Spin fluctuations in metallic magnets

Dakin, Simon John (1992) Spin fluctuations in metallic magnets. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

A detailed study has been undertaken of spin fluctuations in the concentrated Kondo series of intermetallics CeRu₂Si_2-xGe_x, and in the antiferromagnetic systems YMn₂, Y(Mn_0.96Fe_0.04)₂, (Y_0.97Sc_0.03)Mn₂, Y(Mn_0.97Al_0.03)₂, and Y(Mn_0.9Al_0.1)₂. Monte Carlo simulation techniques have been used to separate the magnetic and phonon scattering cross sections in inelastic neutron time of flight experiments. From an initial representation of the single phonon density of states, easily obtained from high wavevector transfer measurements, the total phonon cross section was calculated exactly. Each order of phonon scattering event was derived from the simulation. The single phonon events were found to have a quadratic dependence on the wavevector transfer, whereas the phonon multiple scattering cross section was found to be independent of the wavevector transfer. The cross over from good local moment behaviour in CeRu₂Ge₂ to strongly hybridized heavy fermion behaviour in CeRu₂Si₂ has been investigated. Single crystal SQUID magnetometer measurements on the series of alloys CeRu₂Si_2-xGe_x revealed the magnetic ground state in these alloys to be almost pure mid ± 5/2 > , with very strong anisotropy giving an easy direction along the tetragonal c-axis. The two excited energy doublets from the crystal field split ground state were found to lie at ≈40 meV and ≈80 meV. The lower of these levels was also measured with high energy neutrons. Power neutron diffraction measurements suggested a critical germanium concentration of x= 0.07 for long range magnetic order at low temperatures. CeRu_2Ge_2 was found to have the full saturation moment, with the cerium moment decreasing with increased hybridization. The magnetic ground state was found to be modulated with Q≈1/3 (2π/a). A metamagnetic anomaly was found in high field VSM measurements, associated with a collapse of antiferromagnetic spin correlations. This anomaly was observed at lower applied fields with increasing germanium concentration. Inelastic neutron measurements on this series of alloys gave a linear temperature dependence of the single ion linewidth above 50 K, with a low temperature intercept, taken to be the Kondo temperature. T_K was found to have an exponential dependence on the coupling parameter, as expected from the theory of the Kondo effect. The single ion susceptibility was found to be Curie-like. The linewidth modulation with wavevector transfer revealed the intersite RKKY exchange interactions, J(0), to decrease with decreasing temperature.