\Bibitem{Smi06} \by A.~I.~Smirnov \paper Magnetic resonance modes in spin-gap magnets \jour UFN \yr 2006 \vol 176 \issue 6 \pages 673--676 \mathnet{http://mi.mathnet.ru/ufn330} \crossref{https://doi.org/10.3367/UFNr.0176.200606k.0673} \adsnasa{https://adsabs.harvard.edu/cgi-bin/bib_query?2006PhyU...49..649S} \transl \jour Phys. Usp. \yr 2006 \vol 49 \issue 6 \pages 649--652 \crossref{https://doi.org/10.1070/PU2006v049n06ABEH006077} \scopus{https://www.scopus.com/record/display.url?origin=inward&eid=2-s2.0-33750416689}