A test system and method for high-order magneto-optical Kerr effect in ferromagnetic thin films and a magneto-optical Kerr effect criterion method
By using a simplified vertical incident optical path system and a quadrupole magnet to control the direction of the magnetic field, the QMOKE signal can be directly observed, solving the complex problem of detecting high-order magneto-optical Kerr effect in existing technologies and enabling more intuitive signal measurement and quantitative analysis.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- UNIV OF SCI & TECH OF CHINA
- Filing Date
- 2026-03-09
- Publication Date
- 2026-05-29
AI Technical Summary
Existing methods for detecting high-order magneto-optical Kerr effects are complex and difficult to describe signal changes in a direct and detailed manner. The 8-field method is also difficult to describe in detail the changes in the second-order magneto-optical Kerr effect (QMOKE) signal as the magnetic moment inside the sample changes.
A simplified measurement system with a vertically incident optical path is used. The direction of the applied magnetic field is controlled by a quadrupole magnet. Combined with linear polarization and polarization detectors, a photodetector is used to record the changes in light intensity, simplifying the optical path structure and allowing direct observation of the QMOKE signal.
It simplifies the measurement process of QMOKE signals, provides a more intuitive measurement method and result curves, is suitable for independent experiments, can directly determine the existence of QMOKE effect and perform quantitative analysis, and assists in the study of magnetic domains and internal electronic structure of ferromagnetic thin films.
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Abstract
Description
Technical Field
[0001] This invention belongs to the field of spintronics and magneto-optics, and relates to a testing system, method, and criterion method for higher-order magneto-optic Kerr effect in ferromagnetic thin films. First, a magneto-optic Kerr detection optical path that only requires a perpendicular incident light path is constructed. The magnetic field around the sample is rotated in small steps by a quadrupole magnet to change the direction of the sample's magnetic moment. The intensity change curve of the emitted light is obtained by a photodetector, and this is used as the criterion for judging whether the second-order magneto-optic Kerr effect (QMOKE) property is present. Background Technology
[0002] The concept of electron spin was first proposed in 1924 by Wolfgang Pauli, an Austrian-American physicist. Subsequently, in 1928, British physicist Paul Dirac further elucidated the spin property of electrons in quantum mechanics, successfully using it to explain the fundamental principles and essence of superconductivity and ferromagnetism. However, because the effects caused by electron spin are extremely weak at the micrometer scale, for more than half a century afterward, the electron spin property was not fully understood or applied to electronic devices. In the 1980s, with the gradual advancement of micro- and nano-fabrication technologies, researchers gradually achieved the manipulation of electron spin properties, observing and utilizing more spin-related effects.
[0003] In 1988, German scientists Grünberg et al. discovered that the magnetoresistance of a Fe (12nm) / Cr (1nm) / Fe (12nm) three-layer film structure reached 1.5% at room temperature, far exceeding the anisotropic magnetoresistance (AMR) of a 25nm thick Fe monolayer film. Simultaneously, French scientists Fert et al. discovered that the magnetoresistance of Fe / Cr multilayer superlattice films grown via molecular beam epitaxy reached 17% at room temperature and even 50% at low temperatures, thus naming it GMR. The principle can be simply described as follows: due to the antiferromagnetic coupling in this film structure, the magnetic moments of adjacent ferromagnetic layers spontaneously align antiparallel under zero field, but align parallel under a sufficiently large external magnetic field. Since the two alignments scatter electrons to different degrees, the transition between low-resistance and high-resistance states can be achieved by controlling the relative magnetization direction of the magnetic multilayer film, i.e., the GMR (Giant Magnetoresistance) effect. The GMR effect revealed that the correlation effect of electron spin can significantly alter the resistance of electronic devices, effectively enabling the conversion between spin signals and electrical signals. This greatly improved the detection and manipulation of electron spin, allowing the construction of novel electronic devices based on the strong correlation between electron spin and electron charge. This sparked a surge of research into the influence of electron spin on electron charge transport in ferromagnetic materials. Grünberg and Fert independently discovered this phenomenon and jointly received the 2007 Nobel Prize in Physics, considered a milestone in the birth of spintronics. The GMR effect has already found widespread applications in data storage, sensing, and other fields.
[0004] In 1995, Miyazaki et al. observed a tunneling magnetoresistance (TMR) effect of up to 18% in amorphous Al2O3 barrier layers. Subsequently, researchers reported high TMR effects in numerous tunnel junctions. TMR exhibits a more significant magnetoresistance change compared to GMR, and the current perpendicular to the film surface is more conducive to device integration. Currently, magnetic memories based on the TMR effect are gradually moving towards commercialization. Internet platforms widely used in daily life, such as WeChat and Alipay, rely on high-capacity hard drives manufactured using spintronics technology for cloud support, significantly enhancing the informatization, modernization, and intelligence of human society. In recent years, leading integrated circuit companies have been developing non-volatile magnetic random access memories (MRAMs), which have already been applied in aerospace technology and advanced electronic products. Spintronics technology has now become one of the key technologies in the "post-Moore's Law era." In the near future, with continuous research and development, spintronics will undoubtedly create more innovative achievements, benefiting human society.
[0005] In the current interdisciplinary field of spintronics and magneto-optics, the detection of the magneto-optic Kerr effect remains an important research topic. Current research mainly focuses on the detection of the first-order magneto-optic Kerr effect and the observation of magnetic domain and domain wall motion and changes through magneto-optic measurement systems, while research on higher-order magneto-optic Kerr effects is somewhat lacking. The higher-order magneto-optic Kerr effect, also known as the nonlinear magneto-optic effect, is of great significance for the detection and characterization of its signals in cutting-edge research on crystal symmetry and internal electronic structure. Current methods for detecting the higher-order magneto-optic Kerr effect typically employ a dual-beam, multi-photodetector differential detection method. This method uses an eight-field method (i.e., eight different directions of applied magnetic fields) to separate the linear signal (LMOKE) from the nonlinear signal (QMOKE) to obtain the QMOKE signal. This detection method requires numerous optical components, resulting in a relatively complex measurement system. Furthermore, the eight-field method cannot intuitively and comprehensively describe the changes in the QMOKE signal with variations in the sample's internal magnetic moment. Based on this, the present invention proposes a testing system and method for higher-order magneto-optical Kerr effect in ferromagnetic thin films, aiming to reduce the complexity of the testing system while making the measurement of QMOKE signals more intuitive and detailed. Summary of the Invention
[0006] The purpose of this invention is to propose a testing system, method, and criterion for the higher-order magneto-optical Kerr effect in ferromagnetic thin films. By constructing a simple optical path system for measuring the second-order magneto-optical Kerr effect (QMOKE) signal, a concise light intensity curve is obtained to determine whether a ferromagnetic thin film system has the second-order magneto-optical Kerr effect (QMOKE) property.
[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:
[0008] First, this invention proposes a testing system for the higher-order magneto-optical Kerr effect in ferromagnetic thin films, mainly comprising:
[0009] A ferromagnetic thin film sample is placed at the very center of a quadrupole magnet, with the edges of the sample parallel to the x and y directions of the magnet, respectively.
[0010] A laser emits a laser beam toward a ferromagnetic thin film sample;
[0011] A polarizer is a linear polarizer placed in the optical path between the laser and the ferromagnetic thin film sample. The laser light emitted by the laser is converted into linearly polarized light with a horizontal polarization direction after passing through the polarizer, and then incident on the center of the ferromagnetic thin film sample at an angle perpendicular to the sample surface.
[0012] The analyzer linear polarizer is located in the optical path between the photodetector and the ferromagnetic thin film sample. The light reflected from the surface of the ferromagnetic thin film sample is received by the photodetector after passing through the analyzer linear polarizer, whose polarization direction is at an angle of 0-5° to the polarization direction orthogonal to the polarizer.
[0013] A photodetector records light intensity as QMOKE signal strength data.
[0014] Secondly, this invention also proposes a testing method for the higher-order magneto-optical Kerr effect in ferromagnetic thin films. A magneto-optical Kerr detection optical path is constructed by an incident optical path perpendicular to the sample surface, and the direction of the applied magnetic field is controlled by adjusting the direction of the sample magnetic moment by using a quadrupole magnet with small steps. The intensity change curve of the detection light under the rotating magnetic field is detected by a photodetector, thereby characterizing whether the sample has a second-order magneto-optical Kerr effect (QMOKE) and quantitatively observing its intensity.
[0015] Finally, this invention proposes a criterion for a higher-order magneto-optical Kerr effect, and proposes two different cases based on the different magnetic anisotropies of the samples:
[0016] (1) For ferromagnetic thin film samples with quadriaxial magnetic anisotropy and high-order magneto-optical Kerr effect (such as the Fe sample in the example), as the magnetic moment direction of the sample rotates through the first, second, third and fourth quadrants with the azimuth angle of the applied magnetic field, the QMOKE signal curve will show alternating high and low changes in the angle range corresponding to different quadrants, while the signal remains basically constant within a certain range on both sides of the sample's easy axis (45°, 135°, 225°, 315°);
[0017] (2) For ferromagnetic thin film samples without specific easy-axis magnetic anisotropy but exhibiting higher-order magneto-optical Kerr effects (such as the Co and NiFe samples in the examples), as the azimuth angle of the applied magnetic field rotates through the first, second, third, and fourth quadrants, the QMOKE signal curve will show the same characteristics as... Similar function changes, with a period of .
[0018] Compared with the prior art, the beneficial effects of the present invention are as follows:
[0019] 1. This invention uses a single-beam incident method with perpendicular incidence to construct the QMOKE signal measurement optical path, which is different from the oblique incidence optical path and the dual-beam measurement optical path. The waveform of the QMOKE signal can be directly observed, simplifying the process of separating LMOKE and QMOKE. It provides a more intuitive and concise measurement method and result curve paradigm for experiments that only require the QMOKE signal.
[0020] 2. The optical path of this invention requires only a quadrupole magnet and power supply, a pair of linear polarizers, a common red laser source and a photodetector. The device is simple in structure, reduces the influence of optical components on the polarization state of light, has wide configuration requirements, and is suitable for independent experiments.
[0021] 3. The result curve paradigm given in this invention is relatively intuitive, which can directly determine whether the QMOKE effect exists, and further fit the curve to perform simple quantitative analysis. It can serve as an intuitive basis for determining whether it is a QMOKE signal.
[0022] 4. The test method in this invention uses a small-step (5°) rotation of the external magnetic field for testing, which can observe a more comprehensive effect of the external magnetic field on the QMOKE signal and provide assistance for further research on the changes of magnetic domains and domain walls in ferromagnetic thin film samples, as well as the detection of crystal symmetry and internal electronic structure. Attached Figure Description
[0023] Figure 1 A schematic diagram (a) and a top view (b) of the optical path for detecting high-order magneto-optical QMOKE signals of ferromagnetic thin film samples.
[0024] Figure 2 This is a schematic diagram of the sample and the quadrupole magnet in planar view.
[0025] Figure 3 This is the actual detection optical path diagram.
[0026] Figure 4 The diagram shows the four-axis anisotropy of a single-crystal Fe sample (a) and the QMOKE curve of the four-axis anisotropic sample (b).
[0027] Figure 5 The QMOKE measurement results curve of the Fe sample prepared for the example is shown in the figure.
[0028] Figure 6The QMOKE measurement results curves of Co sample (a) and NiFe sample prepared for the example are shown in Figure (b). Detailed Implementation
[0029] The magneto-optical Kerr effect is defined as follows: when a beam of linearly polarized light is incident on the surface of a magnetic material, the reflected light becomes an elliptically polarized beam due to the different exit velocities of the left-handed and right-handed circularly polarized light. This effect is called the magneto-optical Kerr effect.
[0030] The magneto-optical Kerr effect can generally be characterized by the magneto-optical Kerr signal, which consists of two parts: the magneto-optical Kerr rotation angle and the Kerr ellipsoid. The angle between the major axis of the elliptically polarized light and the polarization direction of the incident light is the Kerr rotation angle, and the ratio of the minor axis to the major axis of the elliptically polarized light is the Kerr ellipsoid.
[0031] The magneto-optical properties of magnetized crystals can be expressed using the dielectric constant tensor. It can be represented as a series expansion of the sample magnetic moment M:
[0032]
[0033] in, It represents the dielectric constant when unmagnetized, i.e., the vacuum dielectric constant; This represents the dielectric constant tensor related to the first power of M. The magneto-optical effect corresponding to this part is called the first-order magneto-optical Kerr effect, also known as the linear magneto-optical Kerr effect (LMOKE). The dielectric constant tensor related to the square of M is represented by this part, and the corresponding magneto-optical effect is called the second-order magneto-optical Kerr effect (QMOKE).
[0034] When considering the second-order term of the dielectric tensor, both LMOKE and QMOKE contribute to the magneto-optical effect. If the magnetization M is aligned parallel to the sample surface and the incident light is perpendicular to the sample surface, according to the aforementioned first-order magneto-optical Kerr effect principle, LMOKE completely disappears, leaving only QMOKE.
[0035] When the magnetic moment of the sample is in-plane anisotropic, and considering both the first-order and second-order magneto-optical Kerr effects, the expression for the magneto-optical Kerr signal can be written as:
[0036]
[0037] in, Let be the component of the magnetization of the magnetic thin film in the direction parallel to the incident plane. That is, the component of magnetization in the direction perpendicular to the incident plane. and The relationship is shown in the following formula:
[0038]
[0039] Since the coefficients in equation (1.2) are only related to the angle between the crystal orientation and the incident plane of the linearly polarized light, they can be considered constants when the test sample and the incident plane remain unchanged. The total magnetization M of the sample is also constant. Therefore, combining equations (1.2) and (1.3), equation (1.2) can be equivalent to:
[0040]
[0041] Equation (1.4) states that, for the same test system, the second-order magneto-optical Kerr effect is... and A linear combination of .
[0042] Based on this, it is possible to characterize and quantitatively analyze whether ferromagnetic thin film systems, especially those with easy-axis anisotropy, possess QMOKE properties. Convenient and rapid measurement and judgment play a significant supporting role in in-depth research based on the nonlinear magneto-optical Kerr effect.
[0043] First, please refer to Figure 1 As shown, this embodiment establishes a magneto-optical Kerr detection optical path and detection method with a single incident mode and perpendicular to the sample surface:
[0044] 1.1 Preparation of ferromagnetic thin film samples with different materials and structures:
[0045] Fe samples were grown by magnetron sputtering and covered with a W anti-oxidation layer, with a structure of W (2nm) / Fe (6nm) / MgO (substrate).
[0046] Co samples were grown by magnetron sputtering and covered with a W anti-oxidation layer, with a structure of W (2nm) / Co (40nm) / Pt (1nm) / Si (substrate).
[0047] NiFe samples were grown by magnetron sputtering and covered with a W anti-oxidation layer. The structure was W(2nm) / NiFe(60nm) / W(10nm) / Si(substrate).
[0048] All samples are 10mm x 10mm in size and approximately 1mm thick. The in-plane magnetic anisotropy of the samples was confirmed by measuring the hysteresis loop.
[0049] 1.2 Place the sample (Fe, Co, NiFe) at the center of the quadrupole magnet, with the sample edges parallel to the x and y directions of the magnet, ensuring that the theoretical input angle of the magnet matches the actual angle. Figure 2 (As shown).
[0050] 1.3 The light source is a common red laser. After the laser is emitted, it is converted into linearly polarized light with a horizontal polarization direction after passing through the polarizer.
[0051] 1.4 Linearly polarized light is incident on the center of the ferromagnetic thin film sample at an angle perpendicular to the sample surface.
[0052] 1.5. The light is reflected by the sample surface and then passes through the analyzer linear polarizer, whose polarization direction is at an angle of 0-5° to the polarizer's polarization direction.
[0053] 1.6 The output light from the analyzer is received by a photodetector, and the light intensity is recorded as QMOKE signal intensity data.
[0054] It is important to note that all components in the constructed optical path should be kept collimated.
[0055] Next, this embodiment proposes a testing method and its result curve:
[0056] 2.1 In the LabVIEW testing software, set the current supplied to the quadrupole magnet power supply to 0.3A, and set the direction of the magnetic field generated by the quadrupole magnet to change in increments of 5°, ranging from 0 to 360°.
[0057] 2.2 Testing in a dark environment ( Figure 3 (As shown), to reduce the influence of external light sources on the detector. Each angle is measured repeatedly 5 times and the average value is taken. This process is repeated after one cycle, with a cycle of 100 times. The resulting average curve is the curve of the QMOKE signal changing with the rotation angle of the applied magnetic field.
[0058] Finally, this embodiment proposes a criterion for intuitively judging whether the deposited ferromagnetic thin film sample has the QMOKE effect based on the result curve. It is especially applicable to samples with easy axis in a specific direction and can be easily quantitatively analyzed.
[0059] 3.1. From equation (1.4), it can be seen that the QMOKE effect can be derived from... and The linear combination representation of , where and The sum of squares is a constant and can be expressed as: and Therefore, the QMOKE signal is and A linear combination of, where The angle between the direction of the sample's magnetization (magnetic moment) and the positive half of the x-axis.
[0060] 3.2 For single-crystal Fe structure samples, there is a definite four-axis anisotropy, with its easy axes distributed along the two diagonals of the square sample, namely 45°, 135°, 225°, and 315°. Figure 4 (a) As shown in the diagram. Due to the existence of the easy axis, the magnetization direction of the sample tends to align with the easy axis. When the direction of the applied magnetic field is close to the direction of the easy axis, the magnetization direction of the sample tends to remain in the direction of the easy axis, and the corresponding QMOKE signal will remain almost unchanged within a certain angular range. When the direction of the applied magnetic field moves away from the current easy axis and gradually approaches the next (counterclockwise) easy axis, the magnetization direction of the sample tends to quickly jump to that easy axis.
[0061] The QMOKE signal value remains constant around the direction angles of the four easy axes. Dividing the coordinate system into four quadrants [0, 360], we can see that within these four quadrants... All are positive values. Based on equations (1.3) and (1.4), this contribution remains unchanged in the QMOKE curve, while in the first and third quadrants... Positive, second and fourth quadrants. If the value is negative, based on equation (1.4), in the QMOKE curve, this term's contribution alternates between positive and negative in the four quadrants. Figure 4 (b) shown).
[0062] 3.3 The QMOKE measurement results curve of the Fe sample prepared in this embodiment is shown in the figure below. Figure 5 As shown, based on the above conclusions, this curve can be regarded as the basis for judging the QMOKE signal of a ferromagnetic thin film sample with a quadraxial anisotropic easy-axis distribution.
[0063] 3.4 In this embodiment, the Co and NiFe samples prepared do not have a specific free axis, and their QMOKE signals are as follows: A periodic sine and cosine oscillation curve (such as...) Figure 6 As shown in the figure, based on the above conclusions, this curve can be regarded as the basis for judging the QMOKE signal of a free-axis ferromagnetic thin film sample without a specific direction.
[0064] Note:
[0065] The reason why the amplitude of the QMOKE signal in the first and third quadrants of the sample test result curve is that the system does not have a completely perpendicular incidence, and the LMOKE has a small component contribution. The positive and negative values of the LMOKE signal are opposite in the first and third quadrants, which causes the amplitude of the result curve to be different.
[0066] In summary, this invention proposes a testing system, method, and criterion for the higher-order magneto-optical Kerr effect in ferromagnetic thin films. Specifically, it involves growing three test samples (Fe, Cu, and NiFe) by magnetron sputtering, constructing a magneto-optical Kerr detection optical path through an incident optical path perpendicular to the sample surface, controlling the direction of the sample's magnetic moment by adjusting the direction of the applied magnetic field using a quadrupole magnet, and detecting the change curve of the detection light intensity under the rotating magnetic field using a photodetector. This is used to characterize whether the sample exhibits the second-order magneto-optical Kerr effect (QMOKE) and quantitatively observe its intensity.
[0067] The method proposed in this invention only requires a single light intensity variation curve obtained from perpendicular incident light as a characterization criterion for whether a sample possesses QMOKE. The measurement method using a small-step (5°) rotating external magnetic field provides a more comprehensive characterization of the influence of the magnetic field direction on the QMOKE signal, and the QMOKE intensity can be calculated based on the curve amplitude. This simplifies the steps of separating the first-order magneto-optical Kerr effect (LMOKE) from the MOKE curve, and simplifies the characterization criteria for the QMOKE effect in ferromagnetic thin film systems, especially Fe structure thin films. At the same time, it provides the relationship between the QMOKE curve of Fe structure samples and their four-axis anisotropy.
[0068] The method and characterization criteria proposed in this invention provide a convenient foundation for subsequent research on magneto-optics and have certain auxiliary value for the application of nonlinear optics in optical communication and other fields.
Claims
1. A testing system for higher-order magneto-optical Kerr effect in ferromagnetic thin films, characterized in that, Mainly includes: A ferromagnetic thin film sample is placed at the very center of a quadrupole magnet, with the edges of the sample parallel to the x and y directions of the magnet, respectively. A laser emits a laser beam toward a ferromagnetic thin film sample; A polarizer is a linear polarizer placed in the optical path between the laser and the ferromagnetic thin film sample. The laser light emitted by the laser is converted into linearly polarized light with a horizontal polarization direction after passing through the polarizer, and then incident on the center of the ferromagnetic thin film sample at an angle perpendicular to the sample surface. The analyzer linear polarizer is located in the optical path between the photodetector and the ferromagnetic thin film sample. The light reflected from the surface of the ferromagnetic thin film sample is received by the photodetector after passing through the analyzer linear polarizer, whose polarization direction is at an angle of 0-5° to the polarization direction orthogonal to the polarizer. A photodetector records light intensity as QMOKE signal strength data.
2. The testing system as described in claim 1, characterized in that, The ferromagnetic thin film samples were prepared as follows: Fe samples were grown by magnetron sputtering and covered with W as an anti-oxidation layer, with a structure of W (2nm) / Fe (6nm) / MgO (substrate); Co samples were grown by magnetron sputtering and covered with W as an anti-oxidation layer, with a structure of W (2nm) / Co (40nm) / Pt (1nm) / Si (substrate); NiFe samples were grown by magnetron sputtering and covered with W as an anti-oxidation layer, with a structure of W (2nm) / NiFe (60nm) / W (10nm) / Si (substrate); the sample size was 10mm*10mm, and the in-plane magnetic anisotropy of the samples was confirmed by measuring the hysteresis loop.
3. A method for testing higher-order magneto-optical Kerr effect in ferromagnetic thin films using the system described in claim 1 or 2, characterized in that, A magneto-optical Kerr detection optical path is constructed by an incident light path perpendicular to the sample surface. The direction of the sample's magnetic moment is controlled by adjusting the direction of the applied magnetic field in small steps using a quadrupole magnet. The intensity change curve of the detection light under the rotating magnetic field is detected by a photodetector. This is used to characterize whether the sample has a second-order magneto-optical Kerr effect (QMOKE) and to quantitatively observe its intensity.
4. The method as described in claim 3, characterized in that, In the LabVIEW testing software, the current supplied to the quadrupole magnet was set to 0.3A, and the direction of the magnetic field generated by the quadrupole magnet was set to change in 5° increments, ranging from 0 to 360°. The testing system was tested in a dark environment, and each angle was measured 5 times and the average value was taken. This process was repeated after one cycle, and the cycle was repeated 100 times. The average result curve obtained is the curve of QMOKE signal changing with the rotation angle of the applied magnetic field.
5. A method for using the test method described in claim 3 or 4 as a criterion for the magneto-optical Kerr effect, characterized in that, Based on ferromagnetic thin film samples with different magnetic anisotropy, two criteria methods are proposed: (1) For ferromagnetic thin film samples with quadriaxial magnetic anisotropy and high-order magneto-optical Kerr effect, as the magnetic moment direction of the sample rotates through the first, second, third and fourth quadrants with the azimuth angle of the applied magnetic field, the QMOKE signal curve will show alternating high and low changes in the angle intervals corresponding to different quadrants. At the same time, the signal remains basically constant within a certain interval on both sides of the sample's easy axis (45°, 135°, 225°, 315°). (2) For ferromagnetic thin film samples without specific easy-axis magnetic anisotropy but exhibiting higher-order magneto-optical Kerr effects, as the azimuth angle of the applied magnetic field rotates through the first, second, third, and fourth quadrants, the QMOKE signal curve will show the same characteristics as... Similar function changes, with a period of .