Extreme anti-reflection enhanced magneto-optic Kerr effect microscopy

Abstract

Magnetic and spintronic media have offered fundamental scientific subjects and technological applications. Magneto-optic Kerr effect (MOKE) microscopy provides the most accessible platform to study the dynamics of spins, magnetic quasi-particles, and domain walls. However, in the research of nanoscale spin textures and state-of-the-art spintronic devices, optical techniques are generally restricted by the extremely weak magneto-optical activity and diffraction limit. Highly sophisticated, expensive electron microscopy and scanning probe methods thus have come to the forefront. Here, we show that extreme anti-reflection (EAR) dramatically improves the performance and functionality of MOKE microscopy. For 1-nm-thin Co film, we demonstrate a Kerr amplitude as large as 20° and magnetic domain imaging visibility of 0.47. Especially, EAR-enhanced MOKE microscopy enables real-time detection and statistical analysis of sub-wavelength magnetic domain reversals. Furthermore, we exploit enhanced magneto-optic birefringence and demonstrate analyser-free MOKE microscopy. The EAR technique is promising for optical investigations and applications of nanomagnetic systems.

Analyzer-free, intensity-based, wide-field magneto-optical microscopy

“…In the recent article 18 , mentioned already in Sect. I, it was shown that the MO contrast of ultrathin magnetic films with perpendicular anisotropy can be significantly enhanced by embedding the magnetic media in between dielectric antireflection coatings, all deposited on a non-magnetic mirror film.…”

“…Although the authors in Ref. [18] talk about MOKE microscopy, the domains were actually imaged by Faraday microscopy as the magnetic films are optically transparent and as the light passes the films twice due to the mirror on the backside thus resembling Faraday microscopy in reflection geometry. The improved Faraday signal made it also possible to realise analyser-free MO microscopy by using circularly-polarised light for illumination rather than plane-polarised light.…”

“…Thus far, rotation-based Kerr-, Faraday-and Voigt microscopy has been exclusively employed for domain imaging in the visible frequency regime [3][4][5] . A recent investigation 18 on ultrathin magnetic films showed that anti-reflection coatings could significantly enhance the MO rotation and further found that a MO contrast could even be detected by illuminating the sample with left-and right-circularly polarised (LCP and RCP) light rather than by plane-polarised light, thus indeed making use of the MCD effect. The latter contrast was enhanced by the extreme anti-reflection coating used in that work, but it indicates the possibility of magnetic domain imaging without the need for an analyser, which would not have any effect on a circularly polarised wave anyway.…”

Band structure analysis of the magneto-optical effect in bcc Fe

“…The magneto-optical (MO) effects are routinely used for characterization of ferro-, ferri-, and antiferromagnetic materials. Magneto-optics can be employed as spectroscopy tool 1 – 4 , microscopy tool 5 – 8 , tool to investigate ultrafast magnetization dynamics 9 – 12 , etc. The MO effects are used in many applications, such as holographic displays 13 – 15 , optical isolators 16 – 18 and photonic crystals 19 – 21 .…”

Kerr effect microscope combined with a pulse magnet to observe high-entropy alloys fabricated using combinatorial technology

“…Commercially available Kerr effect instruments use a constant-current magnet to perform the magnetic domain imaging of ferromagnetic materials at a few micrometers and the hysteresis measurement of the Kerr signal and magnetic field strength at selected points. Some highly sensitive measurement methods for very thin films have been developed [15][16][17] recently. A common bottleneck in miniaturizing the Kerr effect instruments is that the magnets applying the magnetic field occupy most of the instrument space.…”