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 neutro n diffraction experiments were performed on Ce3Al11 single crystals. M agnetization measurements evidence a strong anisotropy with the b axis as the easy magnetization direction and, below 3.2 K, field induced t ransitions are observed. Neutron diffraction experiments in zero magne tic field confirm that all the moments are parallel to b in the ferrom agnetic phase. The moments are reduced, compared to the free ion value and are very different on the two Ce sites (1.27 mu(beta) on Ce-I and 0.24 mu(beta) on Ce-II). Kondo interactions may be invoked to explain such a small value of the moment on the Ce-II site. At T = 3.2 K, the very unusual transition from a ferromagnetic to an incommensurate str ucture at lower temperatures is confirmed. The propagation vector is t au = (0,0,tau(z)), with tau(z) close to 1/3 but slightly larger. This component is temperature dependent and decreases as temperature decrea ses. The magnetic structure below 3.2 K consists of magnetic (001) lay ers with moments parallel to b, which propagate along the c direction. As in the ferromagnetic state, a small value of the moment on the Ce- II 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 transitio ns observed in the magnetization curves below 3.2 K correspond to the change from the incommensurate structure to a ferromagnetic-type struc ture.