Tbx Er1-x Ni5 compounds: An ideal model system for competing Ising- XY anisotropy energies

We have studied Tbx Er1-x Ni5 (x=0, 0.1, 0.2, 0.3, 0.4, 0.6, 0.8, 0.925, and 1.0) compounds by using several experimental techniques such as ac-susceptibility, heat-capacity, and neutron-diffraction measurements. All the compounds are found to crystallize in the CaCu5 -type structure with space group P6/mmm. The a axis shows a linear increase with Tb concentration, whereas the c axis remains almost unchanged over the whole doping range. Our neutron-diffraction studies revealed that samples for 0≤x≤0.8 have a commensurate magnetic structure with k=0, whereas the two samples on the Tb-rich phase (x=0.925 and 1.0) have an incommensurate structure. Of particular interest is that individual Tb and Er moments keep their mutually orthogonal arrangement seen at the end-member compositions over the whole doping range, due to very strong magnetic anisotropy of single-ion nature. We have established a complete magnetic x-T phase diagram of Tbx Er1-x Ni5 to find that two straight lines of the ordering of the Tb and Er subsystems are persistently seen, which intersect at a tetracritical point.