Available experimental data indicate the existence of a tricritical point (tcp) on the line of transition of UPt3 into the superconducting state in the temperature-pressure plane. In particular, at normal pressure the transition has to be of the first order. The analysis of a phase diagram of a superconductor with the tcp in the temperature-magnetic field coordinates is presented here. The diagram does not conform either with the usual behavior of type I nor type II superconductors. The lines Hcm(T) and Hc2(T) intersect at a finite field. The line Hc1(T) branches from Hcm(T) at a finite field too. This diagram can be brought into agreement with that observed for UPt3. Both the existence of a tcp and the related magnetic properties are not accounted for by the weak coupling BCS theory.
Magnetic Structures in Pure Type-II Superconductors