Thermodynamic Measurement of Angular Anisotropy at the Hidden Order   Transition of URu$_2$Si$_2$
  The heavy fermion compound URu$_2$Si$_2$ continues to attract great interest due to the unidentified hidden order it develops below 17.5K. The unique Ising character of the spin fluctuations and low temperature quasiparticles is well established. We present detailed measurements of the angular anisotropy of the nonlinear magnetization that reveal a $\cos^4 \theta$ Ising anisotropy both at and above the ordering transition. With Landau theory, we show this implies a strongly Ising character of the itinerant hidden order parameter. 
Jennifer Trinh, Ekkes Bruck, Theo Siegrist, Rebecca Flint, Premala   Chandra, Piers Coleman and Arthur P. Ramirez
cond-mat.str-el