Fragile magnetic ground state in half-doped LaSr₂Mn ₂O₇

We investigated the orbital and antiferromagnetic ordering behaviors of the half-doped bilayer manganite La_2-2xSr_1+2xMn ₂O₇ (x < 0.5) by using Mn L_2,3-edge resonant soft x-ray scattering. Resonant soft x-ray scattering reveals the CE-type orbital order below T_oo220K, which shows partial melting behavior below T_m165K. We also found coexistence CE- and A-type antiferromagnetic orders. Both orders involve the CE-type orbital order with nearly the same orbital character and are coupled with each other. These results manifest that the ground state with the CE-type antiferromagnetic order is easily susceptible to destabilization into the A-type one even with a small fluctuation of the doping level, as suggested by the extremely narrow magnetic phase boundaries at x < 0.5±0.005.