WO2019057243A1

WO2019057243A1 - Device for determining small magnetic fields using at least one squid sensor

Info

Publication number: WO2019057243A1
Application number: PCT/DE2018/100784
Authority: WO
Inventors: Michael Paulsen; Klaus Kiefer; Dennis MEIER; Michael Fechner; Bastian KLEMKE; Martin PETSCHE; Harry LUCHT
Original assignee: Helmholtz-Zentrum Berlin Für Materialien Und Energie Gmbh
Priority date: 2017-09-22
Filing date: 2018-09-14
Publication date: 2019-03-28
Also published as: DE102017122028A1; DE102017122028B4

Abstract

The invention relates to a device for determining small magnetic fields using a SQUID sensor. The device comprises at least one SQUID sensor (1) having a superconducting detection coil (2) and a flow control loop (13), and a cryostat (4) having a sample chamber (5). A section of the cryostat (3), in which the sample chamber is accommodated, is provided with a shielding (6). The sample chamber (5) can be rotated by means of a drive (9) from outside the cryostat (4) via a sample holder (15). A position of the rotation can be determined by means of an optical encoder (11). The cryostat (4) is arranged in a shielding chamber (17). According to the invention, the drive (9) is a drive from a group consisting of a spring drive, a weight drive, a hydrodynamic drive, a pneumatic drive and a thermodynamic drive and is made from non-magnetic materials. The embedding for the superconducting detection coil (2), the section (3) of the cryostat (4), in which the sample chamber (5) is accommodated, and the sample chamber (5) are produced from non-magnetic materials. The shielding (6) of the part of the cryostat (3), in which the sample chamber (5) is accommodated, is produced from superconducting lead and the optical encoder (11) is equipped with a light source made of a glass fiber cable (12).

Description

designation

Device for determining small magnetic fields with at least one SQUID sensor

Technical area

The present invention relates to the field of determination of

Magnetic fields with SQUID sensors (Superconducting Quantum Interference Device, SQUID, superconducting quantum interference detector) used e.g. for the determination of magnetic material properties.

State of the art

Devices for the determination of magnetic fields using SQUID sensors are known in the art.

SQUID sensors are used i.a. used to determine small magnetic fields caused by magnetic properties of a sample. The threshold of commercially available measuring systems using the SQUID sensors and those for the investigation of magnetic

Material properties used is ~ 10 ^"10 T at a distance from the sample that is less than 1 mm apart.

In some applications it is necessary to determine magnetic fields smaller than those mentioned above ~ 10 ^"10 T (small magnetic fields), eg for the determination of the external net magnetization of antiferromagnets Materials of this kind are eg for components the so-called spintronics interesting. One difficulty that generally arises when measuring small magnetic fields is the influence of electromagnetic interference fields. For example, DE 43 35 486 A1 proposes a SQUID sensor in one

To operate shielding chamber with compensation fields. In the

EP 0 584 866 B1 discloses a shielded SQUID magnetometer. EP 0 503 108 B1 discloses a superconductive shielding element for a connection conductor of a SQUID sensor.

However, electromagnetic interference fields are also particularly affected by the devices surrounding the SQUID sensor, e.g. Sample chambers, cryostats, electrical control units and experimenters, in the form of biomagnetic fields, caused.

In essay 1 of DN Astrov and BN Ermakov (Quadrupole magnetic field of magnetoelectric Cr203, JETP Letters, Vol. 59 (4), 1994, pp 297-300) and the essay 2 of DN Astrov et. AI (External Quadrupole Magnetic Field of Antiferromagnetic Cr203, JETP Lett. 63 (9), 1996, pp. 745-751) describes experiments in which very small magnetic fields given by a quadrupole field of an antiferromagnetic material are determined. The experimental implementation of the experiments is disclosed in terms of measuring principle and basic apparatus. The quantitative indication of the magnetic shielding and in particular the sample movement is not disclosed. Conventional electro-mechanical stepper motors and piezo drives generate magnetic signals that negatively impact high-resolution SQUID measurements. How the authors have overcome this difficulty is not mentioned. task

The object of the invention is to specify a device for determining small magnetic fields with at least one SQUID sensor in which interference fields are minimized and which is inexpensive and easy to manufacture.

The object is solved by the features of claim one. Advantageous embodiments of the invention are the subject of the dependent claims.

The at least one SQUID sensor used in the device according to the invention corresponds to a DC-SQUID (DC = direct current) current or measurement amplifier based on microsystem techniques. Such a SQUID sensor includes a flux control loop, a so-called FLL (Frequency Locked Loop) electronics.

In addition, the SQUID sensor is coupled to a superconducting detection coil to detect magnetic flux changes resulting from sample movements. The sensors and FLL electronics are prior art magnetic

Maximum resolution, on the order of ~ 2.2 μΑ / 0ο and better and magnetic noise behavior, on the order of ~ 10o / VHz and better.

The SQUID sensor with the superconducting detection coil and a

Sample chamber for receiving a sample to be measured are arranged in a cryostat. The SQUID sensor is arranged at the sample location in the sample chamber. Cables for controlling and reading the SQUID sensor are inserted and installed in the cryostats with guides shielded with stainless steel (to shield high-frequency electromagnetic signals such as radio waves). The cryostat can, depending on the used SQUID sensor or

Detection coil material, ie, the superconductor used in each case Helium or a nitrogen cryostat. According to the invention, the cryostat is made of non-magnetic materials. Non-magnetic materials in the context of the invention are those which have a magnetic remanence and

Susceptibility on the order of the material PEEK

(PolyEtherEtherKaton), ie <9.2 * 10 ⁷ emu or 1 * 10 ^-7 emu / g. Materials which fulfill this criterion can be found eg in the article 3 by JC Mester and JM Lockhart (Remanent Magnetization of Instrument Materials for Low Magnetic Field Applications, Czechoslovak Journal of Physics, Vol. 46, Suppl. S5, 1996, pp. 2751-2752 ). The area of the sample chamber in the cryostat is superconducting with a

Lead shield, which is cooled to temperatures below the critical temperature of lead before the FLL electronics is turned on. Thus, the magnetic field generated by the FLL electronics is not frozen, but only the very weak background field of the magnetically shielded environment. Thereafter, the superconducting Bleeschirmung protects the sample from additional, external magnetization z. B. by the FLL electronics when it is turned on. Another object of superconducting lead shielding is to prevent the superconducting detection coil from magnetic noise, even in magnetic shielded ones

Environments exists to protect.

The sample is rotatable via a sample holder from outside the cryostat via a drive. A position of rotation of the sample holder is made with an optical encoder associated with the rotation of the sample holder.

The cryostat is placed in an environment shielded by external magnetic fields, a shielding chamber. This can be realized for example by a shielded, walk-in room or from a shield which includes only the cryostat and the structure may be formed. The shield should at least be sufficient to attenuate or reduce the earth's magnetic field to at least one thousandth of its value Background field of at least ~ 10 ^~ 7T in the shielding chamber too

enable.

According to the invention, the apparatus for determining small magnetic fields is optimized in that the core for the superconducting detection coil, the portion of the cryostat in which the sample chamber is housed and the sample chamber in the form of a tube of non-magnetic materials (eg PEEK, nylon or demagnetized materials ) are made. The sample chamber is advantageously made of a plastic and the sample rod made of glass fiber.

The signal provided by the SQUID sensor is ultimately fed to electronic data processing located outside the shielding chamber.

Light for the optical encoder is introduced with fiber optic cables into the shielded experiment environment and also discharged again for signal processing. Thus, the position determination generates no electrical

Signals in the experiment environment.

According to the invention, the drive for the rotation of the sample rod is made of non-magnetic materials and corresponds to one of the group comprising the spring drive, weight drive, hydrodynamic drive, pneumatic drive and thermodynamic drive.

In one embodiment, the drive corresponds to a weight drive, such as that found in pendulum clocks, wherein a weight compensates for the friction loss of the pendulum. The weight as well as the pendulum itself derive the energy, which causes an advancement by the movement of the pendulum, from the gravitational force. The pendulum movement is the actual drive for a signal box (clockwork), the pendulum movement in a rotary or

Linear motion converts. According to the invention, both a weight drive can be used with or without a pendulum, whereby the version with pendulum is to be preferred. The entire drive with weight, pendulum and interlocking is according to the invention of non-magnetic materials such as preferably plastic but also for example made of wood. A drive that a

Using pendulum motion is about optimizing weight,

Pendulum characteristics (such as length, shape, weight, mounting of the axis of rotation) and the translation in the interlocking extremely precise and vibration-less feasible. The reduction of vibration, magnetic background increases the precision of measurements with SQUID sensors.

Similar to the drive, as it corresponds to a pendulum clock, is also another drive, which is known in particular from the watch technology, the spring drive, according to the invention usable.

A pneumatic drive using compressed air as an energy source is realized via pneumatic cylinders, corresponding to a piston engine, as it corresponds to a further embodiment. The up and down movement or rotational movement of the piston is thereby converted into a rotational or linear movement. The entire drive is also according to the invention here

non-magnetic materials, such as preferably plastic but also e.g. made of wood or ceramic.

According to the pneumatic drive and a hydrodynamic drive can be used.

The drives can be made particularly advantageous with the 3D printing technology and thus adapt to all conditions.

Another drive according to the invention is a thermodynamic drive, as z. B. is given in a Stirling engine. The cold or warm sources are to be arranged outside the shielding chamber. In particular, by the inventive design of the drive made of non-magnetic materials, the drives get along in particular without the use of electrical energy or fuels, the interference fields in the device for determining small metallic fields are advantageously greatly reduced. The remaining embodiments of the optical position decoding and choice of materials for the cryostat and the sample environment further advantageously reduce the influence of noise fields on the highly sensitive SQUID sensors and the samples to be measured. B. comes in conventional SQUID systems solely by the materials used or the earth's magnetic field, among other things, may lead to corrupted signals in certain antiferromagnetic samples. Thus, the proposed design allows the measurement of magnetic material classes, in particular a direct measurement of the magnetic far fields of antiferromagnets, which could not be developed previously.

embodiment

The invention will be explained in more detail in an embodiment and with reference to 4 figures.

The figures show:

Fig. 1: Overview of a device according to the invention for the determination of small magnetic fields.

Fig. 2: drive with gravity as an energy source.

Fig. 3: drive with compressed air as an energy source.

FIG. 1 shows an overview of the structure of a device according to the invention for determining small magnetic fields, as well as a few other components. A SQUID sensor 1 with superconducting Detection coil 2 is arranged in the lower part 3 of a cryostat 4, in which the sample chamber 5 is located. The lower part of the cryostat 3 is equipped with a shield 6. The cryostat 4 has a closure 7 in which a filler neck 8 for cryogenic gases (helium, nitrogen) is integrated. The sample holder 15 is located in a tube 16 which is connected to the lower part 3 of the cryostat 4 and via a

Helium seal 19 features. The sample holder 15 is rotated by a drive 9, which is fed with a physical energy source. On the sample holder 15, a coding disc 10 is arranged on the optically with an optical encoder 1 1 an angle-dependent position of the

Sample holder 15 can be determined. For the function of the optical encoder 1 1 light is conducted with a fiber optic cable 12 in and out of the shielding chamber 17. For signal stabilization of the SQUID sensor 1 FLL electronics 13 is used.

The signals of the SQUID sensor can be processed outside the chamber 17 with a data processing system 18. The light signals from the optical encoder 1 1 can also be detected outside the chamber with a light sensor 14.

In Fig. 2, a drive with gravity as an energy source is shown. The embodiment shown utilizes a weight A and a pendulum B. The weight A is moved by gravity downwards, whereby an axis C is set in rotation, which is transmitted to the pendulum B with gears D. The pendulum movement in turn is transmitted through a signal box on the sample holder 15 (not shown here), so that this along its

Longitudinal axis turns. With a hand crank E can weight A with

Muscle power can be fed back to potential energy. Instead of

Actuation of the crank E with muscle power, other drives are conceivable here, provided that they are arranged outside the shielding chamber 17, such as electric motors. In Fig. 3, a drive with compressed air as an energy source in the embodiment is shown as a piston machine with pneumatic cylinders i. Compressed air is supplied via the openings ii to the cylinders i, whereby, through the decompression, the pistons iii are moved downwards and this movement is converted on a crankshaft iv into a rotational movement which sets an axis v in rotation. This rotation is then transmitted through an interlocking to the sample holder 15 (not shown here) so that it rotates along its longitudinal axis.

In the embodiment, the drive 9 corresponds to a drive with

Gravity as an energy source as shown in FIG. 2. The drive is completely composed of plastic parts for building block plug-model, as they are known as children's toys, composed. The weight A is formed from a PET bottle filled with water. The weight can be adjusted very precisely by the degree of filling with water.

The SQUID sensor 1 is a microsystem-based, single-stage, DC-SQUID (direct current) current amplifier, which is known in the art or available as a commercial product.

The structure of the device according to the invention in the embodiment is made individually and corresponds to that shown in FIG. In this case, the SQUID sensor 1 is surrounded by a small niobium-brass shield and connected to the FLL electronics 13. The cryostat 4 is realized by a Dewar.

In the lower part 3 of the formed of rigid PVC cryostat 4 is the

Sample chamber 5 in the form of a tube made of plastic. A guide for the sample holder 15 is provided on one side with a helium seal 19 and connected on the other side with the sample chamber 5 and thereby formed from a double-walled steel tube 16, in which the cables are guided from and to the SQUID sensor 1 , The superconducting Detection coil 2 is located in an embedding means of PEEK

(PolyEtherEtherKaton). The sample holder 15 is formed of glass fiber. The area around the sample chamber 5 in the cryostat 4 is provided with a superconducting lead shield 6.

The shielding chamber 17 is a passive shielded space, BMSR-1

(BMSR = Berlin Magnetically Shielded Room) of the Physikalisch Technische Bundesanstalt, which is made available to external users. A

A description of the space can be found in the article 4 by J. Bork et al., The 8-layered Magnetically Shielded Room of the PTB: Design and Construction, Proceedings 12 ^th International Conference on Biomagnetism, Edited by J. Nenonen et al. Pp. 970-973).

The encoder disk is a black and white printed film that has been laminated. The fiber optics and lenses are commercially available products.

With the device according to the invention succeeded in an external

To determine quadrupole field of a Cr2Ü3 sample, as in the article 5 of I. Dzyaloshinskii (External magnetic fields of antiferromagnets, Solid State Communications, Vol. 82 (7), 1992, pp 579-580) is postulated. In addition, magnetic spin spirals of an antiferromagnetic phase of TbMn03 could be successfully measured. Magnetic spin spirals have heretofore been able to be used only with e.g. Neutron scattering experiments or

Synchrotron radiation can be characterized. The inventive

Device is a viable and cost effective alternative or

Supplement to the measurement of antiferromagnetic structures with

Large appliances. The estimated magnetic field resolution of

Device according to the invention is better than 10 ^{~ 13} T at a distance of 1 mm from the sample, ie several orders of magnitude more accurate than conventional. The particular suitability of the device for the determination of small magnetic fields is thus demonstrated.

Claims

claims

1 . Device for determining small magnetic fields with a SQUID sensor (1), comprising at least a SQUID sensor (1) with a superconducting detection coil (2) and with a flux control loop (13), a cryostat (4) with a sample chamber (5), wherein a portion of the cryostat (3) in which the sample chamber is housed is provided with a shield (6) and wherein the sample chamber (5) is rotatable via a sample holder (15) from outside the cryostat (4) via a drive (9) and a position of the rotation via an optical encoder (1 1) can be determined and the cryostat (4) is arranged in a shielding chamber (17),

characterized in that

- The drive (9) one of the group spring drive, weight drive, hydrodynamic drive, pneumatic drive and

thermodynamic drive is and non-magnetic

Materials is made,

- An embedding for the superconducting detection coil (2), the

Section (3) of the cryostat (4) in which the sample chamber (5) is accommodated and the sample chamber (5) are made of non-magnetic materials,

- The shield (6), the part of the cryostat (3) in which the

Sample chamber (5) is housed, is made of superconducting lead and

- The optical encoder 1 1 with a light source from a

Fiber optic cable 12 is equipped.

2. Apparatus according to claim 1,

characterized in that

the drive (9) is designed as a weight drive with pendulum.

3. Apparatus according to claim 1, characterized in that

the drive (9) is designed as a pneumatic drive.