Low-temperature two-dimensional order-disorder magnetic transition in TbRu2Si2 within a square-modulated magnetic phase.

Abstract

Low-temperature specific-heat measurements on the tetragonal ${\mathrm{TbRu}}_{2}$${\mathrm{Si}}_{2}$ intermetallic compound show the existence of a magnetic transition at ${\mathit{T}}_{\mathit{c}}$=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. Such a transition, which exhibits all the characteristic features of a \ensuremath{\lambda}-type order-disorder transition with extended fluctuation effects on both sides of ${\mathit{T}}_{\mathit{c}}$ can be explained through a phase shift creating (100) nodal magnetic planes separated by 13 atomic spacings. The observed entropy change \ensuremath{\Delta}S\ensuremath{\simeq}0.4 J ${\mathrm{mol}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{K}}^{\mathrm{\ensuremath{-}}1}$ is very close to the 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.