Sensor system for sensing a magnetic field or magnetic-field-dependent measurement variable

Abstract

The invention relates to a sensor system (1) for sensing a magnetic-field parameter of an outer magnetic field, which has an NV diamond (2), a microwave source (3) arranged and designed to generate a microwave field in the NV diamond (2), an excitation light source (4) arranged and designed to direct an excitation light beam (5) onto the NV diamond (2), at least one detector (6) arranged and designed to sense an optical signal emitted from the NV diamond (2) as a result of the irradiation with the excitation light beam (5), and a magnetic-field generation device (12) arranged and designed to generate an inner static magnetic field which can be detected in the NV diamond (2). The sensor system (1) is characterised in that the magnetic-field generation device (12) comprises a multi-layer arrangement of permanent magnets (7) which are arranged around the NV diamond (2).

Description

[0001] Description

[0002] title

[0003] Sensor system for detecting a magnetic field or a magnetic field-dependent measured variable

[0004] The present invention relates to a sensor system for detecting a magnetic field or a magnetic field-dependent measured variable.

[0005] Background of the invention

[0006] To detect a magnetic field, so-called NV magnetic field sensors are used. These comprise a diamond whose crystal lattice exhibits defects in the form of NV centers. In an NV center, a nitrogen atom occupies the lattice site of a carbon atom, with a defect located directly adjacent to the nitrogen atom - again on the lattice site of a carbon atom. If such a crystal lattice is irradiated with excitation radiation with a wavelength between 490 nm and 575 nm, an electronic transition from a ground state to a 3 A2 into an excited state 3 E induced. From the excited state 3 E relaxes the NV center back to the ground state by emitting fluorescence radiation in a wavelength range between 600 nm and 850 nm 3 A2. The ground state 3 A2 has three magnetic substates with m s =0, m s=±1. The states with m s =0 and m s =±1 are distinguished by an energy difference of 2.87 GHz (zero field splitting). The excited state 3 E also has three magnetic substates with m s =0, m s =±1. When irradiated with the excitation radiation, the NV center is now partially removed from the m s =±1 , 3 A2 ground state in the excited m s =±1 , 3 E state. From there it relaxes predominantly radiationlessly and without spin conserving back into the m s =0, 3 A2 -ground state. At the same time, the excited state m relaxes s =0, 3 E emitting fluorescent radiation also in the m s =0, 3 A2 - Ground state. If the NV center is now in the ground state 3 A2 exposed to microwave radiation with a frequency of 2.87 GHz, the NV center oscillates between the m s =0, 3A2 ground state and the m s =±1 , 3 A2 - Ground state. If the amplitude of the fluorescence radiation is measured as a function of the frequency of the microwave radiation during irradiation with the excitation radiation, a sudden drop in the amplitude of the fluorescence radiation (a so-called dip) occurs at a frequency of 2.87 GHz. The drop in the amplitude of the fluorescence radiation can be explained by the fact that - when microwave radiation is irradiated with a frequency of 2.87 GHz - the transition between the m s =0, 3 A2 ground state and the m s =±1 , 3 A2 ground state is induced, which is excited spin-conservingly by the excitation radiation, but non-radiatively and not spin-conservingly in the m s =0, 3 A2 ground state can relax. If microwave radiation of a precisely matching frequency is radiated in, the m s =0, 3A2 ground state, during the m s =±1 , 3 A2 ground state fills up.

[0007] In a magnetic field, the m s =±1 , 3 A2 ground state into two states with spin quantum number m s =1 and m s =-1 (Zeeman effect). If the amplitude of the fluorescence radiation is now measured while changing the frequency of the microwave radiation, two dips are obtained. The frequencies at which these dips occur depend on the size of the splitting of the m s =±1 , 3 A2 ground state and thus on the field strength and direction (especially the projection onto the respective NV axes) of the magnetic field. In this way, the projection of a magnetic field onto the NV axis can be determined.

[0008] Such an NV magnetic field sensor is described, for example, in US 2019 / 0018091 A1. In this case, an internal, static magnetic field is additionally generated by several permanent magnets, which are designed as individual magnets and arranged in a single-layer Halbach configuration. The individual magnets are held by a frame attached to a base. The NV diamond is also attached to this base. The individual magnets are inserted into the frame along the circumference of the frame, with the frame aligned perpendicular to the base along its main plane.

[0009] The sensitivity of the NV magnetic field sensor depends significantly on the homogeneity of the internal, static magnetic field at the location of the NV diamond. Typical field strengths range from 0.1 pT to several tens of mT, with the internal, static magnetic field having a homogeneity of typically less than 1000 ppm measured in the volume of the NV magnetic field sensor. Typical volumes of NV magnetic field sensors are in the range of 1 mm. 3Typically, the internal, static magnetic field is generated by continuous currents, permanent magnets, or combinations of permanent magnets and continuous currents. The homogeneity of the resulting internal, static magnetic field depends directly on the size of the device that generates it. To generate the most homogeneous magnetic field possible, the largest possible magnetic field generating devices are used—especially in systems with manufacturing tolerances. Therefore, there is a need for devices for generating an internal, static magnetic field that can be used in compact, robust, and sensitive NV magnetic field sensors.

[0010] To generate the internal, static magnetic field, conductors carrying direct current can be used, such as loop currents, solenoids, or Helmholtz coils. By using two concentric, circular current loops arranged along an axis of symmetry at a distance corresponding to the loop radii, fluctuations in the static magnetic field along the axis of symmetry can be compensated. For this reason, the Helmholtz coil configuration is the most commonly used arrangement for generating a homogeneous static magnetic field using continuous currents.

[0011] Alternatively, permanent magnets can be used to generate the static magnetic field.

[0012] Furthermore, the two methods mentioned above (the use of current-carrying conductors (Method 1) and the use of permanent magnets (Method 2)) can be combined. Alternatively, shimming techniques can be applied together with both or one of the two methods. In this case, a base level for the internal, static magnetic field is first generated using Method 1 and / or Method 2. Then, Method 1 and / or Method 2 are used to compensate for fluctuations in the static magnetic field in a target volume. For example, a static magnetic field can be generated using an arrangement of permanent magnets (Method 2) and a specific arrangement of conductors carrying relatively weak currents (Method 1). The homogeneity of the static magnetic field can be further improved by compensating for fluctuations in the static magnetic field in the target volume.This method is called "shimming" and is a widely used tool for improving the homogeneity of the generated static magnetic field. This method can be implemented either passively, actively, or in a hybrid manner.

[0013] In passive compensation, field inhomogeneities are reduced by placing small permanent magnets at predefined positions and / or inducing predefined currents in coils placed at predefined positions. In active compensation, the positions of the compensating permanent magnets and / or the currents of the compensating (shimming) coils are adjusted with regard to the relevant key figure, such as the spectroscopy of detectable resonances, the overall noise of the system, or the overall sensitivity of the system, etc. A variety of methods exist for hybrid compensation. For example, the positions of the compensating permanent magnets are changed during the assembly process while compensating currents are switched off. Later, the compensating currents are used to improve the coefficient of performance.

[0014] The methods described above have several disadvantages. First, a magnetic field based on direct currents requires a direct current supply, which increases the system's power requirements. For example, for static magnetic fields of several mT generated by coils several centimeters in size, the total power consumption can be up to several hundred mW, so the component generating the static magnetic field can become one of the largest power consumers of the entire sensor system. Furthermore, the heat generated by a coil or system of coils, Q, is proportional to the square of the current I flowing in the wire of which the coils are made, so Q = 1 2 R, where R is the total resistance of the wire. The static magnetic field Bo generated by the coils is proportional to I and the total number of windings in the system n, e.g., Bo °< nl.

[0015] However, heat generation cannot be completely reduced by decreasing I and increasing n, because additional turns increase the resistance R, which in turn is proportional to the total length of the wire L and inversely proportional to the cross-sectional area of ​​the wire A, i.e., R = pL / A, where p is the resistivity of the material. The heat generated by the current can also adversely affect the permanent magnets used for shimming purposes or to generate the base level for the static magnetic field. Furthermore, the heat can cause temperature gradients in the permanent magnets or other parts of the sensor, the effects of which on the system are unknown or very difficult to predict and / or compensate for.

[0016] Furthermore, the current required to generate the static magnetic field generates electronic noise of various origins, such as flicker noise and Johnson-Nyquist noise. This additional noise can negatively impact the system's noise floor, affect other components, and ultimately lead to reduced system sensitivity.

[0017] Coils have a high inductance, which allows them to couple with currents in the sensor housing, fields generated by other parts of the system, and external magnetic interference, among other things. This parasitic coupling can impair the sensitivity of the sensor system.

[0018] Furthermore, the static magnetic field generated by the currents in the most common arrangement (Helmholtz coil) leads to a magnetic dipole moment m, which results in a significant stray field outside the arrangement – ​​the arrangement itself, viewed from the outside, becomes a permanent magnet. This stray field amplifies the parasitic influence of soft magnetic materials in the sensor system and in the immediate vicinity of the sensor. This parasitic magnetization of the surrounding structures can manifest itself as offset errors in the measurement and impair the homogeneity of the static magnetic field Bo itself.

[0019] Disclosure of the invention

[0020] The invention relates to a sensor system for detecting an external magnetic field or a measured variable dependent on an external magnetic field. Such a sensor system comprises a diamond with at least one NV center (NV diamond). The sensor system further comprises a microwave source, an excitation light source, and a detector. The microwave source is arranged and configured to generate a microwave field in the NV diamond. The excitation light source can be, for example, a diode laser. The excitation light source is particularly configured to generate an excitation light beam with a wavelength between 490 nm and 575 nm. The excitation light source is further arranged and configured to direct the excitation light beam onto the NV diamond, which, upon irradiation with the excitation light beam, emits an optical signal, in particular fluorescent radiation in a wavelength range between 600 nm and 850 nm.A beam-shaping (e.g., a lens) or beam-guiding (e.g., a deflecting mirror) optical element can be arranged between the excitation light source and the NV diamond. The detector is designed and arranged such that it can detect the optical signal emitted by the NV diamond as a result of irradiation with the excitation light beam. In particular, the detector can be designed as a photodiode that can detect radiation in a wavelength range of the fluorescence radiation emitted by the NV diamond, in particular between 600 nm and 850 nm. In addition, at least one reference detector can be provided for detecting the excitation light beam in order to detect fluctuations in the intensity of the excitation light source and to evaluate the optical signal independently of these fluctuations.Optical filters designed to filter out the wavelength range to be detected from the respectively incoming radiation can be arranged upstream of the detector and / or the at least one reference detector. The sensor system further comprises a magnetic field generating device that is arranged and designed to generate an internal, static magnetic field in the NV diamond. The magnetic field generating device comprises a multi-layer, in particular two-layer, arrangement of permanent magnets arranged around the NV diamond. If the arrangement of permanent magnets comprises two layers, the NV diamond can be arranged centrally between the two layers. The two layers are then arranged at a distance d from one another, with the NV diamond being arranged in particular at a distance d / 2 from the two layers.Such an arrangement can generate a very homogeneous magnetic field in the NV diamond, thereby increasing the measurement accuracy of the sensor system.

[0021] In a further development of the invention, the multi-layer arrangement of permanent magnets comprises at least one, in particular two, ring magnet(s), i.e. at least one, in particular two, permanent magnets are designed as ring magnets. In particular, each of the layers can comprise exactly one ring magnet. The ring magnets can be arranged concentrically to one another. In the case that the arrangement of permanent magnets comprises exactly two layers - and thus in particular exactly two ring magnets - the NV diamond can be arranged centrally between the two ring magnets on an axis that runs through the centers of the two ring magnets. In the context of this application, ring magnets are understood to be permanent magnets designed as hollow cylinders whose magnetization direction varies continuously along their circumference, i.e. which are not a combination of individual magnets.

[0022] In a further development of the invention, in the multi-layer arrangement of permanent magnets, at least two, in particular four, permanent magnets are designed as individual magnets in at least one of the layers, wherein the magnetic dipole moments of adjacent individual magnets point in different directions. The individual magnets of the respective layer can be arranged in a plane, in particular along an imaginary circular line, so that an annular arrangement of permanent magnets is present, which, however, is not referred to as a ring magnet in the context of this application. The individual magnets can have a magnetic dipole moment that is aligned, for example, along one of the edges of a cube- or cuboid-shaped individual magnet. The magnetization of the ring magnet designed as a hollow cylinder can be aligned according to a Halbach arrangement. A plane perpendicular to the main axis of the ring magnet is defined as the xy plane.In the case of an alignment of the magnetization of the ring magnet according to a Halbach arrangement, the magnetization of the ring magnet is aligned in the xy-plane, in particular the magnetization in the direction of the main axis of the ring magnet (z-axis) is zero or almost zero. The magnetization direction of the ring magnet varies in the xy-plane, in particular with twice the polar angle (M (cos2 <p, sin2<p, 0)), wobei der Polarwinkel <p den Winkel in der xy-Ebene und M die Magnetisierung bezeichnet. Dabei variiert insbesondere der Betrag der Magnetisierung in Umfangsrichtung gesehen nicht, so dass das magnetische Gesamtdipolmoment des Ringmagneten gleich null ist (m=0). Es bildet sich somit ein Magnetfeld in der xy-Ebene aus.

[0023] In particular, the sensor system can have two layers, each with a ring magnet, the magnetization of which is aligned according to a Halbach arrangement. In this case, the two ring magnets are offset along the z-axis and arranged concentrically to one another. The ring magnets of different layers have the same orientation with regard to their magnetization in the xy-plane, i.e. sections that are arranged on different ring magnets but have the same coordinates in the xy-plane, i.e. are arranged directly above one another, have the same direction of magnetization. In other words, when the two ring magnets are projected into the xy-plane, one ring magnet is imaged exactly or almost exactly onto the other. In such an arrangement, a maximum magnetic field results inside the arrangement of ring magnets, while the magnetic field outside the arrangement of ring magnets is minimal.

[0024] The multi-layer arrangement of permanent magnets can have three or more individual magnets in at least one of the layers, which are arranged according to a Halbach arrangement. The magnetic dipole moments of the respective individual magnets can have the same value. The individual magnets are arranged in particular in a plane, preferably on an imaginary circular line, wherein the direction in which the respective magnetic dipole moments are aligned is in particular twice the polar angle (m <* (cos2 <p, sin2cp,0)) variiert. Dabei ist insbesondere das Gesamtdipolmoment m der jeweiligen Lage gleich null (m=2mi=0), wobei rrii das magnetische Dipolmoment der jeweiligen Einzelmagnete bezeichnet Eine solche Anordnung hat den Vorteil, dass Komponenten in der Umgebung der Halbach-Anordnung nicht oder weniger stark magnetisiert werden, und so die Empfindlichkeit des Sensorsystems verringert wird.

[0025] The magnetic field generating device of the sensor system has, in particular, two layers, in each of which, in particular, three or more individual magnets are arranged according to a Halbach arrangement. The two layers are arranged, in particular, concentrically to one another, with the individual magnets in each of the two layers being arranged in a circle. The individual magnets of the respective layers are arranged one above the other, in particular as seen in the z-direction, i.e., individual magnets with the same orientation of their magnetic dipole moment in the xy-plane have the same coordinates in the xy-plane and a different coordinate in the z-direction. The magnetic dipole moments of the individual magnets arranged one above the other therefore point, in particular, in the same direction. This results in a maximum internal, static magnetic field inside the two-layer arrangement, while the magnetic field outside is minimal. The homogeneity of the internal, static magnetic field depends, among other things, ondepends on the distance between the two layers in the z-direction. The optimal distance, at which the homogeneity of the internal, static magnetic field is maximized, can be determined through a simulation optimization process.

[0026] The Halbach arrangement of single or ring magnets described above does not exhibit rotational symmetry with respect to the z-axis.

[0027] In a further development of the invention, the multilayer arrangement comprises at least two permanent magnets arranged in an Aubert arrangement around the NV diamond. In particular, the magnetization and / or the magnetic dipole moments of the permanent magnets are aligned according to the Aubert arrangement.

[0028] In such an arrangement, at least one of the layers can have a permanent magnet designed as a ring magnet, the magnetization direction of which in the xy plane varies continuously with the simple polar angle (M°<± (coscp, simp, 0). In particular, the amount of magnetization does not vary in the circumferential direction, so that the total dipole moment of the respective ring magnets is zero (m=0) or almost zero. In particular, the ring magnets have no or almost no magnetization in the z-direction.

[0029] In particular, the magnetic field generating device of the sensor system has two layers, each with a ring magnet. The magnetization direction of a first ring magnet, arranged in a first layer, points outward from the ring center, while the magnetization direction of a second ring magnet, arranged in a second layer, points inward from the ring center. The magnetization direction varies in each of the layers with the simple polar angle (M°<± (coscp, simp, 0)). With such a magnetic field generating device – designed as an Aubert arrangement – ​​a very homogeneous magnetic field is produced in a volume around the center of the arrangement. The inner, static magnetic field is aligned along the z-axis of the two concentrically arranged ring magnets.In particular, the magnitude of the magnetization does not vary in the circumferential direction of the ring magnets, so that the total dipole moment of the respective ring magnets is zero (m=0) or almost zero. The two ring magnets preferably have the same outer radius. The homogeneity of the internal, static magnetic field depends, among other things, on the distance between the two layers in the z-direction. The optimal distance at which the homogeneity of the internal, static magnetic field is maximum can be determined in a simulation optimization process.

[0030] If the respective ring magnets are made of materials with different magnetization strengths, they can also have different radii. The ring magnet made of the less strongly magnetized material has a smaller outer radius, so that an equally strong internal static magnetic field is generated as if the two ring magnets were made of materials with the same magnetization strength and had the same radius.

[0031] In a further development of the invention, at least one of the layers of the magnetic field generating device has at least two, in particular three or more, individual magnets, resulting in an Aubert arrangement for the entire magnetic field generating device. In particular, the magnetic field generating device has two layers of permanent magnets designed as individual magnets, which are arranged as an Aubert arrangement. The individual magnets can be arranged in each of the two layers along an imaginary circular line, wherein the circles delimited by the two circular lines can be arranged concentrically to one another. The z-axis runs in particular through the centers of the two circles.

[0032] In particular, the individual magnets within a layer are arranged at a polar angle according to (pi,i = <pi,o + (j - 1) ■ 360° / ni, <p2,i = <pi>+ (i - 1) ■ 3607^2, where n is the number of individual magnets in the first layer, n2 is the number of individual magnets in a second layer and / denotes the respective individual magnet. The orientation of the magnetic dipole moments of the individual magnets in the xy plane changes from individual magnet to individual magnet with the simple polar angle (m«± (coscp, simp, 0)), while the magnetic dipole moments of the respective individual magnets in the z direction are zero or almost zero. The magnetic dipole moments of the individual magnets within the first layer point outwards as seen from the center of the imaginary circle, while the dipole moments of the individual magnets in the second layer point inwards as seen from the center of the imaginary circle, i.e. towards the center of the circle. This leads to the internal, static magnetic field developing along the z axis.The resulting internal, static magnetic field is highly homogeneous within a volume around the center of the magnetic field generating device, i.e., the point on the z-axis located halfway between the two layers. As in the other two-layer arrangements, the homogeneity of the internal, static magnetic field depends, among other things, on the distance between the two layers in the z-direction. The optimal distance at which the homogeneity of the internal, static magnetic field is maximum can be determined in a simulation optimization process.

[0033] The magnitudes of the magnetic dipole moments of the individual magnets can be equal. In this case, the individual magnets in the respective layers are all arranged along the same imaginary circular line. This results in a total dipole moment of zero in the respective layer (m=Smi=0). In particular, the number of individual magnets per layer can also be the same. Furthermore, the individual magnets in both layers can each be arranged on the same imaginary circular line, whereby the magnitudes of the magnetic dipole moments of the individual magnets in the respective layers are equal and the centers of the two circles formed in this way each lie on the z-axis. The design of the Aubert arrangement just described thus exhibits discrete rotational symmetry with respect to the z-axis, i.e. a rotation of the arrangement by an angle of 360° / n maps the arrangement back onto itself.In such a symmetrical arrangement, each individual magnet experiences the same magnetic field, so that all individual magnets are affected by the same magnetization or demagnetization by their neighboring magnets. This means that—even if a non-zero magnetization or demagnetization occurs—no loss of homogeneity due to this effect occurs.

[0034] In a further development of the invention, the permanent magnets are designed as individual magnets and arranged as an Aubert arrangement comprising at least two layers, wherein the individual magnets of the first layer are arranged offset from the individual magnets of the second layer, so that the first layer is arranged, so to speak, "twisted" relative to the second layer - in particular by any desired angle. The individual magnets of the respective layers are arranged on an imaginary circular line, wherein the circles delimited by the imaginary circular lines each have the same radius and are arranged concentrically to one another. This results in an arrangement in which the individual magnets of adjacent layers are not arranged directly above one another when viewed in the z-direction. Such an arrangement enables a flexible arrangement of other components of the sensor system.It has been found that the angle at which the two layers are rotated relative to each other has little or no influence on the homogeneity of the magnetic field and thus on the sensitivity of the sensor system.

[0035] Alternatively or additionally, the first layer can also differ from the second layer in the number of its individual magnets. In particular, the sum of the magnitudes of the dipole moments of the individual magnets in the first layer is equal to the sum of the magnitudes of the dipole moments of the individual magnets in the second layer. This means that if the magnetic dipole moments of the individual magnets in a layer all have the same magnitude m, rm = m m2 applies. The total magnetic dipole moment of the respective layers is m i = £ mu and m2 = I m 2i , is equal to 0. Here, mu denotes the respective magnetic dipole moments of the individual magnets 9 in the first layer and m 2i where n2 denotes the respective magnetic dipole moments of the individual magnets in the second layer, and n2 denotes the number of individual magnets in the first and second layers. Arrangements designed in this way allow other components of the sensor system, such as the excitation light source and / or the microwave source, to be arranged in a space-saving manner. However, this does not, or almost does not, affect the sensitivity of the sensor system. This allows the sensor system to be designed compactly.

[0036] Compared to the Halbach arrangement, the Aubert arrangement has the advantage that the individual magnets can be arranged more flexibly, allowing the other components of the sensor system to be arranged with more degrees of freedom. Comparing a two-layer Halbach arrangement with a two-layer Aubert arrangement with the same number of individual magnets per layer and the same radius, generating the same internal, static magnetic field—i.e., with the same target magnetic field—the Aubert arrangement requires individual magnets with dipole moments approximately 55% higher than the magnetic dipole moments of the individual magnets in the Halbach arrangement. This has several advantages.Among other things, with the same target magnetic field and provided that the individual magnets are arranged at the same radius as in the corresponding Halbach arrangement, individual magnets with higher dipole moments can be used for the Aubert arrangement. These magnets therefore comprise materials with stronger magnetization and coercivity, making them less susceptible to demagnetization. A further advantage is that with the same target magnetic field and the same radius and material of the individual magnets, individual magnets with higher magnetic dipole moments can be used for the Aubert arrangement, allowing for larger external dimensions. These are easier to handle during assembly of the sensor system and less susceptible to manufacturing tolerances.

[0037] Compared to Halbach arrays with the same radius and the same number of individual magnets, Aubert arrays generate more homogeneous magnetic fields than Halbach arrays. Furthermore, Aubert arrays are less sensitive both to the position of the permanent magnets used and to the magnitude and orientation of the magnetic dipole moments of the permanent magnets used.

[0038] In a further development of the invention, the individual magnets of a layer arranged according to the Aubert arrangement or the Halbach arrangement are arranged along at least two different imaginary circular lines that delimit concentrically arranged circles, i.e. circles of different radii around a common center point. For example, an individual magnet can be arranged on a first circular line and the other individual magnets on a second circular line. Thus, at least one individual magnet is arranged at a different radius than the other individual magnets. The individual magnets arranged on different circular lines have different magnitudes of their respective magnetic dipole moments, with individual magnets with a larger magnetic dipole moment having a larger radius than individual magnets with a smaller magnetic dipole moment.In particular, half or more than half of the individual magnets can differ from each other in the magnitude of their magnetic dipole moments and in their radius. It is understood that the radius is directly linked to the distance of the individual magnets from the NV diamond arranged at the center of the magnetic field generation device, i.e., the larger the radius, the greater the distance of the individual magnets from the NV diamond.

[0039] Provided that the magnitudes of the magnetic dipole moments of the respective individual magnets are known, such an arrangement makes it possible to use individual magnets whose dipole moment deviates in magnitude from the desired value and still generate a homogeneous internal, static magnetic field.

[0040] The individual magnets can alternatively or additionally have a magnetic dipole moment that is not aligned according to their respective geometric orientation. For example, the magnetic dipole moment of a cube- or cuboid-shaped individual magnet can be aligned at an angle between 0° and 90° to at least one of the edges of the cube- or cuboid-shaped individual magnet. Likewise, the magnetic dipole moment of a cylindrical individual magnet can be aligned at an angle between 0° and 90° to the main axis of the cylindrical

[0041] Individual magnets must be aligned, i.e. the geometric alignment deviates from the alignment of the magnetic dipole moment.

[0042] Such a single magnet can be geometrically aligned in the Halbach or Aubert arrangement so that its magnetic dipole moment is aligned according to its intended orientation in the Halbach or Aubert arrangement, i.e. in the case of the Aubert arrangement, the magnetic dipole moment points to the center of the circle along whose circular line the individual magnet in question is arranged, while the edges or the main axis of the individual magnets do not point to the center of this circle.

[0043] Provided that the direction of the magnetic dipole moment of the individual magnets can be accurately measured, it is possible to use inexpensive individual magnets whose magnetic dipole moment has not been precisely specified by the manufacturer in terms of magnitude and direction and nevertheless to generate a very homogeneous magnetic field inside the arrangement.

[0044] The individual magnets can be cube-, cylinder- or cuboid-shaped, in particular with an edge length between 0.1 mm and 5 mm.

[0045] Particularly in the case of arrangements of individual magnets with radii in the range of approximately 5 mm, it can be advantageous if the individual magnets have the same aspect ratio, e.g. are cube-shaped or - if they are cylindrical - are the same or approximately the same in height and diameter.

[0046] The individual magnets can also be arranged on one or more circular lines defining circles with a radius between 2 mm and 200 mm, in particular between 5 mm and 50 mm. This results in distances between the individual magnets and the NV diamond arranged at the center of the magnetic field generating device in a similar size range.

[0047] Magnetic field generating devices in which the individual magnets are arranged in an Aubert array at these radii generate a very homogeneous magnetic field and are less sensitive to deviations in the actual from the assumed position, as well as to deviations in the actual from the assumed value and orientation of their magnetic dipole moment, with regard to the homogeneity of the generated magnetic field. Aubert arrays with the largest possible number of individual magnets offer the same advantages.

[0048] Short description of the drawings

[0049] Fig.1 shows a schematic structure of a sensor system according to an embodiment of the invention.

[0050] Fig.2 shows a magnetic field generating device in which individual magnets are arranged as a Halbach arrangement.

[0051] Fig.3 shows a magnetic field generating device with a total of eight individual magnets arranged as a Halbach arrangement.

[0052] Fig.4 shows a magnetic field generating device in which individual magnets are arranged as an Aubert arrangement.

[0053] Fig.5 shows a magnetic field generating device in which cube-shaped individual magnets are arranged as an Aubert arrangement.

[0054] Fig.6 shows a magnetic field generating device in which cuboid-shaped individual magnets are arranged as an Aubert arrangement.

[0055] Figures 7a) - c) each show magnetic field generating devices in which individual magnets are arranged in a Halbach array at different radii or in different orientations of their respective magnetic dipole moments. For reasons of clarity, only one position is shown.

[0056] Fig.8 shows a magnetic field generating device in which individual magnets are arranged as an Aubert arrangement, wherein the individual magnets of the two layers are offset from each other by the angle a.

[0057] Fig.9 a) - c) each show a magnetic field generating device in which individual magnets are arranged as an Aubert arrangement, wherein in Fig. 9b) the individual magnets of the two layers are arranged at an angle to each other and in Fig.9c) additionally the number of individual magnets of a first layer differs from the number of individual magnets of a second layer.

[0058] Embodiments of the invention

[0059] Figure 1 shows an overview of a sensor system 1 for detecting a magnetic field parameter of an external magnetic field. The sensor system 1 comprises a diamond crystal with NV defects (NV diamond 2), a microwave source 3, an excitation light source 4, at least one detector 6, at least one reference detector (not shown here), and a magnetic field generating device 12. Optical filters (not shown here) can be arranged on the detector 6 and / or the reference detector, each of which is designed to block wavelength ranges that should not or cannot be detected by the detector 6 and / or the reference detector. The detector 6 and / or the reference detector can be designed as photodiodes. The excitation light source 4 is designed to emit an excitation light beam 5. The wavelength of the excitation light beam 5 is in particular in a range between 490 nm and 575 nm.The NV diamond 2 and the excitation light source 4 are arranged such that the NV diamond 2 can be irradiated with the excitation light beam 5. The excitation light beam 5 can be directed onto the NV diamond 2, for example, via a partially reflecting mirror, wherein the reflected portion of the excitation light beam 5 can be directed onto the NV diamond 2 and the transmitted portion of the excitation light beam 5 can be directed onto the reference detector. The microwave source 3 is designed to emit microwave radiation 13 and is arranged such that a microwave field is generated in the NV diamond 2 by the emitted microwave radiation 13. The detector 6 is designed to detect fluorescent radiation, for example in a wavelength range between 600 nm and 850 nm, and is designed, for example, as a photodiode. The detector 6 can be attached directly to a side surface of the NV diamond 2.A filter designed to separate the fluorescent radiation from the excitation light beam can optionally be arranged between the NV diamond 2 and the detector 6. Multiple detectors, in particular multiple photodiodes, can also be provided to detect exclusively the fluorescent radiation or to simultaneously detect the fluorescent radiation and the excitation light beam 5. The at least one detector 6 and / or the at least one reference detector can optionally be preceded by filters that filter out predetermined wavelength ranges from the incoming radiation. The magnetic field generating device 12 is designed and arranged to generate a static, internal magnetic field B in the NV diamond 2. The magnetic field generating device 12 can have two layers of a total of at least two permanent magnets 7, which are arranged such that the NV diamond 2 is located at the center of the magnetic field generating device 12.The permanent magnets 7 can be designed - as shown in Fig.1 - as individual magnets 9 or as ring magnets with a continuously changing magnetization direction.

[0060] Figure 2 shows the magnetic field generating device 12 according to an embodiment of the invention, wherein the permanent magnets are designed as individual magnets 9. In a first layer 14 and a second layer 16, 12 individual magnets 9 are arranged according to a Halbach arrangement 10. The magnetic dipole moments of the individual magnets 9 typically have the same magnitude.

[0061] The permanent magnets can alternatively be designed as ring magnets. In this case, the magnetization—viewed along the circumferential surface of the ring magnets—has the same magnitude. The magnetic dipole moment and the magnetization are related by the relationship m = MV. Where m represents the magnetic dipole moment of the individual magnet, M represents the magnetization, and V represents the volume of the individual magnet.

[0062] In order to generate a magnetic field with a specific magnetic flux density, ring magnets must have a much larger volume than individual magnets, although they can also be made of a less strongly magnetized material than the corresponding individual magnets. The individual magnets 9 of a layer are each arranged in a single plane along a circular line 15, 25, which defines a circle with radius R. The direction of the magnetic dipole moments m; of the individual magnets 9 (or of the magnetization in the case of a ring magnet) changes along the circumferential direction with twice the polar angle m <* (cos 2 <p, sin 2<p, 0). Dabei ist der Polarwinkel der Winkel in der xy-Ebene, wobei die xy-Ebene die Ebene, in der die Einzelmagnete 9 angeordnet sind, bezeichnet. Im Fall, dass es sich bei den Permanentmagneten um Ringmagnete handelt, ist die xy-Ebene die Ringebene.The z-component of the magnetic dipole moments of the individual magnets 9 or the magnetization of the ring magnet is zero or almost zero.

[0063] In such an arrangement, a static magnetic field results inside the ring magnet or the ring-shaped arrangement of individual magnets 9. Outside the ring magnet or the ring-shaped arrangement of individual magnets 9, the static magnetic field disappears.

[0064] The first and second layers are arranged concentrically and offset from each other at a distance d along the z-axis. Magnetic dipole moments of individual magnets 9 arranged directly above each other are directed in the same direction. The homogeneity of the generated, internal, static magnetic field is in a volume around the center of the arrangement r c = (0,0,0) maximum, where r c is the origin of the coordinate system. The center of the arrangement is located on the z-axis, at a distance d / 2 from the two layers. The z-axis runs through the centers of the circles delimited by the circular lines 15, 25, on which the individual magnets of the first and second layers are arranged. The generated magnetic field B runs in the xy-plane. The NV diamond - not shown in Fig. 1 - is at the center of the magnetic field generating device r c = (0,0,0). The above statements apply analogously to the case where the permanent magnets are designed as ring magnets.

[0065] In the magnetic field generating device shown in Fig. 3, a total of eight individual magnets 9 are arranged in two layers, each comprising four individual magnets. The four individual magnets 9 are arranged according to a Halbach arrangement 10 along imaginary circular lines 15, 25. Spacers, which are made in particular of a non-magnetic material, are arranged between directly adjacent individual magnets 9 of the respective layers.

[0066] To ensure that the total dipole moment m per layer is zero (m = Jm; = 0), the Halbach arrangements can have more than three individual magnets or exactly three individual magnets per layer.

[0067] In a magnetic field generating device with two ring magnets, this condition is met if the absolute value, i.e. the magnitude, of the magnetization does not change along the circumferential direction.

[0068] Fig. 4 shows a magnetic field generating device 12 in which the individual magnets 9 are arranged as an Aubert arrangement 11. Here, too, the magnetic field generating device 12 has two layers 14, 16, in each of which individual magnets 9 are arranged along the imaginary circular lines 15, 25. The two layers 14, 16 are arranged concentrically to one another and offset from one another along the z-axis. The individual magnets 9 of the first layer 14 each have a magnetic dipole moment that points outwards from the center of the circle delimited by a first imaginary circular line 15, i.e. the direction of the magnetic dipole moments of the individual magnets 9 in the xy-plane varies with the simple polar angle m <* ± (cos <p, sin <p, 0). Die z-Komponente der magnetischen Dipolmomente ist dabei null oder nahezu null.

[0069] The individual magnets of the second layer 16 each have a magnetic dipole moment which, viewed from the center of a circle delimited by a second imaginary circular line 25, points inwards, ie here too the direction of the magnetic dipole moments of the individual magnets 9 in the xy-plane varies with the simple polar angle m <* ± (cos <p, sin <p, 0). Die Homogenität des erzeugten, inneren, statischen Magnetfelds ist in einem Volumen um den Mittelpunkt der Anordnung und Ursprung des Koordinatensystems r c = (0,0,0). This is also where the NV diamond (not shown in Fig. 3) is located. The generated magnetic field B is therefore aligned along the z-axis. To ensure that the total dipole moment m per layer is zero (m = Jm; = 0), the Aubert arrays can have more than two individual magnets per layer or exactly two individual magnets.

[0070] Alternatively, concentrically arranged ring magnets offset along the z-axis can be configured as an Aubert arrangement. In this case, the magnetization of one of the two ring magnets is directed outward, while the magnetization of the other ring magnet is directed inward. The condition that the total dipole moment m per layer is zero is met if the absolute value, i.e., the magnitude, of the magnetization does not change along the circumferential direction.

[0071] Fig. 5 shows a magnetic field generating device 12 with individual magnets 9 arranged as an Aubert array 11. The magnetic dipole moments of the individual magnets 9 arranged along the imaginary circular line 15 point toward the center 17 of the circle defined by the circular line 15 in the first layer 14; in the second layer 16, the magnetic dipole moments point outward from the center 17. Each of the two layers comprises four individual magnets 9. The individual magnets 9 are cube-shaped.

[0072] Fig.6 shows again the magnetic field generating device 12 from Fig.5, but with cuboid-shaped individual magnets 9.

[0073] Fig.7a) to c) show three different arrangements of individual magnets 9, each in one layer of a magnetic field generating device 12, which in this case is designed as a Halbach arrangement 10. Alternatively, the individual magnets 9 can also be arranged as an Aubert arrangement. Fig.7a) shows the desired positions of the individual magnets 9 in one layer of the Halbach arrangement. All individual magnets 9 have the same magnetic dipole moment (white arrow), which is aligned parallel to the geometric orientation of the respective individual magnets. In this case, the individual magnets are designed as cuboids. The magnetic dipole moment of the respective individual magnets is aligned parallel to the long edge of the cuboid. This corresponds to the ideal case. All individual magnets are positioned on a circle with radius ro. Fig.7b) shows how individual magnets would be positioned if the magnitude of the magnetic dipole moment deviates from the desired value.The individual magnets with a larger magnetic dipole moment are positioned at a larger radius r+, ro, e.g., individual magnet 19 at radius r+. The individual magnets with a smaller magnetic dipole moment, such as individual magnet 21, are positioned at a smaller radius r.. Fig. 7c) shows how individual magnets are positioned when the orientation of the magnetic dipole moment deviates from their respective geometric shape. As an example, individual magnet 20 is rotated slightly so that its magnetization points in the desired direction (corresponding to the target value in Fig. 7a)).

[0074] Fig. 8 shows a magnetic field generating device 12 with individual magnets 9 arranged as an Aubert arrangement. The individual magnets 9 are each arranged along imaginary circular lines 15, 25 around the center of the circles delimited by the imaginary circular lines 15, 25. As is typical for an Aubert arrangement, the magnetic dipole moments of the individual magnets 9 in the first layer 14 point away from the center of the circle, while the magnetic dipole moments of the individual magnets 9 in the second layer 16 point towards the center. However, in contrast to the previously shown magnetic field generating devices 12, the two layers 14, 16 are arranged rotated relative to one another in the xy plane by the angle α. Individual magnets 9 in the first layer 14 are therefore not arranged congruently with individual magnets 9 in the second layer 16, as seen in the z-direction, but differ in their x and y coordinates.

[0075] Fig. 9a) to c) each show a projection along the z-axis onto the magnetic field generating device designed as an Aubert arrangement. Fig. 9a) shows the case in which the individual magnets 9 of the first and second layers are arranged directly above one another. Fig. 9b) shows a rotated arrangement in which the individual magnets 9a of the first layer 14 have different coordinates in the xy plane than the individual magnets 9b of the second layer 16. In Fig. 9a) and 9b), the two layers 14, 16 have the same number of individual magnets 9.

[0076] Fig. 9c) also shows such a twisted arrangement, but the two layers 14, 16 also differ in the number of their individual magnets 9. In particular, the sum of the magnitudes of the magnetic dipole moments of the respective individual magnets is the same in each layer, ie in the case that the magnetic dipole moments of the individual magnets of a layer all have the same magnitude m, rm = n2m2 applies. The total magnetic dipole moment of the respective layers mi = £ mu and m2 = I m 2i , is equal to 0. Here, mu denotes the respective magnetic dipole moments of the individual magnets 9 in the first layer and m 2i where n2 denotes the respective magnetic dipole moments of the individual magnets in the second layer, and n2 denotes the number of individual magnets in the first and second layers. For this purpose, individual magnets with a smaller magnetic dipole moment can be used for the layer in which a larger number of individual magnets are arranged. For example, if individual magnets with a magnetic dipole moment of m are arranged in the first layer, n2=2 individual magnets with a magnetic dipole moment of m / 2 can be arranged in the second layer.< / pi>

Claims

Claims 1 . Sensor system (1) for detecting a magnetic field parameter of an external magnetic field, which • a NV diamond (2), • a microwave source (3) arranged and designed to generate a microwave field in the NV diamond (2), • an excitation light source (4) arranged and designed to direct an excitation light beam (5) onto the NV diamond (2), • at least one detector (6) arranged and designed to detect an optical signal emitted by the NV diamond (2) as a result of irradiation with the excitation light beam (5), • a magnetic field generating device (12) arranged and designed to generate an internal, static magnetic field that is detectable in the NV diamond (2), characterized in that the magnetic field generating device (12) comprises a multi-layer arrangement of permanent magnets (7) arranged around the NV diamond (2).

2. Sensor system (1) according to claim 1, wherein the multi-layer arrangement of permanent magnets (7) comprises at least one, in particular two, ring magnets, wherein the magnetization direction of the at least one ring magnet varies continuously along the ring circumference 3. Sensor system (1) according to claim 1 or 2, wherein the multi-layer arrangement of permanent magnets (7) in at least one layer comprises at least two, in particular four, individual magnets (9) with a magnetic field different from zero dipole moment, wherein the magnetic dipole moments of immediately adjacent individual magnets (9) point in different directions.

4. Sensor system (1) according to one of the preceding claims, wherein the permanent magnets (7) of the two- or multi-layer arrangement are arranged around the NV diamond according to a Halbach arrangement (11) 5. Sensor system (1) according to one of the preceding claims, wherein the permanent magnets (7) of the two- or multi-layer arrangement are arranged around the NV diamond according to an Aubert arrangement (11).

6. Sensor system (1) according to claim 5, wherein a first layer of individual magnets (9) is arranged rotated relative to a second layer of individual magnets (9).

7. Sensor system (1) according to one of claims 3 to 6, wherein the first layer of individual magnets (9) has a different number of individual magnets (9) than the second layer of individual magnets (9), wherein the sum of the amounts of the magnetic dipole moments per layer is the same.

8. Sensor system (1) according to one of claims 3 to 7, wherein of the individual magnets (9) of the same layer, at least one individual magnet (9) differs from the other individual magnets (9) in the magnitude of its magnetic dipole moment and in its distance from the NV diamond (2).

9. Sensor system (1) according to claim 3 to 8, wherein at least one of the individual magnets (9) has a magnetic dipole moment which extends at an angle between 0° and 90° to at least one of the main axes of a cylindrical, cubic or cuboid-shaped individual magnet (9).

10. Sensor system (1) according to one of the preceding claims, wherein the permanent magnets (7) are designed as cube- or cuboid-shaped individual magnets (9), in particular with an edge length between 0.5 mm and 5 mm.

11. Sensor system (1) according to one of the preceding claims, wherein the permanent magnets (7) are designed as individual magnets (9) and are arranged in a Distance between 2 mm and 200 mm, in particular between 5 mm and 50 mm, from the NV diamond (2).

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