Axial phono-magnetic effects

TL;DR

This review assembles the rapidly expanding field of axial phono-magnetism, where circularly polarized (axial) phonons carry angular momentum and couple to electronic spins to generate measurable magnetization. It links phenomenology—via dynamical multiferroicity and Landau theory—to a suite of microscopic frameworks (perturbative, adiabatic Berry-phase, Floquet, and inertial approaches) that all hinge on electron–phonon coupling, yielding consistent expressions for the phonon magnetic moment and related observables. Experimental evidence across diverse materials (from CeF$_3$ and Cd$_3$As$_2$ to SrTiO$_3$ and topological insulators) reveals robust, sometimes giant, phono-magnetic effects including Phonon Zeeman splittings, MOKE signals, and proximity-induced switching, underscoring the universality of the mechanism. The findings point to potential technological avenues in ultrafast magnetic switching, spintronics, and topological materials, while also highlighting fundamental questions about the exact nature of the effective fields and their relation to topology, gauge fields, and inertial couplings. The field is rapidly evolving, with ongoing debates about whether the phonon-induced field obeys Maxwellian dynamics and how best to maximize the effect through material choice and structural design.

Abstract

Axial or circularly polarized phonons are collective lattice vibrations with angular momentum. Over the past decade they have emerged as a promising mechanism for the manipulation of magnetism, in parallel to well established optical protocols. In particular, coherent axial phonons were shown to induce magnetization in materials without spin-ordering, making them a viable tool for ultrafast magnetic switching. The experimental evidence suggests that the size of this magnetization is significant, opening a new research area on the phono-magnetic effect. Remarkably, the coupling of axial phonons to magnetism has been observed a broad class of materials, pointing to a universal nature of the underlying mechanisms. In this review article, we present the recent progress in the field. We give an introduction to the phenomenological perspective and an overview of the experimental evidence for the magnetization emerging from axial phonons, which includes discussing the observations of phonon Zeeman effect, the magneto-optical Kerr effect and the proximity-induced magnetization switching. We present recently proposed microscopic theories for the phono-magnetic effects, based on perturbation theory, adiabatic motion and Floquet theory as well as the emergence of the phonon magnetic moment due to artificial gauge fields or inertial effects. This summary allows us to see correspondences between the seemingly different theoretical approaches, facilitating a more complete perspective of the effect.

Figures (7)

• Figure 1: Schematic representation of dynamical multiferroicity. a) Two reciprocal processes of generating multiferroicity: through spatially varying magnetization or temporally varying polarization. b) Magnetization induced by collective circular motion of ions: two types of ions create local magnetic moments of different magnitude which leads to non-zero net magnetization. Reproduced with permission from juraschek2017dynamical; Copyright 2017 American Physical Society.
• Figure 2: Visualization of the physical phenomena used to detect phono-magnetic effects: The Phonon Zeeman effect, i.e. splitting of a phonon mode into left and right circularly polarized phonons; the magneto-optical Kerr effect which constitutes polarization rotation of a probe field reflected off a magnetized sample; Magnetic switching where circularly polarized phonon modes induce switching of magnetic order in a structure placed on top of the substrate.
• Figure 3: Phonon Zeeman splitting in CeF$_3$ observed using Raman scattering Schaack1976, in Cd$_3$As$_2$ observed using time-domain magnetoterahertz spectrometry cheng2020large, and in PbTe observed with polarization-dependent terahertz spectroscopy baydin2022magnetic. (a, reproduced with permission from Schaack1976; Copyright IOP Publishing. All rights reserved.)(b, reproduced with permission from cheng2020large; Copyright 2020 American Chemical Society)(c, reproduced with permission from baydin2022magnetic; Copyright 2022 American Physical Society)
• Figure 4: Dynamic magnetization induced by axial phonons, measured using the magneto-optical Kerr effect (MOKE). (a)-(c) show results for CeF$_3$luo2023, (d)-(e) show results for SrTiO$_3$Basini2024. (a) Faraday rotation in CeF$_3$ at 10 K after excitation with circularly polarized THz pump. (b) Corresponding sample magnetization computed from (a) using the temperature-dependent Verdet constant of CeF$_3$. (c) Deduced effective magnetic field due to axial phonons, giving rise to the magnetization in (a) and (b). (d) Faraday rotation in SrTiO$_3$ at room temperature after excitation with circularly polarized THz pump. (e) Quadratic dependence of the Kerr rotation (magnetization) on the field strength of the THz laser pulse. (f) Comparison of theoretical and experimental phonon magnetic moment. (a)-(c), reproduced with permission from luo2023; Copyright 2023 The Authors, some rights reserved. (d)-(f), reproduced from Basini2024 under the Creative Commons license.
• Figure 5: Schematic overview of the three categories of microscopic theories (perturbative, adiabatic, Floquet), which are limiting cases of each other.