Electron spin resonance in the doped spin-Peierls compound Cu1-xNixGeO3

Electron spin-resonance studies of the Ni-doped spin-Peierls compound CuGeO3 have been performed for the frequency range 9-75 GHz and temperature interval 1.3-20 K. An anomalous temperature dependence of the g-factor below the spin-Peierls temperature was observed for doped samples. At low temperatures the g-factor is much smaller than the value expected for Cu2+ and Ni2+ ions and is much more anisotropic than for an undoped crystal. This anomaly is explained by the formation of magnetic clusters around the Ni2+ ions within a nonmagnetic spin-Peierls matrix. The formation of magnetic clusters is confirmed by the observation of a nonlinear static magnetic susceptibility at low temperatures. The reduction of the spin-Peierls transition temperature was found to be linear in the dopant concentration x over the range 0 less than or equal to x less than or equal to 3.2%. The transition into the antiferromagnetically ordered state, detected earlier by neutron scattering for x greater than or equal to 1.7%, was studied by means of ESR. For x = 3.2% a gap in the magnetic resonance spectrum is found below the Neel temperature and the spectrum is well described by the theory of antiferromagnetic resonance based on the molecular-field approximation. For x = 1.7% the spectrum below the Neel point remained gapless. The gapless spectrum of the antiferromagnetic state in weakly doped samples is attributed to the small value of the Neel order parameter and to the magnetically disordered spin-Peierls background.