Angular momentum conservation and phonon spin in magnetic insulators

Abstract

We develop a microscopic theory of spin-lattice interactions in magnetic insulators, separating rigid-body rotations and the internal angular momentum, or spin, of the phonons, while conserving the total angular momentum. In the low-energy limit, the microscopic couplings are mapped onto experimentally accessible magnetoelastic constants. We show that the transient phonon spin contribution of the excited system can dominate over the magnon spin, leading to nontrivial Einstein-de Haas physics.

Dynamic magnetoelastic boundary conditions and the pumping of phonons

“…1 with NM|vacuum surface A, M|vacuum surface B, and NM|M interface C. Sound waves in elastic media with frequencies up to tens of GHz have wavelengths much longer than the lattice constants and continuum theory applies. We disregard the effect of global rotations on magnons and phonons [27], assuming the total system size to be macroscopic. We focus on the linear regime in which the material (Lagrangian) and spatial (Eulerian) coordinates coincide [28] and denote them by r, while u(r, t ) is the displacement vector at time t of a volume element with equilibrium position r. M and NM are assumed bonded, with identical displacement on both sides close to the interface C in Fig.…”

Direct Observation of Magnon-Phonon Strong Coupling in Two-Dimensional Antiferromagnet at High Magnetic Fields

“…Although M ↑ and M ↓ carry opposite spins [55,56], they inherit the same Raman tensor from M (opposite spins still cause electrical polarizability in the same direction). The spin of M ↑ is transferred to the TO phonon P 3 via the strong coupling and conservation of momentum [10,56,57], as illustrated in Fig. 3(a).…”

Magnetization control by angular momentum transfer from surface acoustic wave to ferromagnetic spin moments

“…The angular momentum of a rigid body rotation 3 and photon 4,5 can be also used to control the magnetization. One might wonder whether it is possible to control the magnetization via the angular momentum transfer from phonons [6][7][8][9] (Fig. 1b).…”