Origin of long-wavelength switching recovery

Determine the physical origin of the recovery of helicity-dependent magnetization switching on the long-wavelength side of the transverse-optical phonon resonance in MgO-, ZnO-, and Al2O3-based heterostructures, where substrate absorption decreases and the response cannot be explained solely by direct resonant excitation of the identified transverse-optical mode.

Background

The paper investigates helicity-dependent magnetization switching in GdFeCo layers deposited on seven diamagnetic substrates. Although switching generally occurs within substrate Reststrahlen bands, the switching maxima are shifted toward shorter wavelengths relative to the corresponding transverse-optical phonon resonances. Transfer-matrix simulations suggest that near- resonant absorption localizes deposited optical energy near the substrate interface, potentially increasing local heating and suppressing switching.

For MgO, ZnO, and Al2O3 substrates, switching reappears on the long-wavelength side of the resonance even as substrate absorption decreases. The authors identify this recovery as a feature not accounted for by their proposed localized-heating explanation or by direct resonant excitation of the identified transverse-optical mode, leaving its physical origin unresolved.

References

While localized energy deposition provides a possible explanation for the reduction in switching efficiency near the TO-phonon resonance, a remaining unresolved feature is the recovery of switching on the long-wavelength side of the resonance, where the substrate absorption decreases. This behavior is observed for MgO, ZnO and Al$_2$O$_3$, indicating that the switching response outside the Reststrahlen bands differs from that in the resonant regime. The origin of this recovery remains unclear and shows that the spectral response cannot be described solely by direct resonant excitation of the identified TO mode.

— Generalized Helicity-Dependent Magnetization Switching via Substrate Phonons  (2609.05064 - Fennema et al., 4 Sep 2026) in Section Conclusion