Orientation-Controlled Bi26-x-yCoxFeyO40/MgO/Si Thin Films: Enhanced Magneto-Optical Performance

Integrated nonreciprocal optical devices on silicon (Si) remain challenging because interdiffusion at the magneto-optical (MO) film/Si interface degrades film quality and weakens optical performance. Here, we insert an MgO buffer layer between the Si substrate and Bi26-x-yCoxFeyO40 (BCFO) thin films deposited by radio-frequency (RF) magnetron sputtering. Structural and interfacial characterizations show that MgO effectively suppresses interdiffusion of Si and Fe/Co, as well as promotes a preferred orientation in BCFO. Consequently, MgO-buffered films exhibit markedly improved properties: the surface roughness decreases by 38%, the saturation magnetization increases to 1.5 times its original value and the magnetic circular dichroism (MCD) signal increases by approximately fivefold. These results highlight the potential of MgO-buffered BCFO films for practical nonreciprocal optical devices.