Crystal Field and p-f Mixing Effects in Uranium Pnictides
The triuranium tetrapnictides, U3X4, crystallize in a cubic body centered crystal structure of Th3P4 type.1 From the neutron diffraction experiments,2 the uranium in these compounds is in the tetravalent configuration with 5f2, S=1, L=5, J=4 and g=0.8 in the Russell-Saunders coupling. Main features of the magnetic properties are as follows: 1) The value of the ferromagnetic moment is about a half of that for J=4.3 2) The magnetic anisotropy in the ordered state is very large with the easy axis in the <111> direction.4,5 3) The ordered structure is multiaxial and the direction of each sublattice moment tilts about 20° from the <111> direction to each uniaxial direction corresponding to one of the <001> axes.1,6 4) Nevertheless, the increase of the moment under strong external magnetic fields is very small. For example, in U3As4, the increase is less than 1% under 20T at 4.2K.7 5) In U3As4, the spin orientational transition is observed at the external magnetic field of 20T in the <001> direction.5,7 This is very sensitive to a small tilt of the field direction from the <001> direction.7 The theoretical investigation was done by Troé et al.1 according to the point charge model of the crystal field. However, the point charge model can not explain the points 3) and 5).
KeywordsCrystal Field Total Angular Momentum Easy Axis Quantization Axis Neutron Diffraction Experiment
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