The role of ESR in research of low-dimensional antiferromagnets

It is well known that any spin system with antiferromagnetic coupling shows quantum spin fluctuation, which is more dominant at very low temperatures rather than thermal fluctuation. This fluctuation becomes larger as the spin dimensionality reduces and the usual spin ordering is not expected in one-dimensional spin system even at zero Kelvin. Although, however, some materials show phase transitions with strange behaviors due to the quantum effect, spin ordering occurs for most materials usually due to weak interactions with adjacent chains or layers. On the other hand, ESR has played an important role for the investigation of such low dimensional spin systems. Frequency-field relation, line-width, line-shape and their temperature dependence or angular dependence for single crystals gives us much important information. Here we summarize some examples to which we have experimentally concerned.