Vision & Sensors / Vision & Sensors Exclusives

Sensors: Magneto-Optical Sensors

March 1, 2012
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The reliable use of magnetic materials requires exact information of magnetic field distribution, for instance within the production process, as part of the quality management process as well as in the field of research and development.

This image represents Magneto-Optical sensors in various stages: initial substrate, coated with MO and reflective layer (from left to right). Source: Matesy GmbH

Pictured is a diagram of the Faraday rotation, where E=light amplitude, d=distance in the transparent medium, B=magnetic flux density of the sample and B=resulting Faraday rotation. Source: Matesy GmbH

The principles of existing magnetic field measurement systems are based on different physical effects caused by magnetic field influence on electrical parameters, such as voltage and current, within a sensor. From the measured values and the specific material constants, the magnetic field strength and flux density can be analyzed. For example in Hall sensors the Hall-Effect of conductive materials (e.g. semiconductor materials) leads to an output of voltage-the Hall voltage-which proportionally depends on the magnetic flux density. Another widely used type is the magnetoresistance sensor, which uses the magnetic field-dependency of the resistance of the sensor material and thus provides a measurement voltage in relation to the applied magnetic field.

The Matesy GmbH, Jena, Germany, has explored a new magneto-optical sensor type (MO-sensors) for direct field visualization and measurement. Instead of electromagnetic effects, Matesy introduces magneto-optics for two-dimensional magnetic field analysis. The magneto-optical sensor has the technical advantage that the magnetic field and its distribution can be visually recognized over the entire magnetic surface. Therefore, real-time analysis of the magnetic field distribution can be performed, instead of using a time intensive “point to point” scanning with hall probes, which need a precise positioning on the surface.

This is a schematic illustration of the rotated polarization planes of light after traversing a MO-medium subjected to different field strengths. Source: Matesy GmbH


The principle of magneto-optical sensors is the Faraday-Effect. It describes the rotation of the polarization plane of linearly polarized light passing the magneto-optical sensor, which is exposed by a magnetic field, which is parallel to the direction of propagation of the applied light wave.

More specifically, linearly polarized light consists of superposed left- and right-circularly polarized waves with the same frequency and phase. As light passes a MO-medium in which a magnetic field parallel to the direction of the light wave is applied, it disperses into two oppositely rotating circularly polarized waves with different phase velocities. As a result of the phase shifts of these two partial waves-a rotation of the polarization plane of light and the unequal absorption of each component together-leads to an elliptically polarized wave, which is finally an analyzable phenomenon of magnetic field strength and allows having an insight into the magnetic properties of the sample.

This is a characteristic diagram of a MO sensor over the entire sensor surface with a dynamic range from 0.05 up to 30kA/m. Source: Matesy GmbH


To achieve both accurate imaging characteristics as well as best possible resolution, the R&D-facility INNOVENT e.V. from Jena designed-based on a bismuth-substituted rare earth iron garnet compound-a monocrystalline ferrimagnetic layer, which is characterized by enhanced magneto-optical imaging properties.

The manufacturing process of the sensor layers is realized by liquid phase epitaxy, which is ideal for applying functional coatings in micron range on monocrystalline garnet substrates. An additional mirror- and protective-layer is deposited on the raw sensor to ensure long-term functionality of the system. For different fields of applications, the sensors can be designed in customized shapes and sizes.

A CMOS-MagView measuring permanent magnets (e.g. 100% quality control). Source: Matesy GmbH


In order to realize an optical visualization of the magnetic fields, the magneto-optical sensor is brought into direct contact with the magnetic sample material and is illuminated by a polarized light source. The light passes through the transparent sensors, is reflected by the mirror-layer and passes the sensor again. When double-passing through the nonreciprocal MO-medium, the described Faraday-Effect applies proportional to the double layer thickness.

Resulting from the different rotation angles-depending on the local magnetic fields strength-the analyzer-polarization module generates an intensity contrast pattern, which is proportional to the magnetic field distribution of the magnetic material. The result is a visual image that illustrates a two-dimensional intersection of the magnetic stray field. This image recording and analyzing of the normal component over the X-Y plane of the magnetic field takes place in real time and simultaneously over the entire sensor surface, allowing detection and analysis of dynamic magnetic field changes.

This is a visualization of individual local magnetizations of stored bits on a hard drive disk (platter) using an MO sensor under a polarization microscope. Source: Matesy GmbH


Since the sensors-for technical reasons-can be saturated, depending on the strength of the applied magnetic field, different dynamic ranges are covered by different types of MO-sensors. Current MO-sensors can detect magnetic field strengths from 50 A/m up to 500 kA/m in order to perfectly fit to each specific task. Field strengths, which are out of specifications due to the slope of the hysteresis curve, cannot be differentiated. Magneto-optical sensor systems can resolve lateral structures down to 1 micron.

This is an illustration of the magnetic field distribution of a ring magnet with different field inhomogeneities from the CMOS analysis software. Source: Matesy GmbH


The Matesy GmbH developed a new visualization and measurement system for magnetic field distribution at the surface of magnetic materials. The “CMOS-MagView” called measurement system is ideal for studying magnetic properties and for analysis of field distribution and magnetic structures.

The CMOS-MagView can be used furthermore for quality control of magnetic materials such as NdFeB, SmCo, AINCo and hard ferrites. Thus magnetic fields of plastic bonded permanent magnets, encoders, alloys of steel, magnetic stripe cards, magnetic ink (e.g. used on bills, safety tags, tickets, packaging), thin sections of magnetic minerals and even of domain materials-as magnetic shape memory alloy-can be visualized with very high geometric resolutions and investigated by the comprehensive CMOS-MagView software.

MO-sensors can be modified perfectly for each application so that optimal field visualization can be achieved for each material. The CMOS-MagView works with a 12-bit CMOS camera technology and can be easily connected to a computer via USB enabling real-time visualization, analysis and archiving of the magneto-optical recordings.

Picture (bottom) is the magnetic field structure of an unsaturated SmCo magnet cube. (Top) The magnetic welding inspection of a surface-polished sample. Source: Matesy GmbH


The integrated CMOS-MagView software allows illustration and analysis of the geometric magnetic field distribution. The magneto-optical images of the measured magnetic material can be displayed in false colors as well as in a 3-dimensional contour of the local magnetic field strengths. Inhomogeneities of field distribution, geometry and field structures are easily visible. Due to the highly detailed image analysis even hidden material properties-for example detection of welding seams-can be revealed and investigated.

A significant advantage of magnetooptical sensors compared to magnetoresistive (MR) and hall sensors for these kind of applications is that MR and Hall sensors only deliver a punctual information about the magnetic field strength. To detect the size and exact shape of material defects, it is necessary to determine the exact field distribution. For MR and Hall sensors, this is only possible through a very time-consuming scanning of the magnetic surface, which results in achievable resolutions far behind those of the CMOS-MagView.

MO-sensors are capable of visualizing magnetic fields over surface areas of up to three inches in diameter directly. The surface sensor can be enlarged almost without limits by “gluing” several sensor together. Especially in the fields of forensic analysis of counterfeit documents, bank notes and steel testings it is necessary to investigate over larger areas at once. For the characterization of industrial permanent magnets a reliable quick-measurement of the entire strayfield is essential for quality management and its importance will increase in the coming years as the property requirements will continuously increase.

Picture is a CMOS-MagView capturing domains and domain boundaries of a steel alloy. Source: Matesy GmbH


Magneto-optical sensors also provide a variety of applications in research activities regarding to magnetic domain structures, their distribution, orientation and behavior under influence of external fields.

Magnetic domain structures are defined as areas of different magnetization directions-also known as Weiss domains-in a single material. Within a single domain the magnetic moments are aligned parallel to each other. This case corresponds to a locally saturated magnetization of the respective area.

The investigation of the domain structure is essential for a variety of research and manufacturing applications-mainly in the fields of sensor and magnetic storage technologies-especially when the orientation of magnetic moments within a Weiss domain and the displacements of domain walls are functionally determinant parameters. Magneto-optical sensors can identify, analyze and clearly illustrate these information in real time. Due to the compact design an adaption and integration of the CMOS-MagView into existing processes can be easily realized. V&S

Picture (bottom) is the visualization of a meander domain structure. (Top) The visualization of a shape memory alloy. Source: Matesy GmbH

Tech Tips

Current MO-sensors can detect magnetic field strengths from 50 A/m up to 500 kA/m in order to perfectly fit to each specific task.

The CMOS-MagView works with a 12-bit CMOS camera technology.

MO-sensors are capable of visualizing magnetic fields over surface areas of up to three inches in diameter directly.

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