Magnetic order tuned by Cu substitution in Fe1.1−zCuzTe

Abstract

We study the effects of Cu substitution in Fe${}_{1.1}$Te, the nonsuperconducting parent compound of the iron-based superconductor, Fe${}_{1+y}$Te${}_{1\ensuremath{-}x}$Se${}_{x}$, utilizing neutron scattering techniques. It is found that the structural and magnetic transitions, which occur at $\ensuremath{\sim}$60 K without Cu, are monotonically depressed with increasing Cu content. By 10$%$ Cu for Fe, the structural transition is hardly detectable, and the system becomes a spin glass below 22 K, with a slightly incommensurate ordering wave vector of ($0.5\ensuremath{-}\ensuremath{\delta}$, 0, 0.5) with $\ensuremath{\delta}$ being the incommensurability of 0.02, and correlation length of 12 \AA{} along the $a$ axis and 9 \AA{} along the $c$ axis. With 4$%$ Cu, both transition temperatures are at 41 K, though short-range incommensurate order at (0.42, 0, 0.5) is present at 60 K. With further cooling, the incommensurability decreases linearly with temperature down to 37 K, below which there is a first-order transition to a long-range almost-commensurate antiferromagnetic structure. A spin anisotropy gap of 4.5 meV is also observed in this compound. Our results show that the weakly magnetic Cu has a large effect on the magnetic correlations; it is suggested that this is caused by the frustration of the exchange interactions between the coupled Fe spins.