Research on extremely weak remanence measurement system

To accurately obtain the extremely weak magnetic field information of the test piece, this article first introduces the basic principles of the extremely weak residual magnetic field measurement technology. Then, an ultra-high precision magnetic shielding system has been developed, which is made of high permeability materials and equipped with a demagnetization coil system, and it can regularly eliminate the inductance of the entire shielding system. The effective shielding inner diameter of the magnetic shielding system is 0.8 m, the length is 1.6 m, and the magnetic field resolution is 0.0015 nT. This system can provide strong support for the measurement of extremely weak magnetic fields in materials and biomedical magnetic research.


Introduction
The Earth itself is magnetic, so there is a magnetic field in the Earth and near Earth's space.This magnetic field is called Earth's magnetic field [1], and the value of Earth's magnetic field is about 0.5×10 -4 T, its strength and direction also vary with location.The Earth's magnetic field can be approximated as the shape of a magnetic field generated by placing a magnetic rod at the center of the

Basic principles
Magnetic shielding refers to putting two mediums with different permeability into the magnetic field, and the magnetic field will change suddenly at their interface.At this time, the size and direction of Magnetic flux density B will change, that is, it will cause the refraction of the magnetic induction line.When the magnetic induction line enters iron from the air, the deviation of the magnetic induction line from the normal is significant, resulting in a strong convergence effect and the formation of a magnetic shield.
The basic principle of magnetic field shielding [5][6][7][8] is to use a cavity wall made of highpermeability materials and its surrounding air as a parallel magnetic circuit.The magnetic field lines are concentrated inside the magnetic shielding body with low magnetic resistance, as shown in Figure 1.Due to the relative permeability of the air medium approaching 1, and the relative permeability of the shielding shell being at least a few thousand, Rm=l/uS, where Rm is the magnetic resistance, l is the length of the magnetic circuit, u is the magnetic permeability, and S is the cross-sectional area of the magnetic circuit.So the magnetic resistance Ro of the air medium is much greater than the magnetic resistance Rm of the shielding shell.At this time, the magnetic flux mainly forms a circuit along the path of low magnetic resistance.Due to the small magnetic resistance of the shielding shell, when the shielding shell is placed in a magnetic field, the magnetic flux will mainly pass through the shielding shell, and the magnetic flux passing through the air will be greatly reduced, thus playing a role in magnetic field shielding.In practice, achieving complete shielding is extremely difficult.There is always some magnetic field that needs to leak into or run out of the shield.To achieve a better shielding effect, it is necessary to choose materials with high magnetic conductivity, such as permalloy, silicon steel sheet, etc., and not too thin.The structural design of the shielding cover should have as few joints as possible, and the joints should be tight during production to minimize air gaps.In short, the smaller the magnetic resistance of the shield is, the better the shielding effect is.If shielding is required in low-frequency alternating magnetic fields, such as in power transformers, the above principles of magnetic shielding are followed.When the shielding requirements are high, multi-layer shielding can also be used.

System development
Because magnetic shielding cannot completely seal the magnetic flux inside the shielding body, multilayer shielding can be used to solve the problem of magnetic leakage, which can greatly improve the shielding effect.At the same time, to reduce the impact of mechanical stress on the permalloy layer, it is necessary to add a layer of hard aluminum to the inner and outer layers of the magnetic shielding cylinder, which can effectively reduce the mechanical stress borne by the permalloy layer on the magnetic shielding cylinder itself, as well as the shielding coil and the tested instrument.The size design of the shielding cylinder in this article is inner cavity ĳ 800 mm×1600 mm.At the same time, a layer of hard aluminum is added to the inner and outer layers of the magnetic shielding cylinder to effectively reduce the mechanical stress borne by the permalloy layer on the magnetic shielding cylinder itself, as well as the shielding coil and the tested instrument.The main structure is designed as a horizontal structure, with one end sealed at the bottom and the other end as a movable cover.The movable cover is a full buckle type, with measurement holes and wiring holes left at the movable cover end.Both the magnetic shielding cylinder and the movable cover are equipped with support and movement mechanisms, which bear a weight of about 1 T and can achieve the support and horizontal movement of the magnetic shielding cylinder, ensuring that the center height of the shielding cylinder is about 1 m.
For shielding materials, the type of material chosen has a significant impact on their performance and cost.When designing shielding materials, it is important to have a deep understanding of the characteristics of different shielding alloys that are commonly used.Understanding these different properties can enable you to choose suitable materials to meet the target requirements.Magnetic shielding materials should be selected based on their respective characteristics, especially magnetic permeability and magnetic saturation performance.Due to its effectiveness in changing the direction of low-frequency magnetic fields, high-permeability materials (such as Mumetal, a high-permeability iron-nickel alloy containing 80% nickel) are commonly used as shielding materials.
The development of formulas and models for most shielding materials is based on the geometric shape of circular or infinitely long cylinders.In practical applications, the practical shape of the given shielding body is determined by the device structure and the available space of the shielding body itself.When designing a shielding body, it is important to understand that it is difficult to rotate the magnetic field lines by 90°.However, it is easier to change the direction of the magnetic field lines of a circular shield, such as a cylinder or a box with circular angles, than a shield with square angles.Similarly, for magnetic field lines that have entered the shielding material and changed their direction, rounded corners are better than sharp corners.It is important to maintain a simple shape of the shield that can provide a low magnetic resistance path or a "minimum magnetic resistance path" for magnetic field movement.The shielding body should be close to all walls to avoid field leakage.This structure (even rectangular) is the closest to a circle, and it can establish a semi-closed magnetic circuit.In addition, all boxes can obtain shielding characteristics on all axes, which can ensure the best shielding performance.When special performance and import/export requirements arise, movable covers, covers, and doors can be combined into the shielding body design.The magnetic shielding cylinder is shown in Figure 2.  From the simulation results, it can be seen that when a magnetic field of 0.338669 Vxs/m 2 is applied externally, the multi-layer shielding cylinder has a good shielding effect.The magnetic field is mainly concentrated on the shell of the shielding cylinder, with a maximum value of 0.338669 Vxs/m 2 .In the zdirection, the magnetic field is mainly concentrated outside the shielding cylinder, with an internal magnetic field of almost zero.In the y-direction, the magnetic field is mainly concentrated outside the shielding cylinder, with an internal magnetic field of almost zero.In the x-direction, the magnetic field is mainly concentrated outside the shielding cylinder, with an internal magnetic field of almost zero.Through simulation calculation, the internal magnetic field is 7×10 -15 T~1.2×10 -14 T, with extremely high uniformity.

Test verification
At present, the fluxgate magnetometer is a common instrument for measuring weak static magnetic fields and low-frequency vector magnetic fields.The relatively mature fluxgate magnetometer has a measurement range of -100000 nT to 100000 nT, and a resolution of 0.01 nT.However, the product materials measured in this system are less than 0.01 nT, so the fluxgate magnetometer is difficult to meet the requirements.Based on the efficient magnetic shielding system, this article has installed a zero field atomic magnetometer on the non-magnetic translation mechanism, sensitivity<15 fT/Ĝ Hz in 3-100 Hz band, size 15 mm×10 mm×5 mm, can exhibit extremely high sensitivity when the magnetic field background is small.
1) We open the cover of a section of the shielding cylinder and install the zero fields atomic magnetometer on the nonmagnetic translation mechanism inside the system; 2) We close the lid and run the zero fields atomic magnetometer to collect data; 3) We record the data.
The test results are shown in Figure 4.  From the test results, it can be seen that the radial magnetic field is 11.8 pT to 12.8 pT, with a maximum fluctuation of 1 pT; the axial magnetic field is -42.2 pT to -43.0 pT, with a maximum fluctuation of 0.8 pT.The magnetic field value is stable and the shielding effect is good.

Conclusion
This article develops an extremely weak residual magnetic measurement system, with the effective space of the entire shielding system being ĳ 800 mm×1600 mm, and the magnetic field resolution is 0.001 nT.This system provides a strong guarantee for the accurate measurement of magnetic fields in aerospace, new material research, biomedical engineering, and other fields.
(a) Radial magnetic field fluctuation value (b) Axial magnetic field fluctuation value