Magnetic ordering in the presence of fast spin fluctuations: A neutron scattering study of CeIn3

We report results of elastic and inelastic neutron scattering experiments in the trivalent spin-fluctuation metal Ce${\mathrm{In}}_3$. Antiferromagnetic order occurs at 10.23 ± 0.01 K. The magnetic reflections can be indexed on a doubled chemical cell of cubic symmetry. The saturated ordered moment $0.65\pm 0.1{\mu }_B$ per cerium atom is comparable to the value $0.71{\mu }_B$ expected for ordering within the ${\Gamma }_7$ doublet, which is the ground level expected for $J=\frac{5}{2}$ cerium moments in a cubic crystal field. The critical behavior of the order parameter is of the form $\frac{M_{\mathrm{st}}^2\mathrm{}\left(T\right)\mathrm{}}{M_{\mathrm{st}}^2\mathrm{}\left(0\right)\mathrm{}}=A{\left[\frac{(T_N-T)}{T_N}\right]}^{2\beta }$ where $M_{\mathrm{st}}$ is the staggered magnetization, $A=2.2\mathrm{}\pm 0.2$ and $\beta =0.42\mathrm{}\pm 0.02$; in addition the critical fluctuations are extremely weak. We discuss this nearly mean-field behavior in the context of recent theories which describe critical behavior in systems where the critical temperature is much smaller than a characteristic spin-fluctuation temperature. The inelastic scattering measurements provide evidence for the existence of such a characteristic energy. At low temperatures the inelastic cross section is dominated by an anomalously broad magnetic scattering peak, centered near 13 meV and with a half-width of about 10 meV. We interpret the large linewidth as arising from fast spin fluctuations, which arise from strong Kondo-type exchange coupling of the $4f$ spins to the conduction electrons.