Development of a magneto-optical Kerr microscope using a 3D printer

Introduction

In recent years, the “provision of goods and services that meet diverse needs without disparity” in Society 5.0 and “production that does not waste resources” in the Sustainable Development Goals (SDGs) have been of importance in our society.¹ In this respect, the 3D printing technology plays a key role in many occasions from manufacturing and construction to hobbies, in which one can create new objects from conceptions regardless of age, lifestyle, and occupation. In the field of science, 3D printing technology is being used to significantly reduce the cost of experimental equipment and to develop new optical elements such as optical choppers, filter brackets, and rails.² It is, therefore, expected that researchers can conduct more experiments along this research direction with a finite budget since low-cost experimental equipment becomes available and experiments can be conducted without buying expensive equipment. One of the applications of 3D printing technology is to design and build microscopes with various external parameters such as magnetic fields. Combining optical systems and data processing methods with a 3D printed microscope, observations with the in-plane resolution of several hundred nanometers have recently been achieved.^3,4 Furthermore, it has been reported that the sample stage can be moved several tens of nanometers.^5,6 The mechanism of these sample stages was developed in the project called “The OpenFlexure Project”, in which the slight expansion and contraction of the plastic material is adjusted by a rubber band (O-ring). These recent technical developments enable the stage movement along any direction in three dimensions, which is often required for the practical use of optical microscopes. The 3D printed microscopes with nanometer scale in-plane resolution, depth resolution, and precision of stage movement are already easily available for end-users with a 3D printer.

In this article, we report the development of a 3D printed microscope to observe a real-space image of magnetic domains in magnetic materials by means of the magneto-optical Kerr effect (MOKE), i.e., a phenomenon that the polarization of the reflected light is rotated by a magnetic material in response to an applied linearly polarized light. To observe these magnetic domains, large and expensive equipment such as optical tables and polarizing microscopes have routinely been used since deflecting elements are required. However, by redesigning several components of the microscope originally designed by “The OpenFlexure Project” using 3D printing technology, we successfully made a compact and low-cost MOKE microscope without large-scale commercial equipment. The sample observed in this study is a magneto-optical (MO) sensor to demonstrate the feasibility of our new 3D-printed microscope in the present work. We choose the MO sensor particularly because a material with perpendicular magnetic anisotropy enables us to observe magnetic domains and their motions appearing on the surface of a magnetic sample with MOKE. We incorporated an electromagnet into the microscope to scrutinize the motions of magnetic domains caused by magnetic fields. Our 3D-printed microscope to observe magnetic domains can be applied for a range of materials including NiCo₂O₄ thin film samples,^7–9 which are attracting attention towards next-generation electronic device applications.

Hardware design

The microscope device using 3D printing technology was fabricated based on a microscope device development project by the University of Bath and the University of Cambridge (The OpenFlexure Project). Figure 1 shows a 3D-printed MOKE microscope device that we have built in the present study. A key development of our study is to make it possible to build a portable MOKE microscope. The actuator gear is used for the X-Y stage movements. This gear makes the microscope portable because this gear does not require a large space. Specifically, the dimensions of the microscope are 105 × 105 × 160 mm³ and therefore considerably smaller than conventional microscopes. In the following, we will examine details of the 3D printed MOKE microscope and its applications to studies of magnetic materials. Firstly, we elaborate on optical paths, components, and the design of our MOKE microscope referring to the cost and performance of each optical element. Also, we describe actual measurements of magnetic domains and their motions caused by magnetic fields in a magnetic material to demonstrate the performance of our newly developed MOKE microscope.

Figure 1. MOKE (Magneto-optical Kerr effect) microscope fabricated by a 3D printer.

(a-c)(Left) Original 3D modeling data provided from “The OpenFlexure Project”. J. P. Sharkey et al., Rev. Sci. Instrum. 87, 025104 (2016); licensed under a Creative Commons Attribution (CC BY) license. (Right) Modified 3D modeling data of the counterpart components used in this study. The red frames indicate the main parts that we redesigned.

The optical path of the microscope can be seen in Figure 2. The light path from the light source to the sample is as follows: Light-emitting diode (LED) → Condenser Lens → Polarizer1 → Beam splitter → Tube Lens → Objective Lens → Sample. The light reflected from the sample reaches the camera through the following optical path: Sample → Objective Lens → Tube Lens → Beam splitter → Polarizer2 → Camera for imaging. This microscope employs the Köhler illumination method, in which the unevenness in the luminance of the light source does not appear on the illuminated surface.¹⁰ A vital development of this microscope is a polarizing element to observe magnetic domains using MOKE. Another distinctive feature of our design is the stage movement using O-rings and the ability to easily change the design of the device using 3D modeling data taken from OpenFlexure and edited with software Fusion360 (ver2.0.14569). Note that 3D modeling data edited in Fusion360 here can also be edited in other software available for free such as FreeCAD. To move the sample stage, one can rotate the actuator gear shown in Figure 1, making the O-ring expand and contract, which in turn moves the X-Y stage shown in Figure 1. Figure 1(a)-(c) show our upgrades of three components in our microscope. These revised components are important to make the MOKE microscope for practical use. We elaborate on these modifications in the following. The body (Figure 1(a)) was edited to sufficiently change the position of the sample stage along the Z-axis. We got rid of the black part indicated by a red rectangular shown in Figure 1(a) to insert the rods (Figure 1(b)) to rotate the polarizer1. With this modification, the distance from the objective lens to the sample can be adjusted when the objective lens with the different magnification is replaced. As shown in Figure 1(b), we added two rods to rotate the polarizing element because the MOKE microscope requires adjustment of the rotation angle of the polarizing element during measurements depending on the Kerr angles of magnetic materials, which in general differ from sample to sample. Figure 1(c) displays a component to install an electromagnet on top of the sample, enabling the application of a magnetic field perpendicular to the sample. To calibrate the magnitude of the magnetic field, a tesla meter (TM-801, KANETEC) was used. The measuring portion of the tesla meter instrument was placed at the position of the sample to be observed, and the instrument was calibrated after confirming that the sample was illuminated by light. We carefully measured the magnitude of the magnetic field at the position of the sample.

Figure 2. Schematic cross-sectional view of the MOKE (Magneto-optical Kerr effect) microscope.

The reflective geometry is realized using a LED as a light source and a beam splitter. The electromagnet storage and polarizers are additional components to make an ordinary optical microscope into a MOKE microscope.

We refer to the cost and performance of each optical element used to assemble our microscope in Table 1. The total cost of filaments (ingredients of the 3D printing) and all the components to fabricate the MOKE microscope is 20,000 yen. This is an excellent cost performance in comparison with standard commercial MOKE microscopes. For instance, ZEISS Axio Imager costs >2,000,000 yen. Instead of purchasing such an expensive instrument, it is thus possible to access a MOKE microscope whose parameters can easily be improved with better optical elements such as objective lenses and tube lenses depending on the requirements of specific measurements.

The microscope fabricated in this study was used to image patterns of magnetic domains appearing on the surface of a magnetic sample. An electromagnet is placed above the sample, and the magnetic field is controlled by a DC power supply. Images were generated with a USB camera connected to a standard computer. The sample measured in this work is a MO sensor (Matesy GmbH: Magneto-optical sensors with mirror and DLC protection (type-A)), whose magnetic domains of the sample surface can be controlled by applying a magnetic field. The magnetic domain patterns were binarized with image analysis software (Igor 6.0 and ImageJ 1.53e). Such image analyses implemented in Igor can also be carried out in primitive programming languages such as Python. The camera properties such as brightness, vividness, exposure, and backlight correction were specified during imaging using free software called WebCamSetting 1.1.0.0. Specifically, the brightness and vividness were set to the minimum value 0 whereas the exposure and backlight correction were kept as default. This setting allows us to clearly observe MOKE images.

Methods

Here, we describe the methods of data acquisition and analysis. A USB camera is connected to the computer for the data acquisition and camera images were obtained using default camera settings. To binarize the images, we used the iterated binarization method in the Igor software. Since the ratio of dark and bright areas can be quantified with ImageJ, the data format was converted from image data to numerical values of ±1 to quantitatively obtain the Kerr intensity in Figure 3. Care was taken to record the data at the same areas of the sample surfaces while taking images with 10 different magnetic fields (±7.6, ±3.8, ±1.9, ±1.1 and ±0.76 mT) to evaluate magnetic field effects on the images.

Figure 3. (Left) Magnetic field dependence of the Kerr intensity and the magnetic domains of the MO (magneto-optical) sensor observed with our 3D printed Kerr effect microscope and plotted by using Igor6.0. The measurements were carried out at room temperature (~ 290 Kelvin) and 10 different magnetic fields (±7.6, ±3.8, ±1.9, ±1.1 and ±0.76 mT). The Kerr intensity is defined as the differential areas of the respective domains within the scope, and thereby represents the macroscopic magnetization of the sample. The numbers next to the data points in the graph indicate the original images used to estimate the Kerr intensity. (Right) Faraday rotation data provided by Matesy GmbH.

To eliminate the background signals in the image data shown in Figure 3, the darkest image (1) (the single magnetic domain, mirroring the macroscopically saturated magnetization by a sufficient amount of magnetic field) is used as a representing background signal. Magnetic domain images at 10 different magnetic fields are subtracted from the image (1) with a software (ImageJ converts the image data into numerical values).

Results

The data associated in this article are available in Underlying data.¹¹ Figure 3 shows images of the magnetic domains of the MO sensor under magnetic fields varying from −7 mT to +7 mT using the electromagnet placed above the sample. These data were collected at room temperature and a white LED shown in Table 1 was used as a light source. The image is monochromatic dark when a sufficient amount of the field was applied (Figure 3 (1)). This is an expected behavior for ferromagnets when the total magnetization is saturated by the external field. Upon the application of a magnetic field, we clearly observed domains in the bright color (we characterized color as either bright or dark), which indicates that the magnetic domains along the other direction are induced by the field (Figure 3 (2)). It is also appreciable that the entire contrast of the image is drastically changed when the sign of the field is reversed (Figure 3 (2) and (3)). The image is dominated by domains in the bright color and is finally monochromatic bright when the substantial field was applied along the opposite direction to the initial field (Figure 3 (4) and (5)). In light of these real-space observations, we have analyzed the contrast of the images to estimate the magnetic field dependence of the net magnetization of the MO sensor. The Kerr intensity is defined as the differential areas of the respective domains within the scope and thereby represents the macroscopic magnetization of the sample. The magnetic field dependence of the Kerr intensity is consistent with the observations of the original images and reminiscent of the typical M- H curve of ferromagnets. Specifically, the coercive field inferred from the Kerr intensity is 2 mT, which quantitatively agrees with the experimental data of Faraday rotation provided by the company Matesy GmbH in Figure 3 (right), considering the relationship of 1 mT = 0.7958 kAm⁻¹.

Conclusions

In conclusion, we have assembled a MOKE microscope using 3D printing technology. The total price is less than 2% of that of the standard commercial MOKE microscope, based on the comparisons we have made (for example, ZEISS Axio Imager costs >2,000,000 yen). To substantially reduce the size of the MOKE microscope, we utilized the 3D modeling provided by the OpenFlexure Project (and such an attempt to assemble a 3D printed MOKE microscope is for the first time to the best of our knowledge). The feasibility of our 3D-printed MOKE microscope is well confirmed by the measurements of the real-space images of the magnetic domains of the MO sensor under the magnetic fields and the analysis of the macroscopic magnetization estimated from these images.

On the experimental front, one can extend the maximum values of the magnetic field by replacing the electromagnet with the one which can tolerate higher electrical currents or superconducting magnets. The real-space resolution can be improved by increasing the magnification of the objective lens. These amplifications of the external parameters should be straightforward for end-users and can be achieved with reasonable costs compared to the price of conventional commercial MOKE microscopes. We thus believe that MOKE microscopes will be more easily available and customizable in the field of materials science along the direction we present in this work.