We have carried out a systematic magnetic relaxation study, measured after applying and switching off a 5 T magnetic field to polycrystalline samples of La Ca MnO and Nd Sr MnO . The long time logarithmic relaxation rate (LTLRR), decreased from 10 to 150 K and increased from 150 to 195 K in La Ca MnO . This change in behavior was found to be related to the complete suppression of the antiferromagnetic phase above 150 K and in the presence of a 5 T magnetic field. At 195 K, the magnetization first decreased, and after a few minutes increased slowly as a function of time. Moreover, between 200 and 245 K, the magnetization increased throughout the measured time span. The change in the slope of the curves, from negative to positive at about 200 K was found to be related to the suppression of antiferromagnetic fluctuations in small magnetic fields. A similar temperature dependence of the LTLRR was found for the Nd Sr MnO sample. However, the temperature where the LTLRR reached the minimum in Nd Sr MnO was lower than that of La Ca MnO . This result agrees with the stronger ferromagnetic interactions that exist in Nd Sr MnO in comparison to La Ca MnO . The above measurements suggested that the general temperature dependence of the LTLRR and the underlying physics were mainly independent of the particular charge ordering system considered. All relaxation curves could be fitted using a logarithmic law at long times. This slow relaxation was attributed to the coexistence of ferromagnetic and antiferromagnetic interactions between Mn ions, which produced a distribution of energy barriers.


Physics, Astronomy, and Materials Science

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©2001 The American Physical Society

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Physical Review B - Condensed Matter and Materials Physics