Beam guidance unit with magnet and spacer element

Abstract

Scanner that includes: - a mirror (150) designed to reflect light (180), - a beam guidance unit (99) comprising: - at least one first elastic element (101-1,102-1) extending between a first base (141-1) and a first end piece (142-1) arranged adjacent to the mirror (150) and made of a semiconductor material, - at least one second elastic element (101-2, 102-2) extending between a second base (141-2) and a second end piece (142-2) arranged adjacent to the mirror (150), parallel to the at least one first elastic element and made of the semiconductor material, wherein the beam steering unit (99) is configured to deflect the light (180) by means of torsion of the at least one first elastic element (101-1, 102-1) and the at least one second elastic element (101-2, 102-2) in a torsion region (149) between the first and second base (141-1, 141-2) and the first and second end pieces (142-1, 142-2) by reflection at the mirror (150) at different angles, wherein the scanner further comprises, - a magnet (910) made of a ferromagnetic material and arranged between the first end piece (142-1) and the second end piece (142-2), - a spacer element (901) which is arranged adjacent to the magnet (910) towards the first and second bases (141-1, 141-2) offset between the first end piece (142-1) and the second end piece (142-2).

Description

TECHNICAL AREA

[0001] Several examples generally concern a beam guidance unit for light. A magnet is provided that moves with a mirror in order to measure the mirror's position. BACKGROUND

[0002] Measuring the distance to objects is desirable in various technological fields. For example, in the context of autonomous driving applications, it can be advantageous to detect objects in the vicinity of vehicles and, in particular, to determine the distance to those objects.

[0003] One technique for measuring the distance to objects is called LIDAR technology (Light Detection and Ranging; sometimes also LADAR). In this technology, pulsed laser light is emitted from an emitter. Objects in the vicinity reflect the laser light. These reflections can then be measured. By determining the travel time of the laser light, the distance to the objects can be calculated.

[0004] To detect objects in the surrounding area with spatial resolution, it may be possible to scan the laser light. Depending on the beam angle of the laser light, different objects in the environment can be detected. A scanner with a beam steering unit is used for this purpose.

[0005] From publication WO 2018 / 171846 A1, it is known to use a scanner that includes a beam-guiding unit with elastic elements to move a mirror by torsion of these elements. This causes light to be reflected by the mirror at different angles, allowing the light to be emitted differently, for example, for scanning. To determine the respective angle or position of the mirror—and thus the emission angle of the light—a magnet is provided that generates a stray magnetic field. The magnet is mounted close to the mirror and moves with it. The stray magnetic field can be measured by a magnetic field sensor that is stationary relative to the mirror.

[0006] It has been observed that in such a scanner, the resonant frequency of the mass-spring system consisting of the elastic elements, the mirror, and the magnet shifts over time. This means that the scanner ages. This makes reproducible and reliable operation difficult. BRIEF DESCRIPTION OF THE INVENTION

[0007] Therefore, there is a need for improved scanners with an elastically suspended mirror and a magnet that moves with the mirror. In particular, there is a need for robust scanners that do not age, or at least not significantly.

[0008] This task is solved by the features of the independent patent claims. The features of the dependent patent claims define embodiments.

[0009] A scanner includes a mirror designed to reflect light. The scanner also includes a beam-guiding unit. The beam-guiding unit comprises at least one first elastic element and at least one second elastic element. The first elastic element extends between a first base and a first end piece, the first end piece being adjacent to the mirror. The first elastic element is made of a semiconductor material. The second elastic element extends between a second base and a second end piece, the second end piece also being adjacent to the mirror. The second elastic element extends parallel to the first elastic element.The at least one second elastic element is also made of the same semiconductor material as the at least one first elastic element, for example silicon.

[0010] The beam steering unit is designed to deflect the light at different angles by means of torsion of the at least one first elastic element and the at least one second elastic element in an area between the first and second bases and the first and second end pieces by reflection at the mirror.

[0011] The scanner also includes a magnet. The magnet is made of a ferromagnetic material and is positioned between the first and second end pieces. The scanner also includes a spacer element, which is located adjacent to the magnet, offset towards the base, between the first and second end pieces.

[0012] The spacer can therefore separate the first end piece from the second end piece.

[0013] The first end piece, the spacer element and the magnet, as well as the second end piece, thus form a "sandwich structure".

[0014] For example, the scanner could also include an angle magnet field sensor that is fixed in position relative to the mirror, i.e., at a fixed distance from the first and second bases. This sensor can measure the stray magnetic field of the magnet.

[0015] The combination of the magnet and the angle magnetic field sensor makes it possible to precisely monitor the rotation of the mirror due to torsion. This allows for accurate monitoring of the angle at which the light is deflected, or the beam angle.

[0016] By using the spacer element, a particularly robust scanner can be provided that does not age or does not age very much.

[0017] The features set out above and those described below can be used not only in the corresponding explicitly set out combinations, but also in further combinations or in isolation, without leaving the scope of protection of the present invention. BRIEF DESCRIPTION OF THE FIGURES Fig. Figure 1 schematically illustrates a beam guidance unit according to various examples. Fig. Figure 2 schematically illustrates a beam guidance unit according to various examples. Fig. Figure 3 schematically illustrates actuators for exciting degrees of freedom movement of a beam guidance unit. Fig. Figure 4 schematically illustrates a beam guidance unit according to various examples. Fig. Figure 5 schematically illustrates a beam guidance unit according to various examples. Fig. Figure 6 schematically illustrates a reference implementation of a beam guidance unit according to various examples. DETAILED DESCRIPTION OF EXECUTION FORMS

[0018] The properties, features and advantages of this invention described above, as well as the manner in which they are achieved, will become clearer and more easily understood in connection with the following description of the exemplary embodiments, which are explained in more detail in conjunction with the drawings.

[0019] The present invention is explained in more detail below with reference to preferred embodiments and the drawings. In the figures, identical reference numerals denote identical or similar elements. The figures are schematic representations of various embodiments of the invention. Elements depicted in the figures are not necessarily shown to scale. Rather, the various elements depicted in the figures are represented in such a way that their function and general purpose are understandable to a person skilled in the art. Connections and couplings between functional units and elements shown in the figures can also be implemented as indirect connections or couplings. Functional units can be implemented as hardware, software, or a combination of hardware and software.

[0020] The following describes various techniques for (re)directing light (beam steering). These techniques can, for example, enable light scanning.

[0021] In general, such light deflection techniques can be used in a wide variety of applications. Examples include endoscopes, RGB projectors, printers, and laser scanning microscopes. LiDAR techniques can be applied in various applications. LiDAR techniques can be used to perform spatially resolved distance measurements of objects in the environment. For example, LiDAR techniques can involve time-of-flight measurements of laser light between the mirror, the object, and a detector.

[0022] A deflection unit is used to reflect the light. The deflection unit is moved, allowing the light to be deflected at different angles. The deflection unit can be implemented, for example, using a mirror.

[0023] In various examples, several elastic elements are used to move the mirror, which deflects light. These elastic elements can exhibit shape- and / or material-induced elasticity. Therefore, they can also be referred to as spring elements or elastic suspensions.

[0024] The elastic elements can be rod-shaped.

[0025] The elastic elements can extend between a base and an end piece. The base can be fixed in position relative to a mirror. The mirror can be attached to the end pieces, for example, via an angled element.

[0026] In particular, multiple groups of elastic elements can be used. Each group can have a common base and end piece. For example, two groups, an upper group and a lower group, can be used in a sandwich structure.

[0027] The elastic elements can be fabricated using MEMS techniques, i.e., produced from a wafer using suitable lithography processes, for example, by etching. Silicon wafers can be used.

[0028] For example, each group of elastic elements, together with its associated base and end piece, could be produced in a single etching step using a lithographically defined mask.

[0029] One or more actuators can be used to excite the movement of the mirror around its base. For example, the base could be tilted using piezoelectric actuators, thereby exciting a torsion of the elastic elements. This allows the mirror to be rotated. The torsion can be excited resonantly with respect to a mass-spring system that includes at least the mirror and the elastic elements.

[0030] Several examples are based on the understanding that it can be desirable to deflect the light with high accuracy regarding the beam angle. For instance, in the context of LiDAR techniques, the spatial resolution of distance measurements can be limited by inaccuracies in the beam angle. Typically, the more precisely (less precisely) the beam angle of the laser light can be determined, the higher (lower) the spatial resolution is achieved.

[0031] The positioning of the mirror can be determined. From this, the beam angle can then be deduced.

[0032] To determine the mirror's position, a magnet that moves along with the mirror can be used. The magnet (more precisely, a ferromagnet, for example made of a nickel alloy, an iron alloy, or a neodymium alloy) can be attached to the ends of the elastic elements.

[0033] On the other hand, the end pieces (just like the elastic elements) are typically made of silicon or another semiconductor material used in a MEMS manufacturing process.

[0034] Several examples are based on the understanding that the adhesion of the magnet to the silicon components can often be crucial for the robustness and longevity of the scanner. These examples address reliable and robust attachment of the magnet to the silicon components.

[0035] Fig. Figure 1 illustrates aspects relating to a beam guidance unit 99. The beam guidance unit 99 comprises a scan module 100. The scan module 100 comprises a base 141, two elastic elements 101 and 102, and an end piece 142. The elastic elements 101 and 102 are formed in one plane (drawing plane of the Fig. 1) The scan module 100 can also be described as an elastic suspension.

[0036] The base 141, the elastic elements 101 and 102, and the end piece 142 are formed as a single piece. The elastic elements 101 and 102 thus form a group with a common base 141 and a common end piece 142.

[0037] For example, it would be possible to obtain the base 141, the elastic elements 101, 102, and the end piece 142 by etching a silicon wafer (or another semiconductor substrate) using MEMS processes. In such a case, the base 141, the elastic elements 101, 102, and the end piece 142 can be formed as single crystals.

[0038] The beam guidance unit 99 also includes a mirror 150 implementing a deflection unit. In the example of the Fig. 1 is the mirror 150, which forms a mirror surface 151 with high reflectivity (for example, greater than 95% at a wavelength of 950 µm, optionally >99%, further optionally >99.999%; e.g., aluminum or gold with a thickness of 80–250 nm) for light 180 on its front side, and is not formed integrally with the base 141, the elastic elements 101, 102, and the end piece 142. For example, the mirror 150 could be glued to the end piece 142. The end piece 142 can be configured to fix the mirror 150 or the mirror surface 151. For example, the end piece 142 could have a contact surface configured to fix a corresponding contact surface of the mirror 150. To connect the mirror 150 to the end piece 142, one or more of the following techniques could be used, for example: gluing; soldering. The mirror also has a back side 152.

[0039] Using such techniques, large mirror surfaces can be realized, e.g., with a diameter of at least 10 mm, optionally at least 15 mm. This allows for high accuracy and range in conjunction with LIDAR techniques that also use the mirror surface 151 as a detector aperture.

[0040] Fig. Figure 2 illustrates aspects relating to a scan module 100. The scan module 100 comprises a base 141, two elastic elements 101, 102, and an end piece 142. The base 141, the elastic elements 101, 102, and the end piece 142 are formed in one piece.

[0041] The example of Fig. 2 basically corresponds to the example of Fig. 1. In the example of the Fig. However, 2 are - unlike in Fig. 1 - the longitudinal axes 111, 112 of the elastic elements 101, 102 are not oriented perpendicular to the mirror surface 151. In Fig. Figure 2 shows the angle 159 between the surface normal 155 of the mirror surface 151 and the longitudinal axes 111, 112. In this example, the angle 159 is... Fig. 2 45°, but could generally be in the range of -60° to +60°.

[0042] Such a tilting of the mirror surface 151 relative to the longitudinal axes 111, 112 can be particularly advantageous if the torsional mode of the elastic elements 101, 102 is used to move the mirror 150. Then, a periscope-like scanning of the light 180 can be implemented using the beam-guiding unit 99.

[0043] Fig. 1 and Fig. Section 2 explains the principle of beam deflection used according to various examples, although only a single group of elastic elements 101, 102 is present there. Details on beam deflection are given in Fig. 3 shown.

[0044] Fig. Figure 3 illustrates aspects relating to a beam guidance unit 99. The beam guidance unit 99 comprises the scan module 100, which could, for example, be configured according to the various other examples described herein (however, in Fig. 3 (for example, a scan module 100 with only a single support element 101 is shown).

[0045] Fig. Figure 3 illustrates aspects relating to piezo actuators 310 and 320. In various examples, bending piezo actuators 310 and 320 can be used to excite the support element 101. The piezo actuators 310 and 320 can be controlled, for example, by a suitable controller—e.g., via a driver. For instance, a first and a second bending piezo actuator can generally be used. The bending piezo actuators can move the base 141 and, in particular, tilt it—to excite a torsional mode.

[0046] Especially in the example of the Fig. 3. The piezo actuators 310, 320 are designed as bending piezo actuators. This means that applying a voltage to the electrical contacts of the bending piezo actuators 310, 320 causes them to bend or bend along their longitudinal axes 319, 329. For this purpose, the bending piezo actuators 310, 320 have a layered structure (in Fig. 3 not shown and oriented perpendicular to the plane of the drawing). In this way, one end 315, 325 of the bending piezo actuators 310, 320 is deflected relative to a fixing point 311, 321 perpendicular to the respective longitudinal axis 319, 329 (the movement is shown in the example of the Fig. 3. oriented perpendicular to the drawing plane). From the example of the Fig. Figure 3 shows that the connection of the bending piezo actuators 310, 320 to the support element 101 is implemented via the edge regions 146 of the base 141. For example, the fixing points 311, 321 could provide a rigid connection between the bending piezo actuators 310, 320 and a housing of the beam guidance unit 99 (in Figure 3). Fig. 3 not shown) are manufactured and arranged in a fixed location within a reference coordinate system (fixed coordinate system).

[0047] Fig. Section 4 illustrates aspects relating to a beam guidance unit 99. In the example of the Fig. Figure 4 shows a scan module 100, which has a first pair of elastic elements 101-1, 102-1, and a second pair of elastic elements 101-2, 102-2. The first pair of elastic elements 101-1, 102-1 is arranged in one plane; the second pair of elastic elements 101-2, 102-2 is also arranged in one plane. These planes are parallel to each other and offset from each other.

[0048] Each pair of elastic elements is assigned to a corresponding base 141-1, 141-2, and a corresponding end piece 142-1, 142-2, i.e. two groups of elastic elements 101-1, 102-1 and 101-2, 102-2 are used in a sandwich structure.

[0049] A connection to a mirror 150 is established using the two end pieces 142-1, 142-2, either directly or via another angle element.

[0050] The base 141-1, the elastic elements 101-1, 102-1, and the end piece 142-1 are formed in one piece, e.g., manufactured from a single wafer in a common MEMS process. The base 141-2, the elastic elements 101-2, 102-2, and the end piece 142-2 are also formed in one piece, e.g., manufactured from a single wafer in a common MEMS process.

[0051] Out of Fig. Figure 4 also shows that the base 141-1 is not formed integrally with the base 141-2. Furthermore, the end piece 142-1 is not formed integrally with the end piece 142-2. Similarly, the elastic elements 101-1 and 102-1 are not formed integrally with the elastic elements 101-2 and 102-2. In particular, it is possible that the various aforementioned parts are manufactured from different areas of a wafer and subsequently joined together, for example, by adhesive bonding or anodic bonding. Other examples of joining techniques include: fusion bonding; fusion or direct bonding; eutectic bonding; thermocompression bonding; and adhesive bonding.

[0052] As a general rule, the end pieces 142-1 and 142-2 do not need to be directly connected to each other. A magnet can be positioned between the end pieces 142-1 and 142-2. This is in Fig. 5 shown.

[0053] Fig. Figure 5 illustrates aspects relating to the integration of a magnet for measuring the positioning of the mirror 150 into a beam steering unit 99. Fig. Figure 5 is a side view of a beam guidance unit 99 according to various examples. Shown are an "upper" elastic element 101-2 and a "lower" elastic element 101-1, as well as their respective end pieces 142-2 and 142-1. The elastic element 101-2 is connected via the end piece 142-2 to another elastic element 102-2 (in the view of the Fig. 5 (covered) connected. Accordingly, the elastic element 101-1 is connected to another elastic element 102-1 via the end piece 142-1. The elastic elements connected via the respective end pieces 142-1 and 142-2 thus form a group, i.e., components formed in one piece.

[0054] In Fig. Figure 5 also shows a base 141-2 and a base 141-1. The base 141-1 connects the elastic elements 101-1 and 102-1. The base 141-2 connects the elastic elements 101-2 and 102-2. The base 141-1, the elastic elements 101-1 and 102-1, and the end piece 142-1 are formed in one piece, e.g., manufactured from a single wafer in a common MEMS process. The base 141-2, the elastic elements 101-2 and 102-2, and the end piece 142-2 are also formed in one piece, e.g., manufactured from a single wafer in a common MEMS process.

[0055] The end piece 142-1 is connected to the end piece 142-2 via a magnet 910. Furthermore, spacer elements 901 and 902 are provided adjacent to the magnet 910, which have the same thickness (in the Z-direction) as the magnet 910.

[0056] Base 141-1 is connected to base 141-2 via a spacer element 931, which also has this thickness.

[0057] Spacer 901 can be attached to end piece 142-1 and end piece 142-2. Similarly, spacer 902 can be attached to end piece 142-1 and end piece 142-2. Magnet 910 can also be attached to end piece 142-1 and end piece 142-2.

[0058] It is unnecessary for magnet 910 to be glued to spacer element 901 or spacer element 902. This simplifies manufacturing because it enables wafer-level gluing on surfaces oriented parallel to the wafer surface.

[0059] In Fig. 5 also includes an angle element 940, which is attached to the spacer element 902 and the end pieces 142-1, 142-2. The mirror 150 is attached to this, such that the mirror normal and the longitudinal axes of the elastic elements 101-1, 101-2, 102-1, 102-2 form a 45° angle with each other (see Figure 5). Fig. 2).

[0060] The spacer elements are made of silicon, as are the elastic elements 101-1, 101-2, 102-1, 102-2, the base 141-1, the base 141-2 and the end pieces 142-1, 142-2.

[0061] By using the spacer element 901, which is arranged adjacent to the magnet 910, offset towards the base 141-1 and the base 141-2, the following effect can be achieved: The elastic elements 101-1, 101-2, 102-1, 102-2 twist when torsion is excited in the area 149 between the base 141-1, the base 141-2, and the end pieces 142-1, 142-2. The area of ​​greatest material stress during such torsion of the elastic elements 101-1, 101-2, 102-1, 102-2 (this area is in Fig. 5 (marked with the dashed circles) is arranged at a silicon-silicon interface when using the spacer element 901, that is, between the spacer element 901 and the end pieces 142-1, 142-2. It was observed that such a silicon-silicon interface can be bonded more stably with adhesive than an interface between a ferromagnetic material of the magnet 910 and silicon. In reference implementations where no spacer element 901 is present (compare Fig. 6) is the area of ​​greatest stress at the interface between the ferromagnetic material of magnet 910 and the silicon of end pieces 142-1, 142-2; here it was observed that the resonance frequency of the mass-spring system (formed from mirror 150, end pieces 142-1, 142-2, magnet 910 and elastic elements 101-1, 101-2, 102-1, 102-2, etc.) for the torsional mode decreases over time. This is caused by microdefects in the adhesive between magnet 910 and end pieces 142-1, 142-2. Corresponding microdefects or a decrease in the resonance frequency are observed in the scenario of Fig. 5 not observed. This is due to the better adhesion of the silicon component 901 to the silicon of the end pieces 142-1, 142-2.

[0062] In short, in the area of ​​maximum stress – i.e., adjacent to the torsion zone 149 between base 141-1, base 141-2, and end pieces 142-1, 142-2 – a silicon-silicon interface is provided by the spacer element 901 (this also applies in principle to the spacer element 931, although no magnet is provided there anyway). This makes the sandwich structure robust, especially compared to the scenario of Fig. 6. Therefore, the spacer element 901 is arranged in the area of ​​maximum tension adjacent to the torsion area 149.

[0063] To further improve the magnet's adhesion, it could be coated with silicon oxide (e.g., SiO2) or silicon. For example, a layer with a thickness of less than 100 nm could be used. However, this is not strictly necessary, because even without silicon oxide, the magnet's adhesion is sufficient in the example of... Fig. 5 a high degree of robustness is achieved.

[0064] The longitudinal extent of the spacer element 901 (i.e., along the longitudinal axes of the elastic elements 101-1, 101-2, 102-1, 102-2) can be approximately equal to the longitudinal extent of the magnet 910 (i.e., in the range of 90% to 110%). This facilitates so-called "pick-and-place" operations, which involve placing the spacer element 901 next to the magnet 910 using a robot arm.

[0065] For example, area 149 could have a longitudinal extent of 4 mm to 8 mm, and the longitudinal extent of the spacer element 901 and the magnet 910 could be, for example, 1 mm.

[0066] The magnet 910 can be made of a ferromagnetic material. This generates a stray magnetic field which can be detected by an angle magnetic field sensor 980. The angle magnetic field sensor 980 is fixed in position within the scanner's mounting system. Based on the measurement signal from the angle magnetic field sensor 980, the position of the mirror 150 can then be determined, and based on the mirror's position, the deflection angle can be calculated.

[0067] Naturally, the features of the embodiments and aspects of the invention described above can be combined with one another. In particular, the features can be used not only in the combinations described, but also in other combinations or individually, without leaving the scope of the invention.

[0068] For example, implementations of a beam guidance unit using silicon as the semiconductor material have been described. However, other semiconductor materials could also be used in principle, such as gallium arsenide.

[0069] Furthermore, scenarios were shown in which each group contains two elastic elements. It would be conceivable to use more than two elastic elements per group.

Claims

[1] Scanner that includes: - a mirror (150) designed to reflect light (180), - a beam guidance unit (99) comprising: - at least one first elastic element (101-1,102-1) extending between a first base (141-1) and a first end piece (142-1) arranged adjacent to the mirror (150) and made of a semiconductor material, - at least one second elastic element (101-2, 102-2) extending between a second base (141-2) and a second end piece (142-2) arranged adjacent to the mirror (150), parallel to the at least one first elastic element and made of the semiconductor material, wherein the beam steering unit (99) is configured to deflect the light (180) by means of torsion of the at least one first elastic element (101-1, 102-1) and the at least one second elastic element (101-2, 102-2) in a torsion region (149) between the first and second base (141-1, 141-2) and the first and second end pieces (142-1, 142-2) by reflection at the mirror (150) at different angles, wherein the scanner further comprises, - a magnet (910) made of a ferromagnetic material and arranged between the first end piece (142-1) and the second end piece (142-2), - a spacer element (901) which is arranged adjacent to the magnet (910) towards the first and second bases (141-1, 141-2) offset between the first end piece (142-1) and the second end piece (142-2). [2] Scanner according to claim 1, wherein the distance element consists of the semiconductor material. [3] Scanner according to claim 1 or 2, wherein the spacer element has a first longitudinal extension in a direction along the at least one elastic element, wherein the magnet has a second longitudinal extension in the direction along the at least one elastic element, where the first longitudinal dimension is 60% to 140%, optionally 90% to 110% of the second longitudinal dimension. [4] Scanner according to any of the preceding claims, wherein the magnet has a silicon oxide coating or a silicon coating around the ferromagnetic material. [5] Scanner according to any of the preceding claims, wherein the scanner further comprises: - another spacer element (902) which is arranged adjacent to the magnet (910) towards the mirror (150) and offset between the first end piece (142-1) and the second end piece (142-2) and is made of the semiconductor material. [6] Scanner according to one of the preceding claims, wherein the spacer element (901) is attached to the first end piece (142-1) and to the second end piece (142-2) by means of an adhesive. [7] Scanner according to one of the preceding claims, wherein the spacer element (901) is not attached to the magnet (910) by means of an adhesive. [8] Scanner according to one of the preceding claims, wherein the distance element (901) is arranged adjacent to the torsion area (149). [9] Scanner that includes: - a mirror (150) designed to reflect light (180), - a beam guidance unit (99) comprising: - at least one first elastic element (101-1,102-1) extending between a first base (141-1) and a first end piece (142-1) arranged adjacent to the mirror (150) and made of a semiconductor material, - at least one second elastic element (101-2, 102-2) extending between a second base (141-2) and a second end piece (142-2) arranged adjacent to the mirror (150), parallel to the at least one first elastic element and made of the semiconductor material, wherein the beam steering unit (99) is configured to deflect the light (180) by means of torsion of the at least one first elastic element (101-1, 102-1) and the at least one second elastic element (101-2, 102-2) in a torsion region (149) between the first and second base (141-1, 141-2) and the first and second end pieces (142-1, 142-2) by reflection at the mirror (150) at different angles, wherein the scanner further comprises, - a magnet (910) consisting of a ferromagnetic material and arranged between the first end piece (142-1) and the second end piece (142-2), wherein the magnet has a silicon oxide coating or a silicon coating around the ferromagnetic material.

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