Mossbauer study of Fe-doped spin-Peierls cuprates, CuGeO3

The crystallographic and the magnetic properties of the Fe-doped CuGeO3 system, Cu1-xGe1-yFex+yO3 (x, y = 0.00, 0.05), have been investigated by means of X-ray diffractometry, Mossbauer spectroscopy, and superconducting quantum interferometry. The structure of the sample systems is orthorhombic, and only the lattice parameter of the c-axis decreases slightly with increasing Fe concentration. The spin-Peierls transition temperature (T-sp) of the Fe-doped system shifts to lower temperature with increasing Fe concentration under the same magnetic field. The T-sp of the system with x, y = 0.05 shifts to lower temperature with increasing magnetic field, 12.11 K under 7 T and 12.53 K at 4T. The Mossbauer spectra of Cu0.95Ge0.95Fe0.1O3 show a superposition of a doublet due to Fe3+ ions at octahedral (O-h) Cu-sites and two Zeeman sextets due to Fe3+ ions at tetrahedral (T-sp) Ge-sites in the temperature range between 50 K and T-sp (= 12.5 K), which means that the Fe3+ ions at Cu-sites are in a paramagnetic state whereas the Fe3+ ions at Ge-sites are well coupled with the Cu2+ ions via superexchange interactions. The Mossbauer parameters show a prompt discontinuity near Tp, which can be attributed to dimerization and magnetic reordering. Furthermore, the Mossbauer spectra obtained below Tp show that the Cu-O(2)-Fe(O-h) bonds undergo a relaxation below 4.2 K; consequently, there is a magnetic phase transition in the vicinity of Tp, which suggests that the newly formed alternating antiferromagnet can be completely stabilized at temperatures lower than 4.2 K. The measured magnetic ordering temperature of the alternating antiferromagnet, composed of Cu-O(2)-Fe(Oh) bonds, is approximately 12 K, a little lower than T-sp.