New materials and functionality in spintronics devices

The next generation of electronics devices, known as spintronics, which incorporate the spin property of the carriers in combination with their charge degree of freedom is the focus of to-date research. Therefore, exciting new classes of materials have been emerging for the last few years for the development of spintronics devices. This study has been carried out to understand/control various properties of such materials at the fundamental level which is important for the spintronics devices applications. Materials studied here include magnetic semiconductors, magnetostrictive alloys and magnetic tunnel junctions (MTJ) based sensors.;In the first part, a comparative study of the room temperature ferromagnetism of Co doped ZnO and CeO2 is presented with emphasis on the role of dopant, defects and host oxide. Systemic structural, magnetic, and transport analyses reveal that the nature of donor defects and host oxide plays a vital role in establishing ferromagnetism. This study provides an insight into the underlying mechanisms responsible for the ferromagnetism in Co-ZnO and Co-CeO 2. Moreover, the discussed exchange mechanisms are in good agreement with the electronic structure calculation of magnetic impurity ions and defects.;Composite materials with strong magneto-electric (ME) coupling require magnetic thin films with large saturation magnetostriction constant at low magnetic fields. In the second part of this dissertation, we have studied FeGa alloys where changes in their microstructure with the incorporation of boron occur. These changes make this material a soft magnetic alloy (coercivity ∼ 2 Oe) which has a narrow ferromagnetic resonance (FMR) line width, large magnetostriction and high saturation magnetization. The anisotropy values have been extracted from study of the angular dependence of FMR. This work highlights the role of crystalline anisotropy and induced uniaxial anisotropy which determine the magnetic softness and enhanced magnetostriction at small magnetic fields. In addition, the effects of rapid thermal annealing on the structure and magnetic properties of the crystalline as well as amorphous FeGaB thin films have been studied.;Additionally, new electrode materials within the magnetic tunneling junction (MTJ) have been developed using FeGaB which serve as the sensing magnetic layer. This provides a method to measure mechanical strain or stress with high sensitivity. It has been shown that TMR of greater than 12% at room temperature could be achieved in CoFeB/MgO/FeGaB based junctions. This suggests that FeGaB could be a new magnetic electrode for MTJs based pressure devices.;The ability of magnetoresistive (MR) material to sense very weak magnetic fields at room temperature can be used for the magnetic sensor's design. In the third part, the Al2O3 based sensors have been studied where the shape anisotropy in the free magnetic electrode has been observed to results in a linear and hysteresis free magnetoresistance (MR) curve. Moreover, Al2O3 based sensor have 28 - 30% TMR and sensitivity up to 0.4 %/Oe over a magnetic field range of -40 Oe to 40 Oe whereas the MgO-based sensor with superparamagnetic free layer has about 90 % TMR and sensitivity of 1.1 %/Oe over the same field range. This work has been carried out under the supervision of the author's dissertation advisor, Prof. John Q. Xiao.