Antiferromagnetism, ferromagnetism, and magnetic phase separation in (formula presented)

We present results of a study of the magnetism in (formula presented) the Co analog of the high (formula presented) superconductor (formula presented) This system evolves from an antiferromagnetic (AF) insulator to an unusual ferromagnetic (FM) insulator as (formula presented) is reduced from (formula presented) to (formula presented) When (formula presented) is close to 0.5, the Co ions have formal oxidation state 3+ and order antiferromagnetically at (formula presented) The (formula presented) crystal has equal numbers of (formula presented) and (formula presented) and exhibits FM behavior with a moment (formula presented) at 5 T and a Curie temperature (formula presented) (formula presented) Single crystal neutron scattering (both polarized and unpolarized), magnetization, and resistivity measurements have been used to characterize the evolution of the magnetic and transport properties between these two doping limits. For crystals with (formula presented) both FM and AF Bragg peaks are observed with neutrons, above a critical field (formula presented) Field-dependent neutron diffraction measurements confirm that the FM peaks result from ferromagnetic domains, which coexist with antiferromagnetic domains, and have a net moment above the critical field. The suppression of Néel order and accompanying increase in the volume of the FM domains with decreasing (formula presented) is measured for samples with (formula presented) We discuss this behavior in the context of phase separation resulting in a hole rich, (formula presented) AF phase and a hole poor, (formula presented) FM phase. In addition, the rich phenomenology of the interacting magnetic domains can be explained by mapping to a form of the random field Ising model.