Magneto-Optical Enhancement and Chemical Sensing Applications of Perpendicular Magnetic CoPt/Ag Stacked Structures with a ZnO Intermediate Layer

Abstract

The magneto-optical (MO) properties of perpendicular magnetic Co₈₀Pt₂₀/Ag stacked films with ZnO intermediate layers of different thickness were investigated by polar Kerr measurements. The insertion of a thin ZnO layer at the CoPt/Ag interface improved the perpendicular magnetic properties and MO enhancement at the plasma edge of Ag. The CoPt/Ag stacked films with a 2-nm-thick ZnO layer exhibited an ideal square out-of-plane hysteresis loop with a relatively large Kerr angle of approximately 1.4° in the ultraviolet region. The peak position of the MO plasma enhancement shifted to longer wavelength as the thickness of the ZnO layer increased, and a new peak appeared consistent with the band-gap energy of ZnO. Moreover, the CoPt/ZnO/Ag stacked layer acted as a Fabry–Pérot etalon when the thickness of the ZnO layer was the sub-wavelength of incident light. As a result, a MO cavity was realized in the stacked films, and an ideal square MO loop with a large Kerr rotation angle of approximately 20° was obtained in the visible region. The MO enhancement factor reached approximately 200. We demonstrated that the stacked films were sensitive to changes in the optical conditions at the film surface. The developed MO cavity can act as a highly accurate chemical and biological sensing system under simple measurement conditions.