Neutron inelastic scattering studies of magnetic excitations in rare earth compounds and transition metal alloys

Osborn, Raymond (1989) Neutron inelastic scattering studies of magnetic excitations in rare earth compounds and transition metal alloys. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

This thesis presents the results and analysis of neutron inelastic scattering investigations of magnetic excitation spectra in the rare earth compounds NdAl₂ and CeAl₂, and the transition metal alloy, γ-MnNi. The experiments were performed on triple-axis spectrometers at A.E.R.E., Harwell, Riso National Laboratory and the Institut Laue-Langevin, Grenoble. The three systems cover a wide spectrum in the field of magnetism with well-defined local moments in NdAl_2 and itinerant electron antiferromagnetism in γ-MnNi. In CeAl₂, the local moments are weakly hybridized with the band electrons. The contrasting ground states are reflected in very different magnetic excitation spectra. The temperature dependence of the crystal field excitons in the [110] direction has been measured in NdAl₂. The energies of the lowest acoustic and optic `magnon' branches increase with temperature up to 43 K (T_{c =} 77.2 K) across the Brillouin zone. High resolution measurements, using a cold-source spectrometer, show that the energy gap increases from 0.78 meV at 4.2 K to 1.25 meV at 29 K. This unusual behaviour is seen to be consistent with the predictions of a generalised RPA theory and arises from a renormalisation of the mode energies up to the molecular field energy. A detailed analysis yields revised estimates of the crystal field and exchange parameters. The magnetic excitation spectrum in the sinusoidally-modulated antiferromagnet, CeAl2, has been measured along several symmetry directions. Broadened inelastic excitations form `acoustic' and `optic' bands with little dispersion except close to the magnetic satellite wavevectors. The results are discussed in the light of recent theories of excitation in spin density waves. The spin wave dispersion for the nearest commensurable analogue, a four-sublattice antiferromagnet, is calculated and used to estimate the strength of the interionic exchange which is shown to be anisotropic. In the subsequent chapter, it is shown that the sinusoidal modulation produces a strong enhancement of the linear coefficient of the low-temperature specific heat consistent with experiment. The effect of the modulation on other experimental measurements is discussed. Finally, the degree of exchange enhancement of the Ce moment is estimated in the light of the stability of the modulation at low temperatures. The antiferromagnetic magnons in the disordered alloy, γ-Mn_.73Ni._27, are measured up to 50 meV at room temperature and shown to persist above the Nél temperature. A phenomenological method of the transverse susceptibility is convolved with the spectrometer resolution function and fitted to the data. In conjunction with earlier measurements at higher energy, the magnon dispersion is estimated and the strong q-broadening is shown to increase linearly with energy. The results are consistent with estimates from band-structure calculations.

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