Abstract
Nanoporous Co40Fe40B20 (CoFeB) and sandwich tungsten (W)/CoFeB/W thin films were fabricated via an anodic aluminum oxide (AAO) template-assisted magneto sputtering process. Their thickness-dependent magneto-optical Kerr effect (MOKE) hysteresis loops were investigated for enhanced Kerr rotation. Control of the Kerr null points of the polarized reflected light can be realized via the thicknesses of the CoFeB layers and W layers. Simulation of the thickness-dependent phase difference change by the finite element method reveals the existence of the two Kerr null points for W/CoFeB/W thin films, matching the experimental result very well. However, there are two additional Kerr null points for pure CoFeB thin films according to the simulation by comparing with the experimental result (only one). Theoretical analysis indicates that the different Kerr null points between the experimental result and the simulation are mainly due to the enhanced inner magnetization in the ferromagnetic CoFeB layer with the increased thickness, which is usually omitted in the simulation. Clearly, the introduction of non-ferromagnetic W layers can experimentally regulate the Kerr null points of ferromagnetic thin films. Moreover, construction of W/CoFeB/W sandwich thin films can greatly increase the highest magneto-optical susceptibility and the saturated Kerr rotation angle when compared with CoFeB thin films of the same thickness.
Highlights
After summarizing the sensing principle in the magneto-optical Kerr effect (MOKE) spectrum, it is noteworthy that the MOKE-based sensors always exhibit remarkable sensitivity, flexible light phase, polarization modulation, and a large figure of merit (FOM) near the position where the MOKE rotation inversion appears [22,23], which is here called the Kerr null point
This MOKE hysteresis loop inversion can be attributed to an abrupt shift in the phase difference caused by the reflection coefficient change of rps and rss (see Equation (3)) and/or by the in plane magnetization direction change of the thin films since εQ in Equation (4) is equal to the gyration vector (g, as shown in Equation (5)) [54]
Thermore, the introduction of W layers induced a second inversion of the Kerr signa
Summary
CoFeB/AAO thin films showed a critical thickness between 51 and 55 nm, during which a relative inversion of the Kerr hysteresis loops occurred, suggesting that the MOKE rotation angle can be reduced to zero, so called the Kerr null point. This MOKE hysteresis loop inversion (or experimentally, the susceptibility of Kerr rotation angle changes its direction) can be attributed to an abrupt shift in the phase difference caused by the reflection coefficient change of rps and rss (see Equation (3)) and/or by the in plane magnetization direction change of the thin films since εQ in Equation (4) is equal to the gyration vector (g, as shown in Equation (5)) [54].
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