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Pulsed laser deposition of nanostructured magnetic materials
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Type
Thesis
Author
Lin, Jiaji
Supervisor
Rawat, Rajdeep Singh
Abstract
Nanostructured magnetic materials exhibit novel physical and chemical properties which are different from their bulk materials due to the length scale effects, and in turn attract intense research interests in both scientific exploration and technological applications ranging from magnetic data storage to biotechnology. It was reported that there would be at least a 10-fold increase in data storage density from currently Gb/in2 to Tb/in2 range if nanostructured magnetic materials, such as nanoparticles or nanocomposite thin films, can be used instead of multi-grains to store information. Therefore, in this research project, the synthesis and characterization of nanostructured magnetic materials were studied which may find possible applications in future ultra-high density magnetic data storage.
As a part of this research project, magneto-optical Kerr effect (MOKE) magnetometer, which measures the change in polarization of reflected lights from the surface of magnetic materials, together with its automation program were designed and developed to study their magnetic properties. The short laser penetration depth, typically about 10-20 nm, allows MOKE magnetometer to investigate the surface magnetism of metallic thin films at their ultra-thin limit. The effects of operation parameters and environment, such as background room light intensity and chopper frequency, on the signal-to-noise ratio of MOKE magnetometer were studied and its optimum operation conditions were proposed.
Pulsed laser deposition (PLD) system, as another part of this research project, was designed and developed for the synthesis of nanostructured magnetic materials due to its prominent advantages of stoichiometry preservation, versatility and simplicity. The effects of deposition conditions, such as ambient gas pressure and target-substrate geometry, on deposited magnetic materials were studied by investigating plume dynamic as well as Fe thin films deposited with different deposition conditions. Deposited magnetic materials can be tailored among smooth thin films, nanoparticle agglomerates and dispersive flocculeslike nanoparticle networks by controlling the deposition conditions. In addition, as special target-substrate geometry, backward plume deposition (BPD) was employed which can synthesize nanostructured magnetic materials that are similar to those deposited by conventional PLD, and was found to have the feasibility to overcome two major drawbacks of PLD techniques, low deposition rate and large number of laser droplets.
Finally, FePt nanoparticles and FePt based nanocomposite thin films, which are recognized as the most prominent candidates of ultra-high density magnetic data storage media, were synthesized using PLD system and investigated by various characterization techniques, such as MOKE magnetometer, transmission electron microscope (TEM), etc. FePt nanoparticles, in the compact forms of nanoparticle agglomerates and nanoparticle networks, can be synthesized by conventional PLD as well as BPD. Nanoparticle agglomerates deposited by BPD are found to have higher uniformity in terms of agglomerate size and size distribution in addition to their other advantages of higher deposition rate and less laser droplets. In both cases, as-deposited FePt nanoparticles exhibit low Ku face-centered-cubic (fcc) phase and thermal annealing at 600 °C is required for their phase transition to high Ku face-centered-tetragonal (fct), which cause grain growth and agglomeration and in turn increase their exchange coupling effects. To reduce exchange coupling effects, ion irradiation using plasma focus device were employed not only to induce nanostructuring of pulsed laser deposited FePt thin films to form wellseparated nanoparticles but also to reduce their phase transition temperature from 600 °C to 400 °C. Compared to hours of other continuous ion sources, single shot ion irradiation, with pulse duration of few hundreds of nano-second (ns), is a novel fast technique which achieves the desired results in a much shorter time. In order to further minimize the annealing effects and reduce the exchange coupling effects, non-magnetic matrix material, Al2O3, was introduced by simultaneously ablating aluminium oxide target using another laser beam during the deposition of FePt nanoparticles to separate them and to form FePt:Al2O3 nanocomposite thin films. FePt nanoparticles are found to be well separated and uniformly distributed inside non-magnetic matrix material and thermal annealing at 600 °C is required for their phase transition. A large number of laser droplets, which are determined to be Al2O3, is observed on FePt:Al2O3 nanocomposite thin films and can be significantly reduced by introducing a strong magnetic field during deposition, which is defined as magnetic trapping assisted PLD. Furthermore, FePt:Al2O3 nanocomposite thin films deposited by magnetic trapping assisted PLD are found to have lower phase transition temperature of 300 °C due to their high defect density and hence low activation energy for diffusion. Hence, the thesis presents not only the successful synthesis of FePt nanoparticles and FePt based nanocomposite thin films using a variety of techniques but also the schemes to lower down their phase transition temperature to reduce the exchange coupling effects.
As a part of this research project, magneto-optical Kerr effect (MOKE) magnetometer, which measures the change in polarization of reflected lights from the surface of magnetic materials, together with its automation program were designed and developed to study their magnetic properties. The short laser penetration depth, typically about 10-20 nm, allows MOKE magnetometer to investigate the surface magnetism of metallic thin films at their ultra-thin limit. The effects of operation parameters and environment, such as background room light intensity and chopper frequency, on the signal-to-noise ratio of MOKE magnetometer were studied and its optimum operation conditions were proposed.
Pulsed laser deposition (PLD) system, as another part of this research project, was designed and developed for the synthesis of nanostructured magnetic materials due to its prominent advantages of stoichiometry preservation, versatility and simplicity. The effects of deposition conditions, such as ambient gas pressure and target-substrate geometry, on deposited magnetic materials were studied by investigating plume dynamic as well as Fe thin films deposited with different deposition conditions. Deposited magnetic materials can be tailored among smooth thin films, nanoparticle agglomerates and dispersive flocculeslike nanoparticle networks by controlling the deposition conditions. In addition, as special target-substrate geometry, backward plume deposition (BPD) was employed which can synthesize nanostructured magnetic materials that are similar to those deposited by conventional PLD, and was found to have the feasibility to overcome two major drawbacks of PLD techniques, low deposition rate and large number of laser droplets.
Finally, FePt nanoparticles and FePt based nanocomposite thin films, which are recognized as the most prominent candidates of ultra-high density magnetic data storage media, were synthesized using PLD system and investigated by various characterization techniques, such as MOKE magnetometer, transmission electron microscope (TEM), etc. FePt nanoparticles, in the compact forms of nanoparticle agglomerates and nanoparticle networks, can be synthesized by conventional PLD as well as BPD. Nanoparticle agglomerates deposited by BPD are found to have higher uniformity in terms of agglomerate size and size distribution in addition to their other advantages of higher deposition rate and less laser droplets. In both cases, as-deposited FePt nanoparticles exhibit low Ku face-centered-cubic (fcc) phase and thermal annealing at 600 °C is required for their phase transition to high Ku face-centered-tetragonal (fct), which cause grain growth and agglomeration and in turn increase their exchange coupling effects. To reduce exchange coupling effects, ion irradiation using plasma focus device were employed not only to induce nanostructuring of pulsed laser deposited FePt thin films to form wellseparated nanoparticles but also to reduce their phase transition temperature from 600 °C to 400 °C. Compared to hours of other continuous ion sources, single shot ion irradiation, with pulse duration of few hundreds of nano-second (ns), is a novel fast technique which achieves the desired results in a much shorter time. In order to further minimize the annealing effects and reduce the exchange coupling effects, non-magnetic matrix material, Al2O3, was introduced by simultaneously ablating aluminium oxide target using another laser beam during the deposition of FePt nanoparticles to separate them and to form FePt:Al2O3 nanocomposite thin films. FePt nanoparticles are found to be well separated and uniformly distributed inside non-magnetic matrix material and thermal annealing at 600 °C is required for their phase transition. A large number of laser droplets, which are determined to be Al2O3, is observed on FePt:Al2O3 nanocomposite thin films and can be significantly reduced by introducing a strong magnetic field during deposition, which is defined as magnetic trapping assisted PLD. Furthermore, FePt:Al2O3 nanocomposite thin films deposited by magnetic trapping assisted PLD are found to have lower phase transition temperature of 300 °C due to their high defect density and hence low activation energy for diffusion. Hence, the thesis presents not only the successful synthesis of FePt nanoparticles and FePt based nanocomposite thin films using a variety of techniques but also the schemes to lower down their phase transition temperature to reduce the exchange coupling effects.
Date Issued
2009
Call Number
TA418.9.N35 Lin
Date Submitted
2009