Scientists propose a new mechanism of phonon magnetism in magnetically ordered systems

Phonons are meta-excitations that describe the collective vibrations of a lattice in a solid. In general, the orbital magnetic moment generated by phonon motion through ion motion is weak. However, in some materials, phonons can obtain large magnetic moments by coupling magnetic degrees of freedom. The large phonon magnetic moment is conducive to the mutual regulation of magnetic order and lattice vibration, which has attracted the attention of researchers: on the one hand, spin dynamics and macroscopic magnetic order of materials can be controlled by manipulating phonons; On the other hand, the properties of phonons can be manipulated by manipulating the magnetic sequence. At present, scientists have observed large phonon magnetic moments in some paramagnetic systems, but phonon magnetism in magnetically ordered systems is rare in literature, and the interaction between long-range magnetic order and phonons also needs to be explored.

Recently, the Wan Yuan Research Group of the Laboratory of Condensed Matter Theory and Materials Computing of the Institute of Physics, Chinese Academy of Sciences/Beijing National Research Center for Condensed Matter, and Zhang Qi, Wen Jinsheng and Sun Jian of the School of Physics of Nanjing University formed a joint research team to observe large phonon magnetic moments in magnetic ordered systems for the first time, discovered the enhancement effect of magnetic fluctuations on phonon magnetic moments, and proposed a new physical mechanism of phonon magnetism in magnetic ordered systems. The research was published in Nature Physics.

The experimental group combined magneto-optical Raman and inelastic neutron scattering techniques to observe a pair of Raman active degenerate optical phonons in the antiferromagnetic insulator Fe2Mo3O8 (Figure 1). The Zeeman splitting of this pair of phonons under the action of the external magnetic field has an effective magnetic moment of about 0.11 Bohr magnetons, which is hundreds of times the magnetic moment of conventional phonon orbits. The magnetic field was further increased to cleave to 1/4 of the phonon frequency when the system was driven to the ferromagnetic phase (Figure 2). At the same time, the effective magnetic moment of phonons with temperature changes shows abnormal behavior, and their values grow rapidly around the magnetic transition temperature (Figure 3).

Figure 1. Phonon excitation of Fe2Mo3O8

Figure 2. When Fe2Mo3O8 transitions from antiferromagnetic to ferromagnetic phase, it produces a huge phonon cleavage

Figure 3. Critical fluctuations cause the phonon magnetic moment to increase

These new experimental phenomena of phonon magnetism in magnetically ordered systems are difficult to explain by existing theories based on paramagnetic systems. Zhou Jing, a doctoral student supervised by Wan Yuan, associate researcher of the Institute of Physics, proposed a new physical mechanism of phonon magnetism in magnetic ordered systems, and explained the experimental results by constructing an effective model, combining analytical calculation and numerical simulation. The theory states that the degenerate phonon obtains an effective magnetic moment by hybridizing with the magnon/paramagnetic oscillon in the system. At the same time, critical fluctuations in the system near the magnetic transition temperature can further amplify the effective magnetic moment of phonons – a small external magnetic field can induce a large molecular field in the system. The phonon further undergoes Zeeman cleavage under a large molecular field, exhibiting a large effective magnetic moment. This theory can not only explain the anomalous temperature change of the phonon magnetic moment (Figure 3c/d), but also explain the anomalous blue shift of the phonon frequency with temperature.

The above research reveals the unique characteristics of phonon magnetism in the magnetic ordered system, which lays an experimental and theoretical foundation for further research on the mutual regulation of phonon and magnetic order in the future. The research work was supported by the National Key Research and Development Program of China and the National Natural Science Foundation of China. (Source: Institute of Physics, Chinese Academy of Sciences)

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