Spin-1/2 Heisenberg antiferromagnetic chains in the frustrated spinel lattice of GeCu₂O₄

One-dimensional quantum antiferromagnetism has been found in a tetragonal spinel oxide, GeCu₂O₄. Actually the temperature dependence of magnetic susceptibility exhibiting a broad maximum around 80 K can be well-described as that of S = 1/2 Heisenberg antiferromagnetic chains with J/k_B = 135 K. Cooperation of spin frustration caused by the triangular arrangement of the cupric ions at octahedral sites and the Jahn-Teller elongation of the CuO₆ octahedra along the c axis seems to lead to the formation of magnetically isolated CuO₂ chains made of edge-sharing CuO₄ squares. However, a small drop in susceptibility and also a sharp spike in specific heat suggesting the occurrence of long-range antiferromagnetic ordering are observed at 33 K. The effects of Si-for-Ge substitution and Mg-for-Cu substitution are also reported.