The isostructural phase transition and frustrated magnetic ordering in TbPd0.9Ni0.1Al studied by neutron diffraction

By using powder neutron diffraction we provide a detailed structural and magnetic investigation of the pseudo-ternary compound TbPd0.9Ni0.1Al with hexagonal ZrNiAl-type crystal structure and disorder of palladium and nickel atoms on two crystallographic sites. We provide experimental evidence for the occurrence of an isostructural phase transition in TbPd0.9Ni0.1Al at 157 K, which is affected by structural disorder compared to that observed in TbPdAl. In TbPd0.9Ni0.1Al the step of the c/ac/a ratio at the first-order phase transition appears about four times smaller than in TbPdAl. TbPd0.9Ni0.1Al undergoes two successive magnetic phase transitions at TN1=41TN1=41 K and TN2≈21.5TN2≈21.5 K. Two third of non-frustrated ordered Tb moments form commensurate antiferromagnetic chains (k1=[1/2,0,1/2]) and coexist with one third of frustrated ordered Tb moments, which change from a commensurate structure (k1=[1/2,0,1/2]) between TN2TN2 and TN1TN1 to an incommensurate structure (k2i=[0.50(1),0.12(1),1/2]) below TN2TN2. Due to the strong magneto-crystalline anisotropy all ordered Tb moments stay perpendicular to the ab  -plane and reach at 1.5 K saturation values of 8.6 (2) μBμB (non-frustrated) and 7.2 (2) μBμB (frustrated). The symmetry of the magnetic structure of TbPd0.9Ni0.1Al is lower than that of TbNiAl.