Nonlinear conductivity of resistive materials has been a popular topic of investigations over many years. During the past decade great interest was aroused by the dramatic electric-field-induced current switching in some charge-ordered manganites.¹ The thermal theory of non-ohmic conductivity, negative differential resistance and dielectric breakdown goes back over a century.² Nevertheless, most recent I-V characteristics were obtained using DC. Joule heating was supposedly ruled out by calculating the expected temperature increment or monitoring it close to the sample surface, while disregarding the inhomogeneous temperature and current distribution.

An old technique for preventing and confirming absence of Joule heating errors consists of applying pulsed currents and following the time dependence of the response on an oscilloscope. Joule heating is negligible, as long as the response remains independent on time. We investigated pulsed and DC I-V characteristics of various resistive oxides: polycrystalline and granular ferromagnetic manganites, double perovskites, charge ordered manganites and related materials (see Reference 3). The DC I-V characteristics were measured up to the negative resistance regime. In most cases the DC characteristics mask a perfectly ohmic or a moderately non-ohmic conductivity obtained by pulsed measurements. This demonstrates that the widely used DC I-V measurements in the high current regime are usually misleading. A review of the results obtained for the non-ohmic regime of the systems investigated will be presented.

1. for a brief review see: S. Mercone et al, J. Appl. Phys. 98, 23911 (2005).

2. H. Froehlich, Rep. Prog. Phys. 6, 411 (1939); D. M. Kroll, Phys. Rev, B 9, 1669 (1974) and references therein.

3. B. Fisher et al, Appl. Phys. Lett. 88, 152103 (2006).

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