DYNAMICAL SUSCEPTIBILITY OF DILUTE (Mo1-xNbx)Fe ALLOYS

-Inelastic neutron scattering has been used to measure the linewidth of the dynamical response of Fe impurities in Mol-,Nb, alloys. The character of the local moment, inferred from its relaxation rate, does not change significantly up to x = 0.15. Between x = 0.15 and x = 0.30 the response decreases rapidly and has disappeared by x = 0.45. The magnetic character of dilute Fe impurities in solid solution Mol-,Nb, alloys is one of the more interesting and most studied magnetic alloy problems. The susceptibility measurements of Clogston et al. [I] showed that 1 % Fe is non-magnetic when dissolved in Nb, but that a local moment of 2 p~ develops rapidly for x < 0.6. This sudden appearance of a local moment corresponding to two electrons as the host electron concentration changes by ci 0.3 was accounted for, at least qualitatively, by Anderson and Wolff [Z]. In the Anderson model the occurence of a local moment on the impurity depends on the ratio of the on-site Coulomb repulsion to the width of the virtual bound state. The latter increases with the density of states at the fermi energy p (EF) of the host. Thus the local moment of Fe in Mo is seen to be associated with the low p (EF) of Mo, whereas in Nb, which has p (EF) about seven times that of Mo, there is no local moment. This picture of a homogeneous variation of the Fe moment as a function of the host p (EF) was challenged by Jaccarino and Walker [3] who suggested that the magnetic state of the impurity might depend on its local environment. The variation of the local moment with composition could be explained by assuming that those Fe atoms with seven or more Mo nearest neighbours would be magnetic. More recent cluster CPA calculations [4] suggest a more complex behaviour where the local moment depends on both the alloy composition and the configurations of the local environment. Attempts have been made to characterise the local moment with microscopic probes, in particular NMR and Mossbauer experiments [5-71. Maley et al. [7] claim to resolve three types of Fe site in their high field Mossbauer spectra. The most strongly magnetic of these exhibits a hyperfine field which follows a modified Brillouin function B {pH/k (T + 0)) , with both p and 0 varying very little with Nb concentration in the range up to x = 0.15. states of different Fe sites will correspond to different Kondo temperatures and therefore to different relaxation rates. To this end we have examined a range of Mo1-bNb, alloys containing 1 % Fe using the IN6 time of flight spectrometer at ILL Grenoble. The incident neutron energy was 3.14 meV and the 336 detectors were grouped into 19 banks covering a range of elastic scattering vectors Q,1 from 0.25 to 2.05 A-1. Spectra were collected for polycrystalline samples with x = 0, 0.15, 0.30 and 0.45 at temperatures between 5 K and 60 K. As a check on background subtraction procedures we also examined a pure Mo and a Mo0.85Nb0.15 sample without Fe over the same temperature range. After correction for sample independent background, absorption and self-shielding, the samples containing no Fe showed no scattering outside the elastic peak, other than a small phonon peak at high energy transfers in energy gain, at the highest temperatures studied. Both the MoFe and the M ~ ~ . ~ ~ N b o . l s F e alloys displayed a clear quasielastic component of scattering whose energy width varied with temperature. The corrected spectra for these alloys were fitted to a scattering function S (Q, w)