Spin-polarized tunneling spectroscopic studies of the intrinsic heterogeneity and pseudogap phenomena in colossal magnetoresistive manganite La_(0.7)Ca_(0.3)MnO_3

Abstract

Spatially resolved tunneling spectroscopic studies of colossal magnetoresistive (CMR) manganite La_(0.7)Ca_(0.3)MnO_3 (LCMO) epitaxial films on (LaAlO_3)_(0.3)(Sr_2AlTaO_6)_0.7 substrate are investigated as functions of temperature, magnetic field and spin polarization by means of scanning tunneling spectroscopy. Systematic surveys of the tunneling spectra taken with Pt/Ir tips reveal spatial variations on the length scale of a few hundred nanometers in the ferromagnetic state, which may be attributed to the intrinsic heterogeneity of the manganites due to their tendency toward phase separation. The electronic heterogeneity is found to decrease either with increasing field at low temperatures or at temperatures above all magnetic ordering temperatures. On the other hand, spectra taken with Cr-coated tips are consistent with convoluted electronic properties of both LCMO and Cr. In particular, for temperatures below the magnetic ordering temperatures of both Cr and LCMO, the magnetic field-dependent tunneling spectra may be quantitatively explained by the scenario of spin-polarized tunneling in a spin-valve configuration. Moreover, a low-energy insulating energy gap ~0.6 eV commonly found in the tunneling conductance spectra of bulk metallic LCMO at T → 0 may be attributed to a surface ferromagnetic insulating phase, as evidenced by its spin-filtering effect at low temperatures and vanishing gap value above the Curie temperature. Additionally, temperature-independent pseudogap (PG) phenomena existing primarily along the boundaries of magnetic domains are observed in the zero-field tunneling spectra. The PG becomes strongly suppressed by applied magnetic fields at low temperatures when the tunneling spectra of LCMO become highly homogeneous. These findings suggest that the occurrence PG is associated with the electronic heterogeneity of the manganites. The observation of lateral and vertical electronic heterogeneity in the CMR manganites places important size constraints on the development of highdensity nanoscale spintronic devices based on these materials.

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