Electroresistance and electronic phase separation in mixed-valent manganites.

Colossal magneto-resistance (CMR) - a huge decrease in resistivity by the application of magnetic field observed in mixed-valent manganites - has triggered intense scientific activity in recent years; yet, the mechanism of the effect is still not fully understood. One aspect of the phenomenon is a bulk metal to insulator transition tuned by changing spin correlations, but possibly involving other degrees of freedom also. Recent studies however suggest that the largest magnetoresistance in these systems is associated with spatial inhomogeneity related to multiphase coexistence. Multiphase coexistence generically causes a sensitivity of physical properties to external perturbations. We examine the sensitivity to external fields in the case of CMR manganites using PZT-ferroelectric or SrTiO₃-dielectric based field effect configurations with La_0.7Ca_0.3MnO₃ (LCMO), Na_0.7Sr_0.3MnO₃ (NSMO), and La_0.7Ba_0.3MnO₃ (LBMO) channels, subjected to electric and magnetic fields, separately and in conjugation. We find that in our device, modest electric fields (~ 4 x 10^5 V/cm) cause very large changes (~75%) in resistivity in the case of LCMO, but the magnitude of the effect is much smaller (a few percent) in the case of NSMO and LBMO. We argue that these results support a percolative phase separation picture of transport in the LCMO film, with the insulating and metallic domains evenly balanced, while the NSMO and LBMO films are preponderantly insulating and metallic, respectively.

T. Wu†, S. B. Ogale*, J. E. Garrison, B. Nagaraj, Amlan Biswas, Z. Chen, R. L. Greene, R. Ramesh and T. Venkatesan, A. J. Millis, Phys. Rev. Lett. 86, 5998 (2001)

Dependence of resistivity of the LCMO channel on temperature, for field biasing with a PZT gate over an applied gate voltage range from +6 to -6 volts. Inset shows the device configuration.

Dependence of LCMO channel resistivity on temperature for the unbiased (A) and electric field-biased (B, 4x10^5 V/cm) channel, in the absence of magnetic field. The dependence (A) changes to (C), and (B) changes to (D) under a magnetic field of 6 T. The insets show MR Vs T in 6 T for the unbiased and E-field-biased CMR channel, and ER Vs T for the channel with and without a H field of 6 T.

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