Spin dynamics in the mixed valence alloyCe1−xThx

Abstract

Temperature-dependent neutron-scattering experiments have been performed on a polycrystalline sample of ${\mathrm{Ce}}_{0.74}$ ${\mathrm{Th}}_{0.26}$ which undergoes a first-order $\gamma -\alpha$ valence transition at $T_V\sim 150$ K. By a measurement of the temperature dependence of the lattice parameter and use of Végard's law, we estimate the temperature behavior of the valence of Ce. The $Q$ dependence of the magnetic scattering is found to follow the form factor of the ${\mathrm{Ce}}^{3+}$ ion surprisingly well. In the inelastic scans, particular attention has been paid to the subtraction of the phonon background via an inelastic study of an identically sized and shaped sample of the nonmagnetic material ${\mathrm{La}}_{0.73}$ ${\mathrm{Th}}_{0.27}$. The corrected ${\mathrm{Ce}}_{0.74}$ ${\mathrm{Th}}_{0.26}$ spectra have then been expressed in the form of the imaginary part of the susceptibility ${\chi }^{\prime \prime }(\overrightarrow{\mathrm{Q}},\omega )$. The $\gamma$-phase dynamic susceptibility is a broad feature with significant intensity extending beyond 70.0 meV (the limit of our measurement) with a peak near ∼20.0 meV. On cooling below $T_V$ the susceptibility decreases in magnitude and broadens such that the peak is beyond 70.0 meV. These results are compared with the macroscopic magnetic susceptibility which exhibits behavior similar to $\chi \left(\overrightarrow{\mathrm{Q}}\right)$ obtained from ${\chi }^{\prime \prime }(\overrightarrow{\mathrm{Q}},\omega )$ by a Kramers-Kronig analysis.