Unexpected Intermediate State Photoinduced in the Metal-Insulator Transition of Submicrometer Phase-Separated Manganites

At ultrafast timescales, the initial and final states of a first-order metal-insulator transition often coexist forming clusters of the two phases. Here, we report an unexpected third long-lived intermediate state emerging at the photoinduced first-order metal-insulator transition of ${\mathrm{La}}_{0.325}{\mathrm{Pr}}_{0.3}{\mathrm{Ca}}_{0.375}{\mathrm{MnO}}_3$, known to display submicrometer length-scale phase separation. Using magnetic force microscopy and time-dependent magneto-optical Kerr effect, we determined that the third state is a nanoscale mixture of the competing ferromagnetic metallic and charge-ordered insulating phases, with its own physical properties. This discovery bridges the two different families of colossal magnetoresistant manganites known experimentally and shows for the first time that the associated states predicted by theory can coexist in a single sample.

Figure 1

(a) Sketch of the MFM setup and geometry of the device for transport measurements. (b) Two resistivity measurements with different light intensities at 145 K, separated by a thermal cycle to room temperature (the photoinduced changes in $\rho$ are persistent and irreversible, unless a thermal cycle is performed). Green dashed lines denote the switch on-off times. Inset: $\rho$ vs temperature upon warming, with red dots denoting the temperatures where experiments were conducted. (c) Light-intensity dependent $\rho$ measured within a single thermal cycle at 130, 145, and 160 K. The lighting process is in the upper panel.

Figure 2

(a)–(c) $15\mu \mathrm{m}\times 15\mu \mathrm{m}$ MFM images acquired at 130, 145, and 160 K. The scanning area is fixed at each temperature and its displacement between different temperatures is within $1\mu \mathrm{m}$ (for topography see Fig. S2 [35]). Left to right, the light intensity increases from 0 to $4.28\mathrm{W}{\mathrm{cm}}^{-2}$. Blue and red regions are the COI and FMM phases, respectively. The MFM signal (phase shift) ranges are the same for each temperature but tuned to a proper level at different temperatures for better presentation (2.8°, 2.6°, and 1.8° for 130, 145, and 160 K, respectively). Note that the MFM signal is negative (positive) for the magnetic (nonmagnetic) phase right above the LPCMO film (see Method section in [35]). (d)–(e) Statistics from the MFM images vs light intensity at different temperatures: both the area fraction of COI state (d) and average size of COI domains (e) increase rapidly at some critical light intensities (arrows). Error bars are standard (see Sec. II in [35]).

Figure 3

(a) Line profiles (lower panel) obtained from the MFM images at 145 K with $3.16\mathrm{W}{\mathrm{cm}}^{-2}$ light intensity along the lines indicated in the upper panel, showing the coexistence of three states: COI (blue), third state (white), and FMM (red). Line profiles ($1.5\mu \mathrm{m}$ long) are cut across two FMM domains (1 and 2) and two third state domains (3 and 4). The COI phase is shown at the two ends of every line profile (1–4). The MFM signal range is the same as in Fig. 2. (b) Histograms of the MFM signal obtained from the MFM images at 145 K, evolving from a bimodal to a trimodal distribution with increasing light intensity. The emerging third peak corresponds to the third state (green triangles), while the FMM and COI peaks are the red and blue triangles, respectively. The positions of the triangles are determined by multipeak Gaussian fitting. (c) Area fraction of all three states: FMM, third, and COI states vs light intensity for five temperatures, illustrating the mediatory role of the third state.

Figure 4

MOKE measurements before (a) and after (b) photoexcitation at 145 K. Time resolution is 30 ms and the light intensity is $4.68\mathrm{W}{\mathrm{cm}}^{-2}$, where the intermediate state is prominent. Both in (a) and (b), dashed lines are the Kerr intensity levels for zero (demagnetized state), for $M_{650}$, and for $M_R$ [$M_{650}$ and $M_R$ are the Kerr intensities at 650 and 0 Oe, respectively, consistent with those determined from the magnetic hysteresis loops in the insets of (a) and (b)]. After switching off the external magnetic field, the Kerr intensity falls rapidly from $M_{650}$ to $M_R$. Having reached $M_R$, the Kerr intensity is stable before photoexcitation (a) and clearly decays increasing time after photoexcitation (b). The latter is a typical superparamagnetic behavior displayed by nanoscale FMM domains, indicating that the intermediate state is a nanoscale mixture of FMM and COI phases. Kerr intensity was normalized to one for better presentation.

Figure 5

(a) Theoretically predicted generic phase diagram when FMM and COI states compete [53]. The expected micrometer-scale CMR1 and nanometer-scale CMR2 phenomena, see text, are shown. $g$ represents a generic variable needed to transfer the system from one phase to the other [53], such as the tolerance factor. (b) Magnetic field $H$ evolution of $\rho$ vs temperature for CMR1, from the COI to the FMM, involving an abrupt first-order transition and concomitant micrometer-scale phase separation. (c) Same as (b) but for CMR2, involving a percolative process and nanometer-scale phase separation.