Effect of Nd and Rh Substitution on the Spin Dynamics of the Kondo-insulator CeFe2Al10

The dynamic magnetic properties of the Kondo-insulator state in $\mathrm{Ce}{\mathrm{Fe}}_{2}{\mathrm{Al}}_{10}$ (spin gap, resonance mode) have been investigated using polarized neutrons on a single crystal of pure $\mathrm{Ce}{\mathrm{Fe}}_{2}{\mathrm{Al}}_{10}$. The results indicate that the magnetic excitations are polarized mainly along the orthorhombic $a$ axis and their dispersion along the orthorhombic $c$ direction could be determined. Polycrystalline samples of Nd- and Rh-doped $\mathrm{Ce}{\mathrm{Fe}}_{2}{\mathrm{Al}}_{10}$ were also studied by the time-of-flight technique, with the aim of finding out how the low-energy magnetic excitation spectra change upon isoelectronic substitution of a rare-earth element (Nd) on the magnetic Ce site or electron doping (Rh) on the transition-element Fe sublattice. The introduction of magnetic Nd impurities strongly modifies the spin gap in the Ce dynamic magnetic response and causes the appearance of a quasielastic signal. The crystal-field excitations of Nd, studied in both $\mathrm{La}{\mathrm{Fe}}_{2}{\mathrm{Al}}_{10}$ and $\mathrm{Ce}{\mathrm{Fe}}_{2}{\mathrm{Al}}_{10}$, also reveal a significant influence of $f$-electron hybridization (largest in the case of Ce) on the crystal-field potential. As a function of the Rh concentration, a gradual change is observed from a Kondo-insulator to a metallic Kondo-lattice response, likely reflecting the decrease in the hybridization energy.