Driving magnetic order in a manganite by ultrafast lattice excitation

Femtosecond midinfrared pulses are used to directly excite the lattice of the single-layer manganite La${}_{0.5}$Sr${}_{1.5}$MnO${}_4$. Magnetic and orbital orders, as measured by femtosecond resonant soft x-ray diffraction with an x-ray free-electron laser, are reduced within a few picoseconds. This effect is interpreted as a displacive exchange quench, a prompt shift in the equilibrium value of the magnetic- and orbital-order parameters after the lattice has been distorted. Control of magnetism through ultrafast lattice excitation may be of use for high-speed optomagnetism.

Figure 1

(a) Top view of the pump-probe scheme and scattering geometries for the orbital $\left(\frac{1}{4}\frac{1}{4}0\right)$ and the magnetic $\left(\frac{1}{4}\frac{1}{4}\frac{1}{2}\right)$ diffraction peaks, measured on a (110) surface normal crystal of La${}_{0.5}$Sr${}_{1.5}$MnO${}_4$. The Bragg planes are indicated as parallel lines; polarizations of the LCLS x-ray beam and the midinfrared light are horizontal and vertical, respectively. Also shown are the energy dependencies measured at beamline X1A2, National Synchrotron Light Source. (b) and (c) show the diffracted spots, recorded over 180×180 pixels of a fast CCD at LCLS. Images represent the average of 70 shots of the FEL.

Figure 2

(a) Time-resolved changes of the intensity of the scattered x-ray spot at the magnetic $\left(\frac{1}{4}\frac{1}{4}\frac{1}{2}\right)$ diffraction peak for vibrational excitation at 13.8 μm (midinfrared) and photodoping at 800-nm (near-infrared) pump wavelengths. Excitation fluences are 1.2 and 5 mJ/cm${}^2$, respectively. (b) Comparison of the melting of the orbital and magnetic order for 1.2-mJ/cm${}^2$ midinfrared excitation. The red lines are single exponential fits to the data, with time constants of 6.3 and 12.2 ps, respectively. (c) Transient change of the scattering angle 2θ and the width $w$ of the diffracted x-ray spot measured for the magnetic wave vector following the midinfrared excitation.

Figure 3

(a) Schematic of charge- and orbital-order pattern in the $ab$ plane of La${}_{0.5}$Sr${}_{1.5}$MnO${}_4$, together with the orbital-order unit cell (thick light-blue line). Displacements of Mn and O atoms associated with (b) the resonantly driven IR-active $B_{2u}$ mode and (c) the Raman-active $A_g$ mode driven through ionic Raman scattering are also shown. The latter mode relaxes the Jahn-Teller distortions to affect the exchange interaction.

Figure 4

Excitation of the lattice reduces the Jahn-Teller distortion as in Fig. 3b, resulting in an effective shift in the equilibrium value of the orbital- and magnetic-order parameters. The effective force on each order parameter is given by the projection of the gradient along each axis.