ES2536940A1 - Procedure for measuring magnetic parameters and temporal harmonics both in phase and in quadrature of the magnetic moment of small samples excited with alternating or continuous magnetic fields and device for the implementation of the procedure (Machine-translation by Google Translate, not legally binding) - Google Patents

Abstract

Procedure for measuring magnetic parameters and temporal harmonics both in phase and quadrature of the magnetic moment of small samples excited with alternating or continuous magnetic fields. The procedure allows to characterize magnetic materials by applying alternating magnetic fields in a wide range of frequencies using a mechanical resonant system. The characterization is made with magnetic fields created with alternating currents, which allows a great variety of measurements. An alternate field gradient susceptometer is defined that can work with exciter fields of any frequency. To this end, the device consists of a ferrite electromagnet (11) capable of generating both continuous and alternating fields of the order of 0.3 t, a mechanical oscillating micro-tongue resonant system (2), a detection system (4) of position of the resonant system and spools of copper wire (6), capable of generating an alternating magnetic field gradient. (Machine-translation by Google Translate, not legally binding)

Description

OBJECT OF THE INVENTION

The present invention relates to a method and a device for characterizing materials

magnetic applying alternating magnetic fields, in a wide range of frequencies,

using a resonant mechanical system.

More specifically, the invention allows to measure the magnetization of samples very

small, of the order of ~ g, the usual magnetization curves as well as the permeability or

magnetic susceptibility of the sample, both its real part and its imaginary part.

20 This wide variety of measurements can be performed because the characterization is done with magnetic fields created with alternating currents instead of continuous currents as usual.

In addition to the measurement method used, the invention also affects the device for

25 the implementation of said method, a device that materializes in an alternating field gradient susceptometer, which can work with exciter fields of any frequency.

Consequently, the invention is therefore placed in the technical sector of instrumentation

30 electronics, specifically in the measurement systems and characterization of magnetic materials.

BACKGROUND OF THE INVENTION

Currently, the most widespread systems for measuring the susceptibility of magnetic materials, among other properties, are those of the inductive type. These systems use a large coil to create an alternating magnetic field, into which the sample to be analyzed is introduced. In turn, two opposing secondary coils are wrapped around this support, on which an induced tension appears, due to the variation of tension in the primary and in the magnetic material and whose amplitude will be proportional to the magnetic properties of the sample. This voltage usually has a very low amplitude, so it is usual to use a Lock-In amplifier, whose reference signal is obtained from the one used to generate the magnetic field.

The main limitation of this class of systems is the low magnetic field that magnets the sample (around 0.015 Teslas) and the excessive temperature increase in the coil that creates the field as the current increases.

In parallel there are two other systems, the vibrating sample magnetometer (VSM Vibrating Simple Magnetometer) and the force magnetometer produced by an alternating field gradient (AGFM AlternatingGradientForceMagnetometer).

These systems also measure magnetic susceptibility, but are based on a totally different mechanism than the one used in inductive systems. The first of eilos, the VSM, uses an electromagnet to create a continuous magnetic field, which can reach very high values (higher than several Teslas). In the central part of the electromagnet a linear actuator is placed vertically, on which the sample of magnetic material is placed and periodically moved. On both sides of the electromagnet there are reels, on which a voltage due to the movement of the sample appears, whose amplitude is detected using a Lock-In amplifier.

The second system, the AGFM, has a reverse mode of operation to the previous one. It uses an electromagnet to create a very intense continuous field, in whose center a tongue or oscillator is placed on which the sample is placed. In turn, on both sides of the tongue, there are reels in opposition and of equal number of turns and in which an alternating signal of the same frequency as the resonance frequency of the tongue is introduced. In this way the tongue oscillates so that the amplitude of the vibration is detected and the relationship with the magnetization of the sample is obtained.

5 While this second system is much more efficient, the means to produce the magnetizing field, the process to generate the oscillation of the micro tongue and the treatment of the signal are not the most appropriate.

10 DESCRIPTION OF THE INVENTION

The procedure and the device for the implementation thereof, which the invention proposes, solve in a fully satisfactory manner the above-mentioned problem, significantly improving the performance of current systems.

15 For this, the procedure that is recommended is based on carrying out the measurement of the magnetic parameters of small excited samples with alternate magnetic fields

or continuous by detecting the oscillation of a mechanical resonance system excited by an alternating field gradient.

20 When determining the frequency of the alternating field gradient, it may be equal to the sum of the frequency of the exciter field plus the resonant frequency of the resonant mechanical system, or it may be equal to the frequency of the exciter field less the resonant frequency of the resonant mechanical system.

25 This methodology is equally applicable when carrying out the measurement of temporal harmonics both in phase and quadrature, so that when determining the frequency of the alternating field gradient, the procedure provides for the following options:

That the frequency of the alternating field gradient is equal to the frequency of the exciter field minus the resonant frequency of the resonant mechanical system.

That the frequency of the alternating field gradient is equal to the frequency of the exciter field plus the resonant frequency of the resonant mechanical system.

That the amplitude of the alternating field gradient be amplitude modulated with a frequency of multiple or sub multiple modulation of the frequency of the resonant mechanical system.

Regarding the measurement of the magnetic parameters both in phase and in quadrature (real and complex), it is expected that the amplitude of the alternating field gradient be amplitude modulated with a frequency of the modulation multiple or sub multiple of the system frequency resonant mechanic

In parallel, and when carrying out the measurement of temporal harmonics both in phase and in quadrature of the magnetic moment, the amplitude of the gradient of the alternating field can be frequency modulated with a frequency of multiple or submultiple modulation of the system frequency resonant mechanic

This methodology is equally applicable when carrying out the measurement of the magnetic parameters both in phase and in quadrature (real and complex) of the magnetic moment of the excited sample.

The described method is carried out by means of a device with an instrumentation that allows measuring the susceptibility, real and complex, of the magnetic materials.

More specifically, the characterization of the samples is carried out:

Creating a uniform and alternating magnetic field of controllable amplitude and frequency, which acts on the sample. This field induces in the sample a

time-varying magnetization, with a fundamental frequency equal to the frequency of the applied field and whose amplitude depends on the amplitude and frequency of the applied field. The magnetic moment of the sample will be equal to its volume multiplied by its magnetization. The magnetic moment mentioned

5 will also be alternate in time.

Adhering the sample on a well-known and well-known resonance frequency resonant mechanical system.

10 By acting on the sample another non-homogeneous alternating magnetic field, such that in the sample it creates a null field, but nevertheless, the component of the field in the direction of oscillation of the mechanical system exhibits a strong linear gradient in the environment of the sample . This field does not act on the magnetization of the sample, since it is zero in it, but produces a force on

The same, which is proportional to the product of the field gradient by the magnetic moment of the sample, which in turn is proportional to the magnetization of the sample. The frequency of the force is equal to that of the field gradient if the magnetization of the sample remains constant. To achieve this situation, the exciter magnetic field, mentioned in the first point,

20 should be constant instead of alternate. In the event that the aforementioned exciter field is alternate, the force on the sample is complex to evaluate and will be treated later.

By attaching to the system a position detector, which measures the oscillation of the resonant mechanical system 25, so that, at all times, we can measure the amplitude, frequency and other parameters of the oscillation.

If the net force that appears on the sample, and therefore on the mechanical system, has a frequency equal to the resonant frequency of the mechanical system, it will oscillate 30 and the amplitude and frequency of the oscillation can be measured and, through These measures determine the magnetization of the sample. This is easily done if the sample is magnetized by a constant field and therefore has a constant magnetic moment and

if the frequency of the gradient of the field acting on the sample is equal to the frequency

Resonance of the mechanical system. This is the basis of operation of the AGFM. But if the magnetic moment of the sample m is alternate in time (m = mosen (wt », the

net force that appears on it will be zero, since its net magnetization is.

It follows that this is the case, except if the frequency of the magnetic field gradient acting on the sample is equal to the magnetization frequency of the sample +/- the resonant frequency of the mechanical system.

In accordance with the above, the characterization of the samples is performed by exciting the sample with:

An alternating frequency field f1 and applying a magnetic field gradient in the sample, in the direction of oscillation of the mechanical frequency system f2. It is imposed that f2 = f1 +/- fr, where the resonant frequency of the mechanical system is fr. In this way, it is possible to measure the amplitude of the magnetization of the sample under the action of a field of any frequency, even of a continuous field.

An alternating frequency field f1 and applying a magnetic field gradient in the sample, in the oscillation direction of the mechanical frequency system also f1, but amplitude modulated with a modulation frequency equal to the resonance frequency of the mechanical system, fr . By properly adjusting the phases of the carrier and modulator frequency, it is possible to measure the amplitude and phase of the magnetization with respect to the magnetizer field and, therefore, determine the complex values of the magnetization, the power dissipated by the sample and the permeabilities, susceptibilities complex and other parameters that magnetically characterize the sample.

An alternating frequency field f1 and applying a magnetic field gradient in the sample, in the direction of oscillation of the mechanical system, of frequency also f1, but frequency modulated, with a modulation frequency equal to the resonant frequency of the mechanical system fr.

By properly adjusting the phases of the carrier and modulator frequency, it is possible to measure the amplitude and phase of the magnetization with respect to the magnetizer field and, therefore, determine the complex values of the magnetization, the power dissipated by the sample and the permeabilities, susceptibilities

5 complex and other parameters that magnetically characterize the sample.

The physical and mathematical foundations on which the measurement system is based are presented below.

10 If we apply to the sample an exciting magnetic field, H, alternating, amplitude, Ha, Y

Wl frequency

R = Ho (z) sin (w, t) Ú,

15 Ha may not be spatially uniform and therefore depend on z. We also assume that the exciter bonnet, H, also has the z-axis direction.

The magnetic moment induced in the sample, m (t) can be written as:

ñ¡ (t) = ¿;., (Ma, sen (iw, t) + Mb, cos (iw, t)) Ü,

20 Being Ma¡ and Mb¡ the Fourier serial development constants of m (t).

As mentioned, the exciter field, H, can be spatially variable and, by

therefore, the gradient of the z component of the field can also be non-zero, so

a force will appear on the sample

25 F = ii¡ (t) '(1 H, =' (1 H, (z) sin (w, t) ¿.., (Ma, sen (iw, t) + Mb, cos (iw, t) ) Üz)

Whose average value will be:

(t) = t '(1 J-l, (z) Ma,

It is, therefore, a constant force, which will not oscillate the mechanical resonance system.

5

If we introduce a coil system capable of creating a spatially variable alternating magnetic field, so that it creates a zero field in the sample and an alternating field gradient of amplitude go and pulse W2_ g = go (z) sin (w, t) Ü, The force on the sample will be:

+ g, Mb, (cos «(iw, -w,) t-cos« (iw, + w,) t)) Ü,

1 o

Therefore, the resonant mechanical system will only oscillate if:

fifteen

• w1 = 0 and w2 = wr, where the resonance pulse of the mechanical system is wr_ That is, the applied field is constant in time and, therefore, so is the magnetic moment of the sample and the force that appears is : F = go sine w2t) L :: l (Mb¡) Üz The force has the resonant frequency of the mechanical system and, therefore, it oscillates with an amplitude proportional to go r Mbi

20 25

• iw, -w2 = wr or if iw, + w2 = Wr_ Therefore, by properly choosing the frequency of the field gradient, we can measure the harmonic of the magnetic moment we want and the amplitude proportional to go, Mb¡_ Although the resonance can appear by the terms in sine, or by the terms in cosine, or by a combination of both_ This becomes more explicit if w2 = iw, + wr_ In this case, the summation disappears, and: F = g, without «(iw, + w,) t) (Ma, sen (iw, t) + Mb, cos (iw, t) Ü, = Ma, go (sen (2iw, + w,) t-sen (w, t)) + goMb , (cos (w, t) -cos «2iw, + w,) t)) Ü,

The mechanical system will oscillate with the terms that have the resonance pulse.

F = -Ma; go (sen (W ,!)) + goMb, (COS (w ,!))

ft = go sin «(i ,, - \ -IV,)!) (Ma; sen (i ,, - \ t) + Mb, cos (i ,, - \ t)) Ú, 5 = Ma; gO (sen (2i ,, - \ -IV,) t + sen (iIV, t) + goMb; (COS (w, t) -cos «2i ,, - \ -IV,) t)) Ú,

Again, the mechanical system will oscillate with the terms that have the resonance pulse 10 F = Ma, go (sin (IV, t)) + goMb, (COS (IV,!))

Therefore, in both cases, the system will oscillate with a combination of the real and imaginary terms of the magnetic moment and, therefore, of the susceptibility. 15 But if the sample is excited with a gradient

g = go (sen ((i ,, - \ + IV,)!) + sen «(i ,, - \ -IV,) t)) = 2go without (I ~ t) COS (IV, t)

20 That is, with a pulse signal iw1, modulated in amplitude with the resonance pulse, we have F = 2goMb, (cos (IV, t))

that is, the mechanical system oscillates with an amplitude proportional to the real part of the magnetic moment.

While, if the sample is excited with a gradient:

g = go (sen ((i ,, - \ + w,) t) -sen ((i ,, - \ -IV,)!)) = 2gocos (I ~ t) sin (IV,!)

30 That is, with an iwJ pulse signal, modulated in amplitude with the resonance pulse and offset TT / 2 with the previous one, we have left

F = 2goMa, (sen (w, t))

5

The mechanical system oscillates with an amplitude proportional to the imaginary part of the magnetic moment.

the

Therefore, indeterminacy is eliminated and it is demonstrated that the present invention is capable of functioning as a classical AGFM and can also measure with alternate fields and determine all the real and imaginary parameters of the magnetic moment. DESCRIPTION OF THE DRAWINGS

fifteen

To complement the description that will then be made and in order to help a better understanding of the features of the invention, according to a preferred example of practical implementation thereof, a set of drawings is attached as an integral part of said description. where with the illustrative and non-limiting nature, the following has been represented:

twenty

Figure 1 shows a schematic perspective representation of the micro-tongue that is established on the mechanical support that participates in the device of the invention.

25

Figure 2.- Shows a schematic perspective view of the ferrite electromagnet and provided with the corresponding coils that participate in the device of the invention. Figure 3.- Shows a schematic view of a three-dimensional model of the mechanical support that participates in the invention.

30

Figure 4.- Shows a diagram of the situation of the elements that create the magnetic next to the micro-tab of Figure 1. countryside

Figure 5.- It shows a diagram of the situation of the reels generating the field gradient with respect to the electromagnet seen from above, showing the hole on which

The micro-tongue is passed.

Figure 6 shows, finally, a block diagram of the different elements involved in the invention.

PREFERRED EMBODIMENT OF THE INVENTION

As can be seen in Figure 1, to carry out the process of the invention a device or equipment is provided in which a mechanical support (1) participates, in whose middle area a micro-tongue (2) is provided. ) of glass as a resonant system of small thickness, on which a sample of amorphous material is adhered, with high susceptibility and permeability.

As previously stated, the system must allow measuring the amplitude of oscillation of the resonant system as a function of the alternating magnetic field that magnetizes the sample of magnetic material, as well as depending on the field gradient applied.

To measure the position of the micro tab (2) an optical system is used, where the emitter is composed of a modulated red laser with a square signal at a frequency of 4.6 KHz and the receiver is formed by a four-quadrant photodetector together to a series of electronic stages where the four signals are added and subtracted, each corresponding to a quadrant, to finally detect the one that allows to measure the transverse vibration with a lock-in amplifier, which provides a proportional continuous voltage signal to the position of the beam of light with respect to the origin, and with which it is possible to eliminate interference and ambient light.

The way to detect the movement of the micro tongue (2) is by adhering a small mirror (3) at the top and placing the optical system (4) so that the light from the emitter falls on the mirror and in turn is reflected until Get to the detector. To achieve a precise and immune adjustment to mechanical vibrations, a support made of a non-magnetic material has been designed to adjust the position of each element with screws.

Figure 3 shows the three-dimensional model of the support (1) next to the

electromagnet of the optical detector (4), where in the two triangular pieces located in the part

upper central are, attached on their oblique face, a mirror in each

5 to direct the beam of light.

To magnetize the sample, a ferrite core (5) is used, on which wires are wound

copper (6) to make the electromagnet that magnets the sample using a magnetic field

alternate.

10 Said element is shown in greater detail in Figure 3, so that in the iron (7) of the electromagnet is where the micro tab (2) is located, since it is the area where the magnetic field is most constant.

15 To excite the electromagnet, a series resonant circuit excited by a square signal is used, which is generated with a microcontroller (8) Foot 16F887, a signal that is amplified by an amplifier (9), so that with a resonant circuit (10) avoiding having to excite the electromagnet (11) with a sinusoidal signal.

20 In figure 4, a diagram of the situation of the elements that magnetize the sample is shown while in figure 5, the electromagnet (11) and the spools of copper wire (6) that generate the field gradient can be observed , which are manufactured by designing a cylindrical support also of non-magnetic material and winding very fine copper wire around it and at both ends, giving the same number of turns but in the direction

25 opposite to achieve a null magnetic field in the center, where a hole is defined

(12) through which the micro tab (2) is passed. The spools of copper wire (6) are excited using a high precision signal generator (13) through a resistor with which the current flowing is measured.

30 Optionally, the spools of copper wire (6) may have a magnetic core.

Figure 6 shows the block diagram of the system where it can be seen that the microcontroller (8) Pie is controlled by a computer (1 4) through the serial port, which is used to vary the frequency of the signal that excites the electromagnet

5 The first step is to measure the resonance frequency of the micro tab (2) calibrating the optical detector and tapping the mechanical support, obtaining always frequencies of the order of 100 Hz.

Next, the procedure is first to excite the electromagnet with a signal

10 frequency between 400 Hz and 15 KHz and increase the frequency of the high precision signal generator until observing the first maximum vibration at the output of the Lock-In, which gives us the first resonance, so that the micro tab (2) oscillates with an amplitude more than enough to be able to measure it and then continue increasing the frequency until it reaches the second resonance. Subtracting the excitation frequency of the electromagnet with that of the

In 15 cases the mechanical resonance frequency of the micro tongue is obtained in both cases.

Claims (1)

1a.-Procedure for measuring magnetic parameters and temporal harmonics both in phase and quadrature of the magnetic moment of small excited samples 5 with alternating or continuous magnetic fields, characterized in that it consists in the creation of a magnetic field or alternating field gradient , of controllable amplitude and frequency acting on a sample that adheres to a resonant mechanical system of defined and known resonance frequency, so that another non-homogeneous alternating magnetic field is actuated on the sample, such that in the sample it creates a countryside 10 null, with the particularity that the net force generated on the sample is nonzero, after which the oscillation of the resonance system is measured. 2a._ Procedure for measuring magnetic parameters and temporal harmonics both in phase and quadrature of the magnetic moment of small excited samples 15 with alternating or continuous magnetic fields, according to claim 1, characterized in that the frequency of the gradient of the alternating field must be equal to the sum of the frequency of the exciter field plus the resonant frequency of the resonant mechanical system. 3a._ Procedure for measuring magnetic parameters and temporal harmonics 20 both in phase and in quadrature of the magnetic moment of small samples excited with alternating or continuous magnetic fields, according to claim 1, characterized in that the frequency of the gradient of the alternating field must be equal to the frequency of the exciter field minus the resonant frequency of the resonant mechanical system. 4-Method according to claim 18, characterized in that for measuring the temporal harmonics both in phase and in quadrature of the magnetic moment of the small excited samples, the amplitude of the gradient of the alternating field is modulated in amplitude with a frequency of multiple modulation or sub multiple of the frequency of the resonant mechanical system. 5th procedure according to claim 1, characterized in that when it comes to measuring the temporal harmonics both in phase and in quadrature of the magnetic moment of the small excited samples, the amplitude of the gradient of the alternating field is modulated in 5 frequency with a modulation frequency multiple or sub multiple of the frequency of the resonant mechanical system. 6a ._ Device for implementing the procedure of claims 1 to 5, characterized in that it is constituted from an electromagnet (11) capable of generating both continuous and alternate fields, in which a pair of reels (6) participate with or without a magnetic core, capable of generating an alternating gradient of magnetic field, reels (6) that define an air gap (7) in which a resonant mechanical system of oscillating micro-tongue (2) is established, having provided that in correspondence with said oscillating mechanical micro-tongue resonant system (2) is arrange a position detection system (4) of the resonant system.