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 Fe-57 Mossbauer spectroscopy (5-300 K). TbNi2Mnx crystallizes in the MgCu2-type structure (space group Fd (3) over barm). The additional contributions to the magnetic energy terms from transition-metal-transition-metal interactions (T-T) and rare-earth-transition-metal interactions (R-T) in RNi2Mn compounds contribute to their increased magnetic ordering temperatures compared with RNi2 and RMn2. Both the lattice constant a and the Curie temperature T-C 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 T-C = 147 K confirm that the magnetic transition is second order with critical exponents beta = 0.77 +/- 0.12, gamma = 1.09 +/- 0.07 and delta = 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 -Delta S-M around T-C is proportional to (mu H-0/T-C)(2/3) in agreement with the critical magnetic analyses. The Mossbauer spectra above T-C are fitted by two sub-spectra in agreement with refinement of the x-ray data while below T-C three sub-spectra are required to represent the three inequivalent local magnetic environments.