Anomalous magnetic field dependence of T AF in La 0.95 Sr 0.05 MnO 3

The magneto-elastic properties of single-crystalline La_0.95Sr_0.05MnO₃ have been studied ultrasonically. Our investigations focussed on the temperature interval where magnetic ordering starts to evolve and results in a spin canted antiferromagnetic ground state. In detail the experiments revealed that the magnetic order parameter in low-doped manganite is only weakly coupled to lattice strains. Furthermore, the anomalous temperature dependence of the order parameter as found resembles highly that in stoichiometric LaMnO₃. However, the main and most surprising finding is that external magnetic fields favor the spin canted phase in La_0.95Sr_0.05MnO₃. It is unclear at present how the exchange interaction can be tuned by magnetic fields in the way observed and we are not aware of existing theoretical concepts which might give a plausible explanation for the unexpected field dependent behavior of the critical temperature. We believe, however, that this behavior primarily results from the fact that the exchange interaction depends sensitively on the orbital configuration of the manganese d electrons. Received 27 March 2000