Magnetic Resonance in Collective Paramagnets with Gapped Excitation Spectrum

In some magnets, despite the presence of a strong exchange interaction between magnetic ions, conventional magnetic ordering does not occur, but a collective paramagnetic state is formed. If, due to the particular architecture of the exchange bonds, the ground state turns out to be a singlet state and is separated from the triplet excited states by a gap in the energy spectrum, then this state persists down to T = 0. The spin dynamics of collective paramagnets with gap excitation spectrum (spin-gap magnets) at low temperatures can be described as the behavior of a dilute gas of triplet excitations. The application of a sufficiently strong magnetic field can close the gap in the spectrum, which leads to a gapless spin liquid state, or even to the unusual phenomenon of the formation of field-induced antiferromagnetism. The introduction of impurities into a spin-gap magnet leads to the formation of a paramagnetic center in the vicinity of a defect or exchange bonds randomly distributed in the lattice. This review presents the results of the study of several characteristic representatives of the class of quantum paramagnets with gapped excitation spectrum by the EPR spectroscopy method: a quasi-two-dimensional antiferromagnet (C 4 H 12 N 2 )Cu 2 Cl 6 and quasi-one-dimensional magnets of the spin tube, Cu 2 C l4 ⋅ H 8 C 4 SO 2 , and the spin ladder, (C 7 H 10 N) 2 CuBr 4 , types. It has been shown that the electron spin resonance spectra make it possible to find common features in the behavior of these systems: to detect and characterize the fine structure of the energy levels of triplet excitations, to detect multiparticle relaxation processes in a gas of triplet excitations, and to observe the excitation of spin waves in the field-induced antiferromagnetically ordered state. Individual features of different systems are revealed as well.