NMR and other magnetic resonance techniques in unstable magnets

Abstract

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/T1. For temperatures below the characteristic or “Kondo” temperatureT0 these experiments given evidence for (1) modification of the transferred hyperfine field [nonlinearK(χ)]. (2) spin fluctuations with a characteristic fluctuation rate ∼kBT0/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 μB/f atom) in CeAl3, CeCu2Si2, U1−xThxBe13 (x=0.033), and UPt3. There is some controversy concerning the interpretation of 1/T1 well aboveT0 in UBe13; the situation is reviewed.