X-ray photoinduced persistent and bidirectional phase transition enabled by impurity doping in layered manganite

We have found persistent and bidirectional x-ray photoinduced phase transition (PIPT) between the charge-orbital ordered and the ferromagnetic phase in a few percent Fe-doped layered manganite La${}_{0.5}$Sr${}_{1.5}$Mn${}_{1-x}$Fe${}_x$O${}_4$. Although partial x-ray PIPT has been observed in undoped manganite as well, the photoinduced effect has been confirmed to be much enhanced by impurity doping. The partial substitution of Mn ions with Fe ions was found to suppress the long-range charge-orbital order and to form a phase-separated ground state between the charge-orbital ordered and the ferromagnetic phases. Such bi-stability arising from competing phases plays an essential role in persistent and reversible PIPT phenomena.

Figure 1

Schematic of charge-orbital order and CE-type antiferromagnetic order for (a) $x$ $=$ 0 and (b) $x$ $=$ 0.03 in La${}_{0.5}$Sr${}_{1.5}$Mn${}_{1-x}$Fe${}_x$O${}_4$. Graphs of the temperature dependencies of (c) intensities for the (1/4 7/4 0) reflection as normalized by the (0 2 0) reflection, (d) correlation lengths of charge-orbital order phase, and (e) magnetizations for $x$ $=$ 0 and $x$ $=$ 0.03. (f) A graph of the magnetic field ($H\left|\right|a$) dependence of the magnetization at 10 K for $x$ $=$ 0 and 0.03 measured after the field-cool procedure with $H_{\mathrm{ann}}$ $=$ 2 T. (g) A graph of the magnetization curves after subtracting the paramagnetic component measured with a varying annealing magnetic field ($H_{\mathrm{ann}}$ $=$ 2, 5 and 7 T).

Figure 2

Graphs of the x-ray exposure time dependencies of (a) the intensities and (b) the correlation lengths for (1/4 7/4 0) reflections measured at 10 and 70 K in impurity doped La${}_{0.5}$Sr${}_{1.5}$Mn${}_{0.97}$Fe${}_{0.03}$O${}_4$. (c) A graph indicating the dependence of the intensity of (1/4 7/4 0) reflections on the total dose of incident photons measured with various photon fluxes.

Figure 3

(a)A graph of the x-ray exposure time dependence of the real parts of the magnetic ac susceptibility [$\Delta {\chi }^{\prime }\left(t\right)={\chi }^{\prime }\left(t\right)-{\chi }^{\prime }\left(0\right)$] measured at 10 and 70 K in impurity doped La${}_{0.5}$Sr${}_{1.5}$Mn${}_{0.97}$Fe${}_{0.03}$O${}_4$. The ac magnetic susceptibility was measured by a lock-in amplifier (LA) method at 1000 Hz with x-ray irradiation ($3.18\times {10}^{18}$ photons m${}^{-2}$s${}^{-1}$). The inset shows a schematic view of the experimental setting. (b) A graph representing the change of the COO intensity by x-ray irradiation with repeated temperature cycling between 10 and 70 K. X-ray irradiation was off during temperature variation.

Figure 4

Graphs representing the dependence of the COO intensities relative to the incident x-ray photon number in La${}_{0.5}$Sr${}_{1.5}$Mn${}_{0.97}$Fe${}_{0.03}$O${}_4$ at various temperatures in (a) the cooling process and (b) the warming process (see text for details). Graphs representative of the temperature dependence of the COO intensity at the initial state [$I\left(0\right)$] and the final state [$I\left(\infty \right)$], which were obtained by curve fitting of the temporal dynamics of x-ray PIPT for (c) the cooling process and (d) the warming process. Schematic free energy diagrams are shown for before (left) and after (right) x-ray irradiation at a temperature (e) below 35 K and (f) between 40 and 130 K, respectively.