [42][43][44][45]63,64 and propose a magnonic analog of the QSHE [31][32][33][34][35]39,40 for edge states that carry helical edge magnons [ Fig.…”