EPR SPECTROMETER WITH AT LEAST ONE POLE PIECE MADE AT LEAST PARTIALLY OF A FUNCTION MATERIAL

Abstract

An electron paramagnetic resonance (EPR) spectrometer includes a magnet system comprising at least one magnet and at least one pole piece for producing a magnetic field along a pole axis in a field of view in front of the at least one pole piece. A probe head comprising a microwave resonator and at least one modulation coil or rapid scan coil produces an additional, time-varying magnetic field aligned along the pole axis. The probe head is arranged in the field of view, and a respective modulation coil or rapid scan coil is arranged between the microwave resonator and a respective pole piece. For each pole piece, at least a part of said pole piece is made of a function material having an electric conductivity σ.sub.f of 10.sup.4 S/m or less, and having a saturation magnetic flux density BS.sub.f of 0.2 T or more.

Claims

1. An electron paramagnetic resonance (EPR) spectrometer comprising: a magnet system comprising at least one magnet and at least one pole piece for producing a magnetic field in a field of view in front of the at least one pole piece, said magnetic field being generated along a pole axis; and a probe head comprising a microwave resonator and at least one modulation coil or rapid scan coil for producing an additional, time-varying magnetic field aligned along the pole axis, wherein the probe head is arranged in the field of view, and wherein a respective modulation coil or rapid scan coil is arranged between the microwave resonator and a respective pole piece; wherein for each pole piece having a modulation coil or rapid scan coil arranged between said pole piece and the microwave resonator, at least a first part of said pole piece facing said modulation coil or rapid scan coil is made of a function material having an electric conductivity σ.sub.f of 10.sup.4 S/m or less, and having a saturation magnetic flux density BS.sub.f of 0.2 T or more.

2. An EPR spectrometer according to claim 1, wherein σ.sub.f is 10.sup.3 S/m or less, and that BS.sub.f is 0.5 T or more.

3. An EPR spectrometer according to claim 1, wherein the function material is isotropic.

4. An EPR spectrometer according to claim 1, wherein the function material comprises particles of ferromagnetic or ferrimagnetic material dispersed in an electrically insulating matrix material.

5. An EPR spectrometer according to claim 4, wherein the function material comprises soft magnetic particles of a ferrite or a metal or a metal alloy dispersed in a polymer matrix material

6. An EPR spectrometer according to claim 1 wherein, for each pole piece having a modulation coil or rapid scan coil arranged between said pole piece and the microwave resonator, said pole piece is completely made from the function material.

7. An EPR spectrometer according to claim 1 wherein, for each pole piece having a modulation coil or rapid scan coil arranged between said pole piece and the microwave resonator, the first part of said pole piece facing the modulation coil or rapid scan coil is made from the function material, and a remaining part of the pole piece is made from a different material.

8. An EPR spectrometer according to claim 7, wherein said different material is a metallic material.

9. An EPR spectrometer according to claim 7, wherein the first part of the pole piece has a thickness T, measured along the pole axis, with T≥0.5 mm, and/or with T≤12.0 mm.

10. An EPR spectrometer according to claim 7, wherein the first part of the pole piece at least partially overlaps with the complete modulation coil or rapid scan coil in a plane perpendicular to the pole axis.

11. An EPR spectrometer according to claim 7, wherein the first part of the pole piece extends beyond the modulation coil or rapid scan coil in the plane perpendicular to the pole axis.

12. An EPR spectrometer according to claim 7 wherein the first part of the pole piece is an insert held by press fit in a frame structure of the remaining part of the pole piece.

13. An EPR spectrometer according to claim 7 wherein the first part of the pole piece is glued onto the remaining part of the pole piece.

14. An EPR spectrometer according to claim 7, wherein the remaining part of the pole piece, but not the first part of the pole piece, comprises one of a plurality of openings through which a respective screw projects, with the screw being screwed into a yoke structure or a permanent magnet of the magnet system.

15. An EPR spectrometer according to claim 1, wherein each modulation coil or rapid scan coil is fixed to the pole piece it faces.

16. An EPR spectrometer according to claim 1, wherein the EPR spectrometer comprises a pair of mutually opposing pole pieces and the field of view has a width WAG along the pole axis, with 10 mm≤WAG≤100 mm.

17. An EPR spectrometer according to claim 1 wherein, for a distance DPC along the pole axis between a respective pole piece and a modulation coil or rapid scan coil it faces, the following applies: 0≤DPC≤2.0 mm.

18. An EPR spectrometer according to claim 1, wherein said pole piece is one of a pair of pole pieces.

19. Use of an EPR spectrometer according to claim 1, wherein a sample is arranged in the microwave resonator, wherein the magnet system generates a magnetic field in the field of view along the pole axis, and wherein the at least one modulation coil or rapid scan coil generates a time-varying, additional magnetic field in the field of view along the pole axis.

20. Use of an EPR spectrometer according to claim 19, wherein the time-varying, additional magnetic field has a frequency between 5 kHz and 200 kHz.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0053] FIG. 1 shows a schematic cross-section of a first embodiment of an inventive EPR spectrometer, having a pair of modulation coils.

[0054] FIG. 2 shows a schematic cross-section of a second embodiment of an inventive single-sided EPR spectrometer, having one pole piece and a single modulation coil.

[0055] FIG. 3a shows a schematic cross-section of a pole piece for the invention, attached to a yoke structure by gluing, with the pole piece being completely made from the function material.

[0056] FIG. 3b shows a schematic cross-section of a pole piece for the invention, attached to a yoke structure by screwing, with the pole piece comprising a part of function material glued to a remaining part of the pole piece made of iron.

[0057] FIG. 3c shows a schematic cross-section of a pole piece for the invention, attached to a yoke structure by screwing, with the pole piece comprising a part of function material clamped into a recess of a remaining part of the pole piece made of iron.

[0058] FIG. 4a shows a schematic cross-section of the field of view area of an inventive EPR spectrometer, with a pair of modulation coils arranged at a short distance from the parts of function material of the pole pieces.

[0059] FIG. 4b shows a schematic cross-section of the field of view area of an inventive EPR spectrometer, with a pair of modulation coils directly attached to the parts of function material of the pole pieces.

[0060] FIG. 5a shows a schematic cross-section of the field of view region of an EPR spectrometer if iron metal pole pieces are used with short distance to the pole pieces (different from the invention).

[0061] FIG. 5b shows a schematic cross-section of the field of view region of an EPR spectrometer if iron metal pole pieces are used with long distance to the pole pieces (state of the art).

[0062] FIG. 6 shows a diagram illustration of the impedance (Ohms) vs. excitation frequency, and the resonance frequency in a circuit comprising a modulation coil and a capacitor, applying neighboring conventional iron metal pole pieces (dashed curves) and pole pieces made of function material in accordance with the invention (full curves), for different distances of the modulation coil from a pole piece; the various curve labels indicate the distance separating the coil to the pole material.

[0063] FIG. 7 shows a diagram illustrating the Q-factor (indicative of losses in pole material) of the modulation coil of the resonant circuits of FIG. 6 using conventional iron metal pole pieces (dashed curves) and pole pieces made of function material (full curves) as a function of the distances of the modulation coil from the pole piece.

[0064] FIG. 8 shows a schematic cross section of a section of a part of a pole piece for the invention, made from the function material.

DETAILED DESCRIPTION

[0065] FIG. 1 schematically illustrates a first embodiment of an inventive EPR spectrometer 1. The EPR spectrometer 1 comprises a magnet system 2 and a probe head 3.

[0066] The magnet system 2 here comprises three magnets 4, namely a permanent magnet 5 and two electromagnets 6a, 6b. The magnet system 2 further comprises a yoke structure 7 made from a ferromagnetic material, typically based on an iron alloy, and having roughly a C shape, for guiding and enhancing the magnetic flux generated. The yoke structure can alternatively be shaped as a window frame, a pot core or any other suitable structure (not shown). The electromagnets 6a, 6b or their windings, respectively, are arranged on the upper and lower arm of the yoke structure 7 here. At the opposing ends of the yoke structure 7, in FIG. 1 on the right-hand side, the yoke structure 7 is equipped with pole pieces 8a, 8b.

[0067] The permanent magnet 5 is here integrated into the back (in FIG. 1 on the left-hand side) of the yoke structure 7.

[0068] The magnet system 2 generates a magnetic field (also called background magnetic field or static magnetic field or B0 field) in a field of view 9 between the pole pieces 8a, 8b, which corresponds here to an air gap between the mutually opposing pole pieces 8a, 8b. The magnetic field in the field of view 9 is aligned with a pole axis PA, which runs here vertically and perpendicular to the surfaces 12a, 12b of the pole pieces 8a, 8b facing the field of view 9 (“inner surfaces” 12a, 12b). The electromagnets 6a, 6b may be used to slowly alter (or “sweep”) the magnetic field during an EPR measurement by appropriate electric current control; accordingly, the electromagnets 6a, 6b are also referred to as sweeping coils.

[0069] In the field of view 9, there is arranged the probe head 3, which here comprises a microwave resonator 10, a pair of modulation coils 11a, 11b, and here also a housing 13 containing the microwave resonator 10 and the modulation coils 11a, 11b. Inside the microwave resonator 10, which has metallic walls, a sample volume 14 for arranging a sample 14a to be measured is arranged. Microwave radiation (also called B1 field) can be coupled into and out of the microwave resonator 10, typically via a waveguide attached to a microwave source and a microwave detector (not shown here, for simplification).

[0070] In the embodiment shown, between each of the pole pieces 8a, 8b and the microwave resonator 10, there is arranged one of the modulation coils 11a, 11b. The pair of modulation coils 11a, 11b may generate an additional, time-varying magnetic field along the pole axis PA, at least in the sample volume 14.

[0071] Further, in the embodiment shown, a part 15a, 15b of each pole piece 8a, 8b facing a modulation coil 11a, 11b is made from a function material (illustrated with a dotted shading) having a low electric conductivity (e.g., with σ.sub.f=10.sup.2 S/m) and having a high saturation magnetic flux density (e.g. with BS.sub.f of 0.5 T).

[0072] The parts 15a, 15b of the function material prevent or at least minimize the induction of eddy currents in the pole pieces 8a, 8b caused by the time-varying magnetic fields generated by modulation coils 11a, 11b. Note that induced eddy currents would generate not only Ohmic losses, decreasing the efficiency of LC tank, but also magnetic fields opposed to the time-varying magnetic field that caused them, hence decreasing coils' inductance. As a double consequence of the application of the function material, since losses caused by the eddy currents in pole materials are prevented or minimized and only the inductance of coils is favorably increased, the electric currents in the modulation coils 11a, 11b can be chosen relatively small in order to achieve a desired strength of the time varying magnetic field at the EPR sample position, what reduces power consumption with a factor of square. The first important result is that the acoustical vibrations energy, due to currents in the coils 11a, 11b, and the Ohmic loss as thermal energy in the pole pieces 8a, 8b and the coils 11a, 11b are reduced as well, which lead to a favorable decreasing the noise of the EPR measurements on the sample 14a. The second important result is that the modulation coils 11a, 11b can be placed close to the pole pieces 8a, 8b in a compact design of the EPR spectrometer 1, and obtain a higher performance vs. cost score.

[0073] It should be noted that instead or in addition to the modulation coils 11a, 11 b, also rapid scan coils can be employed, in accordance with the invention; the above description applies in analogous way then.

[0074] In FIG. 2, a second embodiment of an inventive EPR spectrometer 1 is shown, which is a single sided EPR spectrometer. This embodiment comprises a magnet system 2 generating a magnetic field in the probe head 3 from only one side of the field of view 9, here from the top side. An arrangement of a magnet system 2 on only one side of the probe head 3 is useful in very compact EPR devices such as chip-based EPR sensors.

[0075] In the embodiment of FIG. 2, the probe head 3 comprises only one modulation coil 11a, here arranged above the microwave resonator 10. In this case, it is sufficient to equip the only one pole piece 8a arranged on the top side with the part 15a of function material. The magnet system 2 can be comprised of an electromagnet or a permanent magnet combined with a sweep coil for CW applications.

[0076] It is however possible to arrange a second pole piece (not shown) on the opposite side of pole piece 8a which is made entirely of conventional iron metal material which has a high electric conductivity (>>10.sup.4 S/m). The arrangement of a second pole piece will increase the magnetic field homogeneity in the field of view 9. It is also possible that the magnet system 2 is equipped with a yoke structure for returning the magnetic field to the magnet. Said opposite second pole piece (not shown) is separated from the modulation coil 11a in the direction of the pole axis PA at least by the thickness of the microwave resonator 10, so the local strength of the time varying magnetic field generated by the modulation coil 11a is generally much lower at the surface of the opposite second pole piece as compared to the surface 12a of the pole piece 8a.

[0077] FIG. 3a illustrates a first type of pole piece 8a for use with the invention, for example in an EPR spectrometer as shown in FIG. 1. In the type shown, the pole piece 8a is entirely made from the function material (illustrated with a dotted shading, as used in other figures). The pole piece 8a is here glued to an end of the yoke structure 7.

[0078] FIG. 3b illustrates a second type of a pole piece 8a for use with the invention, for example in an EPR spectrometer as shown in FIG. 1. Here, the pole piece 8a comprises a part 15a made from the function material, and a remaining part 16 (also called holder) is made from a soft magnetic metal material, here iron. The part 15a made of the function material is glued onto the remaining part 16. The remaining part 16 of the pole piece 8a comprises an opening 17 through which a screw 18 projects. The screw 18 is screwed into a thread 19 located in a yoke structure 7.

[0079] FIG. 3c illustrates a third type of a pole piece 8a for use with the invention, for example in an EPR spectrometer as shown in FIG. 1. Here, the pole piece 8a comprises a part 15a made from the function material, and a remaining part 16 (also called holder) is made from a soft magnetic metal material, here iron. The remaining part 16 forms a frame structure 20, here with a recess having a circumferential edge, into which the part 15a has been pressed. Accordingly, the frame structure 20, which can be spread elastically to some degree, exerts some pressure onto the part 15a in a clamping direction 21, which is perpendicular to the pole axis PA. The remaining part 16 of the pole piece 8a again comprises an opening 17 through which a screw 18 projects. The screw 18 is screwed into a thread 19 located in a yoke structure 7. The part 15a covers here the screw 19 and the opening 17.

[0080] FIG. 4a illustrates the area of the field of view 9 of an inventive EPR spectrometer by way of example, similar to the embodiment shown in FIG. 1. In the example shown, in the direction of the pole axis PA, there is a non-zero distance DPC between the pole piece 8a and the modulation coil 11a it faces. Typically, this distance DPC is 2 mm or less. The distance DPC can be measured between the flat surface 12a of the pole piece 8a or its part 15a, and the part of the modulation coil 11a projecting farthest towards the pole piece 8a. Further, the part 15a made of the function material has a thickness T, measured along the pole axis PA. The thickness T is typically in the range of 1 mm through 6 mm. Such a thickness T is enough to reliably block eddy currents in the pole piece 8a.

[0081] It should be noted that in the direction perpendicular to the pole axis PA, the part 15a reaches beyond the modulation coil 11a, and here also reaches beyond the microwave resonator 10. The width WAG of the field of view 9 (i.e., here of the air gap between the pole pieces 8a, 8b) along the pole axis PA is typically in a range of 10 mm≤WAG≤100 mm, preferably in a range of 20 mm≤WAG≤60 mm.

[0082] FIG. 4b illustrates the area of the field of view 9 of an inventive EPR spectrometer in another of example. In the example shown, the modulation coils 11a, 11b are directly attached to the pole pieces 8a, 8b. In other words, the distance along the pole axis PA between the pole pieces 8a, 8b and the modulation coils 11a, 11b they are facing is zero. This results in a very compact magnet system design.

[0083] As another particularity, in the example shown, on the back side of each part 15a, 15b made of function material, there is attached a foil 22 of a highly conductive material, such as copper, for example with an electrical conductivity of 10.sup.7 S/m or more. In this way, remaining eddy currents in the pole piece 8a can be trapped in a defined way, in particular located at a distance relatively far from the modulation coils 11a, 11b. Therefore, in general, a layer of highly conductive material having an electric conductivity of 10.sup.7 S/m or more can be arranged on a side of at least the part 15a, 15b of the pole piece 8a, 8b facing away from a modulation coil 11a, 11b or rapid scan coil, in accordance with the invention, in particular wherein the layer comprises a foil 22 made e.g., of copper.

[0084] In FIG. 5a, the region of the field of view of an EPR spectrometer similar to the one shown in FIG. 1 is shown, wherein the pole pieces 108a, 108b are not equipped with function material in accordance with the invention, but conventional iron material pole pieces 108a, 108b are used. The magnet system 102 generates a background field B0, which is to be superimposed by a time varying magnetic field 123 generated by the modulation coils 111a, 111b.

[0085] When using conventional iron material pole pieces 108a, 108b at the ends of a magnetic system 102, the time-varying magnetic fields 123 generated by the modulation coils 111a, 111b induce eddy currents 124 in the pole pieces 108a, 108b nearby. The eddy currents 124 generate local magnetic fields which are opposed to the time-varying magnetic field 124, so the electric current strength in the modulation coils 111a, 111b must be increased in order to achieve a desired field strength at least in the sample volume 114. Further, the pole pieces 108a, 108b become warm by Ohmic heating, and the heat may spread over the entire setup, in particular into the modulation coils 111a, 111b and the microwave resonator 110, detuning them.

[0086] In order to avoid the eddy currents in the pole pieces 108a, 108b, one may increase the width WAG′ of the field of view 109 and locate the pole pieces 108a, 108b at a larger distance DPC′ from the modulation coils 111a, 111b, as shown in FIG. 5b. However, in order to achieve the same magnetic field strength in the field of view 109 as before, it is then in general necessary to use a larger magnetic system 102 than before, which increases the costs for the EPR spectrometer considerably.

[0087] In order to demonstrate the benefits of the present invention, a resonant circuit with a capacitor and a modulation has been prepared, and the modulation coil has been brought into the vicinity of a conventional iron metal material pole piece or a pole piece made (entirely) from function material, as it is an option of the invention. As function material, an epoxy resin with non-percolating iron metal particulate inclusions has been used (electrical resistivity 0.5*10.sup.4 Ohm*cm, saturation magnetic flux 2.03 T).

[0088] FIG. 6 plots the measured impedance of the resonant circuit at the upright axis (in Ohms) as a function of the excitation frequency in kHz on the rightward axis. The dashed lines show the results using the conventional iron pole piece, and the full lines show the results for the pole piece made of the function material in accordance with the invention. The distance of the modulation coil to the pole piece was 0 mm for the lines marked with diamonds, 2.5 mm for those marked with squares, 4 mm for those marked with circles, and 6 mm for those marked with triangles.

[0089] As can be seen in FIG. 6, for the iron pole piece, the resonance curves (on the right side) become flatter when the distance becomes smaller. This is a strong sign of increasing dampening by eddy currents in the iron pole piece when the distance becomes smaller. On the other hand, for pole piece made of function material (on the left side), the resonant curves are all sharp and practically independent from the distance.

[0090] The inductances of the modulation coils are influenced by the material nearby. Material allowing considerable eddy currents located nearby tends to lower the inductance, whereas material with ferromagnetic permittivity tends to increase the inductance. For the experiment with iron pole piece, these two effects cancelled each other out to a large extent, whereas for experiments with pole pieces equipped with the function material, the inductance was significantly increased. As a result, the resonance frequencies for the experiments using pole pieces with the function material are significantly lower as compared to the experiments with the iron pole pieces (at about 115 kHz for the conventional pole piece and about 90 kHz for the inventive pole piece). Then the Q-factors were determined for the resonant curves of FIG. 6. A low Q-factor directly indicates a strong dampening and vice versa.

[0091] In FIG. 7, the Q-factor (dimensionless value) is plotted to the vertical axis, and the horizontal axis plots the distance (in mm) of the modulation coil to the pole piece in each case. The Q-factor can be determined with Q=f0/Δf, with f0: resonance frequency (location of the maximum amplitude in the resonance curve) and Δf: width of the resonance curve at an attenuation of 3 dB with respect to the maximum. For the conventional Fe pole piece setup (dashed line), the Q-factor is strongly dependent on the distance, and increases from about 2.5 at distance zero to about 6.5 at a distance of 6.5 mm. In contrast, for the pole piece of the function material, the Q-factor is roughly constant between 7.5 and 8 for all investigated distances.

[0092] FIG. 8 illustrates the function material in a part 15a of a pole piece, comprising a plurality of particles 23 of a ferromagnetic or ferrimagnetic material, e.g., iron, distributed in an electrically insulating matrix material 24, e.g., made from a polymer, such as polymeric resin. The particles 23 do not percolate, i.e., they do not build chains of touching particles 23 but, rather, are isolated from each other by the matrix material 24. Accordingly, the electrical conductivity of the matrix material 24 dominates the electric conductivity of the function material. However, the particles 23 may contribute to the overall magnetic permeability of the function material, proportional to their proportion in the function material. With the particles 23 being distributed homogenously in the matrix material 24, the function material exhibits isotropic electric and magnetic properties.