LOW-TEMPERATURE 2-DIMENSIONAL ORDER-DISORDER MAGNETIC TRANSITION IN TBRU2SI2 WITHIN A SQUARE-MODULATED MAGNETIC PHASE

Low-temperature specific-heat measurements on the tetragonal TbRu2Si2 intermetallic compound show the existence of a magnetic transition at T-c=4.7 K, where a [3/13,0,0] square modulated phase with the magnetic moments directed along the c axis was generally accepted to exist Suc h a transition, which exhibits all the characteristic features of a X- type order-disorder transition with extended fluctuation effects on bo th sides of T-c can be explained through a phase shift creating (100) nodal magnetic planes separated by 13 atomic spacings. The observed en tropy change Delta S congruent to 0.4 J mol(-1) K-1 is very close to t he value R/13 ln2=0.44 predicted for a 1/13 molar fraction of Ising 1/ 2 spins. A two-dimensional isotropic Ising model is shown to reproduce well the main aspects of the reported transition.