The H-T phase diagram of ${\mathrm{UPt}}_3$ has previously been shown to be incompatible with a two-component-vector order parameter. It is studied in this paper within the ‘‘AB’’ model, consisting of two nearly degenerate order parameters, one transforming as an A and the other as a B representation of ${\mathit{D}}_{6\mathit{h}}$. The nature of the different one- and two-order-parameter vortex-lattice phases and the possible transitions between them are examined in detail. A quantitative comparison is made with the experimental diagram, and the parameters in the Ginzburg-Landau free energy are deduced. It is found that the flux lattice evolves continuously in the low-H, low-T phase with no additional transitions between different types of two-order-parameter lattices, and that its orientation is in accord with neutron-diffraction studies near one of the inner transition lines.

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