Correlation Between Microstructure and Properties in Magnetic Multilayer Nanostructures

The properties of layered magnetic nanostructures depend critically on their underlying microstructure. A magnetic tunnel junction (MTJ) is one such multilayer nanostructure that has found technological significance as the read-head sensor in hard disk drives. An MTJ exhibits a difference in the electrical resistance across the junction as a function of the relative orientation of the magnetization in two ferromagnetic electrodes that are separated by a thin dielectric layer [tunnel magnetoresistance (TMR)]. There is, however, a great deal that is not understood about the fundamental correlations between the microstructure and the magnetic and magnetotransport properties of these important nanostructures.;In this thesis, the correlations between the microstructure and the magnetic and magnetotransport behavior of MgO-based MTJs, which exhibit an exceptionally large TMR effect, are studied in detail. Three-dimensional atom-probe tomography and transmission electron microscopy are used to analyze the crystal structure and three-dimensional elemental distribution within CoFe/MgO and CoFeB/MgO MTJs as a function of annealing temperature, deposition conditions, multilayer stack design and layer composition. Annealing between 300--380ºC causes the diffusion of Co, Fe, B and other elements, which affects the symmetry of the tunnel barrier potential. Amorphous CoFeB layers partially crystallize upon annealing, resulting in the diffusion of B out of the crystallized grains of CoFeB. The distribution of B after annealing is found to play a critical role in both the magnetic and magnetotransport behavior of the MTJ. It is found that the B distribution can be controlled to some extent by optimization of the growth parameters and multilayer stack design.;Micromagnetic simulations of the magnetization dynamics in patterned magnetic multilayers, which are similar to those found in MTJs, in response to an applied magnetic field were also performed. The effects of circular exchange bias and interlayer exchange coupling between two ferromagnetic layers on the low-frequency gyrotropic and low-lying eigenmode magnetic responses are described in detail. Increasing the strength of the exchange bias increases the frequency of the dynamic response while the sign and strength of interlayer exchange coupling is found to affect both the static and dynamic behavior of the heterostructure.