  The heavy fermion compound URu2Si2 continues to attract great interest due to
the long- unidentified nature of the hidden order that develops below 17.5K.
Here we discuss the implications of an angular survey of the linear and
nonlinear susceptibility of URu2Si2 in the vicinity of the hidden order
transition. While the anisotropic nature of spin fluctuations and
low-temperature quasiparticles was previously established, our recent results
suggest that the order parameter itself has intrinsic Ising anisotropy, and
that moreover this anisotropy extends far above the hidden order transition.
Consistency checks and subsequent questions for future experimental and
theoretical studies of hidden order are discussed.
