Mössbauer spectroscopy of CuO and its relevance to high-temperature superconductors

Mössbauer emission spectroscopy and magnetic susceptibility studies were performed on ${}_{}{}^{57}{}_{}{}^{}\mathrm{doped}$ antiferromagnetic CuO. Magnetic susceptibility data show that antiferromagnetic ordering occurs at ${\mathit{T}}_{\mathit{N}}$=225 K and that the Co dopant (10–100 ppm) does not influence the bulk magnetic properties of CuO. Mössbauer spectra recorded at T>244 K show a dominant quadrupole-split site and two minor impurity sites. At T=244 K, a line broadening of the dominant site is observed indicating the onset of local magnetic order. The hyperfine interaction parameters at T<${\mathit{T}}_{\mathit{N}}$ were determined. In particular, the sign and the asymmetry parameter η of the electric-field gradient (${\mathit{V}}_{\mathit{z}\mathit{z}}$) and the angle θ formed by ${\mathit{V}}_{\mathit{z}\mathit{z}}$ and magnetic hyperfine field were derived. We find η to be very small (η≤0.09) and 21°≤θ≤28°. The value of θ is in accord with neutron-diffraction results. The ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$ Mössbauer spectroscopy of CuO is discussed in relation to other copper oxides and Fe containing perovskites.