Abstract
We have studied magnetoresistance of a series of La1−x−yCaxSryMnO3+d samples with the Curie temperature (TC) close to room temperature. For these samples, the rhombohedral–orthorombic structural phase transformation temperature (Ts) is slightly higher than TC. On cooling in zero magnetic field, we observe a rapid increase of resistivity at Ts. This increase is followed by a rapid decrease due to the ferromagnetic transition. The applied magnetic field significantly shifts the ferromagnetic transition to higher temperatures and suppresses the structure-related resistivity increase. We show that a combination of these two effects leads to an enhancement of the negative magnetoresistance due to strong spin-lattice coupling. Furthermore, by annealing the samples at appropriate temperatures in air, the enhancement can be optimized to appear in relatively low magnetic fields and at room temperature.
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