Dipolar effects on the critical fluctuations in Fe: Investigation by MIEZE

Iron is one of the archetypical ferromagnets to study the critical fluctuations at a continuous phase transition thus serving as a model system for the application of scaling theory. We report a comprehensive study of the critical dynamics at the transition from the ferro- to the paramagnetic phase in Fe, employing the high-resolution neutron spin echo technique MIEZE. The results show that the dipolar interactions lead to an additional damping of the critical spin fluctuations at small momentum transfers $\bf q$. The results agree essentially with scaling theory if the dipolar interactions are taken into account by means of the mode-coupling equations. However, in contrast to expectations, the dipolar wavenumber $q_D$ that plays a central role in the scaling function $f(\kappa/q,q_D/\kappa)$ becomes temperature dependent. In the limit of small $\bf q$ the critical exponent $z$ crosses over from 2.5 to 2.0.