Photoinduced Heating and Phase Transition in CMR Materials
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Photoinduced Heating and Phase Transition In CMR Materials N. Noginova, E. Arthur, R. Ulysse, E. S. Gillman, C. E. Bonner Center for Materials Research, Norfolk State University, Norfolk, VA 23504 ABSTRACT Fast switching from conductor to insulator induced by laser light illumination has been studied in single crystals and thin films of La1-xSrxMnO3 in the range of the ferromagnetic phase transition. Based on our experimental data on the photoresponse as a function of temperature, electric current, and intensity, we have demonstrated that the switching and relaxation processes are determined by heating and heat conduction processes. The relaxation time, specific heat and the latent heat constants have been estimated.
INTRODUCTION As known, colossal magnetoresistance (CMR) materials1 exhibit a sharp transition from a paramagnetic insulator to a ferromagnetic metal at room temperature range. The charge transport in these manganites is strongly dependent on the mutual orientation of the Mn ions, thus providing the opportunity to control the transport with magnetic field or temperature. It has been shown, CMR materials exhibit a strong response in the electric conductivity to laser light illumination 2, 3. The response in the conductivity is mainly related to the change of the resistance due to the heating of the sample through absorption and redistribution of the pulse energy. The relaxation times in this case are determined by heat conduction processes4. However, it should be noted that oxygen-deficient CMR materials demonstrate an additional contribution to the response, with very long relaxation times, τ > 1 s, related to the increase of the carrier concentration due to photoionization of electron-hole pairs and trapping of electrons in oxygen vacancies5.
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In the case of heating only, the temporal behavior of the resistive response, ∆R, can be described as ∆R(t) = R(T(t)) – R(T0), where R is temperature dependent resistance, T0 is the temperature of the sample before the laser pulse absorption, T is the current temperature. If the sample is below the transition temperature, Tc, heating of the material leads to the increase of the resistance, and the response to pulsed laser light is positive. In the opposite case, when T0> Tc,, the resistance decreases with the temperature increase, and the response has a negative sign. The temporal profile of the response is determined by the relaxation of the temperature to T0, as well as the R(T) dependence, in particular in the case when the photoinduced change in temperature is high4. At low laser power, the amplitude of the resistive response is approximately proportional to the dR/dT. In this case, the temporal profile of the response relaxation is determined by the temperature relaxation only, providing an opportunity to determine the thermal conductivity, specific heat and latent heat constants through the study of the response relaxation in the vicinity of the phase transition.
EXPERIMENTAL CMR single crystals La0.75Sr0.25MnO3 were fabricated by the floating zon
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