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.