AC Susceptibility Studies of Phase Transitions and Magnetic Relaxation: Conventional, Molecular and Low-Dimensional Magnets

The paper presents the review of AC susceptibility measurements in study of magnetic phase transitions and spin dynamics. Examples of several types of phase transitions in conventional and molecular magnets are shown. The phenomenon of magnetic relaxation is briefly ascribed and illustrated with results on relaxation in low-dimensional molecular magnets, such as single molecule magnets, single chain magnets and quasi 1-dimensional systems showing magnetic phase transition to the long-range ordered phase. The following experimental possibilities of the AC technique, which allow to get insight into properties of novel magnetic materials are mentioned: (i) change of the AC field amplitude (in order to distinguish ferro-(ferri) and antiferromagnets); (ii) use of a static field H_{DC} (to conclude on the type of magnetic order and to study the phase diagram); (iii) measurement of nonlinear susceptibilities (to conclude on the type of phase transition); (iv) probing the dependence of both components of susceptibility on frequency of the AC field (to study magnetic relaxation); (v) tests of the DC field effect on the frequency dependence of χ_{AC} (to analyze the possibility of blocking or reentrant spin-glass transition).