Investigation of the effect of a buffer layer on the magnetic properties of soft materials

In this thesis, we study the influence of a buffer layer (Ta, Cr, W, Ti) on the structure formation, crystallographic orientation, resistivity and magnetic properties of NiFe alloy in the form of bilayer and multilayer nanostructures. In addition, we also investigate how these physical properties are affected by the buffer layer thickness (from 50 nm to 200 nm), the sputtering gas pressure and the applied magnetic field during deposition. All nanostructures were prepared by sputtering onto oriented Si substrates. The overall aim of this work is to obtain nanostructures with small coercivities and identify the required relevant structural parameters. The structure of the films was analyzed using symmetric and asymmetric X-ray diffraction (XRD). XRD results showed that NiFe films are formed preferentially with a fcc crystalline structure. The structure does not depend on the buffer layer material. However, we observed changes in texture when the thickness of the tantalum buffer layer was increased. Our experiments showed that the changes in thickness for buffer layers of other materials did not affect the crystallographic orientation of the NiFe films. For a tantalum buffer layer, the (111) texture of NiFe is favored in thin layers and is reduced as layer buffer thickness increases. Magnetic measurements using Vibrating Sample Magnetometer (VSM) also showed that films with a 50 nm Ta buffer layer exhibited the lowest hard axis coercivity of about 1.70 ± 0.06 Oe compared to other buffer layer materials with the same thickness. The effect, on both the structure and magnetic properties of the films, of applying a magnetic field over the substrate during sputtering was investigated. Diffraction pattern results did not show significant structural differences between the samples sputtered with and without applied magnetic field. However, coercivity values were significantly reduced. For instance, in the case of the 100 nm of NiFe deposited on top of 50 nm of tantalum buffer layer, the coercivity reduced from 1.70 ± 0.06 to 0.50 ± 0.02 Oe, without and with an applied magnetic field, respectively. This is due to in-plane magnetocrystalline anisotropy induced by the magnetic field in the films. The control of grain size and crystallographic orientation proved to be effective structural parameters in changing the coercivity of the bilayer nanostructures. This control of these structural parameters was achieved by decreasing the gas pressure from 9 mTorr down to 1 mTorr during sputtering. As a result, coercivity decreased from 6.0 ± 0.2 Oe to 0.50 ± 0.02 Oe , respectively. For a single NiFe layer sputtered with argon gas at a pressure of 1.3 mTorr, we obtained a critical film thickness of 222 ± 3 nm for the formation of strip domains. We showed experimentally that thick multilayer nanostructures with low coercivity could be obtained by adding thin spacer layers of Au or Ti between the NiFe layers as long as the NiFe thickness was kept thinner than the critical thickness of formation of strip domains. Coercivity values as low as 0.06 ± 0.01 Oe, measured at an applied field strength of 1000 G, were obtained for a multilayer structure consisting of Si // [(5 nm) Ti / (150 nm) NiFe]2 / (5 nm) Ti. Our studies with Au spacer layers showed that there is an optimum value for the thickness of the spacer, which leads to low coercivities. Increasing the thickness of the spacer layer from the optimum value increased the coercivity as magnetostatic interaction dominates over exchange interaction.