From Tb3Ni2 to Tb3CoNi: The interplay between chemistry, structure, and magnetism

Formation, crystal structure, and macroscopic and microscopic magnetism of the binary Tb3Ni2 and derivative pseudobinary Tb3CoxNi2-x phases have been investigated using an array of experimental methods.While Tb3Ni2 crystallizes in the monoclinic Dy3Ni2 structure type (mS20, C2/m), the substitution of Co for Ni results in a structural transition into the rhombohedral Er3Ni2 type (hR45, R¯3h) at x(Co) ≈ 0.34 and beyond in the Tb3CoxNi2-x system. In both the monoclinic and rhombohedral phases, the addition of Co leads to an anisotropic change of lattice parameters and unexpected reduction of the cell volume. Measurements of bulk properties reveal that these compounds order ferrimagnetically or ferromagnetically at about 100 K. Complex noncollinear ferromagnetic ordering in the Tb sublattices is weakly dependent on composition. For x > 0.34 the long-range magnetic ordering leads to a strong anisotropic magnetostriction accompanied by a symmetry reduction from rhombohedral to triclinic.