SPIN FLUCTUATIONS AT MAGNETIC PHASE-TRANSITIONS IN CMR MANGANITES

We present the results of an investigation into the nature of magnetic phase transitions in CMR manganites, using small-angle neutron scatte ring and single-crystal neutron and X-ray diffraction. In the perovski te compound, Nd0.5Sr0.5MnO3, we observe a second-order phase transitio n at 250 K to a ferromagnetic phase characterized by a divergent corre lation length and conventional critical exponents. At 155 K, a first-o rder phase transition to an antiferromagnetic phase is observed with e vidence for both magnetic and orbital ordering. In the layered mangani te, La1.2Sr1.8Mn2O7, the measured correlation length is only weakly te mperature dependent above T-c, but may result from nuclear rather than magnetic correlations. Published by Elsevier Science B.V.