Electron spin resonance in the doped spin-Peierls compound

Abstract

Electron spin-resonance studies of the Ni-doped spin-Peierls compound 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 and ions and is much more anisotropic than for an undoped crystal. This anomaly is explained by the formation of magnetic clusters around the 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 %. The transition into the antiferromagnetically ordered state, detected earlier by neutron scattering for %, was studied by means of ESR. For x = 3.2% a gap in the magnetic resonance spectrum is found below the Néel 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 Néel point remained gapless. The gapless spectrum of the antiferromagnetic state in weakly doped samples is attributed to the small value of the Néel order parameter and to the magnetically disordered spin-Peierls background.