A new class of nonreciprocal spin waves on the edges of 2D antiferromagnetic honeycomb nanoribbons

Abstract

Antiferromagnetic two-dimensional (2D) materials are currently under intensive theoretical and experimental investigations in view of their potential applications in antiferromagnetbased magnonic and spintronic devices. Recent experimental studies revealed the importance of magnetic anisotropy and of Dzyaloshinskii-Moriya interactions (DMI) on the ordered ground state and the magnetic excitations in these materials. In this work we present a robust classical field theory approach to study the effects of anisotropy and the DMI on the edge and bulk spin waves in 2D antiferromagnetic nanoribbons. We predict the existence of a new class of nonreciprocal edge spin waves, characterized by opposite polarizations in counter-propagation. These novel edge spin waves are induced by the DMI and are fundamentally different from conventional nonreciprocal spin waves for which the polarization is independent of the propagation direction. We further analyze the effects of the edge structures on the magnetic excitations for these systems. In particular, we show that anisotropic bearded edge nanoribbons act as topologically trivial magnetic insulators with potentially interesting applications in magnonics. Our results constitute an important finding for current efforts seeking to establish unconventional magnonic devices utilizing spin wave polarization.The polarization of a spin wave is determined by the precessing direction of the magnetization and constitutes an important additional intrinsic degree of freedom, beside the spin wave amplitude and phase. Spin wave theory in a collinear antiferromagnet, composed of two magnetic lattices with opposite magnetization, is known to yield two bulk modes. The precession frequencies of these modes are characterized by opposite contributions from the exchange interaction [1-3]. As a consequence, the sublattice magnetizations precess clockwise in one of the modes and anticlockwise in the other [3]. The spin wave modes in an antiferromagnet are hence characterized by opposite polarizations, conventionally termed as right-handed and left-handed spin waves.The important advantages for utilizing spin wave polarization to encode and process information in antiferromagnet-based magnonics has recently received significant attention [4][5][6][7][8]. The Dzyaloshinskii-Moriya interactions (DMI) [9,10] was highlighted as a key ingredient to realize polarization-based magnonic devices. In particular, antiferromagnetic domain walls with DMI have been proposed as spin wave polarizer, retarder and transistor [4,5].

Valley-polarized domain wall magnons in 2D ferromagnetic bilayers

“…The translation symmetry in the system is broken (preserved) along the x -direction ( y -direction). The behavior of is hence dictated by the functional form , where denotes the generalized phase factor in the x -direction 15 , 18 , 20 , 21 , 26 , 27 . For intragap modes confined to the LSW, is real and the wavefunction decays exponentially away from the LSW.…”

“…The recent realization of two-dimensional (2D) materials with intrinsic magnetism 1,2 constituted an important breakthrough in condensed matter physics. This discovery immediately stimulated tremendous interest in the fundamental physics underlying 2D magnets, as well as their technological potentials [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] . With the novel characteristics of their magnetic excitations, 2D magnets are expected to open new horizons in technological fields like magnonics.…”

Magnon magic angles and tunable Hall conductivity in 2D twisted ferromagnetic bilayers

“…Spin dynamics in tFBL. The DMI term in H T can be rewritten in terms of a scalar product 22 which unifies the treatment of the exchange and the DMI parts of H T (Supplementary Notes 1 and 3). The effective exchange fields H α l acting on the sublattice magnetizations M α l can then be derived from the Heisenberg Hamiltonian as [19][20][21][22][41][42][43][44][45][46]…”

“…The DMI term in can be rewritten in terms of a scalar product 22 which unifies the treatment of the exchange and the DMI parts of (Supplementary Notes 1 and 3). The effective exchange fields acting on the sublattice magnetizations can then be derived from the Heisenberg Hamiltonian as 19 – 22 , 41 – 46 where we have used the convention that if then and vice versa. Same convention assumed for and .…”

“…Two-dimensional (2D) materials with intrinsic magnetism has recently been realized 1,2 , opening new horizons in 2D material research [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] . In these bosonic Dirac materials, magnetic anisotropy is found to overcome thermal fluctuations and stabilize the magnetic order at finite temperatures.…”