The magnetic structures of Ce3Al11: a single crystal study

Ce3Al11 (orthorhombic, space group Immm) is an ordered Kondo compound with a ferromagnetic structure in the temperature range 3.2 K<T<6.2 K, and a transition to an incommensurate structure at T=3.2 K. To specify its magnetic structures, magnetization measurements and neutron diffraction experiments were performed on Ce3Al11 single crystals. Magnetization measurements evidence a strong anisotropy with the b axis as the easy magnetization direction and, below 3.2 K, field induced transitions are observed. Neutron diffraction experiments in zero magnetic field confirm that all the moments are parallel to b in the ferromagnetic phase. The moments are reduced, compared to the free ion value and are very different on the two Ce sites (1.27μB on CeI and 0.24μB on CeII). Kondo interactions may be invoked to explain such a small value of the moment on the CeII site. At T=3.2 K, the very unusual transition from a ferromagnetic to an incommensurate structure at lower temperatures is confirmed. The propagation vector is τ=(0,0,τz), with τz close to 1/3 but slightly larger. This component is temperature dependent and decreases as temperature decreases. The magnetic structure below 3.2 K consists of magnetic (00l) layers with moments parallel to b, which propagate along the c direction. As in the ferromagnetic state, a small value of the moment on the CeII site is observed. Furthermore, 3rd order harmonics are present. The determination of the exact structure depends on the phase difference between the moments on the two sites. Neutron diffraction experiments under an applied magnetic field, show that the field induced transitions observed in the magnetization curves below 3.2 K correspond to the change from the incommensurate structure to a ferromagnetic-type structure. ((orig.))