The spin relaxation of Yb³⁺-ions due to their coupling to antiferromagnetic spin waves existing in CuO₂ planes in YBa₂Cu₃O_6+y compound is reported. It is shown that it results in a strong temperature dependence of electron paramagnetic resonance (EPR) linewidth. The temperature dependence of EPR g-factor was also obtained and shows a good agreement with experimental data.

Distortion of the long-range antiferromagnetic order in the YBa₂Cu₃O_6+y is investigated by the electron paramagnetic resonance (EPR) measurements for y=0.1-0.4. In the case of the doping level y=0.2, 0.3 the EPR signal consists of narrow and broad lines, which we relate to formation of the charged domain walls. Our theoretical analysis of the inhomogeneous EPR broadening due to the local antiferromagnetic order distortion in domain walls is well consistent with experimental results for the case of coplanar elliptical domain walls.

We investigated the spin dynamics in the heavy fermion compounds YbRh₂Si₂ and YbIr₂Si₂. The contributions of the resonant and nonresonant parts of the total transverse magnetization to the electron spin resonance (ESR) parameters are analyzed for different orientations of the static and microwave magnetic fields. It is shown that at high temperatures, when the Kondo effect is absent, the nonresonant terms may be essential in the case of the perpendicular orientation of the static magnetic field to the crystal symmetry axis. In the presence of the Kondo effect the nonresonant parts do not make a significant contribution to the ESR parameters for any configuration of the magnetic fields.