Magnetic behaviour of the compound CeNi₂Al₅: double-k magnetic structure

The orthorhombic Kondo compound CeNi₂Al₅ is magnetically ordered below 2.6 K. Earlier neutron diffraction experiments have led to a sine-wave modulated magnetic structure, with a single three-component propagation vector k=(0.500, 0.405, 0.083) and a moment in the (a, b) plane, 8 degrees tilted from the b direction towards the a one. The crystal electric field coefficients, previously deduced from the thermal variation of the susceptibilities along the three principal axes, have been confirmed by neutron spectroscopy. They lead to an easy magnetization direction along b, in agreement with the magnetization curves. The discrepancy between earlier neutron and magnetization results concerning the orientation of the moment was solved by performing a new neutron diffraction experiment under uniaxial stress (an applied magnetic field). In fact, the magnetic structure is a double-k structure with k=(0.500, 0.405, 0.083) and k'=(-0.500, 0.405, 0.083). The resulting moment can then be aligned along the easy magnetization direction b. A deeper insight into the magnetic properties of CeNi₂Al₅ suggests that the magnetic transition at 2.6 K could be of first order. Additional interactions other than isotropic exchange interactions and crystal electric field effects are probably present in this compound.