Growth, Characterization and Magnetic Properties of Epitaxial FePt Nanostructures
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2008-04-08
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The primary objective of this dissertation was to study the growth, microstructure and magnetic properties of FePt nanostructures so as to demonstrate their suitability for high-density magnetic storage applications. The FePt system is an attractive candidate for high-density storage (greater than 100 Gbits⁄in2) due to its extremely high uniaxial magnetocrystalline anisotropy (Ku = 7.0 x 107 erg⁄cm3). The high magnetocrystalline anisotropy permits the use of smaller particles before the onset of superparamagnetism; this directly leads to larger recording densities. As a part of this study, we have pioneered the epitaxial growth of c-axis FePt on single crystal Si (100) substrates. This holds tremendous future promise in terms of integration of our magnetic structures with the existing Si-based technology. Integration of FePt on Si (100) was achieved by using epitaxial TiN as a template buffer. The TiN template controls the FePt growth along the magnetically hard c-axis and it also acts as an effective diffusion barrier. The FePt⁄TiN⁄Si (100) heterostructure was synthesized using pulsed laser deposition. X-ray diffraction results showed strong c-axis growth and significant L10 order. Transmission electron microscopy revealed the following epitaxial relationship; FePt (001) <001> TiN (100) <001>Si (100) <001>. In spite of the large misfit strains (ε) involved (εFePt⁄TiN = 9.5 % and TiN⁄Si = 22 %), epitaxy was achieved in the FePt⁄TiN⁄Si heterostructure by the unified paradigm of domain matching epitaxy (DME).
In this work FePt was synthesized both as continuous thin films and individual discrete nanoparticles. The effect of microstructure on magnetic properties of the epitaxial FePt system was studied in detail. The microstructure was progressively varied from a 9 nm nanoparticle system to a 30 nm thick continuous film. Magnetic hysteresis measurements showed that all the samples were predominantly perpendicularly magnetized with higher coercivity, squareness and remanence when compared to the in-plane loops. The individual nanoparticles, being in a single domain state, showed higher coercivity than the continuous thin film. Within the nanoparticle regime coercivity increased with increasing particle size. The highest coercivity of 13,500 Oe was obtained for a bead-like microstructure, when the individual nanoparticles just begin to merge to form a continuous thin film. The continuous thin film showed least coercivity because of its multi-domain state and lack of defects or pinning sites. The best microstructure in terms of magnetic storage was the 18 nm sized nanoparticle system, with a perpendicular coercivity of 10,000 Oe. For this system, if we assume one bit of information to be stored in each nanoparticle, a storage capacity of 1 Terabit⁄in2 can be realized. All the samples under study showed perpendicular hysteresis loops with remarkable squareness (≥ 0.95). The coercivity of the FePt system can also be controlled via the composition. For the 18 nm system, the perpendicular coercivity was decreased from 10,000 Oe to 3,200 Oe by changing the composition from Fe50Pt50 to Fe41Pt59. A negative magnetoresistance (MR) of 0.57 % was observed at room temperature for the Fe50Pt50 bead-like thin-film system. The MR loops showed hysteretic behavior with maximum resistance at Hc. The values of coercivity in the MR and MH measurements were consistent. The presence of MR in the FePt system was attributed to thin domain walls, whose thickness is comparable to spin diffusion lengths. The negative MR effect was explained based on spin and domain wall dependent electron scattering.
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Magnetic properties, FePt, Epitaxy, nanostructures
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EdD
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Materials Science and Engineering
