Fragile Magnetic Ground State in Half-Doped Manganite LaSr2Mn2O7: Orbital Instability

Recently, a careful doping control study, however, reported that the La_2-2xSr_1+2xMn₂O₇ material has an exotic phase diagram very near the half doping with extremely narrow antiferromagnetic phase boundaries at x∼ 0.5 ± 0.005 -- the CE-type within the boundaries but the A-type outside.
ootnotetextQ. Li et al., Phys. Rev. Lett. 98, 167201 (2007). To understand a complexity on this material's phase diagram, we investigated the orbital and antiferromagnetic ordering behaviors of the half-doped bilayer manganite La_2-2xSr_1+2xMn₂O₇ (x ∼ 0.5) by using Mn L_2,3-edge resonant soft x-ray scattering. We confirmed the predicted CE-type antiferromagnetic order for the true half-doped (x = 0.5) case. Moreover, we found that such a narrow phase boundary is due to the close competition of the two antiferromagnetic ordering phases via 3d Mn e_g orbital instability. Our study reveals the spin and orbital orders of electrons in the sample as well as information about their ground states.
ootnotetextJ.-S. Lee et al., Phys. Rev. Lett. 107, 037206 (2011).