Melting of magnetic correlations in charge-orbital ordered La 1 / 2 Sr 3 / 2 MnO 4 : Competition of ferromagnetic and antiferromagnetic states

The magnetic correlations in the charge and orbital ordered manganite ${\text{La}}_{1/2}{\text{Sr}}_{3/2}{\text{MnO}}_{4}$ have been studied by elastic and inelastic neutron scattering techniques. Out of the well-defined charge exchange (CE)-type magnetic structure with the corresponding magnons, a competition between CE-type and ferromagnetic fluctuations develops. Whereas ferromagnetic correlations are fully suppressed by the static CE-type order at low temperature, elastic and inelastic CE-type correlations disappear with the melting of the charge-orbital order at high temperature. In its charge-orbital disordered phase, ${\text{La}}_{1/2}{\text{Sr}}_{3/2}{\text{MnO}}_{4}$ exhibits a dispersion of ferromagnetic correlations, which remarkably resembles the magnon dispersion in ferromagnetically ordered metallic perovskite manganites.