NMR and other magnetic resonance techniques in unstable magnets

This paper reviews and compares the use of nuclear magnetic resonance (NMR) and related hyperfine techniques muon spin rotation (μSR) and, to a lesser extent, other methods] in the study of 4f and 5f magnetism in “unstable magnets”, i.e., intermediate-valent and heavy-fermion materials. In both NMR and μSR the features of interest are the spectral shape, the frequency shiftK (Knight shift in metals) and the spin-lattice relaxation rate 1/T ₁. For temperatures below the characteristic or “Kondo” temperatureT ₀ these experiments given evidence for (1) modification of the transferred hyperfine field nonlinearK(χ)]. (2) spin fluctuations with a characteristic fluctuation rate ∼k _B T ₀/h, (3) strong energy-gap anisotropy (zeros of the gap along lines on the Fermi surface) in heavy-fermion superconductors, (4) spin-singlet Cooper pairing from the change in muon Knight shift in superconducting UBe₁₃, and (5) very weak static magnetism (10⁻¹–10⁻³ μ_B/f atom) in CeAl₃, CeCu₂Si₂, U_1−x Th _x Be₁₃ (x=0.033), and UPt₃. There is some controversy concerning the interpretation of 1/T ₁ well aboveT ₀ in UBe₁₃; the situation is reviewed.