A survey is given of magnetic excitations in metallic 4f systems when the crystal-field splitting is comparable to or larger than the magnetic interaction between different sites. Particular emphasis is put on the interaction of crystal-field split rare-earth ions with conduction electrons. A number of physical effects is discussed which result from that interaction. The modifications of the magnetic excitations are described which result when the coupling of phonons to the crystal-field levels is so strong that a bound state between the two types of excitations is formed. For a study of these phenomena the method of neutron scattering is an indispensable tool. For that reason it plays a central part in this review.

Magnetic excitations in crystal-field split 4f systems

The contribution that the technique of electron spin resonance has made to our understanding of the behaviour of localized moments in metals is reviewed. The main areas of application of the technique are discussed at a simple theoretical level and its limitations are pointed out. A comprehensive review of the experimental results obtained from electron spin resonance is presented and, in appendices to the article, a comprehensive list of experimental results is given indexed to author, impurity and host and to areas of application. In a final section of the review an attempt is made to suggest those areas in which electron spin resonance still has an important part to play in adding to our understanding of magnetism in metals.

Electron spin resonance of magnetic ions in metals an experimental review

For pt.I, see ibid., vol.9, no.9, p.1745 (1979). Starting from the many-electron angular 4f wave functions the author performed a numerical calculation of the exchange coulombic contribution of the conduction electrons to the crystalline electric field in cubic rare earth intermetallics. The results are applied to the compounds DyRh, DyCu, DyZn, LaAg and YMg. The exchange contribution to the sixth-order CEF parameters arising from the f character of the conduction band is found to be one order of magnitude larger than the direct contribution.

Contribution of the conduction electrons to the crystalline electric field. II. Exchange coulombic part

We report on evidence of crystal-electric-field effects in the low temperature thermal expansion of various rocksalt-type rare earth compounds (PrSb, SmSb, ErSb, TmSb, CeTe, TmTe). From our measurements we deduce the volume dependence of the energy splitting between the lowest crystal field levels and we calculate magnetoelastic coupling parameters involving the volume strain.

Crystal field effect in the thermal expansion of cubic rare earth compounds

The screening effects of conduction electrons on the crystal field coefficients in metals are investigated and compared with an elementary (unscreened) point-charge model. The size of the interaction remains of the same order of magnitude but may be reversed in sign, both effects being in accordance with observations on metallic materials containing rare earth atoms.

Theory of screening effects on crystalline electric fields in metallic rare earth materials

The effect of 5d character on the crystalline electric field in noble metal rare earth alloys and in pure rare earths has been investigated. The change in sign of the fourth order CEF coefficient in the noble metal alloys can be interpreted in terms of this d character. From symmetry this d like component has no effect on the sixth order coefficients whose varying magnitude in these alloys can be explained using the f like contribution. In the pure rare earth metals it is likely that d character will predominate.

J.M. Dixon

Papers

The effect of d electrons on crystal field potentials in rare earth metals and dilute alloys