Generality and origin of nanometer-scale phase separation in the paramagnetic window of mixed-valence perovskite manganites

Scanning tunneling spectroscopy on single crystals of the colossal magnetoresistive manganite Pr$_{0.68}$Pb$_{0.32}$MnO$_3$ revealed metallic clusters of 2–3 nm embedded in an insulating matrix, but only within the narrow temperature window between the Curie temperature ($T_C \approx 210$ K) and the metal-insulator transition ($T_{MI} \approx 255$ K), where the two transitions do not coincide. The authors left open whether this nanometer-scale phase separation is a generic property of mixed-valence perovskite manganites or specific to compounds with non-coincident $T_C$ and $T_{MI}$, and whether intrinsic electronic interactions or quenched A-site chemical disorder (and surface effects) produce it. The subsequent literature has not settled this core: bulk neutron pair-distribution-function analysis of La$_{1-x}$Ca$_x$MnO$_3$ finds no nanoscale phase coexistence away from $T_{MI}$ and questions its ubiquity; tunneling studies of La$_{0.7}$Ca$_{0.3}$MnO$_3$ films show different phenomenology (pseudogap physics and heterogeneity on larger scales); point-defect disorder is known to generate phase-separation-like inhomogeneity; and intrinsic (disorder-free) mechanisms have partial theoretical support. No systematic spatially resolved study across a controlled series of manganite compounds has been performed, so the generality question and the intrinsic-versus-disorder decomposition, as well as the microscopic link between polaron dynamics and phase separation, remain unresolved.

Background

Colossal magnetoresistive (CMR) manganites are mixed-valence perovskite oxides $R_{1-x}A_x$MnO$_3$ (with $R$ a rare earth such as La or Pr and $A$ a divalent alkaline earth such as Ca, Sr, or Pb), in which divalent substitution creates Mn$^{3+}$/Mn$^{4+}$ mixed valence. Their magnetoresistance — an enormous drop of electrical resistivity in modest magnetic fields near the metal-insulator transition — arises from the interplay of the double-exchange mechanism (ferromagnetic alignment of Mn spins enables itinerant $e_g$-electron hopping), Jahn-Teller lattice distortions of the MnO$_6$ octahedra, and electron-lattice coupling that can self-trap carriers as small polarons (a charge carrier plus its accompanying local lattice distortion, here confined to roughly one unit cell). Since the late 1990s, a central debate of the field has been whether the CMR transition proceeds through a spatially inhomogeneous, percolative state rather than a uniform phase transition: bulk magnetization and electron microscopy showed micrometer-scale coexistence of charge-ordered insulating and ferromagnetic metallic regions in (La,Ca)MnO$_3$-based compounds (Uehara et al., Nature 399, 560 (1999)), scanning tunneling microscopy (STM) on thin films found sub-micrometer inhomogeneity through the transition (Fäth et al., Science 285, 1540 (1999)), and theoretical work argued that intrinsic electronic tendencies toward phase separation (PS), amplified by arbitrarily weak quenched disorder near first-order phase boundaries, naturally produce such mixed states (Moreo, Yunoki, and Dagotto, Science 283, 2034 (1999)).

Scanning tunneling spectroscopy (STS) measures the local density of electronic states by recording the tunneling current versus voltage at each point of an STM scan, producing spatial maps of local conductance. Applying STS to cleaved single crystals of Pr$_{0.68}$Pb$_{0.32}$MnO$_3$ (PPMO), Rößler et al. (EPL 83, 17009 (2008)) found: (i) a polaronic insulating state above $T_{MI}$, with stripe-like and zigzag topographic features on a 0.4–0.5 nm scale indicating short-range ordering of one-site polarons; (ii) maps of the zero-bias conductance showing metallic clusters of 2–3 nm coexisting with insulating regions, with a bimodal conductance distribution, but only within the window $T_C \lesssim T < T_{MI}$; and (iii) homogeneous electronic maps deep in the ferromagnetic metallic state and at 300 K. PPMO is notable because its Curie temperature $T_C \approx 210$ K (where ferromagnetic order sets in) lies about 45 K below its metal-insulator transition $T_{MI} \approx 255$ K, so the intermediate window hosts a paramagnetic metal; the authors argued that the nanometer-scale PS is tied to this non-coincidence of $T_C$ and $T_{MI}$, with transport in the window occurring by percolation of the metallic clusters. They explicitly stated that it remains an open question whether this nanometer-scale PS is a generic property of mixed-valence manganites or is specific to PPMO and related non-coincident-transition compounds, noted that the pattern of local density-of-states inhomogeneity is certainly affected by disorder (random Pr$^{3+}$/Pb$^{2+}$ A-site substitution and the associated ionic-size mismatch, i.e. quenched chemical disorder) and by surface effects, and called for similar spatially resolved STS studies on other manganites. This is to be distinguished from the earlier micrometer-scale PS that persists deep into the metallic regime in other compounds: the PPMO phenomenon is nanometric, strictly temperature-limited, and its provenance — intrinsic electronic interaction versus quenched disorder versus surface-specific effects — was left undetermined. A related prior result is the atomic-scale STM observation of charge-ordering textures in another mixed-valence manganite (Renner et al., Nature 416, 518 (2002)), which established atomic-resolution imaging of such correlations but did not address the temperature-window question raised by the PPMO data.

Problem Statement

Determine whether nanometer-scale phase separation — metallic clusters of order 2–3 nm embedded in an insulating matrix, as observed by scanning tunneling spectroscopy on Pr$_{0.68}$Pb$_{0.32}$MnO$_3$ single crystals in the temperature window $T_C \lesssim T < T_{MI}$ between the Curie temperature and the metal-insulator transition — is a generic property of mixed-valence perovskite manganites, or is specific to Pr$_{0.68}$Pb$_{0.32}$MnO$_3$ and to compounds whose $T_C$ and $T_{MI}$ do not coincide. A complete answer must (1) establish, across a controlled series of mixed-valence manganite compounds (varying doping, A-site composition, and the separation $T_{MI}-T_C$), in which compounds the nanometer-scale metallic-cluster phase separation appears in the window $T_C \lesssim T < T_{MI}$, with which length scales and temperature extent, whether by theoretical and model analysis and synthesis of the existing literature or by new measurements; (2) determine the relative contributions of intrinsic electronic interactions (double exchange, polaron formation and short-range polaron ordering, magnetic correlations) versus quenched chemical disorder (random A-site substitution and ionic-size mismatch, quantified for example by the A-site size variance) in producing the clusters, their length scale, and the spatial pattern of the local density of states; and (3) control for surface effects (preparation, termination, and reconstruction) in identifying the bulk phenomenon. These three deliverables serve one common objective: to establish the origin — intrinsic, disorder-driven, or surface-related — and the degree of generality of the nanometer-scale phase-separated state that governs transport in the paramagnetic window of colossal magnetoresistive manganites.

References

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