Electron spin resonance of Ni-doped (formula presented) in the paramagnetic, spin-Peierls, and antiferromagnetic states: Comparison with nonmagnetic impurities

We have performed electron-spin-resonance measurements on single crystals of the doped spin-Peierls compounds (formula presented) and (formula presented) with (formula presented) Mg, Ni (formula presented) The first part of our experiments was performed in the paramagnetic and spin-Peierls phases at 9.5, 95, and 190 GHz. All nonmagnetic impurities (Si, Zn and Mg) were found to hardly affect the position and linewidth of the single line resonance, in spite of the moment formation due to the broken chains. In contrast to Si, Zn, and Mg dopings, the presence of Ni (formula presented) at low concentration induces a spectacular shift toward high fields of the ESR line (up to 40% for (formula presented) together with a large broadening. This shift is strictly proportional to the ratio of Ni to Cu susceptibilities: Hence it is strongly enhanced below the spin-Peierls transition. We interpret this shift and the broadening as due to the exchange field induced by the Ni ions onto strongly exchange coupled Cu spins. Second, the antiferromagnetic resonance was investigated in Ni-doped samples. The frequency vs magnetic-field relation of the resonance is well explained by the classical theory with orthorhombic anisotropy, with g values remarkably reduced, in accordance with the study of the spin-Peierls and paramagnetic phases. The easy, second-easy, and hard axes are found to be a, c, and b axes, respectively. These results, which are dominated by the single ion anisotropy of (formula presented) are discussed in comparison with those in the Zn- and Si-doped (formula presented).