Crystal and magnetic structures of R₂Ni₂In compounds (R = Tb and Ho)

Crystal and magnetic structures of R₂Ni₂In (R = Tb and Ho) have been studied using powder neutron diffraction at low temperatures. The compounds crystallize as orthorhombic crystal structures of the Mn₂AlB₂ type. At low temperatures, the magnetic moments localized solely on the rare earth atoms form antiferromagnetic structures. The Tb magnetic moments, equal to 8.8 (4) μ_B and parallel to the c axis, form a collinear magnetic structure described by the propagation vector k = [½ , ½ , ½]. This magnetic structure is stable up to the Néel temperature T_N = 40 K. For Ho₂Ni₂In a complex, temperature-dependent magnetic structure is detected. In the temperature range 6.1-8.6 K, an incommensurate sinusoidal magnetic structure, described by the propagation vector k₁ = [0.24, 1, 0.52] is observed, while in the temperature interval 2.2-2.5 K a square-modulated magnetic structure, related to k₂ = [0.17,{{5} \over {6}},{{1} \over {2}}] (the component along the a axis slightly differs from the commensurate value) and its third harmonics 3k₂ = [0.50,{{5} \over {2}},{{3} \over {2}}] is found. At 3.1-3.7 K as well as below 2 K, a coexistence of both detected magnetic structures is observed. The Ho magnetic moments remain parallel to the c axis in both the sine- and square-modulated magnetic structures. The low-temperature heat capacity data confirm a first-order transition near 3 K.