F1000ResearchF1000Research2046-1402F1000 Research LimitedLondon, UK10.12688/f1000research.133292.2Research ArticleArticlesDevelopment of a magneto-optical Kerr microscope using a 3D printer
[version 2; peer review: 1 approved, 1 not approved]
UeboKokiConceptualizationInvestigationMethodologyWriting – Original Draft Preparation1ShiokawaYutoConceptualizationInvestigationMethodology1TakahashiRyunosukeConceptualizationInvestigationMethodology1NakataSuguruConceptualizationInvestigationMethodologyWriting – Review & Editing1WadatiHirokiConceptualizationInvestigationMethodologyWriting – Review & Editinghttps://orcid.org/0000-0001-5969-8624a12
Department of Material Science, Graduate School of Science, University of Hyogo, Ako, Hyogo, 678-1297, Japan
Institute of Laser Engineering, Osaka Univerisity, Suita, Osaka, 565-0871, Japanwadati@sci.u-hyogo.ac.jp
Magneto-optical Kerr effect (MOKE) microscopes are powerful experimental tools to observe magnetic domains in magnetic materials. These devices are, however, typically large, unportable, and expensive (∼ several million yen), and therefore prevent many researchers in the field of materials science from easy access to study real-space images of magnetic domains.
Methods
To overcome these issues, we utilized data from “The OpenFlexure Project” developed by the University of Bath and the University of Cambridge. The purpose of this project is to make high-precision mechanical positioning of the studied sample available to anyone with a 3D printer, especially for use in microscopes. We built a low-cost and portable MOKE microscope device with a 3D printer. We redesigned the 3D modeling data of an ordinary optical microscope provided by The OpenFlexure project and incorporated additional elements, such as optical polarizers and an electromagnetic coil into the primarily designed microscope that did not originally have these elements.
Results
We successfully observed magnetic domains and their real-space motions induced by magnetic fields using the palm-sized low-cost MOKE microscope, which costs approximately 30,000 yen in raw materials to construct.
Conclusions
Our methodology to assemble a low-cost MOKE microscope will enable researchers working in the field of materials science to observe magnetic domains more easily without commercial equipment.
The OpenFlexure projectMagneto-Optical Kerr microscope3D printing technologylow-costMEXT Q-LEAPJPMXS0118068681Asahi Glass FoundationThe Japan Science SocietyJapan Society for the Promotion of ScienceJP19H05824This work was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number JP19H05824
(to HW), the Ministry of Education, Culture, Sports, Science and Technology (MEXT) Quantum Leap (Q-LEAP) Flagship
Program under Grant No. JPMXS0118068681 (to HW), the Asahi Glass Foundation (to HW), and the Sasakawa Scientific
Research Grant from The Japan Science Society (to RT)The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.Amendments from Version 1
The main revisions are the following. 1. We find that the OpenFlexure Project was mentioned several times in the introduction and Hardware Design sections. Therefore, we revised some sentences in these sections to avoid clutter. We also revised the English expressions throughout the text. 2. We included the information on the power supply as DC power supply 6,000, Kungber stabilized power supply, 0-30V, 0-5A in Table 1. We also revised the information in this table. The total cost was changed from 20000 to 30000 yen. 3. In Fig. 3, we changed the external field unit of the right loop from kAm-1 to mT by using the relationship of 1 mT = 0.7958 kAm-1. 4. We revised the information in Reference 4 as
Opt. Express. 2022;30(13):23216–23298. 10.1364/OE.461910. 5. We added References 10-12.
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, 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 with the precision of several tens of nanometers.
5,6 The mechanism of these sample stages was developed in the project called “
The OpenFlexure Project”, developed by the University of Bath and the University of Cambridge, 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 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. Nevertheless, we successfully made a compact and low-cost MOKE microscope using 3D printing technology 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 to a range of materials, including thin films of NiCo
2O
4, iron garnet, Pt/Co and CoFeB,
7–12 which are attracting attention towards next-generation electronic device applications.
Hardware design
The MOKE microscope device using 3D printing technology was fabricated based on a microscope device distributed in
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 mm
3. Therefore, they are 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.
MOKE (Magneto-optical Kerr effect) microscope fabricated by a 3D printer.
(a-c)(Left) Original 3D modeling data.
5 (Right) Modified 3D modeling data of the counterpart components used in this study. The red frames indicate the main parts that we redesigned. Note that the angle of the rod for Polarizer 1, shown in panel b, can vary from -75 to 75 degrees in the microscope.
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, where the illuminated light becomes collimated on the sample surface by using the three lenses. This ensures the uniformity of the LED illumination.
13 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 rectangle 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 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. The rod can be mechanically rotated by hand. The angle of the holder can vary from -75 to 75 degrees in the current setup.
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.
Schematic cross-sectional view of the MOKE (Magneto-optical Kerr effect) microscope.
The reflective geometry is realized using an 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 30,000 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.
List of optical elements used in the present work and their performance.
Elements used
Price (yen)
Product name
Performance
LED
10
DiCUNO light emitting diode 5 mm
12 candela
Objective lens
2,100
Wal frontxdsn3102k8
10×, Objective lens length 31.3 mm
Electromagnet
1,600
NaRiKa for general contractors
ϕ0.4 mm (Enameled wire), 400 rolls, Iron (Core material)
Camera
4,100
AliExpress HBVCAM-1710-H264 V33
Pixel size 2.2 μm × 2.2 μm, RGB
Condenser lens
100
Easybuyseller G100008
ϕ17 mm, 3.2 mm (Lens thickness), 28 mm (Focal length)
Tube lens
7,500
THORLABS AC127-050-A
ϕ12.7 mm, 5.0 mm (Lens thickness), 50.0 mm (Focal length)
Half mirror
3,000
Shibuya Optical Co., Ltd. H282
1.5 mm (Mirror thickness), Transmission: Reflectance = 1:1
Polarizer
1,200
THORLABS LPVISE2X2
Extinction ratio 100:1 or more in the visible light range
DC power supply
6,000
Kungber stabilized power supply
0-30 V, 0-5 A
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 of 0, whereas the exposure and backlight correction were kept as default. This setting allows us to observe MOKE images clearly.
Methods
Here, we describe the methods of data acquisition and analysis. A USB camera was 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 obtain the Kerr intensity in
Figure 3 quantitatively. 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.
(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) Magnetic field dependence of the magnetization, which is estimated by the Faraday rotation data provided by Matesy GmbH.
To eliminate the background signals in the image data shown in
Figure 3, the darkest image at −7.6 mT (the single magnetic domain, mirroring the macroscopically saturated magnetization by enough magnetic field) is used as a representing background signal. Magnetic domain images at 10 different magnetic fields are subtracted from the image (1) with software (ImageJ converts the image data into numerical values).
Results
The data associated with this article are available in
Underlying data.14Figure 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 enough magnetic field was applied. 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 (1) and (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 (3) and (4)). 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 (5)). Considering 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).
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.
Data availabilityUnderlying data
The original 3D printing data from “The OpenFlexure Project” is available for download here:
https://openflexure.org/projects/microscope/build.
This project contains the following underlying data:
Modified (3D print files).
Image (image files).
Data are available under the terms of the
Creative Commons Attribution 4.0 International license (CC-BY 4.0).
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10.5281/zenodo.795083510.5256/f1000research.155207.r235789Reviewer response for version 2RodriguezAurelio Hierro1Refereehttps://orcid.org/0000-0001-6600-7801Cascales-SandovalMiguel Angel2Co-referee
University of Oviedo (Ringgold ID: 16763), Oviedo, Asturias, Spain
Technische Universitat Wien, Vienna, Vienna, Austria
Competing interests: No competing interests were disclosed.
The author hasn't included my suggestion in detail.
Is the work clearly and accurately presented and does it cite the current literature?
No
If applicable, is the statistical analysis and its interpretation appropriate?
No
Are all the source data underlying the results available to ensure full reproducibility?
Yes
Is the study design appropriate and is the work technically sound?
Partly
Are the conclusions drawn adequately supported by the results?
No
Are sufficient details of methods and analysis provided to allow replication by others?
No
Reviewer Expertise:
Nanomagnetism, X-ray Transmission Microscopy, Magneto-optics, Magnetic Thin Films
We confirm that we have read this submission and believe that we have an appropriate level of expertise to state that we do not consider it to be of an acceptable scientific standard, for reasons outlined above.
WadatiHirokiUniversity of Hyogo, Japan
Competing interests: No competing interests were disclosed.
1242024
The reviewer should specify which suggestions are required so that we can respond appropriately.
Regards,
Hiroki Wadati
10.5256/f1000research.146271.r215081Reviewer response for version 1RodriguezAurelio Hierro1Refereehttps://orcid.org/0000-0001-6600-7801Cascales-SandovalMiguel Angel2Co-referee
University of Oviedo (Ringgold ID: 16763), Oviedo, Asturias, Spain
Technische Universitat Wien, Vienna, Vienna, Austria
Competing interests: No competing interests were disclosed.
The authors report on the development and implementation of a cheap 3D printed Kerr Microscope based on the main design of “The OpenFlexure Project”. The idea is very interesting and really brings the capability of performing Kerr Microscopy with small and cheap instruments. However, there are several and important flaws in the work, from its redaction to the discussion and conclusions, passing through the experimental details, that prevent us from recommending this manuscript for its publication. Here follows our indications and comments.
As a general first comment, the content is sometimes repetitive which makes the general reading difficult, thus the style should be improved.
In the introduction, current literature on 3D printing is referenced, however, no citations appear about other Kerr Microscope setups for information and comparison with current state of the art techniques. As this work is focused on Kerr Microscopy, it should be very important to have this framework well defined.
Also in the introduction, it is not clear for me what the authors mean with the sentence saying that the stage can be moved several tens of nanometers. Is this indicating the precision of the movement using the 3D printed stage actuators, or is the total displacement possible per axis? If it is the first case, then there is a clear problem as tens of nanometers is a very limited movement range for an optical microscope. Clarify.
Also in the introduction section, what is the meaning of depth resolution at the end of the first paragraph? Is that the depth of focus or the precision of the Z stage? If it is the first, then this strongly depends on the objective used and it is usually in the range of ~1 um.
In the second paragraph of the introduction, the application of the method to be discussed is focused towards a very specific material (NiCo2O4). If this work is devoted to present a general purpose and cheap Kerr Microscope, a wider discussion about systems where it could be used as a useful characterization tool should be cited and discussed briefly.
In the Hardware design section, a more thorough description of the polarization optics should be done as it is the key for a proper functioning of a Kerr Microscope. It would be great if comparison with in-plane magnetization sensitive microscopes would be done, as there are plenty of systems where in-plane magnetization is the main actor.
As a comment, in figure 1 caption, maybe it is not necessary to include the citation for the OpenFlexure Project as it is already in the main text. The caption should focus on the figure itself.
There is a typo in the description in the main text of figure 1(a): “by a red rectangular” should be “by a red rectangle”.
The maximum rotation of the analyser should be indicated in the description of panel b of figure 1 as this is important for the tuning of the microscope. Could it be possible to include a retarder waveplate in the design in addition to the analyser too as it exists in other KM for signal optimization? A comment about this would be helpful as this would indicate that the authors have reviewed current Kerr Microscope setups in detail.
We find the discussion about the electromagnet design very obscure. As it is a very important part of the microscope, it should be described with more detail indicating core material, remanent field, possible heating problems such as 3D printed plastic deformation? Also, stability of the microscope while utilization with the magnetic field on. Interference of the magnetic field with magnetic parts of the objective?
Just a minor comment, probably, the price of the power supply for the electromagnet should be included in the total price of the microscope.
In the discussion about the domain imaging and analysis of the Magneto-optic sensor provided by Matesy Gmbh, we have a lot of doubts.
First of all, why the authors binarize the data for its processing? The intensity of the signal recorded by the camera if it works in a linear manner is very important and can give useful information about the orientation of the magnetization within the system (vector sensitivity inherent to the MO effect). Here it is important also to indicate the dynamic range of the camera used as well as its relation with the smallest signal that the microscope can measure. A thorough discussion about this is mandatory as this work is describing a characterization technique.
Another important point here is the fact that the sample used for demonstrating the proof of concept does not really use the Kerr effect, but the Faraday effect which gives a huge signal when compared with polar Kerr for instance. The rotation indicated in figure 3 by the manufacturer of the sensor indicates a rotation of +/-8 degrees between saturations. This is very easy to measure and is adequate for an initial demonstration, but real samples measured using Kerr configuration will most probably show way smaller rotations of the polarization. In this regard, how are the low extinction ratio polarizers going to work? At this point we think that in order to demonstrate real capabilities of the proposed microscope, a real sample where Kerr effect is used for its characterization is mandatory, as is in this configuration where the microscope should work.
Finally, two comments about figure 3. The external field units of the two loops (half-loops) presented should be the same, and the normalization protocol used for removing background and getting magnetic signal should be better clarified. As it is now, I understand that image 1 (-3.8 mT) is the one used for normalization, which shows stripes. If this is the image taken for normalization, why aren’t these stripes present always in all the images? A fully flat image should be taken to work as reference and then the microscope would be working in differential mode. Have the authors observed strong Faraday effects due to the objective optics in combination with the external field from the electromagnet? Please clarify this point.
Is the work clearly and accurately presented and does it cite the current literature?
No
If applicable, is the statistical analysis and its interpretation appropriate?
No
Are all the source data underlying the results available to ensure full reproducibility?
Yes
Is the study design appropriate and is the work technically sound?
Partly
Are the conclusions drawn adequately supported by the results?
No
Are sufficient details of methods and analysis provided to allow replication by others?
No
Reviewer Expertise:
Nanomagnetism, X-ray Transmission Microscopy, Magneto-optics, Magnetic Thin Films
We confirm that we have read this submission and believe that we have an appropriate level of expertise to state that we do not consider it to be of an acceptable scientific standard, for reasons outlined above.
WadatiHirokiUniversity of Hyogo, Japan
Competing interests: No competing interests were disclosed.
19122023
The authors report on the development and implementation of a cheap 3D printed Kerr Microscope based on the main design of “The OpenFlexure Project”. The idea is very interesting and really brings the capability of performing Kerr Microscopy with small and cheap instruments. However, there are several and important flaws in the work, from its redaction to the discussion and conclusions, passing through the experimental details, that prevent us from recommending this manuscript for its publication. Here follows our indications and comments.
Our Reply: We thank the reviewers for their careful reading of our manuscript and for providing us with constructive comments. In the following, we answer their concerns one by one and point out the changes made in the manuscript to improve both the discussion and the figures following his suggestions.
1. As a general first comment, the content is sometimes repetitive which makes the general reading difficult, thus the style should be improved.
Our Reply: We find that the OpenFlexure Project was mentioned several times in the introduction and Hardware Design sections. Therefore, we revised some sentences in these sections to avoid clutter. We also revised the English expressions throughout the text.
2. In the introduction, current literature on 3D printing is referenced, however, no citations appear about other Kerr Microscope setups for information and comparison with current state of the art techniques. As this work is focused on Kerr Microscopy, it should be very important to have this framework well defined.
Our Reply: We added Refs. 12-14 to include the information on other Kerr Microscope setups for thin films of iron garnet, Pt/Co, and CoFeB.
3. Also in the introduction, it is not clear for me what the authors mean with the sentence saying that the stage can be moved several tens of nanometers. Is this indicating the precision of the movement using the 3D printed stage actuators, or is the total displacement possible per axis? If it is the first case, then there is a clear problem as tens of nanometers is a very limited movement range for an optical microscope. Clarify.
Our Reply: The tens of nanometers indicated the precision of the movement. The movement range is a few millimeters. Now we wrote, Furthermore, it has been reported that the sample stage can be moved with the precision of several tens of nanometers.
4. Also in the introduction section, what is the meaning of depth resolution at the end of the first paragraph? Is that the depth of focus or the precision of the Z stage? If it is the first, then this strongly depends on the objective used and it is usually in the range of ~1 um.
Our Reply: We would like to thank the referee for pointing this out. We removed “depth resolution” in the introduction to avoid confusion since the depth resolution is beyond our interest in the present study.
5. In the second paragraph of the introduction, the application of the method to be discussed is focused towards a very specific material (NiCo2O4). If this work is devoted to present a general purpose and cheap Kerr Microscope, a wider discussion about systems where it could be used as a useful characterization tool should be cited and discussed briefly.
Our Reply: We added Refs. 12-14 to include the information on more general setups for observing magnetic domains of iron garnet, all-optical magnetization switching, and the creation of magnetic skyrmions.
6. In the Hardware design section, a more thorough description of the polarization optics should be done as it is the key for a proper functioning of a Kerr Microscope. It would be great if comparison with in-plane magnetization sensitive microscopes would be done, as there are plenty of systems where in-plane magnetization is the main actor.
Our Reply: We chose
THORLABS LPVISE2X2 with an extinction ratio of 100:1 or more in the visible light range. We also tried
EDMUNDS 8.5" x 5" Linear Polarizing Film but did not succeed due to the lower polarization efficiency of 88%. We agree that there are a number of materials exhibiting in-plane magnetization. However, observations of magnetic domains of in-plane magnetism are beyond the scope of the present study, although this is indeed an interesting research direction in the future.
7. As a comment, in figure 1 caption, maybe it is not necessary to include the citation for the OpenFlexure Project as it is already in the main text. The caption should focus on the figure itself.
Our Reply: We agree that the caption should explain a figure itself. We therefore replaced the detailed information of the original literature and “The Openflexture Project” with Ref. 5. in the figure caption.
8. There is a typo in the description in the main text of Figure 1(a): “by a red rectangular” should be “by a red rectangle”.
Our Reply: We would like to thank the referee for pointing this out. We accordingly changed this part from “by a red rectangular” to “by a red rectangle.”
9. The maximum rotation of the analyser should be indicated in the description of panel b of figure 1 as this is important for the tuning of the microscope. Could it be possible to include a retarder waveplate in the design in addition to the analyser too as it exists in other KM for signal optimization? A comment about this would be helpful as this would indicate that the authors have reviewed current Kerr Microscope setups in detail.
Our Reply: To be confirmed, the analyzer, which we call the polarizer 2 in the text, is not rotatable in the first place. We have added the following sentence in the caption for panel b of Figure 1: “Note that the angle of the rod for the polarizer 1 shown in panel b can vary from -75 to 75 degrees in the microscope.”
10. We find the discussion about the electromagnet design very obscure. As it is a very important part of the microscope, it should be described with more detail indicating core material, remanent field, possible heating problems such as 3D printed plastic deformation? Also, stability of the microscope while utilization with the magnetic field on. Interference of the magnetic field with magnetic parts of the objective?
Just a minor comment, probably, the price of the power supply for the electromagnet should be included in the total price of the microscope.
Our Reply:
We included the information on the core material (iron) in Table 1. The relationship between the magnetic field (mT) and the current (A) is shown below, indicating no remnant field. So far, we have not observed any plastic deformations of the 3D printed objects due to thermal heating of the coil.
Figure R1: Current evolution of the magnetic field of the coil we employed in the present study.
(Please find the updated figure file in above hyperlink Figure R1)
We included the information on the power supply as
“DC power supply 6,000
Kungber stabilized power supply 0-30V, 0-5A”
in Table 1. The total cost was changed from 20000 to 30000 yen.
11. First of all, why the authors binarize the data for its processing? The intensity of the signal recorded by the camera if it works in a linear manner is very important and can give useful information about the orientation of the magnetization within the system (vector sensitivity inherent to the MO effect). Here it is important also to indicate the dynamic range of the camera used as well as its relation with the smallest signal that the microscope can measure. A thorough discussion about this is mandatory as this work is describing a characterization technique.
Our Reply:
For the following reasons, analyses by binarization are well justified.
(a) This thin film has high perpendicular magnetic anisotropy, so the magnetic domains of up and downward spins are distinct.
(b) The width of the domain wall is much smaller than the spatial resolution of the optical microscope.
Figure R2 shows the effect of binarization available with Zenodo of Ref.14. This operation makes the up and down magnetic domains created by strong perpendicular magnetic anisotropy more clearly visible. Domain walls are too small to be observed.
Figure R2: (Left) Original data of magnetic domains in the MO sensor collected with
B = -1.1 mT (Ref. 11). (Right) Binalized data, a part of which is shown in panel 2 of Figure 3 in the main text. Note that the black/white is inverted compared with the left figure for clarity.
((Please find the updated figure file in above hyperlink
Figure R2)
12. Another important point here is the fact that the sample used for demonstrating the proof of concept does not really use the Kerr effect, but the Faraday effect which gives a huge signal when compared with polar Kerr for instance. The rotation indicated in figure 3 by the manufacturer of the sensor indicates a rotation of +/-8 degrees between saturations. This is very easy to measure and is adequate for an initial demonstration, but real samples measured using Kerr configuration will most probably show way smaller rotations of the polarization. In this regard, how are the low extinction ratio polarizers going to work? At this point we think that in order to demonstrate real capabilities of the proposed microscope, a real sample where Kerr effect is used for its characterization is mandatory, as is in this configuration where the microscope should work.
Our Reply:
The right side of Figure 3 is the Faraday effect (obtained by transmission) provided by the company Matesy GmbH to obtain the reference M-H curve. We are conducting microscopy measurements on this sample by using the reflective Kerr effect and have already observed magnetic domain images using this MO sensor as a real sample. Measurements on other samples are currently underway, and we would like to report on them in future publications.
13. Finally, two comments about figure 3. The external field units of the two loops (half-loops) presented should be the same, and the normalization protocol used for removing background and getting magnetic signal should be better clarified. As it is now, I understand that image 1 (-3.8 mT) is the one used for normalization, which shows stripes. If this is the image taken for normalization, why aren’t these stripes present always in all the images? A fully flat image should be taken to work as reference and then the microscope would be working in differential mode. Have the authors observed strong Faraday effects due to the objective optics in combination with the external field from the electromagnet? Please clarify this point.
Our Reply:
We apologize that the original statement was different from our analysis procedure and thereby misleading. We used the darkest image without stripes collected at -7.6 mT (not shown) as a background. This is certainly why these stripes are not present in normalized images. To make this point clearer, we wrote, “the darkest image at -7.6 mT is used as a representing background signal.”
We changed the external field unit of the right loop from kAm-1 to mT by using the relationship of 1 mT = 0.7958 kAm-1.
We confirmed that there are no Faraday effects of the objective lens. By considering the Verdet constant, this is much below 0.1 deg, much smaller than the Kerr rotation of the thin film.
10.5256/f1000research.146271.r189385Reviewer response for version 1MatsuiTatsunosuke1Refereehttps://orcid.org/0000-0003-2179-9432
Department of Electrical and Electronic Engineering, Mie University, Tsu, Mie Prefecture, Japan
Competing interests: No competing interests were disclosed.
This manuscript shows development of an open-source, low-cost and compact 3D printed microscope for the observation of magnetic domains in magnetic materials by the magneto-optical Kerr effect (MOKE). Developments of such open-source and low-cost microscope are actively studied in recent years
1. A lot of design files are freely available and the authors of the current manuscript utilized one of these 3D printable design developed by the OpenFlexure Project
2. In MOKE observation, polarizers have to be introduced to detect rotation of the polarization of the linearly polarized light reflected from a magnetic material under applied magnetic field. Therefore, they redesigned several components of the OpenFlexure microscope to attach electromagnet and polarizers, and they also made the data for the reproduction of them freely available. They also showed experimental results of the MOKE observation using commercially available magneto-optical (MO) sensor as a sample, which is sufficient to guaranty feasibility of their system.
The OpenFlexure microscope families are superior for their extremely high positioning precision with tens of nanometers based on flexure mechanism of the flexible plastics and low-cost stepper motors
3. Several groups have made contribution to the improvement of such OpenFlexure microscope for advanced scientific applications such as, the optical projection tomography (OPT)
4, computational super-resolution imaging system based on the super-resolution radial fluctuation (SRRF) algorism
5, and so on. The studies presented in the current manuscript successfully added new function to the OpenFlexure microscope, which may be useful for the researchers working in the field of materials science, especially in magnetic materials.
We are also one of such contributor and reported about a low-cost fabrication of the structured illumination microscope (SIM) for the optical sectioning based on the OpenFlexure stages
6. I am confident that we are familiar with the basic working principle of the OpenFlexure microscope and are experienced in building 3D-printable microscopes, but I have poor experience on magnetic measurements. I can say that the authors present appropriate procedures for the reproduction of their work, but I think it will be more informative and helpful if they can provide detailed information about polarizer holder. The polarizers are key components in the MOKE observation and have to be rotated depending on the direction of axes of the magnetic material. The authors simply describe that the polarizers can be rotated in the current manuscript, but do not give detailed explanation how it can be done. My other concern is about uniformity of the LED illumination and the applied magnetic field in the field of view. I think it will be better if the authors can provide descriptions on these points.
Is the work clearly and accurately presented and does it cite the current literature?
Yes
If applicable, is the statistical analysis and its interpretation appropriate?
I cannot comment. A qualified statistician is required.
Are all the source data underlying the results available to ensure full reproducibility?
Yes
Is the study design appropriate and is the work technically sound?
Yes
Are the conclusions drawn adequately supported by the results?
Yes
Are sufficient details of methods and analysis provided to allow replication by others?
Yes
Reviewer Expertise:
My expertise is in development of novel optical materials and devices based on organic functional materials such as pi-conjugated materials and liquid crystals. I am also working on metamaterials, plasmonics, photonic crystals, terahertz spectroscopy, and optics in general.
I confirm that I have read this submission and believe that I have an appropriate level of expertise to confirm that it is of an acceptable scientific standard.
References:
The Field Guide to 3D Printing in Optical Microscopy for Life Sciences.Adv Biol (Weinh).2022;6(4) :
10.1002/adbi.202100994e21009943469366610.1002/adbi.202100994The OpenFlexure Project.
Reference source:
A one-piece 3D printed flexure translation stage for open-source microscopy.Rev Sci Instrum.2016;87(2) :
10.1063/1.49410680251042693188810.1063/1.4941068:
OptiJ: Open-source optical projection tomography of large organ samples.Sci Rep.2019;9(1) :
10.1038/s41598-019-52065-0156933166660610.1038/s41598-019-52065-0:
Adapting the 3D-printed Openflexure microscope enables computational super-resolution imaging.F1000Res.2019;8:
10.12688/f1000research.21294.120033251862410.12688/f1000research.21294.1:
Optical sectioning robotic microscopy for everyone: the structured illumination microscope with the OpenFlexure stages.Opt Express.2022;30(13) :
10.1364/OE.46191023208-232163622500610.1364/OE.461910WadatiHirokiUniversity of Hyogo, Japan
Competing interests: No competing interests were disclosed.
19122023
1. This manuscript shows development of an open-source, low-cost and compact 3D printed microscope for the observation of magnetic domains in magnetic materials by the magneto-optical Kerr effect (MOKE). Developments of such open-source and low-cost microscope are actively studied in recent years
1. A lot of design files are freely available and the authors of the current manuscript utilized one of these 3D printable design developed by the OpenFlexure Project
2. In MOKE observation, polarizers have to be introduced to detect rotation of the polarization of the linearly polarized light reflected from a magnetic material under applied magnetic field. Therefore, they redesigned several components of the OpenFlexure microscope to attach electromagnet and polarizers, and they also made the data for the reproduction of them freely available. They also showed experimental results of the MOKE observation using commercially available magneto-optical (MO) sensor as a sample, which is sufficient to guaranty feasibility of their system.
Our Reply: We thank the reviewer for his careful reading of our manuscript and for considering our work worthy of publication. In the following, we answer his concerns one by one, and we point out the changes made in the manuscript to improve both the discussion and the figures following his suggestions.
2. The OpenFlexure microscope families are superior for their extremely high positioning precision with tens of nanometers based on flexure mechanism of the flexible plastics and low-cost stepper motors
3. Several groups have made contribution to the improvement of such OpenFlexure microscope for advanced scientific applications such as, the optical projection tomography (OPT)
4, computational super-resolution imaging system based on the super-resolution radial fluctuation (SRRF) algorism
5, and so on. The studies presented in the current manuscript successfully added new function to the OpenFlexure microscope, which may be useful for the researchers working in the field of materials science, especially in magnetic materials.
Our Reply: Thank you for highlighting the novelty of our present work in comparison with recent studies relevant to our study. Note that we found that Ref. 4 in the original manuscript was not properly cited and thus revised as follows: Opt. Express. 2022; 30 (13): 23208-23216.
3. We are also one of such contributor and reported about a low-cost fabrication of the structured illumination microscope (SIM) for the optical sectioning based on the OpenFlexure stages6. I am confident that we are familiar with the basic working principle of the OpenFlexure microscope and are experienced in building 3D-printable microscopes, but I have poor experience on magnetic measurements. I can say that the authors present appropriate procedures for the reproduction of their work, but I think it will be more informative and helpful if they can provide detailed information about polarizer holder. The polarizers are key components in the MOKE observation and have to be rotated depending on the direction of axes of the magnetic material. The authors simply describe that the polarizers can be rotated in the current manuscript, but do not give detailed explanation how it can be done.
Our Reply: We totally agree that detailed information about the polarizer holder would be helpful for readers. To better understand our system, we have added the following sentence in the section on Hardware design: “The rod can be mechanically rotated by hand. The angle of the holder can vary from -75 to 75 degrees in the current setup.”
4. My other concern is about uniformity of the LED illumination and the applied magnetic field in the field of view. I think it will be better if the authors can provide descriptions on these points.
Our Reply:
To clarify the uniformity of the LED illumination, we rewrote the sentence as
“This microscope employs the Köhler illumination method, where the illuminated light becomes collimated on the sample surface by using the three lenses. This ensures the uniformity of the LED illumination.”
The in-plane component of the magnetic field was 0.1 mT or less, measured by a Tesla meter. Small non-uniformity causes no problem due to the observed small area (~0.5 × 0.5 mm). Moreover, the coercive field estimated from our Kerr intensity in response to the magnetic field is nearly identical to the Faraday rotation data provided by Matesy GmbH (Fig. 3), implying that the magnetic field is essentially homogeneous in our apparatus.