Characterization of magnetic thin films and spintronic devices using magneto-optic Kerr microscopy
image: (A) a single magnetic domain and (B) multiple magnetic domains with opposite polarization. (C) Following the subtraction of image (A) from image (B), a Kerr image with sharp contrast is obtained [39]. (D) The magnetic domains in Ta/CoFeB/Ta stack with in-plane uniaxial anisotropy [40]. (E) The magnetic domains in the surface of polished NdFeB. (F) The magnetic domains in the silicon steel observed by Kerr microscope assisted by MOIF.
Credit: Advanced Devices & Instrumentation
The Magneto-optical Kerr Effect (MOKE) is a physical phenomenon that causes a change in the polarization state of light when it is reflected from the surface of a magnetized material. Kerr microscopy combines the techniques of optical microscopy and the principles of MOKE, enabling high-resolution observation of magnetic orientation on material surfaces. Over the past decades, Kerr microscopy has become one of the most popular techniques for capturing images of magnetic domains and magnetization evolution processes due to its excellent performance, including convenient operations, high sensitivity, relatively high spatial resolution down to 200 nm, adjustable field of view for large samples, non-destructive nature, high dynamic characteristics, and good compatibility with various test conditions such as magnetic fields, electrical tests, and cryogenics. In this review, we focus on the applications of Kerr microscopy in the characterization of magnetic materials and spintronic devices. First, we briefly introduce the physics mechanism of MOKE and the structure of the Kerr microscope. Then, we elucidate the role of Kerr microscopy in observing the magnetic textures in different magnetic materials and discuss its potential applications. We provide an overview of research on defects in magnetic thin films or devices using Kerr microscopy, including defects in magnetic films and those produced during nanofabrication. A commonly used procedure for measuring domain wall (DW) motion velocity using Kerr microscopy is described, and a method for evaluating material quality based on DW dynamics is provided. Additionally, we present the characterization of key parameters of magnetic thin films using Kerr microscopy, such as saturation magnetization, Heisenberg exchange stiffness, and Dzyaloshinskii–Moriya interaction (DMI). Finally, we introduce the applications of Kerr microscopy in studying spin-transfer torque (STT) and spin-orbit torque (SOT) induced domain dynamics in spintronic devices.
Magneto-optical Kerr effect is a convenient and non-destructive method for magnetic characterization, particularly effective for large-scale samples. It offers high sensitivity, high resolution, and high dynamic characteristics in observing domain and domain wall behaviors. In terms of imaging, Kerr microscopy provides high-resolution views of the microstructure of various magnetic materials, aiding in the evaluation of material quality. Additionally, the dynamics of domain wall (DW) propagation can be studied using Kerr microscopy, allowing for the extraction of key parameters of magnetic materials such as depinning field, DW velocity, saturation magnetization (Ms), exchange stiffness (Aex), and Dzyaloshinskii–Moriya interaction (DMI) values. Due to its compatibility with other test conditions, such as magnetic fields, electrical currents, and cryogenics, spintronic devices can also be quantitatively analyzed using this method. For example, by combining a Kerr microscope with a magnetic probe station, the DW dynamics induced by spin-transfer torque (STT) or spin-orbit torque (SOT) effects can be observed and analyzed. In summary, the technique of Kerr microscopy, along with related experimental methods, is continuously evolving and plays an increasingly important role in the study of magnetic materials and spintronic devices.