Studies On colossal magnetoresistance

 La1-xCaxMnO3 (x = 0.25, 0.33 AND 0.45) by ultrasound method


Nor Azah Nik Jaafar, R. Abd-Shukor

School of Applied Physics

Universiti Kebangsaan Malaysia

43600 Bangi, Selangor, Malaysia




La1-xCaxMnO3 which exhibits colossal magnetoresistance (CMR) effect for x = 0.25, 0.33 and 0.45, was subjected to ultrasound investigations. For all samples, temperature-dependent electrical resistivity measurements showed a metal-insulator transition identified by a peak in resistivity, typical transport behaviour of a CMR material. The x = 0.25 sample showed the highest TIM at 260 K, x = 0.33 at 240 K and x = 0.45 at 150 K. For x = 0.45 sample, a large resistivity hysteresis occurs between 90 to 190 K. Temperature-dependent sound velocity measurements (80 to 300 K) showed a large change in longitudinal velocity of ~6 % takes place just below TIM (insulator-metal transition) for x = 0.25 and 0.33 samples. The sudden increase in Dv was also observed in shear mode for both samples. These frequency hardenings which take place below Tc, indicate the prominent role of lattice vibrations on the physical properties of this material. A characteristic behaviour of ultrasonic attenuation, which decrease sharply in the region below Tc, accompanied the anomalous phonon hardenings. These features, which are closely related with the mechanisme at Tc, have been previously predicted from theoretical calculations based on the combined double-exchange and lattice polaron model. Elastic measurements for x = 0.45 sample showed a completely different behaviour compared to the x = 0.25 and 0.33 samples. This sample showed the largest Dv for both modes within 80 to 210 K measurement range (>10 %). Large thermal hysteresis between 130 to 190 K observed in both shear and longitudinal velocity measurements, were attributed to simultaneous occurrence of conducting ferromagnetic state and charge-ordered state.






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