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Nanostructured magnetic material using two different plasma based methods
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Type
Thesis
Author
Wang, Ying
Supervisor
Rawat, Rajdeep Singh
Tan, Augustine Tuck Lee
Abstract
Nanostructured magnetic materials are of high interest in various applications including data storage, biomedicine and catalyst etc. The required properties of magnetic materials vary for different applications. The most important property for applications in data storage and biomedicine is the magnetic property, while electrochemical property is the main concern in catalyst application. Hard magnetism is required for high density data storage application. Inversely, superparamagnetism is preferred in biological and biomedical applications. To achieve certain properties for the special application, various synthesis and processing techniques are studied as it is found that properties of samples depend on both synthesis and processing parameters. In this PhD program, two synthesis techniques, pulsed laser deposition (PLD) and atmospheric microplasma (AMP), are studied.
In the investigation using PLD, the FePt films thinner than 20 nm with hard magnetic properties are targeted for data storage application particularly for perpendicular data recording. However, impurities that degrade formation of hard magnetic properties are detected in FePt thin films. A systematic investigation was carried to determine the factors that might have led to impurity phase formation. The factors include (i) PLD target composition, (ii) substrate material, (iii) annealing parameters such as temperature, duration and ambient gases and (iv) PLD deposition parameters such as chamber ambient gases, laser energy fluence (LEF) and target-substrate distance. Among all these factors, a decrease in the LEF and increase of the target-substrate distance are finally found to result in elimination of impurity phases and enhancement in magnetic and structural properties. The energy of the ablated plasma species, controlled by the LEF and the target-substrate distance, is found to be the main factor responsible for the formation of the impurity phases. Moreover, a further study of the influence of LEF on magnetic properties in FePt thin films is carried out. Magnetic properties of FePt thin films are investigated at three different LEFs of about 51, 136 and 182 J/cm2. Deposition at lower LEF (~51 J/cm2) yields softer out-of-plane coercivity (≤0.4 kG), whereas deposition at higher LEF (~136 and 182 J/cm2) results in harder out-of-plane coercivity (≥5.0 kG). The improved coercivity is found to be attributed to the formation of vacancy defects in thin films, which is indicated by stress change from tensile to compressive form with increasing LEF. Maximum out-of-plane saturation magnetization of 615 emu/cm3 and remanent squareness ratio of 0.88 are achieved for 16 nm thick FePt thin films deposited at moderate LEF of about 136 J/cm2, making them suitable for high density perpendicular data storage applications.
In the study of AMP technique, Pt3Co nanoflowers and Fe3O4 nanoparticles are synthesized and studied. All these nanostructured materials are synthesized at atmospheric conditions, without any thermal treatment and using short discharge duration of less than 10 min, highlighting AMP as novel and versatile plasma technique for synthesis of nanostructured materials. The prepared Pt3Co nanoflower is a new nanostructure of bimetallic Pt3Co. The Pt3Co nanoflower growth is identified to occur in three stages: nucleation and agglomeration of Pt3Co; formation of petals on agglomerates; and petals growth to nanoflowers. The synthesized Pt3Co nanoflowers show electrochemical properties which are suitable to be applied in catalyst, demonstrating the potential use of AMP in catalyst application. Moreover, the prepared Fe3O4 nanoparticles show quite soft coercivity (~3 G) with small particle sizes less than 6 nm, indicating a potential formation of superparamagnetic properties if a further careful study can be carried on. This also makes AMP synthesis technique as a strong candidate for novel plasma based nanofabrication method in biomedical applications.
In the investigation using PLD, the FePt films thinner than 20 nm with hard magnetic properties are targeted for data storage application particularly for perpendicular data recording. However, impurities that degrade formation of hard magnetic properties are detected in FePt thin films. A systematic investigation was carried to determine the factors that might have led to impurity phase formation. The factors include (i) PLD target composition, (ii) substrate material, (iii) annealing parameters such as temperature, duration and ambient gases and (iv) PLD deposition parameters such as chamber ambient gases, laser energy fluence (LEF) and target-substrate distance. Among all these factors, a decrease in the LEF and increase of the target-substrate distance are finally found to result in elimination of impurity phases and enhancement in magnetic and structural properties. The energy of the ablated plasma species, controlled by the LEF and the target-substrate distance, is found to be the main factor responsible for the formation of the impurity phases. Moreover, a further study of the influence of LEF on magnetic properties in FePt thin films is carried out. Magnetic properties of FePt thin films are investigated at three different LEFs of about 51, 136 and 182 J/cm2. Deposition at lower LEF (~51 J/cm2) yields softer out-of-plane coercivity (≤0.4 kG), whereas deposition at higher LEF (~136 and 182 J/cm2) results in harder out-of-plane coercivity (≥5.0 kG). The improved coercivity is found to be attributed to the formation of vacancy defects in thin films, which is indicated by stress change from tensile to compressive form with increasing LEF. Maximum out-of-plane saturation magnetization of 615 emu/cm3 and remanent squareness ratio of 0.88 are achieved for 16 nm thick FePt thin films deposited at moderate LEF of about 136 J/cm2, making them suitable for high density perpendicular data storage applications.
In the study of AMP technique, Pt3Co nanoflowers and Fe3O4 nanoparticles are synthesized and studied. All these nanostructured materials are synthesized at atmospheric conditions, without any thermal treatment and using short discharge duration of less than 10 min, highlighting AMP as novel and versatile plasma technique for synthesis of nanostructured materials. The prepared Pt3Co nanoflower is a new nanostructure of bimetallic Pt3Co. The Pt3Co nanoflower growth is identified to occur in three stages: nucleation and agglomeration of Pt3Co; formation of petals on agglomerates; and petals growth to nanoflowers. The synthesized Pt3Co nanoflowers show electrochemical properties which are suitable to be applied in catalyst, demonstrating the potential use of AMP in catalyst application. Moreover, the prepared Fe3O4 nanoparticles show quite soft coercivity (~3 G) with small particle sizes less than 6 nm, indicating a potential formation of superparamagnetic properties if a further careful study can be carried on. This also makes AMP synthesis technique as a strong candidate for novel plasma based nanofabrication method in biomedical applications.
Date Issued
2015
Call Number
TA418.9.N35 Wan
Date Submitted
2015