Coherence Seminar: Studying Critical dynamics in a second-order ordering transition

Studying Critical dynamics in a second-order ordering transition

The ordering transition in the Heussler AuAgZn2 alloy is equivalent to the second-order ferromagnetic transition of the Ising model. This transition appears as an antiferromagnetic ordering in a simple cubic lattice. The static behaviour follows the well known divergence of the fluctuations S(q) and of the fluctuation length x as measured in clasical (incoherent) X-ray diffuse scattering experiment.

The dynamics of this type of system is considered as equivalent to the Field-theoretical model "A" of Hohenberg et al. A brief discussion of this dynamics and of the existing simulations will be given. The observation of the dynamics of the fluctuations has been carried out by the dynamic measurement of the speckle patterns at the ID10 beamline of the ESRF.

The careful exploitation of the results will be described, and the results will be discussed: for the first time, we obtained a q (scattering vector) and temperature dependence of the fluctuation time t. The results provide an experimental estimate of the divergence of the fluctuation time t, and we show that t~|T-Tc|-nz, with nz=1.246(30), which yields z=1.97(5) from n=0.631. The results agree well with numerical simunations and with the field-theoretical model results of model "A"  (z=2.02). Future improvements will be discussed.