Development of perpendicular magnetic anisotropy property in CoFeB/MgO frames with high thermal annealing stability for recording layer of magnetic tunnel junctions

Abstract

This doctoral dissertation describes an experimental study on development and optimization of perpendicular magnetic anisotropy properties in CoFeB/MgO configurations for a practical application of magnetic tunnel junctions. At first, the magnetic properties of a typical Ta/CoFeB/MgO configuration was investigated along with various annealing temperatures. The perpendicular magnetic anisotropy properties of this configuration were revealed after moderate temperature annealing process. As an increased annealing temperature, the magnetic dead layer and the saturation magnetization were increased. However, at annealing temperature greater than 300 °C, the perpendicular magnetic anisotropy properties were deteriorated. The interface perpendicular magnetic anisotropy energy density was reached to the maximum vales of 1.60 erg/cm2 at 275 °C annealing temperature. Second, the Ta layer was substituted by W layer in CoFeB/MgO frames. Highly stable perpendicular magnetic anisotropy was achieved up to 450 °C at a specific W thickness in both W buffer/CoFeB/MgO and MgO/CeFeB/W capping frames, along with the presence of the dominant perpendicular magnetic anisotropy properties at a relatively thick CoFeB and the achievement of a high effective perpendicular magnetic anisotropy energy density of approximately 5 Merg/cc. Lastly, to achieve outstanding PMA features and desirable annealing stability at temperatures greater than 400 °C, a generic alternative approach of the simple incorporation of tungsten nitride at the interface of Ta/CoFeB was investigated and obtained an enhanced perpendicular magnetic anisotropy features in Ta/CoFeB/MgO frames without thermal degradation. Precise control of the thickness and composition of tungsten nitride was required to ensure stable perpendicular magnetic anisotropy even after a 425 °C annealing process, along with the achievement of a high effective magnetic anisotropy energy density of almost 10 Merg/cc. The secondary ion mass spectrometry and energy-dispersive X-ray spectroscopy confirmed the role of tungsten nitride as a diffusion barrier in Ta/WN/CoFeB/MgO frames.