Tricritical Points in the Underscreened Anderson Lattice Model*

We report calculations on the Underscreened Anderson Lattice Model that has been proposed to describe the Hidden Ordered phase of URu2Si2. Since the proposed Hidden Ordered Phase is associated with a spin-orbital density wave, in which the spins condense in a spontaneously chosen plane, the magnetic properties become anisotropic in the low temperature phase. The ansiostropy is similar to that found by magnetic torque measurements. We examine the field-temperature phase diagram. At zero field, the transition to the Hidden Ordered phase is second order. The application of a magnetic field has the effect of reducing the transition temperature towards zero. However, at a critical value of the applied field, the transition becomes first-order in agreement with high field measurements. We present an analysis of the tricritical point, which shows that the upper critical dimension of the model is reduced to d=3. Therefore, the transition may be reasonably described by a Gaussian approximation.