Magnetic structures ofY2SrCu0.6Co1.4O6.5andY2SrCuFeO6.5

Abstract

The nuclear and magnetic structures of two layered mixed copper transition-metal oxides ${\mathrm{Y}}_{2}{\mathrm{SrCu}}_{0.6}{\mathrm{Co}}_{1.4}{\mathrm{O}}_{6.5}$ and ${\mathrm{Y}}_{2}{\mathrm{SrCuFeO}}_{6.5}$ have been determined from time-of-flight neutron powder diffraction. Both compounds consist of apex-linked pyramidal Cu/M-${\mathrm{O}}_{5}$ double layers, which alternate with oxygen defective ${\mathrm{Y}}_{2}{\mathrm{O}}_{1.5}$ fluorite layers. The Co-containing compound has a simple magnetic structure with a Shubnikov group ${\mathrm{Ib}}^{\ensuremath{'}}{a}^{\ensuremath{'}}m.$ The magnetic moments $(2.2{\ensuremath{\mu}}_{B})$ of this compound are aligned along the crystallographic b axis with antiferromagnetic order between neighboring Cu/Co ions. In contrast, the Fe-analog possesses a complicated noncolinear magnetic structure ${(Pc}^{\ensuremath{'}}{c}^{\ensuremath{'}}n,$ with moments $m\ensuremath{\perp}c),$ which can also be considered as comprising two components with ${I}^{\ensuremath{'}}{\mathrm{bam}}^{\ensuremath{'}}$ and ${\mathrm{Ib}}^{\ensuremath{'}}{a}^{\ensuremath{'}}m$ symmetry. The combination of the two components results in an $82\ifmmode^\circ\else\textdegree\fi{}$ angle between the spins of Cu/Fe ions in neighboring Cu/Fe-${\mathrm{O}}_{5}$ planes within a given double layer. The approximate $90\ifmmode^\circ\else\textdegree\fi{}$ rotation of the moments results from a ferromagnetic interaction between the neighboring Cu/Fe ions along the c axis through the apical oxygen atoms. The ferromagnetic component may originate from the $\mathrm{Cu}{d}_{{z}^{2}}^{2}$-$\mathrm{O}2p$-$\mathrm{Fe}{d}_{{z}^{2}}^{1}$ type exchange interaction. The implication of such an interaction to related layered cuprates is also discussed. By symmetry analysis, the magnetic structure of the Co compound can be classified as a ${G}_{y}g$ type and the appearance of weak ferromagnetism can be attributed to the Dzyaloshinsky-Moria interaction which couples the ${G}_{y}g$ mode and ${F}_{z}f$ mode in the same representation of the $\mathrm{Ibam}$ space group.