Doctor of Philosophy (Ph.D.)
This thesis focuses on the study of the static and dynamic magnetic interactions in ferromagnetic/nonmagnetic heterostructures using interface-specific and time-resolved optical techniques. The goal of this thesis is to elucidate the interface exchange coupling, magnetic anisotropy, and coherent spin dynamics in these advanced material systems, crucial to the realization of high performance spintronic devices.;First, a pronounced exchange bias (EB) phenomenon is observed in Fe/MgO (001) by magnetic second harmonic generation. The bulk magnetization does not, in marked contrast to typical systems where EB is manifested only in the net magnetization. The magnitude of the exchange bias varies with interface oxygen concentration, suggesting that the pinning layer originates from local FeO nanoclusters formation. Temperature and strain dependent studies show that the lattice mismatch between MgO and Fe enhances the FeO nanoclusters blocking temperature above room temperature. Our results have broad implications for understanding ferromagnet/oxide heterostructures, and provide new insights into the interface spin system and exchange bias.;Second, the magnetization reversal process within the first two iron layers at the Fe/GaAs(001) interface is found to be different and independent from the Fe bulk, as measured by magnetic second-harmonic generation and magneto-optical Kerr effect, respectively. The interface magnetization is largely noncollinear from the bulk with an abrupt magnetic boundary and an anisotropic exchange coupling stiffness, weak inter-layer coupling but relatively strong intra-layer stiffness. In contrast, Fe/GaAs(110) exhibits a rigid coupling between interface and bulk magnetization suggesting that the interfacial bonding structure can dramatically change the nature of the exchange coupling. These results are consistent with the observation of noncollinear alignment of interface and bulk magnetization in Fe/MgO(001), and also relevant to other magnetic/non-magnetic interfaces with abrupt chemical bond structures.;Last, the relaxation mechanism of coherent spin precession is investigated in single crystalline Fe/CoO/MgO(001) heterostructure by time-resolved magneto-optical Kerr effect. at 78K, the intrinsic damping property is enhanced by AF spins in CoO layer for thicknesses of 2.5 nm and 4 nm. In contrast, for thicknesses of 1 nm and 1.5 nm or at room temperature, the damping process is dominated by a dephasing effect caused by disordered AF spin clusters.
© The Author
Fan, Yichun, "Optical Characterization of Magnetism in Magnetic/Nonmagnetic Heterostructures" (2013). Dissertations, Theses, and Masters Projects. Paper 1539623362.