Spin fluctuations at magnetic phase transitions in CMR manganites

Abstract

We present the results of an investigation into the nature of magnetic phase transitions in CMR manganites, using small-angle neutron scattering and single-crystal neutron and X-ray diffraction. In the perovskite compound, Nd 0.5Sr 0.5MnO 3, we observe a second-order phase transition at 250 K to a ferromagnetic phase characterized by a divergent correlation length and conventional critical exponents. At 155 K, a first-order phase transition to an antiferromagnetic phase is observed with evidence for both magnetic and orbital ordering. In the layered manganite, La 1.2Sr 1.8Mn 2O 7, the measured correlation length is only weakly temperature dependent above T C, but may result from nuclear rather than magnetic correlations.