Magnetization Reversal In Epitaxial Exchange Biased FeGa/IrMn Heterostructures Controlled By Oblique Deposition

Fe Ga alloys exhibit great promise in application of actuator and sensor due to the moderate magnetostriction of 350 ppm under very low magnetic fields, the low hysteresis, and the high tensile strength. Recently, with the development of magnetostrictive materials applied in multiferroic composite, spintronic, and microwave devices, Fe Ga alloys in thin-film form have attracted extensive attention. In various thin-film structures, exchange bias(EB) heterostructures induced by interfacial exchange coupling between ferromagnetic(FM) and antiferromagnetic(AFM) materials have been employed to stabilize the magnetization in spintronic devices, enhance the ferromagnetic resonance frequency in microwave devices, and induce the magnetoelectric coupling in multiferroic composite. However, due to the average of crystallographic orientations and inherent stresses, polycrystalline Fe Ga thin films exhibit a reduced magnetostriction less than 100 ppm. Parkes et al., have epitaxially grown Fe Ga thin film on single-crystalline substrates and achieved the magnetostriction as large as the value of bulk counterpart. This observation suggests that the single-crystalline Fe Ga films, when integrated into the magnetoelectric materials and devices, could significantly improve their magnetomechanical behaviors, which results in the importance of fabricating the epitaxial Fe Ga/AFM EB heterostructures and controlling their magnetic properties. EB essentially establishes a unidirectional anisotropy and an accompanied uniaxial anisotropy, which breaks the symmetry of magnetization reversal. In epitaxial EB systems, the intrinsic magnetocrystalline anisotropy needs to be further taken into account. Depending on the strengths and the relative orientations, the different configurations of magnetic anisotropies would give rise to the complicate magnetic switching processes. The oblique deposition was used to set a controlled uniaxial anisotropy parallel or perpendicular to the EB. In this thesis, we have systematically investigated the influence of magnetization reversal in epitaxial Fe Ga and Fe Ga/Ir Mn films by oblique deposition inducing a uniaxial anisotropy.(1) We have epitaxially deposited FeGa films onto MgO(001) substrates at an oblique angle φ varying from 0° to 45° and systematically investigated their magnetization reversal. The square and two-step hysteresis loops were observed in the samples deposited at φ = 0° and 15°. The reversed two-step and three-step hysteresis loops were observed in the samples deposited at φ = 30° and 45°, in which a strong uniaxial magnetic anisotropy Ku was induced by the oblique deposition. A model based on domain wall(DW) nucleation and propagation was employed to quantitatively describe the angular dependent behaviors of Fe Ga epitaxial films, which indicates that, for the samples deposited with increasing φ, the 180° magnetic transitions occurring in the magnetic field orientation between-45° and 45° gradually change from the two successive 90° DW nucleations to the 180° DW nucleation.(2) We have fabricated epitaxial EB Ir Mn/Fe Ga bilayers by oblique deposition and systematically investigated their magnetization reversal. Two different configurations with the uniaxial magnetic anisotropy Ku parallel and perpendicular to the unidirectional anisotropy Keb were obtained by controlling the orientation of the incident Fe Ga beam during deposition. A large ratio of Ku/Keb were obtained by obliquely depositing Fe Ga layer to achieve a large Ku whilst reducing the Ir Mn thickness to obtain a small Keb. Besides the previously reported square loops, conventional asymmetrically shaped loops, and one-side and two-side two-step loops, the unusual asymmetrically shaped loops with three-step magnetic transition for the descending branch and two-step transition for the ascending branch and the biased three-step loops were observed at various field orientations in the films of both Ir Mn(tIr Mn = 1.5 to 20 nm)/Fe Ga(10 nm) with Ku ⊥ Keb and Ir Mn(tIr Mn ≤ 2 nm)/Fe Ga(10 nm) with Ku ∥ Keb. Considering the geometries of anisotropies, a model based on DW nucleation and propagation was employed to quantitatively describe the angular dependent behaviors of Ir Mn/Fe Ga bilayers. The biased three-step magnetic switching was predicted to takes place when u90 ebK ?K?? ?, and the EB leads to the appearance of the unusual asymmetrically shaped hysteresis loops.