Intralayer and interlayer exchange tuned by magnetic field in the bilayer manganite (La_0.4Pr_0.6)_1.2Sr_1.8Mn₂O₇ probed by inelastic neutron scattering

The bilayer manganite La_1.2Sr_1.8Mn₂O₇ (LSMO) exhibits a phase transition from a paramagnetic insulating (PI) to a ferromagnetic metallic (FM) state with a colossal magnetoresistance (CMR) effect. Upon 60% Pr substitution, both the magnetic order and the PI to FM transition are suppressed. However, application of a magnetic field induces a first-order transition to a FM state with a CMR effect. We report here inelastic neutron scattering from a single crystal of (La_0.4Pr_0.6)_1.2Sr_1.8Mn₂O₇ under a magnetic field of 3.7 T, applied along the c axis and along the [110] direction. Our data reveal the in-plane exchange J_ab close to that of the Pr-free compound LSMO although the interlayer exchange J_c is much smaller than that found in LSMO. Anisotropic lattice distortion induced by the smaller size of Pr³⁺ ion is considered to explain these results.