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) mu(B) and parallel to the c axis, form a collinear magnetic structure described by the propagation vector k = [1/2, 1/2, 1/2]. This magnetic structure is stable up to the Neel 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 k(1) = [0.24, 1, 0.52] is observed, while in the temperature interval 2.2-2.5 K a square-modulated magnetic structure, related to k(2) = [0.17, 5/6, 1/2] (the component along the a axis slightly differs from the commensurate value) and its third harmonics 3k(2) = [0.50, 5/2, 3/2] 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.