EP3033755B1 - Ferrite device for power application and manufacturing method of device - Google Patents

Description

The field of the invention is that of multilayer magnetic components for inductance and transformer functions in power electronics and more precisely that of components comprising ferrite materials used at high and very high frequencies (between 1 MHz and 100 MHz) which can advantageously present low magnetic losses for large applied powers and for operation over a wide temperature range, typically from -50 ° C to + 150 ° C.

In this context, it has already been proposed to use "low loss" ferrite materials intended for high frequency applications (f> 1 MHz), it may especially be ferrites based on nickel, zinc and copper, cobalt. They are used as magnetic cores of various shapes (cores, pots, rods, etc.) and allow the realization of inductors or wound transformers, the winding portion being made using enamelled copper wire or coaxial conductor.

Their advantages are a low sintering temperature (T <950 ° C), an adjustable permeability between 10 and 1000, low magnetic losses at high and very high frequencies, a possible operation in a wide temperature range and a low manufacturing cost. .

Currently, the development of electronic equipment, both in civilian and military applications, is related to the miniaturization of passive and active components. Among these components, the largest are inductive passive components (inductance or transformer). When the volume of the inductive components is reduced, the power density increases, which leads to an increase in the operating temperature. Yields decrease as well as the life of electronic equipment.

In applications that use high electrical powers, the losses of the inductive component are essentially determined by the magnetic losses, said total losses of the magnetic material used for the realization of the core.

Ferrites based on nickel, zinc, copper, cobalt are particularly well suited because of their adapted magnetic properties and their high electrical resistivity for this type of application. In addition, multilayer components can be produced from this type of magnetic ferrite material by the coffering technology.

In particular, it has already been proposed to use LTCC technology for "Low Temperature Cofired Ceramic" that requires the prior production of cast strips of Ni, Zn, Cu and Co-based ferrite material to develop inductive components. For this, the shaping of the ceramic powder consists of making strips with a thickness of 50 to 300 microns. It is then possible to deposit on these strips conductive inks, in particular based on silver, and then stack several strips to produce integrated components as shown by Figures 1a and 1b . Ferrite layers 10 are covered with tracks, for example silver 11, interconnected from one layer to another by Via. The figure 1b highlights metal terminations connected to the turns 12.

In order to achieve micro-inductances, coils can be made from screen-deposited metal or other thick layer ceramics deposition technique. To avoid magnetic flux leakage, it is essential to magnetically isolate the turns of the winding or coils.

It is then necessary to use dielectric strips compatible with ferrite-silver cofiring at 900 ° C. Nevertheless, problems of interdiffusion and different coefficients of thermal expansion complicate the realization of such components in the presence of intermediate dielectric elements.

In this context, the present invention proposes to use a dielectric material, which can form cast strips for the elaboration of dielectric elements, integrated in the component and does not disturb the composition of the elements in the presence during the operation of co-curing of all the constituents of the component: cast strips of ferrite material, metal tracks, cast strips of dielectric material.

Thus, the proposed solution consists in using a non-magnetic ferrite, of the same family as the magnetic ferrite chosen, so as to keep the same elements in a similar way, even identical, for magnetic and dielectric parts, and so to assemble ceramic materials whose coefficients of shrinkage and expansion are close.

• le ferrite magnétique répond à la formulation chimique : Ni_xMg_yZn_zCu_vCo_wFe_2-δO₄ avec v non nul, 0<δ<0.1 et x+y+z+v+w = 1 ;
• ledit composant comprend :
  • ∘ des pistes en métal noble pouvant être de l'argent, de l'or ou du palladium-argent, réparties sur différents niveaux constitués par les surfaces des couches, pour former à chaque niveau une spire, les spires d'un niveau à un autre niveau pouvant être reliées électriquement ou non ;
  • ∘ des éléments diélectriques à base de ferrite amagnétique, comprenant du nickel, du zinc et du cuivre, positionnés sur au moins une partie desdites pistes de métal noble et entre au moins deux couches de matériau ferrite magnétique, de manière à ce que lesdits éléments diélectriques soient incorporés dans du matériau ferrite magnétique ; le ferrite amagnétique répondant à la formule suivante : Ni_aMg_bZn_cCu_dCo_eFe_2-δO₄ avec : $$\mathrm{c}>0.5,\mathrm{d}>0.1,0<\mathrm{\delta }<0.1\mathrm{et}\mathrm{a}+\mathrm{b}+\mathrm{c}+\mathrm{d}+\mathrm{e}=1;$$
    
  • ∘ un empilement desdites pistes métalliques constituant lesdites spires, dans lequel chaque piste métallique est séparée de la piste métallique du niveau supérieur ou inférieur par un matériau diélectrique en ferrite amagnétique.

More precisely, the subject of the invention is an inductive component comprising a stack of layers, said stack comprising layers based on magnetic ferrite, characterized in that:

• the magnetic ferrite corresponds to the chemical formulation: Ni _x Mg _y Zn _z Cu _v Co _w Fe _2-δ O ₄ with v not zero, 0 <δ <0.1 and x + y + z + v + w = 1;
• said component comprises:
  • ∘ noble metal tracks that can be silver, gold or palladium-silver, distributed on different levels formed by the surfaces of the layers, to form at each level a turn, the turns from one level to one other level that can be electrically connected or not;
  • ∘ non-magnetic ferrite dielectric elements, comprising nickel, zinc and copper, positioned on at least a portion of said noble metal tracks and between at least two layers of magnetic ferrite material, such that said dielectric elements are incorporated in magnetic ferrite material; nonmagnetic ferrite corresponding to the following formula: Ni _a Mg _b Zn _c Cu _d Co _e Fe _2-δ O ₄ with: $$\mathrm{vs}>0.5,\mathrm{d}>0.1,0<\mathrm{\delta }<0.1\mathrm{and}\mathrm{at}+\mathrm{b}+\mathrm{vs}+\mathrm{d}+\mathrm{e}=1;$$
    
  • ∘ a stack of said metal tracks constituting said turns, wherein each metal track is separated from the metal track of the upper or lower level by a dielectric material made of non-magnetic ferrite.

According to a variant of the invention, the dielectric elements are based on a nonmagnetic ferrite ceramic having a sintering temperature of between 800 and 1000 ° C.

According to a variant of the invention, w is non-zero.

The invention also relates to a micro-inductor comprising an inductive component according to the invention, characterized in that it comprises coil elements positioned on at least one set of magnetic ferrite material layers, so as to perform a winding integrated in said component, said coil elements being electrically connected to each other.

The subject of the invention is also a transformer comprising an inductive component according to the invention, characterized in that it comprises at least two series of coil elements positioned on at least one set of layers of magnetic ferrite material, so as to making windings integrated in said component, in each series the coil elements being electrically connected.

The invention also relates to an electronic system comprising at least one inductive component, or a micro-inductor or a transformer and at least one capacitor and an electronic control, characterized in that the inductive component or the micro-inductor or the transformer is according to the invention.

According to a variant of the invention, the capacitor comprises a nonmagnetic ferrite material having a permittivity greater than or equal to 20.

• l'élaboration de bandes coulées de matériau ferrite magnétique répondant à la formule suivante : Ni_xMg_yZn_zCu_vCo_wFe_2-δO₄ avec v non nul, 0<δ<0.1 et x+y+z+v+w = 1 ;
• la réalisation de pistes métalliques à base de métal noble pouvant être en argent, en or ou en palladium à la surface d'au moins une partie desdites bandes coulées de matériau ferrite magnétique ;
• l'élaboration de bandes coulées à base de matériau diélectrique ferrite amagnétique ;
• la découpe d'éléments dans lesdites bandes coulées à base de matériau ferrite amagnétique ;
• le positionnement desdits éléments au niveau des pistes métalliques à la surface desdites bandes coulées de matériau ferrite ;
• l'ajout d'éléments de ferrite magnétique en périphérie des éléments diélectriques et en surface de manière à incorporer lesdits éléments diélectriques dans ledit composant ;
• une opération de cofrittage de l'ensemble des bandes coulées de matériau ferrite magnétique intégrant les pistes métalliques, et des éléments diélectriques de manière à former ledit composant inductif.

The subject of the invention is also a method for manufacturing an inductive component according to the invention, characterized in that it comprises the following steps:

• the production of cast strips of magnetic ferrite material corresponding to the following formula: Ni _x Mg _y Zn _z Cu _v Co _w Fe _2-δ O ₄ with v not zero, 0 <δ <0.1 and x + y + z + v + w = 1;
• the production of metal tracks based on noble metal may be silver, gold or palladium on the surface of at least a portion of said strips of magnetic ferrite material;
• the production of cast strips based on non-magnetic ferromagnetic dielectric material;
• cutting elements in said cast strips based on non-magnetic ferrite material;
• positioning said elements at the metal tracks on the surface of said cast strips of ferrite material;
• adding magnetic ferrite elements at the periphery of the dielectric elements and at the surface so as to incorporate said dielectric elements in said component;
• an operation of cofritting all the cast strips of magnetic ferrite material integrating the metal tracks, and dielectric elements so as to form said inductive component.

According to a variant of the method, the realization of metal tracks is performed by a screen printing operation.

According to a variant of the method, the metal tracks are deposited by ink jet.

According to a variant of the process, the co-sintering operation is carried out at a temperature of between about 800 ° C. and 1000 ° C.

• la réalisation d'encres de ferrite magnétique, de ferrite amagnétique et de métal noble ;
• la réalisation des couches dudit empilement par dépôts par jets utilisant lesdites encres.

The subject of the invention is also a method for manufacturing an inductive component according to the invention, characterized in that it comprises the following steps:

• the production of magnetic ferrite inks, nonmagnetic ferrites and noble metals;
• making the layers of said stack by jet deposits using said inks.

• les figures 1a et 1b illustrent une micro-inductance de l'art connu ;
• les figures 2a et 2b illustrent une structure de transformateur de l'invention comprenant un empilement de spires métalliques réalisées à la surface de bandes coulées de ferrite magnétique, lesdites spires étant recouvertes de diélectrique ferrite amagnétique ;
• la figure 3 illustre les pertes mesurées au niveau du primaire d'un transformateur dont la structure comprend un empilement de bandes cofrittées avec un diélectrique classique et avec un diélectrique ferrite amagnétique, en fonction de la tension appliquée ;
• la figure 4 illustre une variante de composant selon l'invention, dans lequel l'élément diélectrique recouvre intégralement un élément de spire ;
• les figures 5a et 5b illustrent un exemple de composant selon l'invention mettant en évidence l'isolation de deux spires via des éléments de ferrite amagnétique ;
• les figures 6a et 6b illustrent un exemple de schéma de raccordement de 2 spires, respectivement côté primaire et côté secondaire dans un exemple de transformateur ;
• la figure 7 illustre les pertes mesurées au niveau du primaire d'un transformateur comprenant un composant similaire à celui illustré en figure 4, en fonction de la tension appliquée ;
• la figure 8 illustre les cotes d'un noyau de transformateur classique du commerce servant à établir des comparaisons de performances avec un composant de l'invention ;
• la figure 9 illustre un schéma de principe d'un système électronique à base de substrat céramique multifonctionnel.

The invention will be better understood and other advantages will become apparent on reading the description which follows given by way of non-limiting example and by virtue of the appended figures among which:

• the Figures 1a and 1b illustrate a micro-inductance of the known art;
• the Figures 2a and 2b illustrate a transformer structure of the invention comprising a stack of metal turns made on the surface of magnetic ferrite cast belts, said turns being covered with nonmagnetic ferrite dielectric;
• the figure 3 illustrates the losses measured at the primary level of a transformer whose structure comprises a stack of strips cofired with a conventional dielectric and with a nonmagnetic ferrite dielectric, as a function of the applied voltage;
• the figure 4 illustrates a component variant according to the invention, wherein the dielectric element completely covers a coil element;
• the Figures 5a and 5b illustrate an example of a component according to the invention highlighting the insulation of two turns via non-magnetic ferrite elements;
• the Figures 6a and 6b illustrate an example of a connection diagram of 2 turns, respectively primary side and secondary side in an exemplary transformer;
• the figure 7 illustrates the losses measured at the primary level of a transformer comprising a component similar to that illustrated in FIG. figure 4 , depending on the applied voltage;
• the figure 8 illustrates the ratings of a conventional commercial transformer core for performing performance comparisons with a component of the invention;
• the figure 9 illustrates a block diagram of an electronic system based on a multifunctional ceramic substrate.

In general, the inductive component of the present invention is a component comprising layers of magnetic ferrite material having the formula: Ni _x Mg _y Zn _z Cu _v Co _w Fe _2-δ O ₄ with v not zero, 0 <δ <0.1 and x + y + z + v + w = 1.

It may advantageously be a material based on Ni, Zn, Cu and Co but without limitation.

• des pertes très faibles à haute fréquence.
• la possibilité d'être frittés à basse température, aux environs de 900°C soit 400°C en-dessous de la température de frittage des ferrites conventionnels, ce qui permet de les cofritter avec des diélectriques et des métaux comme l'or ou l'argent.

Indeed, multilayer components made from NiZnCuCo ferrites have two important advantages:

• very low losses at high frequency.
• the possibility of being sintered at low temperature, at about 900 ° C or 400 ° C below the sintering temperature of conventional ferrites, allowing them to be co-cured with dielectrics and metals such as gold or silver.

Thus, in an inductive component of the invention, the coils can be made from silver deposited by screen printing or another technique, for example by ink jet. It should be noted that all the ceramic deposits can also use the inkjet technique.

To avoid magnetic flux leakage, a non-magnetic ferrite, also based on Ni, Zn and Cu (nonmagnetic ferrite may also comprise Co) is used so as to retain the same chemical elements for the magnetic and non-magnetic parts, and so to assemble ceramic materials whose expansion coefficients are close.

For this, by adjusting the composition of the ferrite, it is possible to modulate the Curie temperature, so as to lower it and that below the ambient temperature. It is thus possible to render a non-magnetic ferrite above ambient temperature and to enable it to provide an insulation function and thus make it possible to prevent magnetic flux leakage by adjusting the composition thereof.

The windings are made from silkscreened silver and covered with non-magnetic ferromagnetic dielectric elements to prevent magnetic flux leakage.

The combination of NiZnCuCo ferrites with low-temperature sintering dielectrics and silver-based metallizations thus enables the production of high-performance, cost-effective magnetic components.

It is thus easy to realize inductance and transformer functions that are necessary for the proper functioning of energy converters or power amplifiers.

Embodiment of a transformer according to the invention

A multilayer transformer was made from a cast NiZnCuCo ferrite. The schema of the structure is given in Figures 2a and 2b . Stack 4 layers as described in top view figure 2b alternating the side where the silver tracks come out. We close then with thicker layers of ferrites. We finally end with the connections of the turns between them.

The component thus comprises a ferrite core 20, around which is deposited a coil element 30, covered with a dielectric element 40, of the invention is a nonmagnetic ferrite, and then are deposited in a complementary manner magnetic ferrite elements 21 around of the dielectric element 40 of nonmagnetic ferrite.

• de matériau ferrite magnétique, sur lequel sont sérigraphiés les éléments de spire en argent ;
• de matériau ferrite amagnétique, diélectrique ;
• on découpe des éléments de matériau diélectrique et de matériau ferrite magnétique, disposé dans un même plan.

To achieve this type of structure, cast strips are produced:

• magnetic ferrite material, on which are printed the elements of spiral silver;
• non-magnetic, dielectric ferrite material;
• cutting elements of dielectric material and magnetic ferrite material, disposed in the same plane.

The whole is cofritté at 900 ° C under air for 2 hours so as to realize the transformer.

After sintering, electrical measurements were made at the input inductance (called the primary) and the output inductance (so-called secondary), in terms of power losses. More precisely, a power is injected into the component and the dissipated power is measured.

Two types of structures were tested after production according to the method described above: one using commercial dielectric strips (reference: ULF140 from AFM Microelectronics Inc.), the other, nonmagnetic ferrite strips, of composition: Ni _0.05 Cu _0.2 Zn _01.75 Fe _1.96 O ₄ Transformer "Dielectric" material Primary inductance Leakage inductance YOUR ULF 140 dielectric L _p = 1.1 μH L _f = 0.8 μH TB Non-magnetic ferrite L _p = 1.26 μH L _f = 1 μH

It is found that the primary inductances are low and quite comparable between the two transformers, and the leakage inductances relatively large, these leakage inductances accounting for the coupling between the primary and secondary.

The evolution of the losses at 2 MHz, as a function of the applied rms voltage, was measured for these two transformer structures, on the primary side. The results are given in figure 3 . The curve 3a is relative to the structure with the ULF dielectric 140, the curve 3b relates to the structure with the nonmagnetic ferrite dielectric.

It is found that the losses are higher when ULF140 dielectric is used, which can be explained by the presence of stresses generated by the difference of the shrinkage coefficients.

To reduce the leakage inductance and further improve the coupling, the Applicant has made structures for which the output of the silver conductors does not pass through the magnetic ferrite, for a transformer referenced TC, as illustrated in FIG. figure 4 . According to this variant, the dielectric element 40 'covers the entirety of the coil element 30.

The Figures 5a and 5b show two sectional views of an inductive component in an elementary or more elaborate form, comprising tracks forming two turns isolated by means of nonmagnetic ferrite elements. Such a configuration can be achieved by a succession of magnetic ferrite layers C _fmi , the nonmagnetic ferrite elements 40 thus isolate the two turns 30 _i and 30 _{i + 1} from each other.

The Figures 6a and 6b illustrate an example of a connection diagram of the two turns respectively on the primary side and secondary side, illustrated representatives of the portions of solenoids.

The primary and leakage inductances have been measured, the values obtained are given below for the CT transformer: TC Non-magnetic ferrite L _p = 1.68 μH L _f = 0.165 μH

There is a clear decrease in leakage inductance indicating better coupling.

The power losses were measured for the CT transformer under the same conditions as for transformers TA and TB. We here too, a clear improvement, with a gain of more than a factor of 3 compared to the previous best result. The Applicant was then able to measure up to 15 V rms.

The figure 7 shows the total losses for the CT transformer, as a function of the applied rms voltage, measured on the primary side.

• le transformateur TC étant de petite dimension, les valeurs d'inductance sont faibles et pour obtenir des valeurs similaires avec un noyau plus gros, il est nécessaire de choisir une perméabilité encore plus faible, ce qui empêche l'utilisation de ferrites Mn-Zn ;
• ce ferrite est un ferrite de puissance haute fréquence donc parfaitement adapté à la présente comparaison ;
• ses pertes à 3 MHz et 10 mT, données par le fabricant, valent 200 mW/cm³.

To evaluate the performance of this component in relation to the state of the art, the Applicant has chosen to compare it with a 2: 2 transformer, having 2 turns at the primary and 2 turns at the secondary made with copper wire of diameter. 300 μm on a Ferroxcube ferrite core 4F1. This choice is justified by the following remarks:

• since the CT transformer is small, the inductance values are low and to obtain similar values with a larger core, it is necessary to choose an even lower permeability, which prevents the use of Mn-Zn ferrites;
• this ferrite is a high frequency power ferrite therefore perfectly suited to the present comparison;
• its losses at 3 MHz and 10 mT, given by the manufacturer, are worth 200 mW / cm ³ .

The Ferroxcube 4F1 ferrite core retained is a core of type E22, the letter E corresponding to the shape of the core as shown in the diagram of the figure 8 , illustrating the ribs.

Dimensions are given in mm. The effective parameters for a core consisting of 2 E contiguous are shown in the table below: Effective length 31.6 mm Effective section 78.5 mm ² Effective volume 2480 mm ³

The E22 core at 4F1 was wound with 2 turns for the primary and 2 turns identical to the secondary. The Applicant measured the primary and leakage inductances that were compared to those of the CT transformer. The coupling coefficient was also measured in comparing primary and secondary voltages for fixed primary excitation. Transformer Primary inductance Leakage inductance coupling TC L _p = 1.68 μH L _f = 0.165 μH 66% E22 / 4F1 L _p = 1.7 μH L _f = 0.205 μH 59%

The results are similar, with slightly better performance for the cofired CT transformer.

• 540 mW pour le transformateur TC cofritté ;
• 480 mW pour le transformateur en 4F1.

To finalize the comparison, the Applicant measured the losses at 2 MHz according to the applied voltage. For an effective voltage of 15 V (sinusoidal signal), the results are as follows:

• 540 mW for the cofired CT transformer;
• 480 mW for the transformer in 4F1.

Similar losses (about 10% less for the transformer in 4F1) were obtained but for a cofritted structure of volume 6 times lower. If one compares neither the real volumes but the apparent volumes, one obtains a gain of a factor 10, demonstrating by the same, the potential of the cofired components realized.

This type of inductive component of the present invention which is efficient, of small size, can advantageously be integrated into more complex systems comprising all the passive functions based on ceramics (inductances, transformers, capacitors, filters) in a substrate thanks to the use of compatible materials and coffering technology. On one side of the substrate can be reported discrete components and on the other side, a cooled plate.

Such a system is illustrated in figure 9 which shows on a ceramic substrate 100, a cofired multilayer structure 101, elementary layers 102, a cooled plate 300, an electrical control 400 and a function to be fed 500. The set of constituent layers of the inductive components and capacitors are so advantageously made of magnetic ferrite material and non-magnetic ferrite.

Claims (12)

1. An inductive component comprising a stack of layers, said stack comprising magnetic ferrite based layers, characterised in that:

   - the magnetic ferrite has the chemical formula:
   Ni_xMg_yZn_zCu_vCo_wFe_2-δO₄, with v being non-zero, 0 < δ < 0.1 and x + y + z + v + w = 1;

   - said component comprises:

   o tracks made of noble metal that can be silver, gold or palladium-silver distributed over various levels formed by the surfaces of the layers, to form a turn on each level, the turns from one level to another level being or not being able to be electrically connected;

   ∘ non-magnetic ferrite based dielectric elements, comprising nickel, zinc and copper, positioned on at least one part of said noble metal tracks and between at least two layers of magnetic ferrite material, so that said dielectric elements are incorporated in the magnetic ferrite material; the non-magnetic ferrite having the following formula: Ni_aMg_bZn_cCu_dCo_eFe _{2 -δ}O₄, with: $$c>0.5,d>0.1,O<\delta <0.1\mathrm{und}a+b+c+d+e=1;$$

   

   ∘ a stack of said metal tracks forming said turns, in which each metal track is separated from the metal track of the upper or lower level by a dielectric material made of non-magnetic ferrite.
2. The inductive component as claimed in claim 1, characterised in that the dielectric elements are based on a non-magnetic ferrite ceramic having a sintering temperature between 800°C and 1000°C.
3. The ferrite based inductive component as claimed in any one of claims 1 or 2, characterised in that w is non-zero.
4. A micro-inductor comprising an inductive component as claimed in any one of claims 1 to 3, characterised in that it comprises turn elements positioned on at least one set of layers of magnetic ferrite material, so as to produce a winding integrated in said component, said turn elements being electrically connected together.
5. A transformer comprising an inductive component as claimed in any one of claims 1 to 3, characterised in that it comprises at least two series of turn elements positioned on at least one set of layers of magnetic ferrite material, so as to produce windings integrated in said component, the turn elements being electrically connected together in each of the series.
6. An electronic system comprising at least one inductive component or one micro-inductor or one transformer and at least one capacitor and an electronic command, characterised in that the inductive component or the micro-inductor or the transformer is in accordance with any one of claims 1 to 5.
7. The electronic system as claimed in claim 6, characterised in that the capacitor comprises a non-magnetic ferrite material with permittivity that is greater than or equal to 20.
8. A method for manufacturing an inductive component as claimed in any one of claims 1 to 3, characterised in that it comprises the following steps:

   - developing cast strips of magnetic ferrite material having the following formula: Ni_xMg_yZn_zCu_vCo_wFe_2-δO₄, with v being non-zero, 0 < δ < 0.1 and x + y + z + v + w = 1;

   - producing metal tracks based on noble metal that can be silver, gold or palladium on the surface of at least one part of said cast strips of magnetic ferrite material;

   - developing cast strips based on non-magnetic ferrite dielectric material;

   - cutting elements in said non-magnetic ferrite material based cast strips;

   - positioning said elements on metal tracks on the surface of said cast strips of ferrite material;

   - adding elements of magnetic ferrite on the periphery of the dielectric elements and on the surface so as to incorporate said dielectric elements into said component;

   - performing an operation of co-sintering all the cast strips of magnetic ferrite material, which integrate the metal tracks, and the dielectric elements, so as to form said inductive component.
9. The method for manufacturing an inductive component as claimed in claim 8, characterised in that the metal tracks are produced by a screen printing operation.
10. The method for manufacturing an inductive component as claimed in claim 8, characterised in that the metal tracks are deposited by an ink jet.
11. The method for manufacturing an inductive component as claimed in any one of claims 8 to 10, characterised in that the co-sintering operation is performed at a temperature between 800°C and 1000°C.
12. The method for manufacturing an inductive component as claimed in any one of claims 1 to 3, characterised in that it comprises the following steps:

    - producing inks of magnetic ferrite, of non-magnetic ferrite and of noble metal;

    - producing layers of said stack by jet deposits using said inks.

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