Sequential Spin Polarization of the Fermi Surface Pockets in URu 2 Si 2 and Its Implications for the Hidden Order

Using Shubnikov\char21{}de Haas oscillations measured in ${\mathrm{URu}}_{2}{\mathrm{Si}}_{2}$ over a broad range in a magnetic field of 11\char21{}45 T, we find a cascade of field-induced Fermi surface changes within the hidden order phase I and further signatures of oscillations within field-induced phases III and V [previously discovered by Kim et al., [Phys. Rev. Lett. 91, 256401 (2003)]. A comparison of kinetic and Zeeman energies indicates a pocket-by-pocket polarization of the Fermi surface leading up to the destruction of the hidden order phase I at $\ensuremath{\approx}35\text{ }\text{ }\mathrm{T}$. The anisotropy of the Zeeman energy driving the transitions in ${\mathrm{URu}}_{2}{\mathrm{Si}}_{2}$ points to an itinerant hidden order parameter involving quasiparticles whose spin degrees of freedom depart significantly from those of free electrons.