The Origin of Bulk Magnetoresistivity in Manganites

Abstract

On substitution of Ca2+ for La3+ in LaMnO3, the compound La1−xCaxMnO3 becomes a ferromagnetic metal for 0.18≤x<0.5. The hopping of an electron from Mn3+ to O2− is associated with simultaneous hop from the latter to Mn4+. The probability of the hopping by this double exchange is highest when both hopping electrons have the same spin orientation, requiring Mn ions to be ordered ferromagnetically. The metallic behavior is synergistically associated with the ferromagnetic order through electron hopping. The resistivity increases considerably near the Curie temperature (TC) and application of a magnetic field causes a dramatic decrease in resistivity due to regeneration of metallicity. The origin of this colossal magnetoresistivity is rather enigmatic and has attracted lot of attention from researchers. We have addressed this problem using emission Mössbauer spectroscopy where the local probe 57Co/ 57Fe substituting Mn atoms senses the microscopic behavior of the material. We find that the long-range ferromagnetic order breaks down anomalously below TC and the material degenerates into small spin clusters which fluctuate rapidly and which exhibit superparamagnetic-like behavior. On a time scale of 10−8 s, the spin clusters exhibit minimal lattice distortions. These spin clusters survive well above TC and would constitute very effective scattering centers for charge carriers due to their rapid fluctuations. On application of an external magnetic field, the small magnetic clusters coalesce to form larger ones with more ordered spins, and the material shows enhanced conductivity through percolation. We also observe superparamagnetic-like behavior below TC in the pyrochlore, Tl2Mn2(57Co)O7, which does not exhibit double exchange electron hopping. It seems that the breakup into small rapidly fluctuating ferromagnetic spin clusters near TC (exhibiting superparamagnetic-like behavior) is a prerequisite for observing bulk magnetoresistivity.