No description available.

A system for mass-selective capture of ions by a Paul trap was designed and tested. / An Paul trap and associated electronics have been assembled and tested, selectively trapping the ionic clusters C$ sp{+} sb2$ and C$ sp{+} sb3$ from a pulsed laser ion source. The ions were injected at 90 eV, trapped, extracted and detected, several parameters related to these processes were investigated. Simple models of the injection mechanism employed and for evaluating the pseudo-potential well depth of the ion trap are proposed. The overall efficiency of the ion injection system has been evaluated and compared to other methods of ion capture by an RFQ trap from an external source.

No description available.

The buffer gas cooling process and the properties of the cooled beam in a novel rf quadrupole ion beam guide with axial field was studied. The operation principle of the axial field quadrupole ion beam guide is similar to the quadrupole mass filter operating in AC only mode but the rods were segmented and differential DC biases were applied to produce the axial field. It was operated in buffer gas. / A Monte Carlo simulation was developed and the simulation results were compared with the experimental results. The beam distribution in a thermal equilibrium was assumed as the thermal equilibrium distribution in a simple harmonic motion with rf distortion and the assumption was confirmed by the Monte Carlo simulation. / The temperature of the cooled beam was measured with a beam profile, measurement system which consists of a miniature quadrupole ion beam guide aid a rotating semi-circular Faraday plate. The temperature of the cooled beam was measured to be 0.0424 +/- 0.0026 eV in the experiment while that of the simulated beam was 0.0372 +/- 0.0009 eV. The variation of beam temperature with beam current was 0.0153 +/- 0.0026 eV/nA. The maximum achieved ion transmission was 80%. The transmitted ion beam's energy spread was measured with a retarding potential energy analyzer to be less than 0.1 eV. The cooling time was 0.7 ms with 1V/ cm axial field and was observed to be very dependent on the ion current when there was no axial field.

No description available.

Motivated by studies of non-magnetic dopings of pyrochlore spin ice and the experimental realization of the hyperkagome lattice in Na4Ir3O8, this work studies the dipolar ice model on the hyperkagome lattice. This is a local <111> Ising model with classical spins featuring an antiferromagnetic neighbour exchange as well as a long range dipolar interaction, previously studied on the pyrochlore lattice. A hybrid single spin flip/loop algorithm Monte Carlo code has been developed to address ergodicity issues seen at low temperatures. This algorithm agrees with analytical results for the smallest system size and has been extended to larger system sizes. A phase diagram very similar to that of the pyrochlore lattice is found with an antiferromagnetic region as well as regions with a spin ice crossover and a low temperature ordering transition. An additional charge-ordered state, similar to that in the recently studied kagome spin ice, was also found.

No description available.

The persistent photoconductivity (PPC) effect in the diluted magnetic semiconductor Cd(_1-x)Mn(_x)Te:In has been studied in detail. Electrical transport measurements have been made on a large number of samples to build up an understanding of the phototransport properties of this material. In particular, the compositional dependence of the phototransport parameters has been measured up to x ≈ 0.2. Several samples exhibit an elevated temperature PPC effect which has been interpreted in terms of the formation of multiple DX centres. These samples can have a quenching temperature of up to 190 K, suggestmg that Cd(_1-x)Mn(_x)Te:In could be an interesting material in terms of applications of room temperature persistent photoconductors. The low field magnetoresistance has been measured and analysed quantitatively in order to attempt to identify the origin of the magnetoresistive effects in insulating and metallic samples. The positive magnetoresistance has been found to be linked with the magnetization of the sample. An anomalous negative magnetoresistance has been observed tinder certain experimental conditions. This negative magnetoresistance has been interpreted in terms of the formation of bound magnetic polarons and their contribution to spin-disorder scattering. The main body of this thesis is concerned with the study of the Metal-Insulator Transition (MIT). The PPC effect allows us to study the MIT in a continuous fashion by fine timing the carrier density by illumuiation. In this way we have made the first zero magnetic field study of the MIT in a magnetic semiconductor. The critical behaviour has been found to be consistent with the scaling theory of electron localization, which predicts a critical form σ = σ(_0)(n/n(_c) – 1)(^v). The critical conductivity exponent, v was determined to be close to one, while the critical carrier density, n(_e), was found to be ~ 2 x 10(^17) cm(^-3), for x = 0.08. The temperature dependence of the conductivity has been quantitatively analysed m both the metallic and insulating phases. On the insulating side of the transition, variable range hopping (VRH) conduction has been observed at low temperatures (down to 300 mK). The temperature dependence is consistent with VRH conduction with electron-electron interaction effects taken into account. In the metallic phase the temperature dependence of the conductivity (up to ~ 1 K) is consistent with a model where the zero temperature value of the conductivity is corrected by electron-electroninteraction effects, and the effects of weak localization. The magnitudes of these corrections are found to be in reasonable agreement with theoretical predictions. The electrical transport has also been studied in the weakly localized regime in Cd(_1-x)Mn(_x)Te:In and Cd(_1-x)Mn(_x)Te:In, Al. A rapid decrease in the conductivity occurs at low temperatures ( < 1.5 K). This is interpreted in terms of the effect of the s-d exchange interaction, which leads to the formation of bound magnetic polarons. It is suggested that this drop in conductivity can only be observed in the paramagnetic phase, and that spinglassordering has a significant effect on the temperature dependence of the conductivity at low temperatures.

This thesis focus on the magnetic behavior, from single atoms to bulk materials. The materials considered in this thesis have been studied by computational methods based on ab initio theory, density functional theory (DFT), including treatment of the spin-orbit coupling, non-collinear magnetism, and methods capable of treating discorded systems. Furthermore strongly correlated materials have been investigated using the dynamical mean field theory (DMFT). The uniaxial magnetic anisotropy energy (MAE) of the Fe2P was investigated using the full-potential linear muffin tin orbital (FP-LMTO) method. Based on a band structure analysis, the microscopical origin of the large magnetic anisotropy found for this system is explained. It is also shown that by straining the crystal structure, the MAE can be enhanced further. This opens up for the possibility of obtaining a room temperature permanent magnet based on the Fe2P. The spectral properties of Fe impurities in a Cs host have been investigated, for both surface and bulk systems, by means of combination of density-functional theory in the local density approximation and the dynamical mean-field theory (LDA+DMFT), using two different impurity solvers, the Hubbard I approximation (HIA) and the Exact Diagonalization (ED) method were used. It is shown that noticeable differences can be seen in the unoccupied part of the spectrum for different positions of Fe atoms inside the host. The calculations show good agreement with the experimental photoemission spectra. The stability of the 12-fold metal-phosphorous coordination, existing in the meteorite mineral melliniite has been investigated trough total energy calculations using the coherent potential approximation (CPA) combined with an analysis of the chemical bonds, performed by balanced crystal overlap population (BCOOP). It was shown that its uniquely high metal–phosphorous coordination is due to a balance between covalent Fe–P binding, configurational entropy and a weaker nickel–phosphorus binding. Supported clusters have drawn a lot of attention as possible building blocks for future data storage applications. This topic was investigated using a real space noncollinear formalism where the exchange interactions between Co atoms were shown to be tuned by varying the substrate surface composition. Furthermore the spin dynamics of small Co clusters an a Cu(111) surface have been investigated and a new kind of dynamics, where magnetization switching can be accelerated by decreasing the switching field, has been found. A method for calculating the electronic structure for both ordered and disordered alloys, the augmented space recursion (ASR) method, have been extended to treat non-collinear magnetic order. The method has been used to investigate the energy stability of non-collinear arrangements of MnPt and Mn3Rh alloys.

Naval submarines and surface ships are regularly subjected to a treatment called &quotdeperming&quot that seeks to design the vessel???s permanent magnetisation for optimal magnetic camouflage. A scaled model of a magnetic treatment facility (MTF) has been established as a valid system to simulate deperming and used to investigate various aspects of the deperm process including: magnetic anisotropy and demagnetising fields as factors in the physical modelling of magnetism in whole vessels; a comparison of current and alternative deperm procedures; the application of theoretical models of bulk magnetisation to calculate deperm outcomes in the physical model and in actual vessels. A &quotlaboratory MTF&quot was constructed to imitate the applied field geometry at a naval MTF. The system was calibrated and it was determined that the laboratory MTF could make magnetic measurements on a CU200T-G steel bar sample with an equivalent accuracy (error = ??5%) to that of standard magnetometric equipment. Experiments were conducted with emphasis on a holistic approach to modelling the deperm process and describing magnetisation changes in whole objects. The importance of the magnetic anisotropic changes to steel with cold rolling was confirmed. In CU200T-G steel sheet the initial susceptibility (ci) was found to increase by a factor of 3 ??0.1 in the rolling direction, from a value of ~ 110 in the un-rolled steel sheet (thickness dependent). ci in the rolled sheet transverse to the rolling direction was decreased by a factor of 0.94 ??0.09 to ci in the un-rolled sheet steel. Previous studies on hull steel have neglected to account for this transformation through cold work. The demonstration on mild steel here is expected to have an analogy in the final state of the hull sheet steel as it resides in a submarine pressure hull. Future studies either on hull material or on modelling whole vessels should include the same or similar magnetic anisotropic properties in the steel(s) under investigation. Hollow circular tubes made from CA2S-E and CU200T-G steel sheet were selected as models for vessels. It was shown that these steel tubes were a good choice in this regard: minimising the complexity of the experiment whilst maintaining the validity of a deperm simulation. During a deperm there was an excellent qualitative likeness in the permanent longitudinal magnetisation (PLM) for the steel tubes to PLM in both a submarine and a surface vessel. Permanent vertical magnetisation (PVM) deperm results from the tubes displayed a close qualitative match with PVM in a submarine but not in a surface vessel. A theoretical treatment for demagnetisation factors (Nd) in hollow ellipsoids was used in conjunction with a geometrical approximation to calculate Nd for finite hollow objects of revolution. Subsequent theoretical calculations correlated well with experimental results for measured effective ci (ceff) in hollow circular CU200T-G steel tubes of various lengths and aspect ratios. Using an estimate of 100 as ci for submarine hull steel, the same analysis produces Nd for the axial and transaxial directions in a submarine equal to 5.97??10-3 and 0.0142 respectively. Three items for potential improvement were identified in the current deperm protocol used on naval vessels (Flash-D): redundancy in the protocol; the duration of the deperm and a theoretical basis for predicting the final magnetisation or changes in magnetisation during a deperm. Simulations of a novel &quotanhysteretic deperm&quot method, designed to combat these issues, compared favourably to the Flash-D protocol. The standard deviation (s) of the final PVM from 30 Flash-D deperms on steel tubes was 206 A/m; for the final PVM from 30 anhysteretic deperms of the same duration, this was 60 A/m. The s for the final PLM for Flash-D and anhysteretic deperms of the same duration were 416 A/m and 670 A/m respectively. The conclusion is that adopting the anhysteretic deperm on actual vessels would improve the reliability of the PVM outcome. Though the procedure would demand the same duration as Flash-D, there is the advantage of saving time by not having to repeat deperms to obtain the desired result. Additionally the anhysteretic deperm is considerably more amenable to theoretical analysis. A modified version of Langevin???s equation was used to predict the final PLM and PVM results for anhysteretic deperms and to provide a useful analysis of the anhysteretic processes in the Flash-D procedure. Using a Preisach analysis of hysteresis, a mathematical description of bulk magnetic changes that occur to a specific object, within a deperm, has been developed. Theoretical calculations of PLM in a steel tube during and after both types of deperm are in excellent agreement with experimental data. The same theoretical approach was also used to retrospectively model PLM results from previous Flash-D deperms on a submarine with equal success. With this analysis it is proposed that anhysteretic deperm outcomes could be predicted a priori. The influence of magnetic cargo on hull magnetisation was demonstrated to be of significance during and after deperming. &quotSympathetic deperming&quot occurs where a magnetic source is located close to the hull during a deperm. It was found that a vessel or model vessel hull could still be demagnetised even when they contain magnetic cargo that would normally resist the direct application of the same magnetic fields. This was explained using the principles of demagnetising fields and anhysteretic magnetisation. A possible explanation was provided for a PVM measurement anomaly common to the model and vessel deperm results. From measurement, alternating longitudinal applied fields apparently induce corresponding changes in the PVM. This effect could be explained by the depermed object being offset longitudinally from the position expected by the measurement system. This offset could be estimated using an analysis of the changes to PLM and PVM after a longitudinal applied field. The offset displacements calculated for the vessels were too small to be verified experimentally (&gt 0.1m), but the predicted offset for the steel tubes coincided with the limit of precision for their placement in the laboratory MTF = 0.5mm The aim of this work was to look at the deperm process with reference to a system that demonstrated qualitative similarities to deperms on actual vessels. The laboratory MTF is a unique facility, permitting a useful practical analysis of deperming based on sound magnetostatic measurements The experimental and theoretical results gained here have direct application to future deperms on naval vessels with particular reference to submarines.