Local Spin Disorder in the Magnetic Kondo Compounds CeNi1-xCux

Five members (x = 0.9, 0.6, 0.5, 0.4 and 0.2) of the strongly correlated electron compounds CeNi1-xCux have been studied using µSR spectroscopy. For 0.8 ≥ x ≥ 0.4 bulk magnetic data had indicated the existence of a spin-glass-like intermediate state between the long-range magnetic ordered (LRO) state at low temperatures and paramagnetism at higher temperatures. The measurements reported in this paper show that this intermediate magnetic state is unusual, its exact nature not being fully resolved. The µSR spectra for T ≤ 0.4 K are in all five compounds compatible with LRO, but show large transverse relaxation rates. In antiferromagnetic CeNi0.1Cu0.9 (TN = 2.1 K, μord = 1.45 µB) this can be explained as originating from a complex spin structure. A second phase transition at 1.3 K was found, probably involving a spin reorientation. For x = 0.6 neutron diffraction data suggest collinear ferromagnetism while the large transverse relaxation indicates an additional substantial local spin disorder. Strong non-monotonic variations of the local field with x suggest different ordered spin structures at different compositions. In CeNi0.6Cu0.4 the µSR spectra indicate a spin arrangement containing a modulated spin component. The ordered Ce moment for CeNi0.8Cu0.2 is estimated to be ≈ 0.1 µB, confirming the reduction of moment with increasing Ni content due to the enhancement of the Kondo interaction. In all compounds except the x = 0.5 alloy an intermediate state, which is characterized in its µSR response by a high sensitivity to zero field cooling (ZFC) vs. field cooling (FC) history, was found. The dominant effect of FC is a slowing down of spin fluctuations. The µSR spectra further revealed that the intermediate magnetic regime is neither a spin frozen state, nor a unique dynamically short-range correlated random spin state, but is characterized by a magnetic inhomogeneity with coexistence of an ordered and a non-ordered fraction. The latter increases strongly with temperature. The observation of a field history dependence is unusual in µSR studies of magnetically disordered materials and appears to be related to magnetic inhomogeneities. The magnetically diluted sample La0.25Ce0.75Ni0.8Cu0.2 was also studied. For this alloy, freezing into a randomly oriented spin state below 0.7 K was detected but no subsequent transition to LRO. Also for this alloy large field history effects were found. The local magnetic properties of CeNi1-xCux are discussed in terms of a cluster-type model.