Electron Spin Resonance of Ni-doped CuGeO$_3$ in the paramagnetic, spin-Peierls and antiferromagnetic states: Comparison with non-magnetic impurities

摘要

We have performed Electron Spin Resonance measurements on single crystals ofthe doped spin-Peierls compounds CuGe$_{1-y}$Si$_y$O$_3$ andCu$_{1-x}$M$_x$GeO$_3$ with M = Zn, Mg, Ni ($x, y\leq 0.1$). The first part ofour experiments was performed in the paramagnetic and spin-Peierls phases at9.5, 95 and 190 GHz. All non-magnetic impurities (Si, Zn and Mg) were found tohardly affect the position and linewidth of the single line resonance, in spiteof the moment formation due to the broken chains. In contrast to Si, Zn andMg-doping, the presence of Ni (S = 1) at low concentration induces aspectacular shift towards high fields of the ESR line (up to 40% for x=0.002),together with a large broadening. This shift is strictly proportional to theratio of Ni to Cu susceptibilities: Hence it is strongly enhanced below thespin-Peierls transition. We interpret this shift and the broadening as due tothe exchange field induced by the Ni ions onto the strongly exchange coupled Cuspins. Second, the antiferromagnetic resonance was investigated in Ni-dopedsamples. The frequency vs magnetic field relation of the resonance is wellexplained by the classical theory with orthorhombic anisotropy, with $g$ valuesremarkably reduced, in accordance with the study of the spin-Peierls andparamagnetic phases. The easy, second-easy, and hard axes are found to be $a$,$c$, and $b$ axes, respectively. These results, which are dominated by thesingle ion anisotropy of Ni$^{2+}$, are discussed in comparison with those inthe Zn- and Si-doped CuGeO$_3$.