Magnetic resonance in quantum spin chains with competing exchange interactions

Based on a previous review on magnetic resonance in quantum spin chains [H.-A. Krug von Nidda, N. Büttgen, and A. Loidl, Eur. Phys. J. Special Topics 180, 161–189 (2010)] we report on further development in this field with special focus on transition–metal oxides and halogenides consisting of quasi one– dimensional spin systems, where both intra– and inter–chain exchange interaction may give rise to frustration effects and higher–order anisotropic exchange contributions like the Dzyaloshinskii–Moriya interaction become decisive for the formation of the magnetic ground state. Selected examples show how NMR and ESR contribute valuable information on the magnetic phases and exchange interactions involved: LiCuVO4 with competing nearest neighbour and next–nearest neighbour intra–chain exchange, LiCu2O2 with complex zig–zag chains, and Cs2CuCl4 where the chains form a triangular lattice with the inter–chain interaction weaker but of the same order of magnitude than the intra–chain interaction. The so called paper–chain compound Ba3Cu3In4O12, where each successive pair of CuO4 plaquettes is rotated by 90° with respect to its predecessor along the c–direction like in a paper–chain, provides an interesting topology of frustrated intra–chain exchange interactions. Finally, a few dimer systems are considered.