  An enhanced susceptibility is a natural consequence of the "heavy fermion"
(HF) state rendering the possibility of measurably large nonlinear
susceptibilities. In recent work a universal behavior of the peaks observed in
the linear (\Chi_1) and the third order (\Chi_3) susceptibility in HF
metamagnets has been identified. This universality is well accounted for by a
single energy scale model considering on-site correlations only. A prediction
of this model is a peak in the fifth order susceptibility, \Chi_5, as well. In
the first measurements on a HF metamagnet, UPt3 reported herein, we find that
Chi_5 rather than attaining a peak, saturates at low temperatures and is
positive. The thermodynamic implications of these towards the stability of the
metamagnetic HF state are discussed.
