Enhancement of the low-field magnetoresistance of La[sub 1-x-y]Ca[sub x]Sr[sub y]MnO[sub 3+d] at room temperature.

We have studied magnetoresistance of a series of La[sub 1-x-y]Ca[sub x]Sr[sub y]MnO[sub 3+d] samples with the Curie temperature (T[sub C]) close to room temperature. For these samples, the rhombohedral-orthorombic structural phase transformation temperature (T[sub s]) is slightly higher than T[sub C]. On cooling in zero magnetic field, we observe a rapid increase of resistivity at T[sub s]. 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. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]