Magnetic resonance of defects in spin-Peierls and Haldane magnets

The electron spin resonance (ESR) study of defects in the spin gap magnets of the spin-Peierls type (CuGeO3) and of the Haldane type (PbNi2V2O8) are reported. The magnetic resonance spectra in the paramagnetic state reveal the multi-spin nature of defects originating near the impurity atoms. Below the Neel temperature of the impurity-induced antiferromagnetic phases a transformation of the ESR spectrum was found, indicating an antiferromagnetic resonance mode of spin precession. At small concentrations of the diamagnetic impurities the signals of antiferromagnetic resonance were found to coexist with the signal of the paramagnetic resonance indicating a microscopic separation of the magnetic phases.