Photoinduced Melting of Antiferromagnetic Order in ${\mathrm{La}}_{0.5}{\mathrm{Sr}}_{1.5}{\mathrm{MnO}}_4$ Measured Using Ultrafast Resonant Soft X-Ray Diffraction

We used ultrafast resonant soft x-ray diffraction to probe the picosecond dynamics of spin and orbital order in ${\mathrm{La}}_{0.5}{\mathrm{Sr}}_{1.5}{\mathrm{MnO}}_4$ after photoexcitation with a femtosecond pulse of 1.5 eV radiation. Complete melting of antiferromagnetic spin order is evidenced by the disappearance of a $(\frac{1}{4},\frac{1}{4},\frac{1}{2})$ diffraction peak. On the other hand, the $(\frac{1}{4},\frac{1}{4},0)$ diffraction peak, reflecting orbital order, is only partially reduced. We interpret the results as evidence of destabilization in the short-range exchange pattern with no significant relaxation of the long-range Jahn-Teller distortions. Cluster calculations are used to analyze different possible magnetically ordered states in the long-lived metastable phase. Nonthermal coupling between light and magnetism emerges as a primary aspect of photoinduced phase transitions in manganites.

Figure 1

(a) Energy dependence of the $(\frac{1}{4},\frac{1}{4},\frac{1}{2})$ diffraction peak intensity before (open circles) and 200 ps after photoexcitation (closed red circles) using a laser fluence of $5.5\mathrm{mJ}/{\mathrm{cm}}^2$. The lines represent smoothed fits to the data. (b) Energy dependence of the $(\frac{1}{4},\frac{1}{4},0)$ diffraction peak intensity before (open circles) and 200 ps after photoexcitation (closed red circles) using a laser fluence of $10\mathrm{mJ}/{\mathrm{cm}}^2$. The lines represent smoothed fits to the data.

Figure 2

(a) Time dependence of the $(\frac{1}{4},\frac{1}{4},\frac{1}{2})$ diffraction peak intensity recorded at $h\nu =640.25\mathrm{eV}$ with a laser fluence of $5.5\mathrm{mJ}/{\mathrm{cm}}^2$ (open squares) and the $(\frac{1}{4},\frac{1}{4},0)$ diffraction peak intensity recorded at $h\nu =641.5\mathrm{eV}$ (closed circles) with a laser fluence of $10\mathrm{mJ}/{\mathrm{cm}}^2$. (b) Fluence dependence of the $(\frac{1}{4},\frac{1}{4},\frac{1}{2})$ (open squares) and $(\frac{1}{4},\frac{1}{4},0)$ (closed circles) diffraction peak intensity 200 ps after photoexcitation. (c) Temperature dependence of the $(\frac{1}{4},\frac{1}{4},\frac{1}{2})$ diffraction peak intensity measured statically. Fluence and temperature dependences are aligned by calculating the sample temperature increase for each irradiation fluence. The red lines are guides to the eye.

Figure 3

(a) Time dependence of the $(\frac{1}{4},\frac{1}{4},\frac{1}{2})$ diffraction peak intensity measured at $h\nu =640.25\mathrm{eV}$ in low-$\alpha$ mode. The red line shows a fit to the data using an error function with a FWHM of 10 ps. (b) Energy dependence of the $(\frac{1}{4},\frac{1}{4},\frac{1}{2})$ diffraction peak intensity before (open circles with line) and 20 ps after photoexcitation (closed red circles with line). (c) Decay of the photoinduced state (closed circles), together with a double exponential fit (red line). The schematic diagrams for the free energy landscape depict the existence of a CE ground state (left side) and of a metastable state (spin-disordered or ferromagnetic) protected by a kinetic barrier.

Figure 4

(a) Schematic of charge, spin, and orbital order in ${\mathrm{La}}_{0.5}{\mathrm{Sr}}_{1.5}{\mathrm{MnO}}_4$ indicating the ferromagnetic zigzag chains which are antiferromagnetically coupled to each other. After laser photoexcitation, using 1.5 eV light, defects are created due to the transfer of $e_g$ electrons from the ${\mathrm{Mn}}^{3+}$ sites to the ${\mathrm{Mn}}^{4+}$ sites along the ferromagnetic zigzag chains. (b) Calculated energy dependence of the $(\frac{1}{4},\frac{1}{4},0)$ diffraction peak intensity for the ground state (black line) and for two types of metastable states: ferromagnetic alignment (red line) and a spin-disordered state (blue line).