1.1 Introduction 1.2 Localisation, valence and magnetism 1.3 From the Kondo impurity to the Kondo lattice 1.4 The Doniach model 1.5 Spin fluctuations and the non Fermi liquid properties 1.6 Quantum phase transition 1.7 Fermi surface 1.8 Comparison with He 1.9 Experiments : crystals, pressure, method 1.9.1 Materials, measurements 1.9.2 From transport measurement to heavy fermion properties

On the Heavy Fermion Road

The heavy fermion properties are reviewed on selected examples often studied in Grenoble. The physical problems are presented as example of exotic matter at low temperature with three major insights: the localisation of the 4f electrons, the collapse of the long range magnetic order and the entrance in an intermediate valence regime. The question on the first or second order nature of the quantum phase transition will be discussed. In the first example, the Ising Kondo lattice of CeRu2Si2, experiments seem to be described with the picture of 4f itinerant electron dominated by their spin fluctuations; the strong feedback, induced in magnetic field, between magnetostriction and switch in the nature of the magnetic interactions illustrates the proximity of a quantum critical field. Comparison is made with CeCu6 or YbRh2Si2 series where the idea of local criticality seems to emerge. Basic arguments are discussed for the breakdown of the idea that Yb heavy fermion systems are the the hole analogue of the cerium ones. Special focus is given on the interplay between unconconventional superconductivity and magnetism either for dominant antiferromagnetic or ferromagnetic correlations. Huge electronic Gruneisen parameter leads often to strong concomitant 4f density and spin fluctuations >.The main boost given by the discovery of new materials as quasi-bidimensional Ce 115 compounds or recently by CePt3 Si or skutterudites is spectacular for the theoretical developments. Finally, the now canonical cases of the UPt3, URu2Si2, UPd2Al3 and UBe13 give now a sound basis in the rich domain of unconventional superconductors.

Publisher Summary The muon spin rotation/relaxation/resonance (μSR) spectroscopy has become an important tool for probing local static and dynamic magnetic properties. In over two decades a quite substantial body of μSR data has been collected from rare-earth and actinide materials, the latter, however, mostly restricted to samples containing uranium. μSR is the measurement of the temporal development of the spatial orientation of the spins of muons, which have been implanted in the material of interest with all spins initially fixed in one direction (complete muon spin polarization). μSR can be performed either with positive or negative muons. The latter behave like heavy electrons and will be captured into Bohr orbits by the atoms in the sample material. They will quickly reach the ls ground state, whose orbital radius, however, is comparable to nuclear radii. Negative muons thus sense predominantly the effects of the protonic charge distribution and the result is primarily of interest to nuclear physics. The magnetic field at the site of the muon in a magnetic compound is, at least in part, created by the dipole moments on neighboring paramagnetic atoms or ions.

Chapter 206 μSR studies of rare-earth and actinide magnetic materials

Chapter 2 Magnetism of compounds of rare earths with non-magnetic metals

The Hall coefficient RH, resistivity ρ, and Seebeck coefficient S of the CeAl2 compound with fast electron density fluctuations were studied in a wide temperature range (from 1.8 to 300 K). Detailed measurements of the angular dependences RH(ϕ T, H≤70 kOe) were performed to determine contributions to the anomalous Hall effect and study the behavior of the anomalous magnetic RHam and main RHa components of the Hall signal of this compound with strong electron correlation. The special features of the behavior of the anomalous magnetic component RHam were used to analyze the complex magnetic phase diagram H-T determined by magnetic ordering in the presence of strong spin fluctuations. An analysis of changes in the main contribution RHa (H, T) to the Hall effect made it possible to determine the complex activation behavior of this anomalous component in the CeAl2 intermetallic compound. The results led us to conclude that taking into account spin-polaron effects was necessary and that the Kondo lattice and skew-scattering models were of very limited applicability as methods for describing the low-temperature transport of charge carriers in cerium-based intermetallic compounds. The effective masses and localization radii of manybody states in the CeAl2 matrix were estimated to be (55–90)m0 and 6–10 A, respectively. The behaviors of the parameters RH, S, and ρ were jointly analyzed. The results allowed us to consistently describe the transport coefficients of CeAl2.

Genesis of the anomalous Hall effect in CeAl2

The previously accepted spatially modulated magnetic structure of CeAl2 is shown to be incorrect. Neutron diffraction measurements in an applied magnetic field imply that the zero-field magnetic structure is spiral: however, the moments on the two Ce sublattices rotate in opposite senses. In the author's model, the magnitude of the Ce ordered moment varies little from site to site; a non-magnetic ground state for the Ce3+ ion is no longer required, and the extreme weakness of the harmonics is explained.

https://iopscience.iop.org/article/10.1088/0953-8984/2/50/026/pdf

B D Rainford

Papers

The magnetic structure of CeAl2 is a non-chiral spiral