Mossbauer spectroscopy of ferromagnetic oxygen-rich dicalcium cuprate Ca2CuO3+delta

Stoichiometric Ca2CuO3 qualifies as a low-dimensional antiferromagnet. Nonetheless, oxygenated sintered samples of high-purity Ca2CuO3+delta, delta congruent to 0.17, exhibited remarkable spontaneous ferromagnetic hysteresis at ambient conditions {[}1-3]. In this work, Mossbauer fitting of spectroscopic data on Ca2Cu0.985 Fe-57(0.015) O3+delta revealed large magnetic hyperfine fields at two sites, indicative of Fe III 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 Fe-57 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 Delta(EQ) = 0.3 mm/s, is assigned to in-grain substitution in dicalcium cuprate, while a small-intensity doublet, having Delta(EQ) = 2.2 mm/s and CS = 0.6 mm/s, is assigned to 57Fe(x)(II)Cu(1-x)O.