Magnetic correlations and crystal-field levels in the superconductor (Ce0.73Ho0.27) Ru2

Abstract

Neutron scattering studies have been carried out to investigate the atomic magnetic properties of the "magnetic" superconductor (${\mathrm{Ce}}_{0.73}$ ${\mathrm{Ho}}_{0.27}$) ${\mathrm{Ru}}_2$. At low temperatures we observe the appearance of elastic or quasielastic magnetic scattering at small momentum transfers, indicating the development of ferromagnetic correlations. The temperature and wave-vector dependence of this scattering can be described to a good approximation by an Ornstein-Zernike correlation function over the entire range of wave vectors (0.035-0.20 ${\mathrm{\AA }}^{-1\mathrm{}}$) and temperatures (0.05-4.2 K) explored. The range of the spatial correlations $\xi (=\frac{1}{\kappa })$ increases smoothly with decreasing temperature and suggests the onset of ferromagnetism at ∼ 0.5 K. However, below 0.5 K, $\xi$ ceases to increase, saturating at a value of 80 Å with no detectable change in the scattering below that temperature. Thus there is no transition to conventional long-range ferromagnetic order. There is also no indication in the magnetic scattering of the onset of superconductivity at 1.6 K. Measurements of the inelastic magnetic scattering reveal a number of crystal-field transitions, demonstrating that the crystalline electric field removes the 17-fold degeneracy of the $J=8\mathrm{}$ ${\mathrm{Ho}}^{3+}$ free ion. The nature of the splittings can be understood on the basis of a crystal field with cubic symmetry, and the ground state is found to be the triply degenerate ${\Gamma }_5$ state, which possesses a magnetic moment. At low temperatures additional magnetic inelastic scattering is observed at low energies; this suggests that there are substantial exchange effects even though the characteristic magnetic temperature (0.5 K) is very small.