Magneto-optic Kerr effect CCD imaging with polarization modulation technique Satoru Nakayama, Makoto Okano, Yukio Nozaki, and Shinichi Watanabe Citation: AIP Advances 7, 056802 (2017); doi: 10.1063/1.4974023 View online: http://dx.doi.org/10.1063/1.4974023 View Table of Contents: http://aip.scitation.org/toc/adv/7/5 Published by the American Institute of Physics Articles you may be interested in Magneto-optical color imaging of magnetic field distribution AIP Advances 7, 056803056803 (2017); 10.1063/1.4974024 Nanopatterning spin-textures: A route to reconfigurable magnonics AIP Advances 7, 055601055601 (2016); 10.1063/1.4973387 Vibrating sample magnetometer 2D and 3D magnetization effects associated with different initial magnetization states AIP Advances 7, 056801056801 (2017); 10.1063/1.4973750 Sub-nano tesla magnetic imaging based on room-temperature magnetic flux sensors with vibrating sample magnetometry AIP Advances 7, 056626056626 (2017); 10.1063/1.4974016 AIP ADVANCES 7, 056802 (2017) Magneto-optic Kerr effect CCD imaging with polarization modulation technique Satoru Nakayama, Makoto Okano, Yukio Nozaki, and Shinichi Watanabea Department of Physics, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan (Presented November 2016; received 23 September 2016; accepted 25 October 2016; published online 10 January 2017) We have developed a magneto-optic Kerr effect (MOKE) imaging system with a charge-coupled-device (CCD) camera by using the rotating compensator technique We chose optimal conditions of the rotation frequency of the compensator with stable rotation along with a CCD camera frame rate that allowed precise control of the exposure timing in order to link with the angle of the compensator Precise timing management of the CCD exposure enables us to carry out repeated experiments, which greatly improves the signal-to-noise ratio of the longitudinal MOKE signal We applied the technique to the material characterization of the Ni81 Fe19 thin film and its microstructure, and succeeded in evaluating the spatial variation of the complex magneto-optic constant Q of the sample Because of its attractive advantages such as high-speed and compactness, the present method provides a novel platform for investigating the domain structures in various magnetic materials © 2017 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4974023] I INTRODUCTION Because of their unique magnetic domain structure and their dynamical behavior,1–3 magnetic microstructures of ferromagnetic materials are of great interest,4–6 especially for application to emergent data-storage devices In order to understand the static and dynamical behavior of magnetic domain structures, it is important to study their spatial distribution and spatio-temporal dynamics Thus, establishing a technique for visualizing the spatio-resolved magnetization in magnetic microstructures is necessary It is well known that magneto-optic Kerr effect (MOKE) microscopy7,8 is a powerful tool to study magnetic domains and their dynamics The polarization change due to the MOKE is a consequence of surface reflection from ferromagnetic materials with complex refractive index N and complex magneto-optic constant Q due to spin-orbit interaction in these materials.9–12 Numerous studies using MOKE microscopy have been carried out to characterize the magnetic properties of ferromagnetic thin films.13–16 Recently, MOKE microscopes with array detectors, such as a charge-coupled-device (CCD), which are useful to obtain the spatial distribution of magnetization, have been developed and utilized.17–23 However, there are no reports about the quantitative estimation of Q from MOKE images Fast characterization of the complex magneto-optic constant Q using the MOKE imaging system with CCD cameras will open up new research possibilities for characterizing magnetic microstructures In this paper, we report the spatio-resolved quantitative evaluation of Q from longitudinal MOKE (L-MOKE) measurement with a CCD camera We utilized the rotating compensator polarimetry technique, a conventional technique in ellipsometry, for quantitative determination of the polarization state of light in terms of the orientation and ellipticity angles From the dependence of the orientation a Author to whom correspondence should be addressed Electronic mail: watanabe@phys.keio.ac.jp 2158-3226/2017/7(5)/056802/7 7, 056802-1 © Author(s) 2017 056802-2 Nakayama et al AIP Advances 7, 056802 (2017) and ellipticity angles on the angle of incidence in the L-MOKE configuration, the value of Q of the magnetic materials can be derived quantitatively We have successfully obtained the spatial image of Q using our technique II METHOD Figure shows the schematic setup of the L-MOKE measurement using the rotating compensator technique The polarization state of reflected light from the surface of the sample changes with respect to the polarization state of incident light due to the L-MOKE The s- and p-polarized electric-field p p amplitudes reflected from the surface (Ers , Er ) are described in terms of the incident light (Eis , Ei ) and the complex Fresnel reflection coefficients, r jk (j,k = s,p), as follows: p (1) p (2) Ers = rss Eis + rsp Ei , and p Er = rps Eis + rpp Ei p In this work, we consider the incident light to be completely linearly s-polarized; therefore, Ei = The Fresnel coefficients r ss and r ps with N and Q are given by10 (cos φi − N cos φt ) , (cos φi + N cos φt ) (3) iQ cos φi sin φi , cos φt (cos φi + N cos φt )(N cos φi + cos φt ) (4) rss = and rps = where φi and φt are the angle of incidence and the complex angle of refraction, respectively The term cos φt is approximated to unity (cos φt 1) in our experimental condition In the case when the incident light is linearly s-polarized, the initial orientation angle θ is 90◦ and the initial ellipticity angle ε is zero before reflection from the surface of the magnetic materials Both θ and ε represent the polarization state of the light After reflection, both θ and ε change, and are expressed in terms of N and Q as p θ− π E iQ cos φi sin φi + iε = − rs = − Er (cos φi − N)(N cos φi + 1) (5) In this work, we determine the parameter Q for a given N from Ref 10 using Eq (5) Determination of θ and ε in our experimental system is carried out as follows As shown in Fig 1, the reflected light from the surface of the sample passes through the rotating compensator and the linear polarizer before detection by the CCD camera In this system, the detected signal intensity I, is described as a function of the angle of the compensator, θ m , as,24,25 I(θ m ) ∝ −2S3 sin 2θ m + S1 cos 4θ m + S2 sin 4θ m + const., (6) FIG Schematic view of the L-MOKE measured by a rotating compensator polarimeter with a CCD camera RC, rotating compensator (quarter-wave plate); P, polarizer Inset shows definitions of the polarization directions and orientation angle θ The dotted line represents the major axis of the polarization ellipse of light 056802-3 Nakayama et al AIP Advances 7, 056802 (2017) where S , S , and S are the three Stokes parameters which represent the polarization state of the light One can determine the parameters through Fourier analysis of Eq (6) The Stokes parameters are related to θ and ε, and are given by, θ= S2 tan−1 , S1 (7) and ε= −1 sin S3 (8) S12 + S22 + S32 The parameter Q is obtained by substituting Eqs (7) and (8) in Eq (5) III MATERIALS AND SETUP A Sample preparation In this work, we used two forms of the Ni81 Fe19 sample with a thickness of 30 nm deposited on a silicon substrate by electron beam evaporation One is a Ni81 Fe19 thin film sample, and the other is a rectangular Ni81 Fe19 microstructure sample with a dimension of 15 àm ì 15 àm (see Fig 2(a)) We fabricated the rectangular Ni81 Fe19 microstructure sample using a standard lift-off process B Optical setup and experiment Figure 2(b) shows the experimental setup of our L-MOKE imaging system We used a picosecond laser diode (wavelength: 406 nm) as a light source (LS) The light beam from LS is focused on a rotating glass diffuser D with a rotating frequency of 41 Hz by using lens L1, to reduce the effect of speckle on the L-MOKE image.20 After collimating the light beam by lens L2, the light beam passes through polarizer P1 and a half-wave plate (HWP) so that the light is s-polarized After reflection by a beam splitter BS, the light beam irradiates the sample via lens L3 and an objective lens (OL) with a numerical aperture of 0.8 The light beam was focused on the back focal plane of OL by L3 for Kăohler illumination The incident angle φi is controlled ranging from -45◦ to +45◦ by moving L3 mounted on a translation stage The magnetic field is applied to the sample using an electromagnet (EM), which is controlled by an external voltage source The reflected light beam from the surface of the sample was collected by OL and passed through BS The reflected light beam passed through a quarter-wave plate (QWP) mounted on a hollow-shaft motor with a rotating frequency (f m ) of Hz and polarizer P2 The transmission axis of P2 was set to the direction of p-polarization The reflected light was imaged in a CCD camera using lens L4, where its intensity was measured In addition, we controlled the exposure timing of the CCD camera by an external trigger input with a repetition frequency of f Shutter = (29/10) f m = 8.7 Hz The exposure time of the CCD camera is set to be about 45 ms FIG (a) 15 µm × 15 µm rectangular Ni81 Fe19 microstructure sample with a thickness of 30 nm (b) Schematic experimental setup of L-MOKE imaging with a CCD camera LS, picosecond pulse laser; L1, L2, L3, and L4, plano-convex lenses; D, glass diffuser; P1 and P2, linear polarizers; HWP (QWP), half (quarter) wave plate; BS, beam splitter; OL, objective lens with a numerical aperture of 0.8; EM, electromagnet 056802-4 Nakayama et al AIP Advances 7, 056802 (2017) FIG (a) Timing chart of the L-MOKE imaging system with a rotating compensator Colored circles in the middle panel represent the measured time when the CCD camera is exposed (top panel) After measuring the data for (10/f m ) seconds (f m = Hz) and reordering the measured data, we obtained the sequence of data at even angular intervals over a 360 degree rotation Each colored circle in the bottom panel corresponds to the same colored circle in the middle panel (b) and (c) correspond to the experimental data before and after reordering of the data respectively Solid curve in (c) represents the curve given by Eq (6) by using 2θm and 4θm frequency components (d) Amplitude spectrum of the experimental data derived from (c) The method of data accumulation and analysis in our L-MOKE measurement system is shown in Fig 3(a) In our system, we acquire the image by the CCD camera with a time interval of (10/(29f m )) The angle of the QWP at the 30th image is identical with that at the 1st image In other words, the 29 intensity data sets with different θ m are repeatedly obtained with a time interval of 10/f m Thus, we can accumulate and average the intensity signal by repeating the experiments N repeat times to increase the signal-to-noise ratio (SNR) Note that f Shutter /f m is not an integer, which means that θ m (0 ≤ θ m