Investigation of Interfacial Effects in Ferromagnetic Thin-Films

Abstract

The magnetic behaviour in thin-film structures has attracted considerable interest and also has importance for wide ranging technological applications. As the dimension of magnetic films reduce, they are able to exhibit different electrical and magnetic properties, where interfacial magnetism become more important. This thesis is centered on the interfacial effects in ferromagnetic thin-film structures with various adjacent materials. Within this framework, the ferromagnetic materials Co and CoFeB:Ta alloy have been investigated.

A detailed investigation of the structural, magnetic and anisotropic magnetoresistance (AMR) properties of Co thin-films with Cu and Ir overlayers as a function of Co thickness was performed. Magnetic characterization of thin-films was performed to determine possible magnetic dead layer formations in these thin-films, where no magnetic dead layers were found to be present within these structures. Electrical resistivity measurements showed that the AMR is dependent upon on Co film thickness, where it decreases with decreasing of Co thickness, and it tends toward zero for Co thicknesses below 6 nm. The contribution to the AMR from a single Co/Ir interface is presented where the AMR is shown to vary inversely proportional to the Co film thickness with a Co/Ir interface.

Interface magnetism and magnetic dead layers in amorphous CoFeB:Ta alloy thin-film multilayers were studied using polarized neutron reflectometry. Temperature dependent variations in the effective magnetic thickness of the film are found, and correlated with structural intermixing at interfaces. At the interface between ferromagnetic film and capping-layer the structurally graded interface appears to cause a concomitant grading of the local Curie temperature, and at the interface between ferromagnetic film and GaAs(001) substrate local interfacial alloying also creates a region where a magnetic dead-layer forms. The thickness of the magnetic dead layer at the ferromagnet-semiconductor interface is shown to be temperature dependent, which may have significant implications for room-temperature operation of hybrid ferromagnetic metal-semiconductor spintronic devices.

Enhancement of Gilbert damping in Co thin-films of various thicknesses with Cu or Ir overlayers is studied under ferromagnetic resonance to understand the role of local interface structure in spin-pumping. Structural analysis indicates that Co films less than 6 nm have fcc(111)-dominated texture while thicker films are dominated by hcp(0001) structure. The intrinsic damping for Co thicknesses above 6 nm is weakly dependent on Co thickness for thin-films with both overlayers, and below 6 nm the Ir overlayers show higher intrinsic damping enhancement compared to Cu overlayers, as expected due to spin-pumping. The interfacial spin-mixing conductance is significantly enhanced in structures where both Co and Ir have fcc(111) structure in comparison to those where the Co layer has subtly different hcp(0001) texture at the interface.