EP0082752A1 - Broad-band, high-power non-reciprocal microwave device, and application thereof - Google Patents

Abstract

1. Non-reciprocal, wide frequency band, high power level microwave device comprising at least one member of ferrimagnetic material and means for applying a magnetic polarization field whereby the material is saturated, characterized in that the material is a monocrystal and in that the monocrystal is oriented along a crystallographic axis determined in such a manner that the variation of the anisotropy field as a function of the temperature compensates for the variations of the saturation magnetization and of the applied magnetic field as a function of the temperature, thus permitting to maintain stable as a function of the temperature the resonance line width of the monocrystal and the minimum frequency of said band.

Description

The present invention relates to a non-reciprocal microwave device with electromagnetic waves, such as for example a Y-gate circulator with three doors, intended to operate simultaneously over a very wide frequency band and at a high power level, and this in a wide range of temperatures.

A non-reciprocal device is a device whose transmission characteristics change according to the direction of propagation of the waves through said device.

In general, for non-reciprocal microwave devices comprising at least one piece of ferrimagnetic or gyromagnetic material, such as for example junction circulators, it is known to apply a continuous polarizing magnetic field, known as a saturating static field. the material, and lower than the gyromagnetic resonance field so as to obtain very low magnetic losses.

On the other hand, various structures of a junction circulator are already known using a ferrimagnetic material polarized below the gyromagnetic resonance and in the saturated state. One of them, of the triplate type, is described in the French certificate of addition no. 2,344,141 granted to the Applicant, concerning: "Junction circulator for transmission at high power level at microwave frequency". According to this certificate of addition, the junction circulator mainly comprises a conductor with three branches inserted between two discs of polycrystalline ferrimagnetic material, and two ground planes disposed respectively on either side of the two discs.

However, such a junction circulator operates on a very low frequency band, the ratio of extreme frequencies being of the order of 1.08.

• - à l'existence de l'anisotropie magnétocristalline se traduisant par un champ d'anisotropie dont l'influence dépend de l'orientation de chaque cristallite par rapport au champ appliqué de polarisation ;
• - à l'existence de champs démagnétisants locaux dus en particulier à l'effet de porosité dans le matériau et à la forme et dimensions de chaque cristallite.

This low bandwidth is linked in particular to the determination of the minimum operating frequency which depends on the maximum value of the saturation magnetization. Thus, for a polycrystalline ferrimagnetic material, it is known that the width of the resonance line is relatively large due to the known phenomenon of so-called natural gyromagnetic resonance, so that said minimum operating frequency must be significantly greater than the frequency of said resonance, which has the effect of reducing the bandwidth of the circulator. In addition, in a polycrystalline material, there are known causes of widening of the resonance line, leading to a "loss" of the desired operating band, observed at the bottom of the frequency band when the material is at just saturated state. These causes are mainly due to:

• - the existence of magnetocrystalline anisotropy resulting in an anisotropy field whose influence depends on the orientation of each crystallite with respect to the applied polarization field;
• - the existence of local demagnetizing fields due in particular to the porosity effect in the material and to the shape and dimensions of each crystallite.

There is also known a triplate junction circulator as described in the French addition certificate cited above, that is to say using two discs of polycrystalline ferrimagnetic material, which operates over a wide frequency band, the ratio of extreme frequencies being of the order of 2.25, in a large temperature range, between -40 ° C and + IOO ° C, but this only at low level of peak and average powers, the peak power being of l '' order of 100 W.

Obtaining a low microwave power level for such a polycrystalline ferrite circulator can be explained as follows.

As is known, the peak power, proportional to the square of the microwave field, must not reach a level critical beyond which the transmission is affected by non-linear effects resulting in additional magnetic losses, thereby destroying the performance of the circulator. These nonlinear effects are due to the fact that the electronic spins do not remain parallel to each other in their movements and that there are first and / or second order spin waves, the simultaneous excitation of the first and second spin waves. second orders occurring in an area surrounding the gyromagnetic resonance.

A minimal microwave critical field is thus defined from which such effects appear, this critical field being a function of the applied static field.

Given that this circulator operates over a wide frequency band, and that the damping on the edges of the resonance for a polycrystalline ferrimagnetic material disappears only slowly due to the causes of widening of the resonance line mentioned above, for certain crystallites there is a simultaneous excitation of the first and second order spin waves. Under these conditions, the minimum microwave critical field takes on a very low value, so that the circulator can only withstand a low level of peak power in a large part of its operating band.

The object of the present invention is to provide a non-reciprocal microwave device, such as a junction circulator, operating at the same time over a very wide frequency band, the ratio of the extreme frequencies being greater than 2.75, at a high level. peak power, greater than 2 kW, and in a wide temperature range, between -40 ° C and + 100 ° C.

To this end, the subject of the invention is a non-reciprocal microwave device with a wide frequency band and a high power level, comprising at least one piece of ferrimagnetic material and means for applying a polarizing magnetic field saturating the material, characterized in that the material is a single crystal, and in that the single crystal is oriented along a determined crystallographic axis so that the variation of the anisotropy field as a function of the temperature compensates for the variations of the saturation magnetization and of the applied magnetic field as a function of the temperature, thus making it possible to maintain in a stable manner as a function of the temperature the width of resonance line of the single crystal and the minimum frequency of said band.

The invention also relates to a use of the microwave device according to the invention, this use being characterized in that the device constitutes a Y-junction circulator of the triplate type.

• - la figure 1 représente des courbes explicatives de phénomènes physiques liés à un ferrite monocristallin ;
• - la figure 2 représente des courbes des pertes magnétiques respectivement pour un ferrite monocristallin et un ferrite polycristallin ;
• - la figure 3 est une vue en coupe d'un circulateur à jonction Y du type triplaque réalisé selon l'invention ; et
• - la figure 4 est une vue en coupe selon la ligne IV-IV de la figure 3.

Other characteristics and advantages of the invention will appear better in the detailed description which follows and refers to the appended drawings, given only by way of example and in which:

• - Figure 1 shows explanatory curves of physical phenomena related to a monocrystalline ferrite;
• - Figure 2 shows the magnetic loss curves respectively for a monocrystalline ferrite and a polycrystalline ferrite;
• - Figure 3 is a sectional view of a Y-junction circulator of the triplate type produced according to the invention; and
• - Figure 4 is a sectional view along the line IV-IV of Figure 3.

où µ_o est la perméabilité du vide
[_X] est définie comme la susceptibilité magnétique tensorielle du ferrite, appelée tenseur de Polder.First of all, it will be recalled that a ferrimagnetic or gyromagnetic material, constituted by a ferrite belonging to the class of ferrites for microwave, to which a microwave magnetic field h of pulsation ω = 2 π f is applied, is the seat of a field magnetization M linked to the field h by the matrix relation:

where µ _o is the permeability of the vacuum
[ _X] is defined as the tensorial magnetic susceptibility of ferrite, called the Polder tensor.

In addition, we know that due to the magnetic losses presented by the magnetization field M, the magnetic susceptibility _X is a complex quantity written:

où µ_o est la perméabilité du vide

• B est l'induction magnétique
• h est le champ magnétique hyperfréquence.

In addition, it is known to characterize microwave ferrites by what is called the magnetic permeability of ferrite µ defined by the matrix relationship:

where µ _o is the permeability of the vacuum

• B is magnetic induction
• h is the microwave magnetic field.

où µ "représente les pertes magnétiques dans le ferrite.As we know that the magnetic induction B also presents magnetic losses, the magnetic permeability u is also a complex quantity written:

where µ "represents the magnetic losses in the ferrite.

It will be noted that the magnetic permeability µ is linked to the magnetic susceptibility X by the relation:

où µ_o est la perméabilité du videOn the other hand, it is known that for a ferrite, for example in the form of a disc of axis of symmetry Oz, to which is applied a continuous magnetic field of polarization H, said static field, and capable of saturating the ferrite, it is created a field inside the ferrite, H uniform and directed according to Oz. As is known, the internal field H _o is equal to:

where µ _o is the permeability of the vacuum

M is the saturation magnetization, in S.I. units.

N _z is the demagnetizing factor in the direction Oz.

soit

où Y est le rapport gyromagnétique, Y étant négatif.To this internal field H _o is associated a pulsation ω _o defined for a ferrite in the just state saturated by the relation:

is

where Y is the gyromagnetic ratio, Y being negative.

FIG. 1 represents, as a function of the pulsation w, on the one hand in solid lines the variation of µ '(real part of the magnetic permeability u), and on the other hand in dotted lines the variation of the losses µ "(imaginary part permeability µ), for a monocrystalline ferrimagnetic material in the saturated state, polarized below the gyromagnetic resonance, and intended for the production of a non-reciprocal microwave device, such as for example a Y junction circulator of the type triplate, or of the waveguide type, according to the invention.

As it can be seen in this figure, the shape of the curve µ "characterizes the known phenomenon of gyromagnetic resonance in region I, called the resonance loss zone, the resonance pulsation w _eff corresponding to µ" _max . The width of the resonance line ΔH is defined as the width at mid-height of the curve µ ".

To obtain a Y-junction circulator with low magnetic losses µ ", it is necessary to determine a minimum operating frequency F _{min '} and therefore a minimum pulse ω _min , located outside of the gyromagnetic resonance, that is:

où les notations sont les mêmes que celles utilisées précédemment, avec 2N_t + N_z = 1.As is known, we define the resonance pulsation ω _eff for a monocrystalline ferrite in the form of a disc of axis of symmetry Oz, by the relation:

where the notations are the same as those used previously, with 2N _t + N _z = 1.

Consequently, relation (2) becomes:

With the saturation magnetization M expressed in SI units, we associate a pulsation ω _M such that:

Y being the gyromagnetic ratio (negative γ).

We also know that the efficiency of a ferrite is directly conditioned by the magnetization which must be chosen as large as possible. Nevertheless, from relations (1) and (3), there is a limitation of the maximum value of the saturation magnetization for the minimum operating frequency F _{min '} such that we have:

It will be noted that the maximum operating frequency F _max also depends on the saturation magnetization M ₅ ; in general, the maximum pulsation w is substantially equal to 2 ω _M.

For a Y-junction circulator comprising two ferrites, and intended to operate on a wide frequency band, a field of polarization known as weak field is applied to each ferrite, defined as being the field less than that necessary to create the gyromagnetic resonance in the useful function band ment; thus region II represented in FIG. 1, and in which the circulator operates, is said to be a weak field operating zone.

FIG. 2 represents, as a function of the pulsation w, on the one hand in solid lines the variation of the losses µ "for a polycrystalline ferrite, and on the other hand in dotted lines the variation of these same losses µ" for a monocrystalline ferrite.

As it appears in this figure, and as is known, the width of resonance line AH of a polycrystalline ferrite is significantly greater than that of a monocrystalline ferrite of the same composition. This is due to the fact that each crystallite of a polycrystal resonates at a different frequency, thus each defining a specific resonance line width, so that the overall resonance line width of the polycrystal is equal to the sum of the line widths of crystallite resonance.

Under these conditions, the use of two identical monocrystalline ferrites in a Y-junction circulator according to the invention makes it possible to determine a minimum operating frequency F _min significantly lower than that of a circulator with two identical polycrystalline ferrites according to the prior art , and therefore makes it possible to widen the useful operating band of the circulator, the maximum frequencies in the two cases being substantially equal. In practice, the minimum frequency F _min of a circulator with monocrystalline ferrites will be determined so that we have:

To transmit electromagnetic waves at a high level of microwave power, it is known on the one hand that the average power is limited by the magnetic losses of the saturated material, these losses producing a heating of the material, and on the other hand that the power of admissible peak must not reach a critical level beyond which the transmission is affected by nonlinear effects.

où ω_M = - γ M_s

avec γ négatif et M exprimé en unités S.I.More precisely, the peak power is directly linked to the critical microwave field h from which non-linear effects (first and / or second order spin waves) appear. This critical microwave field h takes a minimum value for a so-called subsidiary static field, and tends towards a very large value for a limit static field beyond which there are no longer first order spin waves. We know that the existence of spin waves is related to the damping of the spin movement, these waves occurring all the more easily as this movement is less damped. To characterize this damping, we introduce, as is known, a line of wave lines of spin AH _k . Thus, for example for a circulator using flat ferrite discs, the minimum critical field (h _c ) _min is defined as a function of the line width of the spin waves ΔH _k by the expression:

where ω _M = - γ M _s

with negative γ and M expressed in SI units

In general, the first and second order spin waves are excited simultaneously in an area surrounding the gyromagnetic resonance, depending on the dimensions of the ferrite, and for which the minimum critical field (h _c ) _min is very small, of such that a wide-frequency Y-junction circulator can only support a low peak power level there. Consequently, it proves essential to operate such a circulator outside this zone so as to avoid the simultaneous excitation of the first and second order spin waves.

For example, in the case of a ferrite disc of axis of symmetry Oz, the zone in which the coincidence between the first and second order spin waves occurs, extends from ω _eff to ω _eff + N _t ω _{M '} with 2N _t + N _z = 1, N _z being the factor d _ma - gnétisant following the direction Oz.

As a monocrystalline ferrite has a smaller resonance line width than that of a polycrystalline ferrite of the same composition, the passage from a polycrystalline ferrite to a monocrystalline ferrite makes it possible to move down the upper limit of the area in which produces the coincidence between the first and second order spin waves, so that the monocrystalline ferrite circulator, according to the invention, and operating outside said zone, has a wider useful band than that of a circulator with polycrystalline ferrites, and can simultaneously withstand a relatively high peak power.

sellon la relation (1).To obtain a very high peak power, each monocrystalline ferrite is doped with relaxing ions, that is to say ions increasing the linewidth of spin waves ΔH _k , thereby increasing the minimum value of the hyper-frequency critical field (h _c ) _min for an applied static field H equal to N _z M _s

sell relation (1).

For a monocrystalline ferrite such as for example nickel ferrite, pure or substituted for example with aluminum or zinc, the ions of cobalt will be chosen as relaxing ions. For other monocrystalline ferrites, rare earth ions will be used, for example such as Dysprosium or Holmium ions, making it possible to increase the line width of spin waves ΔH _k .

On the other hand, we know that a ferrite single crystal is anisotropic, that is to say that its properties depend on the direction envisaged. This anisotropy results in the fact that spontaneous magnetization has a natural tendency to orient itself in certain privileged crystal directions. Thus, to make the spontaneous magnetization take a direction different from these privileged directions, it is necessary to provide a certain work, called magnetocrystalline energy.

où M_s est l'aimantation à saturation exprimée en unités S.I. et avec K variant avec la température, de sorte que le champ d'anisotropie H_anis varie en fonction de la température.This anisotropy effect can be described in terms of an equivalent magnetic field, called the H anisotropy field. , collinear with the applied static field H, and whose influence depends on the orientation of each crystallite with respect to said applied static field. For a detailed explanation of this field of anisotropy, one will refer for example to the work "Handbook of microwave ferrite materials" published in 1965 by Wilhelm H. von Aulock, pages 32 to 38. The study of this field of anisotropy shows that, as a first approximation, it only depends on an anisotropy constant K and on an angle θ such that θ = (H _a , [100]) where [100] is one of the main crystallographic directions of cubic crystal lattice of a ferrite. The anisotropy field H _{anis is} written:

where M _s is the saturation magnetization expressed in SI units and with K varying with temperature, so that the anisotropy field H _anis varies as a function of temperature.

For example, the anisotropy field for a Nickel ferrite is negative, while it is positive for a Cobalt ferrite. Under these conditions, for a nickel ferrite doped with Cobalt ions, from a certain doping rate and at a temperature dependent on this doping, the anisotropy field of negative origin becomes positive.

• où H_a est le champ statique appliqué
• N_z est le facteur démagnétisant suivant la direction Oz
• M est l'aimantation à saturation en unités S.I. s
• µ est la perméabilité du vide.

Due to this anisotropy field H _anis , the magnetic field H _o inside a ferrite in the form of a disk with an axis Oz is equal to:

• where H _a is the applied static field
• N _z is the demagnetizing factor in the direction Oz
• M is the saturation magnetization in SI units s
• µ is the permeability of the vacuum.

As the applied static field H _a and the saturation magnetization M _s vary as a function of temperature, this results that the width of resonance line AH of a monocrystalline ferrite and the minimum operating frequency F _min are not stable as a function of the temperature. This instability of the resonance line width and the minimum operating frequency is troublesome for a Y-junction circulator with a very wide frequency band and a high power level, since there is a risk of simultaneous excitation of the spin waves. of the first and second orders which is to be avoided for such a circulator. Under these conditions, it proves essential to keep the positions of said resonance line width and of said minimum operating frequency substantially constant.

To this end, according to the invention, the variations of the applied field H and of the saturation magnetization M _s are compensated as a function of the temperature by the variations of the anisotropy field H _anis as a function of the temperature, the field d anisotropy thus playing the role of an element for adjusting the stability of the resonance line width, and therefore the minimum operating frequency, as a function of the temperature. Consequently, the minimum microwave critical field (h _c ) _min at the minimum operating frequency F _min is kept constant, ensuring the maintenance of the high level of the admissible peak power.

This compensation for the different variations as a function of temperature by the anisotropy field H _anis is obtained by a particular orientation of the monocrystalline ferrite with respect to the applied static field H. This orientation of the single crystal is carried out along a determined crystallographic axis so that it corresponds to the most stable value as a function of the temperature of the magnetic field H inside the ferrite, defined by:

The crystallographic orientation axis of the single crystal to be retained is obtained experimentally or by calculations, and is different depending on the type of monocrystalline ferrite used. Thus, for example for pure nickel ferrite, doped with less than 1% of Cobalt ions at room temperature, the preferred orientation of the single crystal is the axis [00] in the cubic lattice.

The single crystal is then cut for example in the form of one or more discs whose axis of symmetry is oriented relative to the crystallographic axes of the medium. The surface condition of the discs can be obtained either by fine lapping or by optical polishing.

Figures 3 and 4 show sectional views of a Y-junction circulator of the triplate type.

This circulator comprises two ground planes 10 and 11 respectively disposed on either side of two discs (13, 14) made of monocrystalline ferrite, such as for example nickel ferrite, doped with Cobalt ions, and oriented for example along the crystallographic axis [100]. Note that the two discs have the same crystallographic orientation, so that their respective resonant line widths are superimposed, thereby obtaining the lowest possible minimum operating frequency.

For the sake of clarity, the cross-sections of the ground planes 10 and 11 shown in FIG. 3 have not been hatched.

Between the two ferrite discs 13 and 14 is inserted a central conductor 15 with three branches forming between them an angle of 120 °.

As it appears in FIG. 4, each branch of the conductor 15 ends in a tongue 17 intended to be fixed, for example by welding, to a connector 19 of which only one has been shown in this figure.

The polarization magnetic field applied to the two ferrite discs 13 and 14 is established by a permanent magnet constituted by discs 21 housed in recesses 22 and 23 formed respectively in the ground planes 10 and 11.

There are shown at 25 crowns of dielectric material each inserted between a ground plane and the central conductor, these crowns surrounding the ferrite discs.

Claims (10)

1. A non-reciprocal microwave device with a wide frequency band and a high power level, comprising at least one piece of ferrimagnetic material and means for applying a polarizing magnetic field saturating the material, characterized in that the material is a single crystal, and in that the single crystal is oriented along a determined crystallographic axis such that the variation of the anisotropy field as a function of the temperature compensates for the variations of the saturation magnetization and of the applied magnetic field as a function of the temperature, allowing thus to maintain stably as a function of the temperature the resonance line width of the single crystal and the minimum frequency of said band. 2. Device according to claim 1, characterized in that the single crystal is doped with ions increasing its width of the spin wave line, thus making it possible to increase the peak power of the device. 3. Device according to one of claims 1 and 2, characterized in that the single crystal consists of a ferrite. 4. Device according to claim 3, characterized in that the ferrite is a nickel ferrite doped with cobalt ions. 5. Device according to claim 4, characterized in that the crystallographic axis of orientation of the ferrite is the axis [100]. 6. Device according to one of the preceding claims, characterized in that the single crystal is produced in the form of a disc whose axis of symmetry is oriented relative to the crystallographic axes. 7. Use of a microwave device according to any one of the preceding claims, characterized in that the device constitutes a junction circulator comprising two parts made of ferrimagnetic material. 8. Use according to claim 7, characterized in that the circulator is of the Y-junction three-plate type with three doors. 9. Use according to claim 8, characterized in that the circulator comprises a central conductor (15) with three branches forming between them an angle substantially equal to 120 °, the conductor being inserted between two discs (13, 14) made of ferrimagnetic material , two ground planes (10, 11) arranged on either side of the two discs, respectively, the magnetic polarization field being established by permanent magnets (21) housed in recesses made in the ground planes. 10. Use according to claim 9, characterized in that the two discs (13, 14) made of ferrimagnetic material have the same crystallographic orientation.

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