Antiferromagnetism, ferromagnetism, and magnetic phase separation in Bi₂Sr₂CoO_6+δ

We present results of a study of the magnetism in Bi₂Sr₂CoO_6+δ, the Co analog of the high T_c superconductor Bi₂Sr₂CuO_6+δ. This system evolves from an antiferromagnetic (AF) insulator to an unusual ferromagnetic (FM) insulator as δ is reduced from ∼0.5 to ∼0.25. When δ is close to 0.5, the Co ions have formal oxidation state 3+ and order antiferromagnetically at TN∼250 K. The δ∼0.25 crystal has equal numbers of Co²⁺ and Co³⁺ and exhibits FM behavior with a moment ∼1.5μ_b/Co at 5 T and a Curie temperature T_c ∼100 K. 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 0.25<δ<0.5, both FM and AF Bragg peaks are observed with neutrons, above a critical field H_c. 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 δ is measured for samples with 0.25<δ<0.5. We discuss this behavior in the context of phase separation resulting in a hole rich, Co³⁺ AF phase and a hole poor, Co²⁺-Co³⁺ 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.