Antiferromagnetism, ferromagnetism and magnetic phase separation in Bi₂Sr₂O₆₊[delta]

Abstract

Neutron scattering, magnetization and transport measurements were performed on single crystals of Bi2Sr2CoO6+[delta] to study the evolution of the magnetic properties as a function of the oxygen content . The oxygen content was varied by annealing single crystals in either a reducing or oxidizing environment to obtain an experimental range of 0.25 =/< [delta] =/< 0.5 and a corresponding average Co valence between +2.5 and +3. We show that the as-grown samples, which are oxygen rich ([delta approximately] 0.5) and therefore contain mostly Co3+ ions, enter an antiferromagnetic (AF) phase with a Neel temperature - 250 K. On the other hand, annealing as-grown crystals in vacuum to reach [delta approx.] 0.25 destroys the AF phase; these samples exhibit predominantly ferromagnetic (FM) behavior with Tc [approx.] 100 K. At intermediate doping, 0.25 < [delta] < 0.5, we find evidence for co-existence of FM and AF domains, which are characteristic of the [delta] = 0.25 and [delta] = 0.5 phases, respectively. The signature of the co-existence is the presence of simultaneous FM and AF magnetic Bragg peaks in the neutron diffraction pattern. Polarized neutron scattering measurements confirm that the FM and AF peaks do not arise from different components of a canted antiferromagnet.

(cont.) The FM regions give rise to a ferromagnetic-like peak in the susceptibility at the same temperature as the spins in the AF phase order. In addition, the FM regions exert a random field in the AF phase, above a critical field Hc. We explain the field dependence of the two-phase samples with a microscopic model. We propose that the FM clusters within the AF phase are the result of regions which are rich in Co2+. Furthermore, we suggest that oxygen facilitates the formation of electronically inhomogeneous regions.