Unraveling Microscopic Origin of Nb 4d Transition-Metal-Induced Magnetic Anisotropy Evolution in W/CoFeB Heterostructures

The tuning of magnetic anisotropy in magnetic thin films is the key aspect in the condensed matter physics research field to develop materials useful for practical applications. In the present work, we have investigated the microscopic origin of magnetic anisotropy evolution in W-buffered (CoFeB)_100–xNb_x (x = 0, 3, 5, 10) alloy films induced by a 4d transition metal (Nb). All films were prepared at an elevated growth temperature of 500 °C using the magnetron sputtering technique. Nb-free films (x = 0) exhibit a polycrystalline structure, magnetically isotropic nature, and soft magnetism (coercive field ∼30 Oe). Systematic addition of Nb (from x = 0 to 10) leads to microstructural transformation from polycrystalline to nearly amorphous structure, with a reduction in grain size (from ∼9 to ∼3 nm), surface smoothening, enhanced soft magnetism (coercive field decreases from ∼30 to ∼6 Oe), and more importantly, the emergence of magnetic anisotropy in CoFeB films. In the anisotropic state, angular variation of coercivity reveals that the magnetization reversal process is consistent with a two-phase model. Remarkably, the orbital and spin magnetic moments of Fe and Co atoms were quantified using an element-specific technique of X-ray magnetic circular dichroism. The correlation between the orbital-to-spin moment ratio and observed magnetic anisotropy provides insight into the role of Nb 4d transition metal in inducing the magnetic anisotropy in amorphous/polycrystalline CoFeB thin films, which is vital for advancing their application in spintronics devices.