Correlation length in La1−xCaxMnO3 (abstract)

Abstract

Small-angle neutron scattering (SANS) has been used to measure the ferromagnetic correlation length in La0.67Ca0.33MnO3. This is of interest in these materials because the transport mechanism responsible for colossal magnetoresistivity involves valence fluctuation on the Mn sites, and thus, spin fluctuations. There should then be a direct relationship between the magnetoresistivity and the spin–fluctuation correlation length. The magnetic SANS was determined by substracting 15 K data from the data at and above TC (250 K). When the quasistatic approximation is valid, the magnetic SANS measures the static equal-time two-spin correlation function. In an ideal isotropic ferromagnet, the fluctuations that drive the phase transition are the transverse (below TC) spin waves, which have a divergent correlation length at and below TC. Our measurements show that the magnetic scattering function for temperatures above TC is approximately Lorentzian. However, the extracted correlation length is anomalously small, starting at about 6 Å at 400 K and increasing only to about 11 Å at the nominal TC=250 K. The correlation length continues to increase below TC to about 14 Å at 225 K. Inelastic scattering measurements suggest that there are significant diffusive fluctuations near TC in addition to the spin waves. We are performing SANS measurements in a magnetic field to try to separate the correlation lengths of these two types of fluctuations.