NONLINEAR SUSCEPTIBILITY MEASUREMENTS IN HEAVY-FERMION SYSTEMS

We discuss the nonlinear susceptibility (chi3) as a probe of higher-or der spin correlations in heavy fermion materials, with particular emph asis on U Ru2Si2 and U Be13. The sharp discontinuity of chi3 in U Ru2S i2 is contrasted with two other antiferromagnets U2Zn17 and U0.95Th0.0 5Pt3 with larger moments. Nonlinear magnetic properties of URu2Si2 can be understood simply in terms of Landau-Ginzberg theory for a large h idden, itinerant order parameter. In UBe13 we discuss how chi3 can be used to probe the presence of quadrupolar fluctuations.