Resonant optical control of the structural distortions that drive ultrafast demagnetization inCr2O3

Abstract

We study how the color and polarization of ultrashort pulses of visible light can be used to control the demagnetization processes of the antiferromagnetic insulator Cr2O3. We utilize time-resolved second harmonic generation (SHG) to probe how changes in the magnetic and structural state evolve in time. We show that, varying the pump photon-energy to excite either localized transitions within the Cr or charge transfer states, leads to markedly different dynamics. Through a full polarization analysis of the SHG signal, symmetry considerations and density functional theory calculations, we show that, in the non-equilibrium state, SHG is sensitive to both lattice displacements and changes to the magnetic order, which allows us to conclude that different excited states couple to phonon modes of different symmetries. Furthermore, the spin-scattering rate depends on the induced distortion, enabling us to control the timescale for the demagnetization process. Our results suggest that selective photoexcitation of antiferromagnetic insulators allows fast and efficient manipulation of their magnetic state.

Magnetic ground state ofSr2IrO4and implications for second-harmonic generation

“…In FeBO 3 , this leads to a change in the anisotropy of the probe polarization that has C 1 symmetry [38]. The laser can also generate changes in the symmetry of the lattice on this time scale, again due to the photodoped carriers, as demonstrated recently for Cr 2 O 3 [39]. There, the symmetry corresponded to an even parity mode, but coupling of excitations to an odd parity mode was recently suggested in Sr 2 IrO 4 [34] for an energy corresponding to the pump energy of Zhao et al [5], which would again lead to a C 1 distortion of the SHG signal.…”

Model of ultrafast demagnetization driven by spin-orbit coupling in a photoexcited antiferromagnetic insulator Cr2O3

“…Fitting the curves using kinetic rate equations, the rise time constant of the sum of the two doublet states are around 400 fs, which agrees well with the experiments by the time-resolved second harmonic generation. 24 The decay time of a photoexcited state strongly depends on the ratio of the energy gap to the electronphonon self-energy difference, which has been demonstrated in transition-metal complexes. 34 When the ratio ∆/ε ranges from 0.5 to 1.5, the fastest decay occurs.…”

“…We simulate the time evolution of the excited states following 1.8 eV and 3.0 eV light illumination and find that the decay times from the highspin quartet states to the low-spin doublet states range from 300 femtoseconds (fs) to 450 fs, in line with the experiments. 24 We show that the ratio of the energy gap to the electron-phonon self-energy has a marked impact on the demagnetization times. The decay times are also influenced by the hybridization of atomic orbitals in the first photoexcited state.…”

“…Owing to the symmetry of Cr 2 O 3 , there are seven Raman modes, two with A 1g symmetry and five with E g symmetry, and the longer wavelengths corresponding to the E g modes. 24 We take the E g mode valuehω = 0.075 eV, which dominates the relaxation at the 1.8 eV pumping, and A 1g modē hω = 0.065 eV, the main damping phonon at 3.0 eV photon excitation. 41 The 1.8 eV photoexcitation results in the transition from the 4 A 2 ground state to the 4 T 2 excited state.…”

“…[15][16][17][18][19][20][21][22][23] However, the ultrafast dynamic demagnetization processes were not probed until recently. 13,[24][25][26] The timeresolved second harmonic generation is applied to probe the time evolution of the magnetic and structural state following laser illuminations in the AFM insulator. Variations in the pump photon-energy lead to either localized transitions within the metal-centered states of the Cr ion or charge transfer between Cr and O.…”