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Title: Spin fluctuations in manganese intermetallics
Author: Horn, Clare Jane
ISNI:       0000 0001 3581 7027
Awarding Body: University of Southampton
Current Institution: University of Southampton
Date of Award: 2007
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Magnetic ground states and dynamics in manganese systems have been investigated for two different types of structure, the binary ~-Mnl.xMx and ternary RMn4AIs intermetallics where M represents cobalt or ruthenium and Ris a rare earth elemenVyttriumlscandium. For ~-Mn alloys containing cobalt impurities, astatic ground state of spin glass-like character has been stabilised for concentrations above 5% Co; there is an absence however of long range order in these systems, up to the highest measured concentration of 25at.% Co. The ~-Mnl.xRux alloys enter aspin glass-type state for all measured concentrations (above x=0.03), in conjunction with a higher-temperature transition to along range antiferromagnetically ordered state for x= 0.12, coexistent with the spin glass phase. These findings are evidenced by peaks in the muon spin relaxation rate, with acharacteristic stretched exponential relaxation function observed below the spin glass transition temperature. Further support is provided by susceptibility and polarized neutron measurements. Clear magnetic Bragg peaks in neutron powder diffraction data, in addition to specific heat anomalies, confirm the presence of the long-range ordered state in the ruthenium alloys. Neutron Iineshapes in anumber of samples of both series may also imply Non-Fermi liqUid scaling. These results are of particular significance in view of the fact that cobalt and ruthenium solutes are shown to substitute onto Site I of the ~-Mn crystal structure, as opposed to the more strongly magnetic Site II as for previously reported spin glass alloy systems, in which the impurity lifts magnetic frustration by removal of configurational degeneracy. Expansion appears to playa key role in ~-Mnl.xRux, with the introduction of ruthenium appearing to drive the system to an intermediate state between the weak itinerant and localized moment regimes. In the RMn4Als compounds, a range of interesting behaviour has been observed: this includes the first evidence of a magnetic phase transition in LaMn4Als, in the form of a peak in the low temperature J.lSR data together with weak Bragg peaks in polarized neutron measurements; PrMn4AIs has shown an oscillatory muon response, indicative of long range order in the Mn sublattice since inelastic neutron data shows Pr3+ to be in a singlet ground state. For systems with heavy rare earths (R = Er, Tb, Dy, Gd and Ho), interaction between the Rand Mn sublattices has been implied by J.lSR measurements, in which separate relaxation contributions have been ascribed to each species. For the RMn4Als with R= Y, Ce and Sc, aspin gap reported previously has been directly observed with inelastic neutron scattering. This is reflected in atwo-featured response in LaMn4Als, and may have its origins in aquasi-one dimensional Mn sublattice.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available