Uncharacteristic phase separation trends with the ionic size in cobaltites

Using elastic neutron scattering on single crystals of La_1−xA_xCoO₃ (A=Ca²⁺, Sr²⁺, and Ba²⁺), we found the development of magnetic superstructures below the global magnetic transition to be strongly dependent on the size of the A-site dopant, ⟨r_A⟩, in an unusual way. Upon reducing the ⟨r_A⟩ (i.e., as with Ca doping), only a commensurate ferromagnetic cluster phase is evident. On expanding the ⟨r_A⟩, the tendency toward coexistence of competing ferromagnetic and antiferromagnetic orders increases giving rise to an inhomogeneous ground state. The antiferromagnetic ordered state, initially incommensurate, continuously strengthens and becomes commensurate with long-range order and a characteristic cubic wave vector of Q⃗_c=(0.25,0.25,0.25) with x. The two competing order parameters become comparable in magnitude indicative of the phase-separated nature of the cobalt perovskite system.