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Atoms Spin and Flip a Magnet

Original: "Generalized Helicity-Dependent Magnetization Switching via Substrate Phonons"
· F. G. N. Fennema, H. Damas, H. Yoshikawa, A. Tsukamoto, C. S. Davies, A. Kirilyuk
arXiv:2609.05064v1 · 2026-09-04 · CC BY 4.0 · 1 min
Materials
Circular vibrations of atoms in a substrate switch the magnetization of a film, but strong light absorption hinders this due to heating.

Atoms in a crystal are always vibrating. If you make them move in circles, they can transfer their rotation to a magnetic layer on top and flip its magnetization. Previously, this trick had been demonstrated only on two materials. Now physicists tested it on seven different crystals to see how universal it is.

For the experiment, they used a thin film of a gadolinium-iron-cobalt alloy and an infrared laser that can twist light. After irradiation, the magnetic state was checked by how it rotates the polarization of light. Switching efficiency varied widely across substrates, from 30% to 95%.

The most intriguing part: switching did not correlate with where the crystal absorbs light most strongly. On magnesium oxide, the best result occurred at a wavelength of 20.8 micrometers, while the absorption resonance lies near 25 micrometers. Calculations showed that near resonance, almost all the energy is deposited in the first 200 nanometers of the substrate, right at the interface with the magnetic layer. This local heating seems to hinder switching. So the key is to find a balance: the light must excite the atoms but not overheat the interface.

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Effective magnetic fields generated by circularly polarized phonons can exceed tens of tesla, comparable to the fields of the most powerful pulsed magnets.

\omega_{LO}^2 - \omega_{TO}^2 = \frac{n e^2 Z^{*2}}{\varepsilon_0 \varepsilon_\infty \mu}
The difference of the squared frequencies of longitudinal and transverse optical phonons is proportional to the square of the Born effective charge and inversely proportional to the reduced ion mass. A larger splitting means stronger coupling of the phonon to macroscopic polarization.
M = \sum_i \gamma_i L_i, \quad \gamma_i = \frac{e Z_i^*}{2 M_i}, \quad L_i = \sum_{\alpha,\beta} Q_{i,\alpha} \times \dot{Q}_{i,\beta}
The phonon magnetic moment is the sum of ion contributions, each determined by the gyromagnetic ratio (which depends on effective charge and mass) and the angular momentum of the circular ion motion.
Concepts
phonon magnetism magnetoresistance alternating magnetism magnetophotonic effects magnetic field effects ferroelectric external magnetic field gravitational redshift domain wall demagnetization antiferromagnet magneto optical kerr effect magnetization magnetosphere Faraday effect