Spin-resonance modes of the spin-gap magnet ${\mathrm{TlCuCl}}_3$

Three kinds of magnetic-resonance signals were detected in crystals of the spin-gap magnet ${\mathrm{TlCuCl}}_3.$ First, we have found antiferromagnetic resonance absorption in the field-induced antiferromagnetic phase above the critical field $H_c.$ This resonance frequency is strongly anisotropic with respect to the direction of the magnetic field. The field dependence of the induced order parameter is determined. Second, we have observed the microwave absorption due to the transitions between the singlet ground state and the excited collective triplet states. Finally, the thermally activated resonance absorption due to the transitions between the spin sublevels of the triplet excitations was found. These sublevels are split by the crystal field and external magnetic field.