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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Light scattering in ferroelectric crystals

Ryan, John Francis January 1973 (has links)
No description available.
2

Conduction properties of thin metallic films

Paterson, Charles January 1973 (has links)
No description available.
3

Magnetic excitations in transition metal alloys and roton-roton interactions in superfluid helium

Smith, Alastair James January 1976 (has links)
Neutron inelastic scattering experiments have measured the spinwave dispersion relations in an ordered (Pd3Fe) and a disordered (Pd0 75 Ni 0.25) transition metal alloy.
4

Magnetic properties of the jarosites : model kagome antiferromagnets

Pettigrew, Keith G. January 2002 (has links)
The long-range ordered ground state of KFe<sub>3</sub>(SO<sub>4</sub>)<sub>2</sub>(OH)<sub>6</sub> has been determined unambiguously through Spherical Neutron Polarimetry on a natural single crystal. The magnetic arrangement has been shown to be the q = 0 structure with triangles of only positive chirality. A new transition to a state of higher magnetisation has been identified in a synthetic sample at T ~ 7K. The driving force for this has been attributed to anisotropy in the system and theory provides a possible ordering mechanism with weak ferromagnetic order. Magnetic neutron scattering confirms an intermediate phase at 52K < T < 65K with the coexistence of Bragg peaks and significant diffuse scattering. D<sub>3</sub>OFe<sub>3</sub>(SO<sub>4</sub>)<sub>2</sub>(OD)<sub>6</sub> has been shown to exhibit spin-glass type behaviour with a freezing temperature, T<sub>f</sub>, that is dependent on the timescale of the measurement. Intentional dilution of the magnetic lattice with Al<sup>3+</sup> induces a transition to long-range order at a temperature dependent on the level of doping. Mössbauer spectroscopy shows two equally-weighed magnetic sites with local field value and probability in excellent agreement with theoretical predictions. A new mixed-cation compound, H<sub>3</sub>OFe<sub>2.4</sub>Cr<sub>0.3</sub>(SO<sub>4</sub>)<sub>2</sub>(OH)<sub>6</sub>, has been successfully prepared and represents the first experimental example of mixed magnetic ions on a kagome lattice. Field-cooled (FC) and zero-field-cooled (ZFC) dc susceptibility data cross at a temperature dependent on the magnitude of the applied field and hysteresis measurements suggest the presence of ferrimagnetism. High resolution neutron diffraction on D<sub>3</sub>OCr<sub>3</sub>(SO<sub>4</sub>)<sub>2</sub>(OD)<sub>6</sub> has shown that the crystal structure is slightly distorted from the <i>R ?m </i>to the <i>R 3 m </i>space group. A magnetic transition is observed at 2K through a sharp peak in ac susceptibility accompanied by a small FC/ZFC splitting. Polarised neutron diffraction shows no change in the magnetic correlation length, ξ, on cooling from 15K to 0.25K where only short-range order exists with ξ ~ 6.6Å, about half the width of a kagome star.
5

The design and characterisation of devices based on epitaxial magnetic garnets

Solomons, Brian January 1978 (has links)
The development of growth techniques for epitaxial film is shown to yield similar methods for growing bubbly domain material and films of yttrium-iron garnet (YIG) for delay lines. Growth by liquid-phase epitaxy is examined in detail. The development of sophisticated bubble films and YIG delay lines is discussed and shown to be a direct consequence of the ease with which this technique can be implemented. The characteristics of bubble domain films are discussed. The composition of different films which were grown in an attempt to meet optimum bubble characteristics is discussed together with the film defects. The deficiences of popular bubble propagation techniques are examined and a novel propagation technique is introduced and explained. This technique exploits the attributes of a bubble lying in an etched channel. The dynamics of such a bubble are studied in terms of its total internal energy. To identify the primary energy and force requirements which determine the stability of a bubble lying in the etched channel, theory is developed for, and experiments conducted on, bubbles lying in simple etched disc. A theoretical model of the bubble's movement in the etched channel is derived, and experiments conducted to verify this model. A possible manner of extending the etched channel propagation technique into a working device incorporating input/output functions is outlined. The wave equation for spin waves in YIG is derived and its solution shown to yield two types of spin waves - the surface and the volume wave. The dispersion characteristics of those waves is examined and those situations which elicit non-dispersive behaviour are identified. A YIG delay line which exploits this non-dispersive behaviour has been designed and built. The delay of this line is studied experimentally and its characteristic insertion loss measured. To obtain efficient transduction of the input r.f. signal to the magnetic spin wave, it is necessary to reduce the insertion loss. The criteria which have to be met to effect a reduction are shown to be dependent on the radiation resistance of the transducer. A model of the transducer radiation resistance is derived and experiments are carried out to verify this model.
6

The influence of magnetostatic interactions on the magnetic properties of magnetite

Virdee, D. January 1999 (has links)
The effect of magnetostatic interactions on the magnetisation structure and magnetic properties of magnetite grains for the size range 0.03 - 0.15 microns has been examined. The understanding of magnetostatic interactions is important because magnetite that occurs naturally by some precipitation process and in sedimentary rocks containing magnetosomes may form as magnetostatic interacting clumps of grains. This has implications in the way we perceive magnetic signals are recorded in rocks, bacterial magnetosomes found in sedimentary rocks, and also in magnetic recording media. The examination of magnetostatic interactions has been achieved by using a three-dimensional micromagnetic model which considers arrays of 4x4x4 cubic magnetite grains with uniaxial shape anisotropy and arrays of 6x6 or 4x4 cubic grains, arranged analogous to arrays of particles that may be produced by a method of electron beam lithography. The grains have a maximum spacing of one grain width, when they are seen to be non-interacting: or a minimum where they are almost touching, when they are interacting. Using this method it is possible to see how grain interactions alter the magnetic domain structure of uniform and non-uniform grains. The effect of interactions on bulk properties, such as coercivity and remanence can be examined from simulated hysteresis cycles. Properties are examined when the grains are arranged within the arrays such that their easy axes of magnetocrystalline anisotropy are aligned parallel with every other grain in the array, and in the case where the axes are in a randomly differing orientation from every other grain within the array. Hysteresis parameters for non-interacting and interacting arrays of grains are calculated, and these theoretical values are compared with previous theoretical micromagnetic models and experimental work. The results from this study are in good agreement with previous work.
7

The effects of screw dislocations on the magnetic properties of magnetite

Newcombe, Lee January 1998 (has links)
The values of the hysteresis parameters produced by micromagnetic models are usually lower than those found experimentally, and in the case of grains which have been subjected to stress the simulated values can be as much as an order of magnitude too low. It has been suggested that the presence of crystalline defects within these stressed grains may be responsible for raising the coercivity and saturation remanence values in comparison with unstressed grains. Grains of magnetite containing a regular array of screw dislocations are modelled for different grain sizes and different dislocation densities using a micro-magnetic model which considers the exchange, demagnetising, magnetoelastic, magnetocrystalline anisotropy and external field interactions. It is found that the values obtained from the new model for coercivity and saturation remanence rise with increasing dislocation density in line with experimental results. Models containing dislocations are found to have the magnetic properties of defect-free grains of smaller grain size, again in line with experiment. The unblocking temperature of a 1μm grain of magnetite in zero applied field is found to decrease in an approximately linear manner with increasing dislocation density. This thesis shows that by considering the magnetoelastic energy arising from the presence of screw dislocations the discrepancy between the hysteresis parameters of magnetite obtained theoretically and experimentally may be resolved.
8

Structural and magnetic studies of strongly correlated electronic systems

Oakley, Gareth S. January 2000 (has links)
Understanding of strongly correlated systems is of great importance in our understanding of fundamental solid-state science, and in the design and improvement of many technologically useful magnetic systems. In this thesis studies of two such systems are presented. The first system is the jarosite mineral family AM<sub>3</sub>(SO<sub>4</sub>)<sub>2</sub>(OH)<sub>6</sub> (where A = H<sub>3</sub>O, K; M = Fe, Cr) which is an experimental manifestation of a kagome lattice antiferromagnet. Such a lattice displays unusual magnetic behaviour which may be of direct relevance to high temperature superconducting materials. A variety of neutron experiments have been performed to investigate the nature of the spin dynamics in the case of the hydronium iron salt, which is unique to the iron series in not exhibiting long range magnetic order. Single crystal studies have been used to probe the nature of the ground state of the potassium salt, and the first unambiguous determination of the magnetic structure is presented. Neutron diffraction studies and muon measurements have been performed on the hydronium chromium salt, the behaviour of which appears to contrast with that of the iron analogue. The second system of study is the series of compounds La<sub>l-x</sub>M<sub>x</sub>MnO<sub>3</sub> (where M = Ca,Pb) which are of interest due to their potential application in read-write head devices. A combination of both dc susceptibility measurements and neutron diffraction studies have been used to investigate the magnetic behaviour of both these systems in key areas of the temperature-composition phase diagrams. The electronic fluctuations in the calcium system have been studied using muon spin relaxation techniques.
9

Studies of a dilute Heisenberg antiferromagnet and the ferroelectric transition in CsH2AsO4

Meyer, Gerald M. January 1976 (has links)
An alternative method of analysing coupled oscillator fits of Raman spectra is presented. It is compared with conventional methods of obtaining the temperature dependence of the ferroelectric mode in KDP type ferroelectrics. As continuous structural phase transitions are approached, an increase in both the Debye-Waller factor and the nuclear quadrupole coupling coefficient is predicted. The predicted form for uniaxial ferroelectrics is shown to differ from that at other transitions because of its macroscopic electric field. It is shown to be consistent with the temperature dependence of the quadrupole resonance in CsDA. The form at other transitions allows the critical exponent to be obtained. Previously reported structure in DCsDA is shown to result from multiple Bragg scattering in which two ferroelectric domains are involved. Variations in the hydrogen-deuterium concentration are believed to cause the 60K smearing of the transition. Measurements of the 12 acoustic mode are consistent with the elastic constant 066 being small. Measurements of the temperature dependence of the shear angle show that the transition in CsDA is first order. The neutron scattering from the magnetic excitations in the substitutionally disordered antiferromagnet Mn0_68Zn0 32F2 have been measured at 6K. The results show structure in the line shape which is similar to that predicted by some of the calculations based on the coherent potential approximation and to that predicted in recent computer simulations. Some of the critical exponents pertaining to the antiferromagnetic transition in the dilute disordered system, Mn 0.63Zn0 312, have been obtained by measuring the wave vector and temperature dependence of the neutron scattering. These are compared with the exponents found in the pure system, Mn?2, and also with some of the recent predictions of the renormalisation group.
10

A three-dimensional micromagnetic investigation of the magnetic properties and structures of magnetite

Thomson, Leslie Campbell January 1993 (has links)
This work is a three-dimensional micromagnetic study of the magnetic properties and structures of magnetite. Large magnetite crystals in the size range 10 - 50 μm were modelled using a relatively low resolution model. The vortex configuration was found to be stable for this size range. A flower type structure was also obtained with large, uniformly magnetised domains lying approximately in magnetocrystalline easy directions as in figure 6.1. A global optimisation algorithm called simulated annealing (SA) was used to minimise the total free magnetic energy and hence find stable structures for crystals in the sub-micron size range. The resolution of the model was limited by computer constraints when using SA for optimisation. The structures obtained were interpolated to a higher resolution and input as the initial configuration for a fast local optimisation technique called the conjugate-gradient method. This combination of techniques allowed high resolution models in the ground state configuration to be obtained. Below ˜ 0.06μm, nearly uniform structures magnetised in the magnetocrystalline easy directions were obtained. Between 0.06 μm and 1.0μm, only the vortex configuration was obtained. These are the lowest energy states found by any method to date. A modified SA algorithm was used to introduce thermal fluctuations into a micromagnetic model, and hence to determine blocking temperatures of grains up to 0.1 μm in size. The model gave results consistent with Néel's single domain thermoremanent magnetisation theory for grains up to 0.065 μm. Between this size and 0.07 μm a reduction in blocking temperature was observed to take place for cubic grains.

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