Laser Pulses Unlock Magnetic Hematite
New research suggests that ultrafast laser pulses can selectively excite high-momentum magnetic modes in hematite, leading to amplified and shifted frequencies of lower-energy modes at the Brillouin zone center. This nonthermal mechanism, detailed in a recent study, indicates a pathway toward precise optical manipulation of quantum materials, with encouraging implications for advanced technologies.
Magnetic Control in Solids
A key challenge in condensed-matter physics involves manipulating magnetic properties at ultrafast timescales—think femtoseconds, or quadrillionths of a second—without unwanted heating. Traditional methods often rely on thermal effects or limited to zone-center excitations, where waves have minimal momentum. Results indicated that overcoming these constraints could enable breakthroughs like inducing magnetic phase transitions or enhancing superconductivity optically.
In the study, researchers focused on hematite (α-Fe₂O₃), a common antiferromagnetic mineral that exhibits weak ferromagnetism at room temperature. Its magnetic spectrum includes gamma-point modes—quasi-ferromagnetic (q-FM) and quasi-antiferromagnetic (q-AFM)—along with a two-magnon (2M) mode, a collective excitation of high-momentum magnon pairs detectable via mid-infrared absorption.
Resonant Pumping with Femtosecond Lasers
The team employed a pump-probe setup to investigate these dynamics. Femtosecond mid-infrared pump pulses, tunable from 37 to 53 THz, targeted the 2M mode either resonantly (at 45 THz) or off-resonantly. A near-infrared probe beam then measured changes in ellipticity, sensitive to in-plane spin precessions via magneto-optical effects like magnetic linear birefringence.
To ensure stroboscopic conditions, a 200 mT in-plane magnetic field saturated the sample's magnetization, as confirmed by SQUID measurements. Fluences ranged up to 100 mJ/cm², with polarization dependencies tested to distinguish mechanisms. Supplementary characterizations, including absorption spectra and electric-field waveforms, validated the resonant interaction.
Amplification and Frequency Shifts
Off-resonant pumping triggered coherent q-AFM and q-FM modes via conventional impulsive stimulated Raman scattering (ISRS), with frequencies at 167 GHz and 21.5 GHz, respectively. Resonant excitation of the 2M mode, however, markedly enhanced amplitudes—up to fivefold for q-AFM and 3.5-fold for q-FM—while renormalizing frequencies: a 4% blue shift for q-AFM and 3% red shift for q-FM at 6 mJ/cm² fluence.
Experiments varying duty cycles ruled out thermal origins, as frequency and amplitude changes persisted despite reduced average heating. Polarization studies aligned with nonthermal ISRS-like behaviors, such as phase reversals. Fluence dependencies showed linear trends for resonant cases, contrasting negligible shifts off-resonance.
Two-Magnon Resonant Raman Scattering
To explain these effects, the researchers proposed two-magnon resonant Raman scattering (2MRRS), where the 2M mode acts as a resonant intermediary. In this framework, light absorption by the 2M excitation—described by a spin-correlation function—nonlinearly couples to gamma-point modes, enhancing Raman efficiency via inverse Faraday and Cotton-Mouton effects.
Analytical theory beyond linear spin-wave approximations predicted shifts but with opposite signs, prompting atomistic spin dynamics simulations. Using an ab initio-parametrized Hamiltonian, simulations mimicked high-momentum excitation with a fictitious staggered field, reproducing the observed blue and red shifts. In contrast, gamma-point pumping yielded mismatched results, underscoring the role of high-momentum modes.
As corresponding author Davide Bossini notes in the study, this coupling "results in the amplification of the gamma-point magnons" through nonlinear interactions across momentum space.
Promising Pathways
The findings highlight a novel route for optically controlling magnetic spectra, potentially enabling coherent instabilities or room-temperature magnon Bose-Einstein condensation in antiferromagnets. Exploring similar drives in cuprates could offer insights into light-manipulated superconductivity.