WO2008145794A1 - Method and device for measuring magnetic gradient and magnetic susceptibility of a material - Google Patents

Abstract

The invention concerns a method and a device for measuring the magnetic gradient and/or the magnetic susceptibility of a material. A magnetically hard material is securely fastened to a vibrating element, and by means of any known technique the assembly is vibrated at the resonance frequency or the harmonics thereof and the frequency, phase or amplitude variations which occur when an external gradient acts on the magnetically hard material are measured.

Description

 METHOD AND DEVICE FOR THE MEASUREMENT OF MAGNETIC GRADIENT AND MAGNETIC SUSCEPTIBILITY OF A MATERIAL

D E S C R I P C I Ó N

OBJECT OF THE INVENTION

The present invention relates to a device and a method for measuring the magnetic gradient.

The proposed device can also work as a susceptometer, since it can measure the magnetic field gradient produced by a magnetized magnetic material located near the system.

BACKGROUND OF THE INVENTION

The measurement of the magnetic gradient is important in many scientific fields such as medical applications (measurement of neuronal activity), geological (deposits and surveys) or military applications (detection of antipersonnel mines). There have been several techniques used for this type of measurement. For very low gradient values, meters based on SQUID magnetometers (superconducting quantum interference devices) have been proposed. However, these have the disadvantage of working at cryogenic temperatures, so the resulting system is very complex.

Magnetic gradient measurements have also been proposed using fluxgates, optical fibers attached to magnetostrictive materials or partially coated with them, magnetometers with magnetostrictive material cores, etc. In all cases, the measurement is based on the magnetic field difference in the positions of the physically separated magnetometers. This means that the measure implies an error in the gradient determination.

Gradient meters with a single transducer element have been suggested. In this case, a resonant system is used to which a magnetically soft material is added that magnets with a frequency that coincides with the resonance of the system. However, it has several drawbacks. The first is that you need a uniform magnetic field control system since, if you do not have it, the magnetically soft sample could be magnetized uncontrollably. This control system may become unfeasible if the value of the surrounding magnetic field is high. In addition, the magnetization of the sinusoidal material is not trivial if we take into account the hysteresis of the process.

Some known patents related to this sector of the technique are: EP0773449, US2003222649, US4549135, and US4918371.

DESCRIPTION OF THE INVENTION

A magnetic gradient meter measures directional magnetic field derivatives but is insensitive to homogeneous magnetic fields. In

The present invention measures the magnetic gradient through variations in the frequency, phase or amplitude of resonance of a system composed of a resonant structure such as a tongue, strap, a membrane or a spring, on which a jointly fixed hard magnetic material.

The vibrating hard-structure material assembly is vibrated at its resonance frequency by means of the application of a magnetic field gradient on the hard magnetic material, produced by magnetic field generating means, for example a series of coils that act on the material magnetic. The application of an external gradient It produces a force on the magnetic material that is transmitted to the structure and causes the resonance frequency, the phase and the amplitude to change.

One of the advantages of the present invention is that it is not necessary

¡Send the magnetic material alternately to get the system to resonate.

The device object of the invention comprises a mechanical element on which a magnet or any magnetically hard material is fixed in solidarity, thereby understanding that material that has a constant value of the magnetization when subjected to magnetic field variations of less than 10 ^{" 2} T. When an alternating magnetic force is applied to said magnetic material the assembly vibrates and the resulting vibration, which can be detected by any of the conventional methods, is related to the force applied on the magnetically hard material and, therefore, with the magnetic gradient that generates this force and the moment of the magnetically hard material.

When a magnetic field H acts on a magnetic material in vacuum, a pair defined by:

r = μ ₀ mx H

and a force acts on it defined by:

F = μ _or grad (m H)

where μ ₀ is the magnetic permeability in a vacuum, m the magnetic moment of the material and H the magnetic field that acts on it. Therefore, if m is constant, a force will only be exerted on the system if the magnetic field H presents spatial variations in Ia position of magnetically hard material or magnet. Developing the previous equation we obtain that the value of the force is:

BH _x dH _and dH _z dz dz dz

Therefore, the application of a magnetic field that presents spatial variations, that is, the application of a magnetic gradient will cause a force to act on the magnet or magnetically hard material. If we assume that the magnetization of said material is oriented in the Z direction, the force on that material will be:

, dH, dH, dH _^

F = μ _or m dx d and dz

In the present invention the magnetically hard material will be located in such a way that the vibration detected is that produced by the force exerted in the Z direction, that is,

This magnetically hard material or magnet is fixedly attached by any of the conventional techniques to a vibrating structure that can be a spring, a tongue, a membrane, etc. These techniques can be glued by means of a glue, resin, etc. Alternatively, the direct growth of the magnetically hard material or magnet can be performed on any of the aforementioned vibrating structures (sputtering, etc.). Another possible fixing technique of the hard magnetic material to the vibrating structure, it can be by another magnetic material located on the other side of the structure, either grown directly on the vibrating structure or glued by magnetic force to the first magnet.

In order to optimize the movement produced by the magnetic gradient on the oscillating system, said system is vibrated at its mechanical resonance frequency or at any of its harmonics. For this, magnetic excitation is used, by means of the application of a field that can be null in the place where the magnetically hard material is located, but with a high gradient at said point.

Another aspect of the invention relates to a method for measuring the magnetic moment of a material, which comprises arranging a hard magnetic material on a vibrating element, and applying a magnetic field on said vibrating material and element.

Said magnetic field is determined to make the vibrating structure formed by the magnetic material and the vibrating element vibrate in the resonance frequency of the assembly or some of its harmonics. In the method, the variation of the vibration characteristics of the vibrating structure is measured, when said structure is subjected to an external magnetic gradient produced by a magnetic sample.

DESCRIPTION OF THE DRAWINGS

To complement the description that is being made and in order to help a better understanding of the characteristics of the invention, according to a preferred example of practical realization thereof, a set of drawings is attached as an integral part of said description. Illustrative and not limiting, the following has been represented: Figure 1 shows a general scheme that illustrates the technique object of this invention for the measurement of magnetic gradient or the magnetic moment of a sample.

Figure 2 shows a schematic representation of a preferred embodiment of the device object of the invention. The crosses and points on the coils denote the direction of the generated field.

Figure 3 shows a representation similar to the previous one with the calibration system. The crosses and points on the coils denote the direction of the generated field.

Figure 4 shows a schematic representation of the device when applied on a magnetic sample. The crosses and points on the coils denote the direction of the generated field.

Figure 5 shows a schematic representation of the device together with an example of a vibration detection system. In the upper right part of this same figure, an enlarged and plan view of the optical fiber used for the detection of vibration has been represented. As in the preceding figures, the crosses and points on the coils denote the direction of the generated field.

PREFERRED EMBODIMENT OF THE INVENTION

In figure 1 a general scheme of the invention is shown, in which magnetic field generating means (1) for applying an excitation (2) on a vibrating structure (3), which is composed of a magnetically hard material is shown (5) and a vibrant element

(4). Due to an external magnetic disturbance (6), a response (7) of the system, measured by means of measurement of variation of the vibration (8).

Some examples of magnetically hard materials that can be used are: SmCo or NdFeB.

Figure 2 shows a practical example to bring the vibrating structure (3) to its mechanical resonance. A hard magnetic material (5), is joined in solidarity with a vibrating element (4), in this case an elastic membrane (9), which in turn is mounted on a fixed element such as a frame or frame (14).

In order to know the resonance frequency of the vibrating structure, the assembly can be vibrated by any known technique, performing a frequency scan to detect the maximum of the amplitude of the signal.

The field generating means consist in this example of a first, second, third and fourth coils (10, 11, 12,13), properly arranged and fed to generate a magnetic field that produces an alternating magnetic gradient in the place where it is located the hard magnetic material (5). An alternating electric current is passed through the coils (10,11, 12,13), which generates a field that is null in the place occupied by the magnetically hard material or magnet (5), but which creates a high peak value at peak magnetic gradient.

More specifically, it can be considered that the elastic membrane (9) is located in an imaginary horizontal plane, and the coils are arranged in pairs with their axis perpendicular to said plane.

Figure 3 is a representation similar to the previous figure, in which a calibration system is involved, formed in this case by a loop (15) of known magnetic moment that is located on the axis of the system, in this case arranged coaxially with the magnetic material (5), and at a suitable distance.

The system calibration can be done by using a small loop (15) through which an electric current passes. By applying a current intensity to the loop (15), it generates an inhomogeneous magnetic field that creates a gradient of known value in the position of the magnetic material (5). A suitable calibration by means of the current loop gives us the relationship between variation of frequency, phase or amplitude and the magnetic gradient.

The device is inserted into a magnetic field shield. This loop (15) is located at a distance from the device such that it can be assumed that the magnetic field produced in the system is that of a magnetic dipole:

where r is the distance between the center of the loop and the center of the magnet, m is the magnetic moment of the loop and whose value is m = I.S, where / is the electric current and S the area of said loop. If the system is designed to measure the directional derivative in the magnetization direction Z of the magnet, then the magnetic field value in that direction is given by:

2π _r

And the gradient produced will be: dB _z (r) ₌ 3μ ₀ m δz 2π r ⁴

If the loop is located close enough to the system so that the dipole approximation does not work, the exact equations of the value of the magnetic field generated by a loop that can be found for example in the publication E.Durand would be used,

Magnétostatique, Masson et Cié, Paris, 1968.

Figure 4 shows the effect of a magnetic sample (16) near the system, which creates a magnetic field gradient that modifies the vibration parameters of the system. When the system is subjected to an external magnetic field gradient, different from that created by the coils, the resonance frequency, the phase and also the amplitude of oscillation of the system change. The measurement of the variation of any of these magnitudes can be used in the detection of said gradient.

If a ferromagnetic material approaches the system, it can generate a magnetic field gradient similar to that of a magnetic dipole, depending on the relationship between its size and the distance to the system. In this way, the proposed system can also be used as a susceptometer, presenting the advantage of not having to move the sample during the measurement in front of the vibrating sample magnetometer (VSM) or the alternating gradient magnetometer (AGM).

The system for detecting the vibration of the system can be carried out by any of the techniques known so far, for example, piezoelectricity, variations in electrical capacity or optical methods.

Figure 5 shows one of the possible systems for detecting the vibration of the system. On the system axis, at an appropriate distance of the magnet or magnetically hard material (5) or of the membrane (9) an optical fiber (17) is located, preferably formed by one or several elements suitable for measuring the deflection of the structure by optical procedures. As detailed in the upper right part of this figure, the optical detection method consists of a coaxial arrangement of optical fibers (17). A central fiber transmits light to the vibrating element and the lateral fibers collect the light reflected on its surface. The measurement of the reflection of the surface without contact and without electrical means, supposes a great advantage respect to other conventional means such as piozorresistors, strain gauges, etc.

The sensitivity of the system will depend on the vibrating system used (its elastic constants) as well as the magnetization and the volume of magnetically hard material used.

Various possibilities of practical embodiments of the invention are described in the attached dependent claims.

In view of this description and set of figures, the person skilled in the art may understand that the embodiments of the invention that have been described can be combined in multiple ways within the object of the invention. The invention has been described according to some preferred embodiments thereof, but for the person skilled in the art it will be evident that multiple variations can be introduced in said preferred embodiments without exceeding the object of the claimed invention.

Claims

1.- Device for measuring the magnetic gradient, characterized in that it comprises: a vibrating structure formed by a vibrating element and a hard magnetic material joined in solidarity with said element, generating means of a magnetic field adapted to vibrate said vibrating structure in its resonance frequency or some of its harmonics, Means for measuring the variation of some of the vibration characteristics of the vibrating structure. 2. Device according to claim 1 characterized in that said generating means of a magnetic field, consist of at least one coil adapted to generate an alternating magnetic field on the vibrating structure. 3. Device according to claim 2 characterized in that the magnetic field is null in the place occupied by the magnetic material. 4. Device according to any of the preceding claims characterized in that the vibrating element is selected from: a tongue, a cantilever strap, a flexible membrane, or a spring. 5. Device according to any of the preceding claims characterized in that the measuring means of variation of the vibration characteristics are selected from: at least one optical fiber, piezoelectric element, or an element with variable electrical capacity. 6. Device according to any of claims 1 to 4, characterized in that the means for measuring variation of the vibration characteristics consist of a group of optical fibers arranged coaxially. 7. Device according to claim 6 characterized in that a central optical fiber is adapted to transmit light towards the vibrating element, and the lateral fibers are adapted to collect the reflected light. 8.- Method for measuring the magnetic moment of a sample of material, characterized in that it comprises: disposing a hard magnetic material joined in solidarity with a vibrating element formed a vibrating structure, vibrate said vibrating structure at its resonance frequency or any of its harmonics, subject the vibrating structure to an external magnetic gradient produced by said sample of material, measuring the variation of some of the vibration characteristics of the vibrating structure after being subjected to said external gradient. 9. Method according to claim 8, characterized in that the measurement of the variation of the vibration characteristics of the vibrating structure comprises the measurement of the resonance frequency of the assembly, the phase and / or the amplitude of the vibration of the vibrating structure. .