The method of rare earth (RE) spectroscopic probe is known to deliver information on magnetic interactions and properties of magnetic materials. The use of high resolution Fourier transform spectroscopy enabled us to broaden substantially the possibilities of this method and to obtain a number of new results. In this lecture, the work of the author's group is summarized. We studied, mainly, the cuprates and nickelates related to high-Tc superconductors. Magnetic ordering, low dimensional magnetic correlations, the arrangement of magnetic moments of transition metal ions, and peculiarities of spin reorientation phase transitions were investigated. The case of a RE alone magnetic compound (LiHoF4) and that of coupled pairs of RE ions in a diamagnetic host are briefly mentioned.
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