Critical behaviour of the magnetic susceptibility at marginal dimensionality in LiTbF4

The parallel susceptibility of LiTbF 4 has been measured in the critical region and is well described by the law chi c (t)= Gamma t -1 mod ln(t 0 /t) mod 1/3 where t is the reduced temperature; t=T/T c -1 over the range 2*10 -4 <t<1.1*10 -1 with T c =2.8850K, Gamma =(3.87+or-0.05)*10 -2 emu cgs g -1 , t 0 =5.5+or-2.5 ( Gamma N =0.910+or-0.012 in dimensionless units). A detailed discussion for the choice of t 0 and of the exponent of the logarithmic correction z= 1 / 3 is given. The universal ratio R c between the critical amplitudes of the magnetic susceptibility, the specific heat and the spontaneous magnetisation, R c =T c AB -2 Gamma , is tested.