Surface Magneto-Optic Kerr Effect

In 1845, Michael Faraday found that the polarization plane of linearly polarized light rotates as the light is transmitted through a magnetized medium. Thirty-two years later the magneto-optic Kerr effect (MOKE) was discovered by John Kerr when he examined the polarization of light reflected from the polished surface of an electromagnet. These two monumental works heralded the beginnings of magneto-optics and form the foundation of its modern utilization. As the field of surface and thin-film magnetism has emerged and blossomed in recent years, so has the need for innovative approaches to study magnetic phenomena on the nanometer-thickness scale. The application of the Kerr effect to study the surface magnetism was introduced by Moog and Bader (1985) along with the acronym SMOKE to denote the surface magneto-optic Kerr effect. Since then SMOKE has emerged as a premier surface magnetism technique of choice in many laboratories worldwide. The broad acceptance of the SMOKE technique stems from its simplicity and its ability to generate the “universal currency” in magnetism—the hysteresis loop. In addition, there are almost no materials limitations to this technique, as long as the sample surface is smooth enough to generate optical reflection. However, the magnitude of the SMOKE signal depends on the materials properties and on the optical wavelength. Although in this article the SMOKE technique for visible light with fixed wavelength is described, it is easy to extend the technique to wavelength-dependent measurements such as magneto-optic spectroscopy and spatially resolved measurements such as magneto-optic microscopy. Nevertheless, SMOKE has been applied successfully to address various contemporary topics in low-dimensional magnetism. The aim of this article is to provide general background about the basic principles and experimental methods of the SMOKE technique. While much of the discussion is directed to nonspecialists, it is structured with the inclusion of mathematical exposition to describe magneto-optics of magnetic multilayers. There are many challenges left to overcome in the quest to provide a complete magnetic characterization of ultrathin-film structures. Significant progress has been made recently to develop second-harmonic-generation (SHG) MOKE to distinguish the response of the surface and/or interfacial layer from the interior-layer or bulk response. A future direction could involve the combination of SMOKE with other techniques to enhance both spatial and time resolution so that small-scale processes, such as domain wall dynamics, can be investigated. The purpose of this article is to provide background on the SMOKE technique with an emphasis on magnetic multilayers. As described in the next section, the magneto-optic effect originates from the spin-orbit interaction. Therefore SMOKE should be regarded as one of many possible versions of this interaction. Others include the Faraday effect technique, which is usually applied to optically transparent materials, and core-level magnetic dichroism, which is an element-specific measurement. Keywords: magneto-optic; quantum description; ferromagnet; kerr effect; practical aspects; method automation; cobalt/copper lattices; ultrathin limit; medium boundary matrices; medium propagation matrices