Critical points in magnetic systems, a neutron diffraction study of CsCoCl32D2O

A review is given of the present phenomenological theories of critical points in terms of the scaling, the universality, and the smoothness hypothesis. The experimental part describes an investigation by means of thermal neutron scattering to the magnetic phase transitions in the metamagnet CsCoCl3.2D20, T/sub N/ = 3.30-0 K. Along the line of critical points, that separates the antiferromagnetic region from the paramagnetic region in the HT-phase diagram, the critical behavior is found both qualitatively and quantitatively in agreement with the predictions of the smoothness hypothesis. A tricritical point is located at T/sub t/ = 1.85-0 K and H/sub t/ = 2.70 k0e, and a full set of tricritical exponents is obtained. It is demonstrated that these exponents are in agreement with the predictions of the scaling hypothesis at a tricritical point, but differ from the classical exponents predicted by recent renormalization group techniques.