Evolution of magnetic order in the anisotropic Kondo insulator Ce(Ni,Cu)Sn

The characteristic feature of a Kondo insulator is the energy gap forming in the low temperature (T) coherent regime. An example is CeNiSn, which exhibits sharp gaps at particular wave vectors in the magnetic excitation spectrum as observed by neutron scattering and reduced low energy excitations as observed in thermodynamic quantities such as specific heat (C) and magnetic susceptibility ($\chi$) best described in terms of a pseudo gap due to the anisotropic hybridization in this compound. We present C, $\chi$ and neutron data of CeNi$_{1-x}$Cu$_{x}$Sn to see how magnetic order forms in such an anisotropic Kondo insulator upon doping. The magnetic gap seems to have collapsed already for x$=$0.13 as the magnetic correlation have become quasielastic. The long range magnetic order below T$_{N}=$1.4K is described by the same wave vector as the dominant inelastic correlations in x$=$0. For x$=$0.078, $\chi$ (T) shows still a shoulder at 4K reminiscent of a pseudo gap. At lower T, $\chi$ (T) and C(T) exhibit a maximum at T$_{N}=$0.4K indicating magnetic order, confirmed by neutron scattering. The magnetic order occurs at x$_{c}=$0.07 before the gap vanishes at x$_{g}\approx $0.1. The transition at x$_{c}$ shows signs of a disordered quantum phase transition.