An unusual magnetic structure in Sr2FeO3F and magnetic structures of K2NiF4-type iron(iii) oxides and oxide halides, including the cobalt substituted series Sr2Fe1−xCoxO3Cl

Abstract

An unusual k = [½ ½ ½] magnetic structure is reported in Sr₂FeO₃F at low temperature. The magnetic structures of LaBaFeO₄, LaSrFeO₄, LaCaFeO₄, Ca₂FeO₃Cl, Ca₂FeO₃Br, Sr₂FeO₃F, Sr₂FeO₃Cl, Sr₂FeO₃Br and solid solutions in the series Sr₂Fe_1−xCo_xO₃Cl have been examined and only Sr₂FeO₃F exhibits this structure. The La₂CuO₄-type spin arrangement dominates in all these phases, with antiferromagnetic ordering in the xy planes. The k = [½ ½ ½] magnetic phase in Sr₂FeO₃F occurs below 100 K and requires La₂CuO₄-type stacking across the SrO layers and La₂NiO₄-type stacking across the SrF layers. It has been possible to show that this phase grows in domains which have purely La₂CuO₄-type stacking at higher temperatures. In the Sr₂Fe_1−xCo_xO₃Cl series the La₂CuO₄-type magnetic structure in Sr₂FeO₃Cl and the La₂NiO₄-type structure in Sr₂CoO₃Cl coexist and structural and magnetic behaviour indicate phase segregation into cobalt- and iron-rich regions.