Static and dynamic evidence for a transition at Tc = 0 in the Ising spin glass Fe0.3Mg0.7Cl2

The critical behaviour of the Ising spin glass Fe 0.3 Mg 0.7 Cl 2 has been studied by d.c. magnetisation, a.c. susceptibility and specific heat measurements. We have extended the range of our previous a.c. susceptibility measurements down to 0.02 Hz by using a new set up which utilises the possibilities of a 16 bit analog to digital converter. The relaxation time τ and the coefficients a 3 and a 5 of the non linear susceptibilities are powers of each other and diverge like exponential functions of θ/T. This confirms our former statement that Fe 0.3 Mg 0.7 Cl 2 is of a class different from that of traditional 3d systems and appears as an example of a real 2d Ising spin glass with T c =0. We claim that the essential L'etude du comportement critique du verre de spin Ising Fe 0.3 Mg 0.7 a ete realisee a l'aide de mesures d'aimantation, de susceptibilite alternative et de chaleur specifique. Les mesures de susceptibilite alternative ont ete effectuees jusqu'a de tres basses frequences (0.02Hz