The magnetic properties of epitaxial Fe100-xGax thin films: the role of Gallium composition and film thickness

Magnetostrictive smart materials in the form of thin films hold a lot of promise because of their utilisation in a variety of microelectromechanical (MEMs) and nanoelectromechanical (NEMs) based devices, magnetic memory and logic technology. A promising candidate is Galfenol which is an alloy of Fe and Ga, and has been demonstrated to have a very high magnetostriction coefficient in bulk single crystal form. Recently, the first measurement of magnetostriction in epitaxial thin films was reported by Parkes et al. who found that epitaxial thin films of Fe81Ga19 show magnetostriction values as large as the bulk material, making the epitaxial layers a prospective candidate for applications. It is therefore important to understand the role of Ga composition and film thickness in determining the magnetic properties of epitaxial Galfenol. This thesis investigates the structural, magneto-static, magneto-dynamic and magneto-transport properties of epitaxial sputtered grown thin films of Galfenol. An elaborated and detailed study of this material in thin-film form was done by growing Fe100-xGax films with x ranging from 7% to 30% as well as growing Fe79Ga21 films with thickness ranging from 5nm to 95nm. All the films discussed in this thesis were epitaxially grown on GaAs(001) substrate due to its low misfit in a cube on cube geometry when grown using sputtering. The presence of a strong crystalline and epitaxial growth of the samples is demonstrated by analysing the structural properties of these samples. This was then observed to be consistent with a strong cubic magnetocrystalline anisotropy in samples with lower Ga concentration revealed by SQUID magnetometry measurements. The samples, apart from having a cubic anisotropy, also presented a weak uniaxial magnetic anisotropy. A clear dependency of anisotropy on Ga concentration is presented in this thesis, which correlates with a reduction of the crystalline structure with increase in Ga concentration. Field rotation transport measurements for current passed along different crystalline direction revealed crystalline and non-crystalline contributions to the anisotropic magnetoresistance(AMR). A closely comparable pattern was observed between the crystalline magneto-transport coefficients and magnetocrystalline anisotropy constants for varying Ga concentration. Also, magnetocrystalline constants obtained from ferromagnetic resonance measurements followed a similar pattern. The Gilbert or intrinsic damping as a function of Ga concentration was also investigated. A low Gilbert damping comparable to previously reported values makes them a competitive candidate for application into microwave devices. An investigation of the ferromagnetic resonance linewidth gives insight into the relative magnitude of the different contributions to the magnetic damping as a function of Ga concentration. Similar investigations were also performed on Fe79Ga21 samples by varying the film thickness. Samples depicted a good crystalline and epitaxial sputtered growth with strong cubic magnetocrystalline anisotropy. The weak uniaxial anisotropy changes in magnitude for thicker samples. The thicker samples tend to deviate from the patterns observed for thinner samples. Field rotation transport measurements represented four different contributions to the AMR with corresponding unique symmetries. The crystalline contributions agreed with the pattern observed for magnetocrystalline anisotropy constants

The magnetic properties of epitaxial Fe100-xGax thin films: the role of Gallium composition and film thickness

Magnetostrictive smart materials in the form of thin films hold a lot of promise because of their utilisation in a variety of microelectromechanical (MEMs) and nanoelectromechanical (NEMs) based devices, magnetic memory and logic technology. A promising candidate is Galfenol which is an alloy of Fe and Ga, and has been demonstrated to have a very high magnetostriction coefficient in bulk single crystal form. Recently, the first measurement of magnetostriction in epitaxial thin films was reported by Parkes et al. who found that epitaxial thin films of Fe81Ga19 show magnetostriction values as large as the bulk material, making the epitaxial layers a prospective candidate for applications. It is therefore important to understand the role of Ga composition and film thickness in determining the magnetic properties of epitaxial Galfenol. This thesis investigates the structural, magneto-static, magneto-dynamic and magneto-transport properties of epitaxial sputtered grown thin films of Galfenol. An elaborated and detailed study of this material in thin-film form was done by growing Fe100-xGax films with x ranging from 7% to 30% as well as growing Fe79Ga21 films with thickness ranging from 5nm to 95nm. All the films discussed in this thesis were epitaxially grown on GaAs(001) substrate due to its low misfit in a cube on cube geometry when grown using sputtering. The presence of a strong crystalline and epitaxial growth of the samples is demonstrated by analysing the structural properties of these samples. This was then observed to be consistent with a strong cubic magnetocrystalline anisotropy in samples with lower Ga concentration revealed by SQUID magnetometry measurements. The samples, apart from having a cubic anisotropy, also presented a weak uniaxial magnetic anisotropy. A clear dependency of anisotropy on Ga concentration is presented in this thesis, which correlates with a reduction of the crystalline structure with increase in Ga concentration. Field rotation transport measurements for current passed along different crystalline direction revealed crystalline and non-crystalline contributions to the anisotropic magnetoresistance(AMR). A closely comparable pattern was observed between the crystalline magneto-transport coefficients and magnetocrystalline anisotropy constants for varying Ga concentration. Also, magnetocrystalline constants obtained from ferromagnetic resonance measurements followed a similar pattern. The Gilbert or intrinsic damping as a function of Ga concentration was also investigated. A low Gilbert damping comparable to previously reported values makes them a competitive candidate for application into microwave devices. An investigation of the ferromagnetic resonance linewidth gives insight into the relative magnitude of the different contributions to the magnetic damping as a function of Ga concentration. Similar investigations were also performed on Fe79Ga21 samples by varying the film thickness. Samples depicted a good crystalline and epitaxial sputtered growth with strong cubic magnetocrystalline anisotropy. The weak uniaxial anisotropy changes in magnitude for thicker samples. The thicker samples tend to deviate from the patterns observed for thinner samples. Field rotation transport measurements represented four different contributions to the AMR with corresponding unique symmetries. The crystalline contributions agreed with the pattern observed for magnetocrystalline anisotropy constants

Protected Long-Distance Guiding of Hypersound Underneath a Nanocorrugated Surface

In nanoscale communications, high-frequency surface acoustic waves are becoming effective data carriers and encoders. On-chip communications require acoustic wave propagation along nanocorrugated surfaces which strongly scatter traditional Rayleigh waves. Here, we propose the delivery of information using subsurface acoustic waves with hypersound frequencies of ∼20 GHz, which is a nanoscale analogue of subsurface sound waves in the ocean. A bunch of subsurface hypersound modes are generated by pulsed optical excitation in a multilayer semiconductor structure with a metallic nanograting on top. The guided hypersound modes propagate coherently beneath the nanograting, retaining the surface imprinted information, at a distance of more than 50 μm which essentially exceeds the propagation length of Rayleigh waves. The concept is suitable for interfacing single photon emitters, such as buried quantum dots, carrying coherent spin excitations in magnonic devices and encoding the signals for optical communications at the nanoscale

Quantification of site disorder and its role on spin polarization in the nearly half-metallic Heusler alloy NiFeMnSn

The electronic structure and magnetism of the quaternary Heusler alloy NiFeMnSn are studied using the full-potential linearized augmented plane-wave (FPLAPW) method. The calculation for the perfectly LiMgPdSn-type ordered crystal structure (type I) of NiFeMnSn shows a high spin polarization (∼76%) with a ferromagnetic ground state. The total spin magnetic moment is in good agreement with the Slater-Pauling rule. The structural investigations using neutron diffraction at 500 K, and Mössbauer spectroscopy at 300 K on the NiFeMnSn alloy, prepared using an arc melting, show the presence of atomic site disorder. The electronic structure calculation for the disordered structure shows that the site disorder destroys the nearly half-metallic nature of this alloy. The magnetization measurements indicate that the Curie temperature is well above room temperature (∼405 K) as desired for the spintronics application