Moessbauer spectroscopy of ferromagnetic oxygen-rich dicalcium cuprate Ca2CuO3+δ

Stoichiometric Ca2CuO3 qualifies as a low-dimensional antiferromagnet. Nonetheless, oxygenated sintered samples of high-purity Ca2CuO3+δ, δ≅ 0.17, exhibited remarkable spontaneous ferromagnetic hysteresis at ambient conditions [1-3]. In this work, Moessbauer fitting of spectroscopic data on Ca2Cu0.98557Fe0.015 O3+δ revealed large magnetic hyperfine fields at two sites, indicative of FeIII charge state, combined with fairly large electric quadrupole interaction parameters, as well as two central quadrupole-split doublets. One sextet, having B=49 T, corresponds to the tetragonal copper ferrite spinel. The other sextet, with unusual B=42 T and greater relative intensity variability following annealing in vacuum and post-oxygenation treatments, allows us to assign it to 57Fe substitution in dicalcium cuprate grain surfaces. This also supports the tentative interpretation in [1] for the ferromagnetism in pristine phase as excess holes-driven double exchange. One of the central doublets, with δEQ= 0.3 mm/s, is assigned to in-grain substitution in dicalcium cuprate, while a small-intensity doublet, having δEQ= 2.2 mm/s and CS= 0.6 mm/s, is assigned to 57FexIICu1-xO