Enhancement of the low-field magnetoresistance of La1-x-yCaxSryMnO3+d at room temperature

We have studied magnetoresistance of a series of La1-x-yCaxSryMnO3+d sample s with the Curie temperature (T-C) close to room temperature. For these sam ples, the rhombohedral-orthorombic structural phase transformation temperat ure (T-s) is slightly higher than T-C. On cooling in zero magnetic field, w e observe a rapid increase of resistivity at T-s. This increase is followed by a rapid decrease due to the ferromagnetic transition. The applied magne tic field significantly shifts the ferromagnetic transition to higher tempe ratures and suppresses the structure-related resistivity increase. We show that a combination of these two effects leads to an enhancement of the nega tive 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 te mperature. (C) 2001 American Institute of Physics.