Intrinsic Inhomogeneities in Manganite Thin Films Investigated with Scanning Tunneling Spectroscopy

Thin films of ${\mathrm{L}\mathrm{a}}_{0.7}{\mathrm{S}\mathrm{r}}_{0.3}{\mathrm{M}\mathrm{n}\mathrm{O}}_3$ on MgO show a metal insulator transition and colossal magnetoresistance. The shape of this transition can be explained by intrinsic spatial inhomogeneities, which give rise to a domain structure of conducting and insulating domains at the submicrometer scale. These domains then undergo a percolation transition. The tunneling conductance and tunneling gap measured by scanning tunneling spectroscopy were used to distinguish and visualize these domains.

Figure 1

Topographic STM images for LSMO and LCMO, $500\times 500{\mathrm{n}\mathrm{m}}^2$, at 87 K (LSMO) and 96 K (LCMO).

Figure 2

(a) Two different I-U characteristics for LSMO in metallic and insulating regions. (b)–(d) Conductance maps, $500\times 500{\mathrm{n}\mathrm{m}}^2$: black, insulating regions ($dI/dV<5.9\times {10}^{-3}\mathrm{n}\mathrm{A}/\mathrm{V}$; white, metallic regions ($dI/dV>5.9\times {10}^{-3}\mathrm{n}\mathrm{A}/\mathrm{V}$) for 87, 150, and 278 K. Area of insulating domains grows with increasing temperature.

Figure 3

Three-dimensional image of LCMO, $170\times 170{\mathrm{n}\mathrm{m}}^2$, at 186 K. The topographic STM image is shown with the coloring of the conductance map: black: insulating regions; red: conductive regions (max $0.021\mathrm{n}\mathrm{A}/\mathrm{V}$).

Figure 4

Electrical resistance (full line, transport), normalized tunneling resistance (circles, STS), and normalized network resistance (stars, simulation) as a function of temperature for LSMO (left scale). Additionally, the magnetization curve (dotted line) together with the relative part of the area of the metallic domains (squares) is shown.