Complex ferromagnetic state and magnetocaloric effect in single crystalline Nd0.7 Sr0.3 MnO3

The magnetocaloric effect in single crystalline Nd0.7 Sr0.3 MnO3 is investigated by measuring the field-induced adiabatic change in temperature (Δ Tad) which reveals a single negative peak around 130 K well below the Curie temperature (TC =203 K). In order to understand this unusual magnetocaloric effect, we invoke the reported Mn55 spin-echo nuclear magnetic resonance, electron magnetic resonance and polarized Raman scattering measurements on Nd0.7 Sr0.3 MnO3. We show that this effect is a manifestation of a competition between the double exchange mechanism and correlations arising from coupled spin and lattice degrees of freedom which results in a complex ferromagnetic state. The critical behavior of Nd0.7 Sr0.3 MnO3 near Curie temperature is investigated to study the influence of the coupled degrees of freedom. We find a complicated behavior at low fields in which the order of the transition could not be fixed and a second-order-like behavior at high fields. © 2007 The American Physical Society.