Critical magnetic transition in TbNi2Mn-magnetization and Mossbauer spectroscopy

The structural and magnetic properties of the TbNi2Mnx series (0.9 ≤ x ≤ 1.10) have been investigated using x-ray diffraction, field- and temperature-dependent ac magnetic susceptibility,DC magnetization (5-340 K; 0-5 T) and 57Fe Mössbauer spectroscopy (5-300 K). TbNi2Mnx crystallize in the MgCu2-type structure (space group Fd-3m). The additional contributions to the magnetic energy terms from transition metal-transition metal interactions (T-T) and rare earthtransition metal interactions (R-T) in RNi2Mn contribute to their increased magnetic ordering temperatures compared with RNi2 and RMn2. Both the lattice constant a and the Curie temperature TC exhibit maximal values at the x=1 composition indicating strong magnetostructural coupling. Analyses of the ac magnetic susceptibility and DC magnetization data of TbNi2Mn around the Curie temperature TC=147 K confirm the magnetic transition is second order with critical exponents β=0.77±0.12, γ=1.09±0.07 and δ=2.51±0.06. These exponents establish that the magnetic interactions in TbNi2Mn are long range despite mixed occupancies of Tb and Mn atoms at the 8a site and vacancies. The magnetic entropy -ΔSM around TC is proportional to (μ0H/TC)2/3 in agreement with the critical magnetic analyses. Mössbauer spectra above TC are fitted by two 2 subspectra in agreement with refinement of the x-ray data while below TC three sub-spectra are required to represent the three inequivalent local magnetic environments.