Femtosecond Dynamics of the Collinear-to-Spiral Antiferromagnetic Phase Transition in CuO

We report on the ultrafast dynamics of magnetic order in a single crystal of CuO at a temperature of 207 K in response to strong optical excitation using femtosecond resonant x-ray diffraction. In the experiment, a femtosecond laser pulse induces a sudden, nonequilibrium increase in magnetic disorder. After a short delay ranging from 400 fs to 2 ps, we observe changes in the relative intensity of the magnetic ordering diffraction peaks that indicate a shift from a collinear commensurate phase to a spiral incommensurate phase. These results indicate that the ultimate speed for this antiferromagnetic reorientation transition in CuO is limited by the long-wavelength magnetic excitation connecting the two phases.

Figure 1

Magnetic structures in CuO, projected onto the $(010)$ plane [17, 35]. Left: The magnetic structure of the CM phase; right: that of the ICM phase. Gray (red) arrows indicate the orientation of the magnetic dipoles.

Figure 2

Images of resonant x-ray diffraction. Upper panel: 1 ps before excitation. Lower panel: 100 ps after excitation with a femtosecond laser pulse at an incident fluence of $39\mathrm{mJ}/{\mathrm{cm}}^2$. The false-color scale for each image is normalized to better show the relative intensities.

Figure 3

(a) Time-dependent normalized diffracted intensity for an excitation fluence of $28\mathrm{mJ}/{\mathrm{cm}}^2$. Inset: Longer time-scale behavior. (b) Relative phase population. The curve is a fit described in the text. Inset: Dependence of the onset time $t_p$ on the excitation fluence. The curve is a guide to the eye. The dashed line shows the time for a $1/4$ period of a long-wavelength magnetic excitation in the CM phase (400 fs). See 33 for additional data.

Figure 4

Schematic of the transition. The ordinate is free energy and the abscissa is a configurational coordinate. Stronger excitation causes both a decrease in the relative energy difference between the ICM and CM phases and a decrease in the height of the energy barrier. At high excitation the change in configurational coordinate is a $1/4$ cycle of a low momentum spin wave.