Development of magnetic tunnel junctions with half-metallic Fe3O4 and novel various nitrogen-doped ferromagnetic thin films

Abstract

This dissertation describes an experimental study on development of magnetic tunnel junctions with half-metallic Fe3O4 and novel various nitrogen-doped ferromagnetic thin films. At first, in order to develop the optimum magnetic tunnel junctions (MTJs), various oxidation or nitridation techniques were systematically carried out to control structural and electrical properties of insulating barrier mainly for obtaining device performance of MTJs. The remote RF plasma techniques was chosen to increase the plasma density of radical oxygen or nitrogen at low operation power, and to reduce ion bombardment effects, and to have more shaper interface between the insulating barrier and ferromagnetic electrode. The experimental surface plasmon resonance spectroscopy (SPRS) analysis suggested that the decrease of the TMR ration starting at 150 K was a cause of residual Al metallic defects distributed within the insulating AlOx barrier. In addition, the conventional sputtering method of MgOx thin films has rich oxygen radicals from residual gas, so that MgOx thin films showed low quality. Second, the temperature dependence of the exchange bias in the single MTJs was measured by using a cryogenic tunneling magneto-resistance (TMR) system and a superconducting quantum interference device (SQUID) at temperatures from 2 K to 300 K. The Hex of the typical exchange bias system was gradually increased up to about 10 K for both as-grown and ex-situ-annealed samples. And, some unusual spin-polarized transport of asymmetric double MTJs (ADMTJs) was investigated. The conductance curves and the TMR ratio clearly oscillate with applied bias voltage, indicating the presence of Coulomb Blockade (CB) effects due to isolated FM-NPs for parallel configuration in the ADMTJs. The oscillation period of them is about 1.5 mV at 2 K Third, the magnetic behaviors of was half-metallic Fe3O4 thin films grown by zero field growth (ZFG) and field growth (FG) techniques during the sputtering process were investigated. Structural observations obtained by high resolution TEM measurements clearly depicted a significant reduction of the grain boundaries and voids in the Fe3O4 films grown under FG conditions, thus explaining the saturated magnetization of the Fe3O4 films at about 0.01 T. In addition, the zero-field-cooled magnetization of the ZFG samples showed an abrupt change at about 285 K, confirming the existence of defects or other phases in the ZFG films. After the Ar plasma treatment process, the root mean square of as-deposited Fe3O4 thin film was decreased. And then, TMR ratio of the MTJs with a Fe3O4 electrode was found to be 11 % at RT. Finally, the dependence of the structure and magnetic properties of novel various nitrogen-doped ferromagnetic thin films were analyzed to investigate the material use as ferromagnetic electrode in MTJs. The addition of a small amount of nitrogen into the ferromagnetic film was strongly associated with variations in the amorphous or nano-crystalline phased of the films and promoted the formation of low coercivity (Hc) and low magnetization (Ms). Particularly, magnetic tunnel junction using new CoFeNx electrodes and AlNx barrier were introduced for use in spintronic applications, achieving TMR ratios of 8 % as grown, and 49 % at 300 ℃.