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Magnetic materials synthesis using plasma focus devices
Author
Pan, Zhenying
Supervisor
Rawat, Rajdeep Singh
Abstract
Magnetic nanoparticles have become a particularly interesting class of nano-materials for ultra-high density magnetic data storage applications where nanoparticles with as small size as possible but high magnetocrystalline anisotropy constant (i.e. magnetically as hard as possible, for stability of data bit) are most desirable. Similarly, for the most effective writing/reading of ultra-high density data, the magnetic write/read head is required to be magnetically as soft as possible with high resistivity and high saturation magnetic flux density. This thesis presents three key experimental investigations, using high energy density plasma focus (DPF) devices, on these important issues that include the synthesis of L10 phase hard magnetic nanoparticles of FePt and CoPt with narrow-sized distribution for possible applications in high density recording media and deposition of soft magnetic thin films of nanophase FeCo for possible applications in reading/writing of ultra-high density magnetic data. Additionally, the theoretical work on the simulation of thermal evolution of materials upon energetic pulsed ion beam irradiation from the pulsed DPF device was undertaken and presented in the thesis.
The theoretical simulation provides us a better understanding of the interaction between high energy pulsed ion beam and the irradiated material. The corresponding simulation codes is developed by MATLAB using finite difference method with respect to the geometry and physical characteristics of the ion beam, the ion-solid interaction process and the thermal properties of materials. The simulation results show that a strong thermal shock, consisting of a fast heating and quenching, takes place during the ion beam irradiation.
The first experimental investigation reports the use of UNU/ICTP DPF device as an energetic pulsed ion irradiation source to irradiate PLD grown FePt thin film samples to achieve the nanoparticles formation and lower annealing temperature for phase transition from low Ku fcc phase to high Ku fct. Two set experiments of the ion irradiation were performed: (i) using different numbers of focus shots and (ii) using different distances of irradiation from the anode top. It is found that the PLD synthesized FePt thin films are nanostructurized successfully with lower annealing temperature of 400 °C for phase transition and significantly enhanced magnetic properties, especially high coercivity.
The second key experimental work was undertaken with an aim to directly synthesize the CoPt nanoparticles of as small particle size as possible with narrow size distribution using an NX2 DPF device. To achieve this goal, the NX2 DPF device was operated in two different modes: (i) the optimized focus mode and (ii) the non-optimized focus mode. In the optimized focus mode, it is observed that the morphological features (nanostructure formation, shape and size) depend strongly on the filling gas pressure and the number of plasma focus deposition shots. The phase transition took place at an annealing temperature of 600 °C with coercivity being enhanced significantly. In the non-optimized focus mode, the more uniform and small sized CoPt nanoparticles were achieved directly in the magnetically hard (001) oriented fct structured L10 phase without any post deposition annealing.
Finally, the synthesis of magnetically soft granular FeCo thin films using a 120 J ‘FMPF-1’ (Fast Miniature Plasma Focus-1) device was undertaken and reported. It is worth to highlight that the miniature plasma focus device has been used for the first time for thin film deposition. The experiments were performed using two different species of filling gases (hydrogen and nitrogen) on Si(100) substrate and on three different substrates (i.e. Si(100), MgO(100) and amorphous Al2O3) to optimize the gas and substrate species. The FeCo films deposited on Si(100) with hydrogen as the filling gas were found to have (110) texture, narrow grain size distribution and soft magnetic properties. The magnetic properties of granular FeCo thin films have strong correlation with the stress and the grain size. The experimental coercivity value for this sample is also consistent with the theoretical estimation done using ripple theory. Hence, it was demonstrated that the DPF device, as convenient and copious source of energetic ions and electrons, can be used as an effective facility for ion irradiation and deposition of magnetic material synthesis.
The theoretical simulation provides us a better understanding of the interaction between high energy pulsed ion beam and the irradiated material. The corresponding simulation codes is developed by MATLAB using finite difference method with respect to the geometry and physical characteristics of the ion beam, the ion-solid interaction process and the thermal properties of materials. The simulation results show that a strong thermal shock, consisting of a fast heating and quenching, takes place during the ion beam irradiation.
The first experimental investigation reports the use of UNU/ICTP DPF device as an energetic pulsed ion irradiation source to irradiate PLD grown FePt thin film samples to achieve the nanoparticles formation and lower annealing temperature for phase transition from low Ku fcc phase to high Ku fct. Two set experiments of the ion irradiation were performed: (i) using different numbers of focus shots and (ii) using different distances of irradiation from the anode top. It is found that the PLD synthesized FePt thin films are nanostructurized successfully with lower annealing temperature of 400 °C for phase transition and significantly enhanced magnetic properties, especially high coercivity.
The second key experimental work was undertaken with an aim to directly synthesize the CoPt nanoparticles of as small particle size as possible with narrow size distribution using an NX2 DPF device. To achieve this goal, the NX2 DPF device was operated in two different modes: (i) the optimized focus mode and (ii) the non-optimized focus mode. In the optimized focus mode, it is observed that the morphological features (nanostructure formation, shape and size) depend strongly on the filling gas pressure and the number of plasma focus deposition shots. The phase transition took place at an annealing temperature of 600 °C with coercivity being enhanced significantly. In the non-optimized focus mode, the more uniform and small sized CoPt nanoparticles were achieved directly in the magnetically hard (001) oriented fct structured L10 phase without any post deposition annealing.
Finally, the synthesis of magnetically soft granular FeCo thin films using a 120 J ‘FMPF-1’ (Fast Miniature Plasma Focus-1) device was undertaken and reported. It is worth to highlight that the miniature plasma focus device has been used for the first time for thin film deposition. The experiments were performed using two different species of filling gases (hydrogen and nitrogen) on Si(100) substrate and on three different substrates (i.e. Si(100), MgO(100) and amorphous Al2O3) to optimize the gas and substrate species. The FeCo films deposited on Si(100) with hydrogen as the filling gas were found to have (110) texture, narrow grain size distribution and soft magnetic properties. The magnetic properties of granular FeCo thin films have strong correlation with the stress and the grain size. The experimental coercivity value for this sample is also consistent with the theoretical estimation done using ripple theory. Hence, it was demonstrated that the DPF device, as convenient and copious source of energetic ions and electrons, can be used as an effective facility for ion irradiation and deposition of magnetic material synthesis.
Date Issued
2011
Call Number
QC765 Pan
Date Submitted
2011