RU2118834C1 - Device measuring weak magnetic fields ( versions ) - Google Patents

Abstract

FIELD: measurement of weak magnetic fields, specifically magnetic anomalies, representation of functions of brain, prospecting for minerals, measurement of weak currents. SUBSTANCE: device incorporates sensitive element made in the form of conductor from ferromagnetic material surrounded by inductance coil. Conductor is connected in series to A C source. Inductance coil is connected to voltmeter across its ends. Conductor of sensitive element has magnetic anisotropy perpendicular to its longitudinal axis and magnetic structure in skin layer with resulting magnetic moment directed perpendicular to its longitudinal axis in absence of external magnetic field. A C source has such current amplitude which realizes condition

, where

is field of current on conductor surface; M_A=H_K+N_Z•M_O is effective field of conductor anisotropy; H_K= 2K₁/M_O; K₁ is constant of conductor anisotropy; M_O is saturation magnetization of conductor; N_Z is demagnetization factor along axis of conductor. In agreement with second version ends of conductor of sensitive element are connected to voltmeter and A C source is connected to inductance coil and displays current amplitude for which condition

is realized , where

is longitudinal field created by current in coil on conductor surface. Use of invention makes it possible to increase sensitivity to constant field to 10^-7÷10^-8 Oe and to variable field with frequency up to 1.0 GHz to 10^-5÷10^-6 Oe at upper edge of frequency range. Range of measured fields amounts to several oersteds. Output signal of device is strictly proportional to intensity of measured magnetic field in entire range. Measurement rapidity amounts to 10^-8 s. Device can be used under conditions of impact loads in temperature range from - 50 to + 150 C. EFFECT: expanded application field, increased sensitivity of device. 2 cl, 5 dwg

Description

 The invention relates to measuring technique and can be used to measure weak magnetic fields, in particular, when detecting magnetic anomalies, displaying brain functions, exploration of deposits, measuring low currents, etc.

The main requirement for the measuring device is high sensitivity to the field, reaching 10 ^-6 - 10 ^-8 Oe. Under the conditions of the Earth's magnetic field fluctuating in time (fluctuations are ≈ 10 ^-4 Oe) an important task is to create a pair of sensors with identical characteristics to construct a differential measurement schemes when a spatially uniform magnetic field acting on a pair of sensors is excluded as in-phase interference. In this case, the magnetic field generated by the localized source can be measured. Thus, the second requirement for a sensitive element is the non-critical dependence of its sensitivity characteristics on such external parameters as geometric dimensions, magnetostatic parameters, magnitude of the measured magnetic field, etc. Known devices with such sensitivity to the field are either bulky and require complex additional equipment (SQUID, quantum optical magnetometer), or have a critical dependence of sensitivity on the parameters of the ferromagnetic conductor (GMI).

 A device for measuring magnetic field strength [1] is known, comprising a sensor made of a ferromagnetic material with magnetic anisotropy oriented in the plane of the element perpendicular to its longitudinal axis, the ends of the sensor being connected to the output terminals of an adjustable constant current source. Measuring the resistance to alternating current passed through the sensor in a measured magnetic field directed along the axis leads to a proportional measurement of the voltage at its terminals, which is the output signal of the device.

 This device has a limited range of fields to the area of positive slope of the curve of the dependence of resistance on the field. To realize the maximum sensitivity of the device, this area should be as small as possible. In practice, it is the share of oersted. In addition, the amplitude of the alternating voltage at the terminals of the magnetically sensitive element has a disproportionate dependence on the magnitude of the measured field, so the sensitivity of the sensor depends on the magnitude of this field.

Closest to the claimed is a device for measuring weak magnetic fields [2], containing a sensing element made in the form of a ferromagnetic conductor surrounded by an inductor and connected in series with an alternating current source, while the ends of the inductor are connected to a voltage meter. The principle of its operation is based on the nonlinearity of the ferromagnet used as a conductor of the sensitive element. The device allows you to determine the absolute value and direction of the magnetic field, however, with low sensitivity (10 ^-2 - 10 ^-3 Oe). In this device, the amplitude of the alternating current supplying the conductor of the sensing element must be greater than a certain critical value in order to ensure magnetization reversal of the ferromagnet along the complete hysteresis loop. This leads to high energy consumption, which is a disadvantage of the device. In addition, the device can operate only at low frequencies of the measured magnetic field, when the thickness of the skin layer in the conductor is greater than the minimum size of the conductor. At high frequencies, the sensitivity of this device drops sharply. The dimensions of the device are large.

 The present invention expands the arsenal of devices for measuring weak constant and variable magnetic fields and solves the problem of creating a compact device for measuring weak magnetic fields with high sensitivity.

 Variants of the invention relate to objects of one purpose - highly sensitive devices for measuring magnetic fields in a wide range compared to devices with comparable sensitivity and are based on one principle - linear intermode transformation of alternating current in the conductor of the sensing element.

Using the proposed device will increase the sensitivity to a constant field to 10 ^-7 - 10 ^-8 Oe, and to alternating fields with a frequency of up to 1 GHz to 10 ^-5 - 10 ^-6 Oe at the upper edge of the frequency range. The range of the measured fields will be several oersteds, while the output signal of the device is strictly proportional to the strength of the measured magnetic field in the entire range. In addition, the measurement performance will be 10 ^-8 s. The operation of the device under shock loads and in the temperature range (-50) - 150 ^o C.

где

- циркулярное поле тока на поверхности проводника;
H_A = H_K + N_ZM_O - эффективное поле анизотропии проводника;
H_K = 2K₁/M_O;
K₁ - константа анизотропии проводника;
M_O - намагниченность насыщения проводника;
N_Z - размагничивающий фактор вдоль оси проводника.The essence of the invention lies in the fact that in a device for measuring weak magnetic fields containing a sensing element made in the form of a conductor of ferromagnetic material surrounded by an inductor, the conductor being connected in series with an alternating current source, and the inductor connected to a voltage meter at its ends , the conductor of the sensing element is made with magnetic anisotropy perpendicular to its longitudinal axis, and a magnetic structure in the skin layer with the resulting magnetic moment in the absence of an external magnetic field directed perpendicularly to its longitudinal axis, and the AC source is made with a current amplitude such that the condition

Where

- circular current field on the surface of the conductor;
H _A = H _K + N _Z M _O is the effective anisotropy field of the conductor;
H _K = 2K ₁ / M _O ;
K ₁ is the anisotropy constant of the conductor;
M _O is the saturation magnetization of the conductor;
N _Z is the demagnetizing factor along the axis of the conductor.

где

- продольное поле, создаваемое током в катушке на поверхности проводника;
H_A = H_K + N_ZM_O - эффективное поле анизотропии проводника;
H_K = 2K₁/M_O;
K₁ - константа анизотропии проводника;
M_O - намагниченность насыщения проводника;
N_Z - размагничивающий фактор вдоль оси проводника.The essence of the invention lies in the fact that in a device for measuring weak magnetic fields containing a sensitive element made in the form of a conductor of ferromagnetic material surrounded by an inductor, an alternating current source and voltage meter, the sensor conductor is made with magnetic anisotropy perpendicular to its longitudinal axis and magnetic structure in the skin layer with the resulting magnetic moment in the absence of an external magnetic field directed perpendicular to its the longitudinal axis, while its ends are connected to a voltage meter, and the AC source is connected to the inductor and is made with a current amplitude for which the condition

Where

- the longitudinal field created by the current in the coil on the surface of the conductor;
H _A = H _K + N _Z M _O is the effective anisotropy field of the conductor;
H _K = 2K ₁ / M _O ;
K ₁ is the anisotropy constant of the conductor;
M _O is the saturation magnetization of the conductor;
N _Z is the demagnetizing factor along the axis of the conductor.

 The condition for selecting a conductor material with the indicated magnetic anisotropy and magnetic structure is necessary for converting energy from the transverse vibrational mode excited by the alternating current source to the longitudinal vibrational mode, and the coefficient of said transformation is a measure of the measured magnetic field. If this condition is not met with the specified amplitude of the exciting current from the current source, the necessary conversion does not occur.

необходимо для создания процессов высокочастотного перемагничивания линейного (обратимого) характера. Для этого достаточно, чтобы переменное поле в окрестности проводника чувствительного элемента, создаваемое током в проводнике или, в другом варианте, в катушке индуктивности, было по крайней мере меньше эффективного поля анизотропии H_A, задаваемого формулой.The specified condition for selecting the current amplitude in the conductor or in the inductance coil of the sensing element and the associated amplitude of the alternating magnetic field

necessary to create processes of high-frequency magnetization reversal of a linear (reversible) nature. For this, it is sufficient that the alternating field in the vicinity of the conductor of the sensing element created by the current in the conductor or, in another embodiment, in the inductor, be at least less than the effective anisotropy field H _A given by the formula.

H _A = H _K + N _Z M _O , H _K = 2K ₁ / M _O ,
Where
K ₁ is the anisotropy constant of the structure;
M _O is the saturation magnetization;
N _Z is the demagnetization factor along the long axis of the ellipsoid.

 Compliance with the proposed conditions for the choice of amplitude allows you to get the maximum sensitivity of the device to the measured field. If this condition is not met, the nature of the processes will be non-linear and the sensitivity will drop sharply.

 Thus, the implementation of the conductor with the specified anisotropy and structure, as well as the choice of the amplitude of the alternating current provide high sensitivity of the device.

 The invention is illustrated by drawings, in which Fig. 1 shows a diagram of a device with a sensing element conductor connected to a current source, Fig. 2 shows a diagram of devices with an inductor connected to a current source; figure 3 shows an example of a magnetic structure of the conductor of the sensing element; figure 4 shows a graph of the dependence of the power transfer coefficient from an alternating current source into the measuring circuit from the applied external field; figure 5 shows a graph of the dependence of the output voltage of the device from the measured magnetic field.

 A device for measuring weak magnetic fields contains a sensitive element made in the form of a ferromagnetic conductor 1, surrounded by an inductor 2, an AC source 3, a voltage meter 4.

 In this case, the conductor 1 has a magnetic anisotropy oriented perpendicular to its longitudinal axis, and a magnetic structure in the skin layer with the resulting magnetic moment directed perpendicular to its longitudinal axis in the absence of an external field.

где

- поле тока на поверхности проводника;
H_A = H_K + N_ZM_O - эффективное поле анизотропии проводника;
H_K = 2K₁/M_O;
K₁ - константа анизотропии проводника;
M_O - намагниченность насыщения проводника;
N_Z - размагничивающий фактор вдоль оси проводника.In the first embodiment of the device, the ends of the conductor 1 of the sensing element are connected to an AC source 3, and the inductor 2 is connected to a voltage meter 4. The amplitude of the current in the conductor 1 from the current source 3 is selected with the possibility of the condition

Where

- current field on the surface of the conductor;
H _A = H _K + N _Z M _O is the effective anisotropy field of the conductor;
H _K = 2K ₁ / M _O ;
K ₁ is the anisotropy constant of the conductor;
M _O is the saturation magnetization of the conductor;
N _Z is the demagnetizing factor along the axis of the conductor.

где

- продольное поле, создаваемое током в катушке на поверхности проводника;
H_A = H_K + N_ZM_O - эффективное поле анизотропии проводника;
H_K = 2K₁/M_O;
K₁ - константа анизотропии проводника;
M_O - намагниченность насыщения проводника;
N_Z - размагничивающий фактор вдоль оси проводника.In the second embodiment of the device, the ends of the sensor conductor 1 are connected to a voltage meter 4, and the inductor 2 is connected to an alternating current source 3. The alternating current source 3 is connected to an inductor 2, and the amplitude of the current in the coil 2 from the current source 3 is selected to be implemented conditions

Where

- the longitudinal field created by the current in the coil on the surface of the conductor;
H _A = H _K + N _Z M _O is the effective anisotropy field of the conductor;
H _K = 2K ₁ / M _O ;
K ₁ is the anisotropy constant of the conductor;
M _O is the saturation magnetization of the conductor;
N _Z is the demagnetizing factor along the axis of the conductor.

 As the material of the conductor 1 of the sensing element, materials based on ferromagnetic alloys with a small negative magnetostriction constant can be used, which, as a result of thermomagnetic processing, acquire the necessary magnetic anisotropy and magnetic structure. Examples of such materials are amorphous CoFeSiB cobalt-based alloys with a known stoichiometric composition, Finemet, Permalloy, and other alloys.

The compactness of the device is ensured by the small size of the sensitive element (1 • 1 • 5 mm ³ ), low power consumption of the AC source.

 The performance of the device is ensured by the possibility of operating it at a high frequency of alternating current (up to several gigahertz), while the upper harmonic frequency of the measured field can reach several hundred megahertz.

 The device operates as follows. The indicated connections of the conductor 1 and inductor 2 with the current source 3 and voltage meter 4 make it possible to excite oscillations of the magnetic field orthogonal to the AC field and, as a result, record the voltage induced by them, which is a measure of the measured external field.

 When the device is operated according to the first embodiment, the alternating current from the source 3, flowing through the conductor 1 of the sensing element, is transformed into alternating current in the inductor 2 due to the gyrotropic properties of its material, and the transformation coefficient depends on the magnetic state of conductor 1 sensitive element, which is determined by the measured magnetic field. The induction EMF at the terminals of the coil 2 is recorded by a voltage meter 4.

 When the device according to the second embodiment is operated, an alternating current from a source 3 flowing through an inductor 2 excites an alternating magnetic field directed along the axis of conductor 1. Due to the gyrotropic properties of its material, which are sensitive to the measured external field, a component of a magnetic field that is perpendicular to the surface of the conductor is excited axis of the conductor 1, which induces a current causing a potential difference at the ends of the conductor 1, recorded by the voltage meter 4.

где
ζ_zφ(H) - зависящая от внешнего измеряемого поля H компонента тензора поверхностного импеданса чувствительного элемента в осях, в которых индекс φ соответствует оси в направлении переменного магнитного поля тока, а z - ей ортогональной и лежащей в плоскости, касательной к поверхности проводника 1 чувствительного элемента.The magnetic permeability of the anisotropic conductor 1 of the sensing element, as well as the surface impedance, are a tensor whose off-diagonal component in the axes associated with the main axes of the element is responsible for the appearance of an alternating magnetic field orthogonal to the current field. This field in the first embodiment of the device excites the voltage at the ends of the inductor 2, the amplitude of which depends on the magnitude of the measured external field; in the second embodiment, the resulting alternating field leads to a voltage at the ends of the conductor 1, which also depends on the magnitude of the measured field. In the general case, the coefficient of energy transformation from one of the above modes to another, which is orthogonal to it, is proportional to the component of the surface resistance tensor of the metal,

Where
ζ _zφ (H) is the component of the surface impedance tensor of the sensitive element depending on the external measured field H in the axes in which the index φ corresponds to the axis in the direction of the alternating magnetic field of the current, and z to it is orthogonal and lying in a plane tangent to the surface of the sensitive conductor 1 item.

. Его проекциями в цилиндрической системе координат {ρ,φ,z} являются {0,M_φ,M_z}. Момент

направлен под углом θ к направлению легкой оси в присутствии внешнего поля

, направленного вдоль оси проводника, и перпендикулярен оси проводника в отсутствие поля. Для случая коаксиальной линии передачи, нагружаемой коаксиальным короткозамкнутым устройством, в котором проводник 1 чувствительного элемента с магнитной структурой, изображенной на фиг.3, является центральным проводником, а внешний проводник устройства имеет размер внешнего проводника коаксиальной линии, коэффициент трансформации в условиях сильного скин-эффекта в проводнике чувствительного элемента
ξ(H) = Reζ_zφ(H){P+Reζ_φφ(H)}^-1, (3)
где
ζ_φφ(H) - диагональная компонента тензора поверхностного импеданса. Параметр P характеризует внешнюю по отношению к проводнику 1 электрическую цепь: P = (a/l)ln(b/a_o), a_o - радиус центрального стержня коаксиальной линии передачи, l - длина проводника 1 чувствительного элемента, b - радиус внешнего проводника коаксиальной линии. Компоненты тензора поверхностного импеданса весьма чувствительны к величине поля H в диапазоне полей, когда направление магнитного момента на поверхности проводника 1 чувствительного элемента изменяется на противоположное. Этим обусловлена высокая чувствительность данного устройства.In FIG. 3 shows an example of the magnetic structure of the conductor 1 of the sensing element. The case of a single-domain structure of the surface layer with the resulting magnetic moment is shown here.

. Its projections in the cylindrical coordinate system {ρ, φ, z} are {0, M _φ , M _z }. Moment

directed at an angle θ to the direction of the easy axis in the presence of an external field

directed along the axis of the conductor, and perpendicular to the axis of the conductor in the absence of a field. For the case of a coaxial transmission line loaded with a coaxial short-circuited device, in which the sensor conductor 1 with the magnetic structure shown in FIG. 3 is the central conductor, and the device’s external conductor has the size of the external coaxial line conductor, the transformation coefficient under strong skin effect in the sensor conductor
ξ (H) = Reζ _zφ (H) {P + Reζ _φφ (H)} ^-1 , (3)
Where
ζ _φφ (H) is the diagonal component of the surface impedance tensor. The parameter P characterizes the electric circuit external to the conductor 1: P = (a / l) ln (b / a _o ), a _o is the radius of the central rod of the coaxial transmission line, l is the length of conductor 1 of the sensing element, b is the radius of the external conductor coaxial line. The components of the surface impedance tensor are very sensitive to the magnitude of the field H in the field range when the direction of the magnetic moment on the surface of the conductor 1 of the sensing element is reversed. This is due to the high sensitivity of this device.

Число витков в катушке индуктивности длиной 1,5 мм и диаметром 1 мм было 15.Figure 4 shows graphs of the dependence of the power transfer coefficient excited by the alternating current source 3, on the applied external field in the circuit of figure 1 (option 1) and in the diagram of figure 2 (option 2) at an AC frequency from a current source of 50 MHz. An amorphous wire of an alloy (Fe ₆ Co ₉₄ ) _72.5 Si _12.5 B ₁₅ with a length of 5 mm and a diameter of 30 μm was used as conductor 1 of the sensitive element in both versions. The negative value of the magnetostriction constant of this alloy λ≈-10 ^-7 caused the circular magnetic anisotropy of the surface layer of the conductor, corresponding to that shown in Fig.3. The effective anisotropy field of the conductor H _A = 2K ₁ / M _O + N _Z M _O , taking into account the anisotropy constant K ₁ = 200 erg / cm ³ , the saturation magnetization M _o = 500 G, and the demagnetizing factor N _Z = 0.001, amounted to 1.511 Oe. Circular the current field from source 3 with an amplitude of 50 μA in the conductor 1 on its surface in option 1 and the longitudinal field created by the current from the source with an amplitude of 0.4 mA in inductor 2 in option 2 did not exceed 0.05 Oe, which satisfies the condition

The number of turns in the inductor 1.5 mm long and 1 mm in diameter was 15.

 The graph shows that the conversion coefficient varies sharply in the field range -1 - + 1E, which allows to obtain a high sensitivity device for measuring magnetic fields.

 Figure 5 shows the dependence of the output voltage of the device, made according to the scheme of figure 1 from the magnitude of the measured field.

The conversion coefficient was 10.5 V / O, and the dependence of the output voltage on the measured field in the range 0 - 0.4 Oe is linear within the measurement error of 0.5%. Linear dependence makes it possible to create identical magnetic field sensors for differential measurement methods without special circuit solutions, including negative feedback. Evaluation of thermal noise, which is the main factor affecting the signal-to-noise ratio for a given magnetic system, gives a threshold sensitivity value of 10 ^-7 - 10 ^-8 E. Experimental achievement of the threshold sensitivity is possible under conditions of filtering out magnetic fluctuating noise.

Sources of information:
1. RF, patent N 2059259, "Magnetosensitive element", publ. in BI, 1996, N 12, MKI: G 01 R 33/02.

 2. I.L. Bershtein. About one new type of magnetometer (erstedmeter) .- Proceedings of the Academy of Sciences of the USSR, Physical Series, VIII, 1944, No. 4, p. 189-193.

Claims (2)

1. A device for measuring weak magnetic fields, containing a sensitive element made in the form of a conductor of ferromagnetic material surrounded by an inductor, the conductor being connected in series with an AC source, and the inductor connected to a voltage meter at its ends, characterized in that the conductor the sensitive element is made with magnetic anisotropy perpendicular to its longitudinal axis, and a magnetic structure in the skin layer with the resulting magnetic moment in of an external magnetic field, directed perpendicular to its longitudinal axis, and AC source is configured such current amplitude, for which the condition is realized

Where

current field on the surface of the conductor;
H = H _k + N ₂ M _o is the effective anisotropy field of the conductor;
H _k = 2K ₁ / M _o ;
K ₁ is the anisotropy constant of the conductor;
M _o is the saturation magnetization of the conductor;
N _z is the demagnetizing factor along the axis of the conductor. 2. A device for measuring weak magnetic fields, containing a sensitive element made in the form of a conductor made of ferromagnetic material surrounded by an inductor, an alternating current source and a voltage meter, characterized in that the sensitive element conductor is made with magnetic anisotropy perpendicular to its longitudinal axis, and a magnetic structure in the skin layer with the resulting magnetic moment in the absence of an external magnetic field directed perpendicular to its longitudinal axis, while Its cells are connected to a voltage meter, and the AC source is connected to an inductor and is made with a current amplitude such that the condition

Where

- the longitudinal field created by the current in the coil on the surface of the conductor;
H _a = H _k + N _z M _o is the effective anisotropy field of the conductor;
H _k = 2K ₁ / M _o ;
K ₁ is the anisotropy constant of the conductor;
M _o is the saturation magnetization of the conductor;
N _z is the demagnetizing factor along the axis of the conductor.

Images