Field of the invention The present invention refers to a composite structure and its preparation process for improving the critical current density in type II superconducting materials. In particular, the invention refers to a composite structure and its obtaining process, which allows pinning of the vortexes in high-temperature superconductors.

Background of the invention Attainment of high currents in superconducting materials is seriously hindered because of the movement of the magnetic flux lines'network (also named vortexes) formed inside these materials due to the presence of a magnetic field. During this movement, the electrons that are in the core of the vortexes undergo a dispersion process that results in of a non-zero electric resistance since said core is in a normal state, i. e., non- superconductor.

The existence of a non-zero electric resistance involves a dissipation of the electrical energy, which produces a strong reduction of the superconducting currents.

Therefore, with the purpose of suppressing the dissipation of electrical energy it is necessary to prevent the movement of the magnetic flux lines'network within the material, due to the presence of a magnetic

field.

With said aim, numerous works whose scope is the pinning of the normal core of the vortexes with various types of defects in the material exist.

In the high-temperature superconductors, the pinning centers are basically punctual defects such as, for example, oxygen gaps in the superconducting planes of copper oxide. However, the interaction forces between the vortexes and this type of defects are very weak, for what the pinning usually is not too effective.

On the other hand, materials comprising yttrium- based oxides show the existence of composition planes that constitute extensive defects along which an important pinning of the conveniently oriented flux lines can take place. As it is known, the composition planes are planes that separate two regions of the material in which the crystallographic axes in two directions perpendicular to each other are inverted.

However, the most used process to attain a high level of pinning of vortexes in high-temperature superconductors consists in the creation of columnar defects.

This way, for example, the US patent No. 5.683.967 refers to a process for improving the critical current density of type II superconducting materials, which consists in the formation of regular defects in the material so that the mean density of the defects is approximately similar to the density of the vortexes at a selected magnetic field, without detriment to the superconducting properties of the material. The process

described in US-5.683.967 consists in: a) the formation of a type II superconducting-material layer ; and b) the creation of a superimposed two-dimensional regular lattice of punctual defects by using the lithography technique at a nanometer scale, so that the diameter of the punctual regular defects corresponds to the diameter of the vortexes in the superconducting material.

In said US patent, the generation of a regular pattern of defects for the pinning of the vortexes is described, wherein the density of the pinning sites matches the density of the vortexes produced by a magnetic field selected in a certain superconducting material.

On the other hand, the US patent No. 5.627.140 describes a process for improving the pinning of the magnetic flux lines in superconductors, which comprises the introduction of nanotubes in a superconducting matrix.

These nanotubes are of carbon and simulate the structure, size, and shape of the columnar defects induced by metallic ions in a superconductor of the family of bismuth-based cuprates, especially Bi2Sr2CaCu208+x. These nanotubes are not affected by high temperatures, i. e., of the order of about 800°C, in inert or oxygen-containing atmospheres.

The nanotubes are introduced by wet action to allow the contact, at atomic scale, between them and the superconductor, and this way to simulate the conditions occurring in the columnar defects created by the trajectory of the heavy metal ions.

Therefore, in the prior art different techniques are described whose scope is to improve the critical current density in a superconducting material. However,

all these techniques refer to the introduction of defects in the superconducting material itself, either by metal ion bombardment, by introduction of defects in the form of carbon nanotubes having a shape, structure and size similar to those that a metallic ion can produce, by the defects produced by the oxygen gaps or by the defects produced by the addition of certain oxides into the superconducting material.

So far, the most used processes to attain a high level of pinning of the vortexes have consisted in the formation of columnar defects whose aim is to exhibit a diameter similar to that of the normal core of the vortexes, i. e. of some few nanometers. These defects pin the individual magnetic flux lines along their longitudinal dimension and, at the same time, they destroy a minimum volume fraction of the superconducting material.

With these defects, very high values for the superconducting current density are attained, which diminish very little as temperature increases, contrary to what happens with the punctual defects, which are actually influenced by the temperature increase, since the thermal weakening of the pinning potential takes place in a much slower way in the first case than in the second one.

However, the techniques used have only referred to the pinning of the flux lines of the normal core.

Therefore, it does not still exist in the prior art a process that allows to pin the whole area defined by the magnetic flux distribution associated to a vortex.

The penetration depth () of this magnetic field area in a superconducting region is defined by the following formula:

wherein m* is the effective mass of the charge carriers responsible for the superconductivity, nu is the density of said carriers, c is the velocity of light in the vacuum and e is the electron charge. At temperature zero, XL is of the order of some thousands A for the type II superconductors.

Summary A first object of the present invention is to provide a composite structure for improving pinning of the magnetic flux lines associated to a vortex, which not only allows the pinning of the normal core but also the magnetic pinning of the vortex. The superconducting- current density obtained with the composite structure of the invention is higher than 101° A/m2, preferably higher than 1011 A/m2 With the composite structure of the invention, it is intended not only pinning the magnetic flux lines of the normal core of a vortex, the sole aim treated so far, but also pinning the whole magnetic flux distribution associated to a vortex, whose penetration depth (L) in the superconducting region has been defined above, by means of a magnetic pinning.

A second object of the present invention is to provide an improved process of pinning of the magnetic flux lines associated to a vortex, which is based on a non-invasive process, i. e., that does not introduce any

defect type in the superconducting material itself.

Description of the invention With the purpose of attaining the first object of the invention, the composite structure is based on the combination of a layer of a high-temperature superconductor material with a layer of a highly anisotropic magnetic material, so that the first layer, which is at the top of the structure, benefits from the features of the magnetic structure of the second layer to increase, this way, the pinning of the vortexes with respect to the case of a pure superconductor.

In the present invention, by magnetic material is understood a ferromagnetic or ferrimagnetic material.

Although at microscopic level, the ferromagnetic and ferrimagnetic materials exhibit important differences in the arrangement of the magnetic moments of the atoms that constitute them, at macroscopic level both types of materials behave in a very similar way as for what refers to their magnetic properties (high magnetic-saturation moment and anisotropy density), for what both can be good candidates for their incorporation to the composite structure of the invention.

In particular, the pinning of the vortex lattice by means of defects in the superconducting material is enhanced by the pinning in the structure of magnetic domains that characterizes the magnetic material. The magnetic moments of the atoms of the material are arranged parallelly or antiparallelly to the direction of the applied magnetic field, so that these two possible orientations define geometric areas, called domains, that extend along some few micrometers and that alternate in

the material. These dominion areas are separated by other transition areas, more narrow, called dominion walls, in which the magnetic moments undergo a 180° reversal to pass from an area to the following one. The distance between two of these dominion walls is called dominion length (1).

Advantageously, the presence of a magnetic material layer not only allows the anchoring or pinning of the vortexes by means of the pinning of its normal core in the defects of the superconducting material, but also by means of the pinning of the whole magnetic flux distribution associated to a vortex. In the first case, it is the"normal pinning"of an object that measures some few nanometers in some anchoring centers whose associated energy is of the order of some tenths eV, for what the object is easily releasable. On the contrary, in the second case it is a"magnetic"pinning of an object that measures some tenths of micrometer, in some anchorage centers whose associated energy is one or even two orders of magnitude greater, which greatly hinders its release.

The composite structure of the invention comprises a superconducting material layer (SC) having a thickness dsc, a magnetic material layer (M) having a thickness which presents a high magnetic saturation moment and uniaxial anisotropy, and a base substrate (SB) having a thickness dSB which functions as a support, this superconducting material and said magnetic material being separated by a barrier-sheet or insulating layer (I) having a thickness (I), so that: disc, dM << 1 dSCZ dM « a, L dI << dsc, dM

where 1 and XL represent the dominion length and the penetration depth of the magnetic field in the superconducting region, respectively, as previously defined.

Two types of superconducting materials exist; on one hand, type I superconducting materials characterized because they lose their superconducting properties as soon as the magnetic field penetrates in its interior and they pass to the so called normal state and, on the other hand, type II superconducting materials that, contrarily to the first ones, keep their superconductivity, which goes getting lost in a progressive manner as the magnetic field is intensified, until reaching a value, the superior critical field, above which it disappears completely.

The appropriate high-temperature superconductors for the composite structure of the invention are a particular case of type II superconductors and they have a high industrial interest because they can sustain very strong superconducting current densities, of the order of at least 109 A/m2, in the temperature range of nitrogen liquefaction (77 K).

The magnetic material is selected from a ferrimagnetic or a ferromagnetic material that possesses high uniaxial anisotropy and magnetic saturation moment.

The appropriate magnetic materials for the invention exhibit a magnetic saturation value superior to 0.5 A/m and an energy density value of uniaxial anisotropy of at least 104 J/m3.

Therefore, the ferrimagnetic and ferromagnetic materials having the above-mentioned magnetic saturation and uniaxial anisotropy values are especially useful for

the composite structure of the invention. Mainly, they are ferrites or metallic alloys of transition metals and rare earths. The most utilized magnetic material has been the ferrimagnetic oxide barium hexaferrite, BaFe120l9, a type M hexagonal ferrite.

On the other hand, the material that acts as a base substrate should present a crystalline structure similar to that of the magnetic material so as the growth of said magnetic material on this base substrate is epitaxial and, this way, it entirely keeps the magnetic properties that it presents in the monocrystalline form.

In the present invention, by"similar crystalline structure"is understood a structure that allows growth of the magnetic material in accordance with its own crystalline structure, although the first binding layers exhibit a transition crystalline structure.

In order to achieve that the composite structure of the invention has the appropriate properties for obtaining the magnetic pinning of the vortexes, all the layers making up said structure should be deposited by means of an epitaxial growth.

Therefore, the base substrate should meet the following requirements: a) The substrate composition should be inert to the magnetic material layer in order to avoid the chemical reaction layer-substrate and the interdiffusion of elements at the high temperatures at which the deposition of magnetic material is carried out. These temperatures range from 650°C to 900°C, especially 800°C ;

b) The crystalline structure (type and lattice parameter) should be compatible with that of the magnetic material to be deposited, in order to facilitate the epitaxial growth; and c) The differences in the thermal expansion coefficients between both materials should be small so as not to produce microcracks.

In preferred embodiments of the invention, the most utilized substrates have been sapphire and yttrium oxide-stabilized zirconium oxide (ZrO2-10% Y203), shortened YSZ, although other commercially available materials exist that can be used as base substrate.

Once the magnetic material growth on the substrate has been achieved, a barrier sheet should be deposited in order to facilitate the epitaxial growth of the superconducting material, and to prevent the chemical reaction and the interdiffusion of elements between the magnetic material and the superconducting material.

Otherwise, the epitaxial growth of the superconducting material is hindered, its properties being able to be very affected, especially if iron diffusion from the magnetic material to the superconducting material occurs.

Consequently, the selection of the barrier-sheet material should be done taking into account the crystalline compatibility with the two layers, the non- interaction between said layers (chemical reactions and interdiffusion of elements), the thermal expansion difference of the two layers and the easiness to adopt the appropriate morphology (uniform growth and low particle density that, depending on the deposition technique, could be formed).

Preferably, the barrier-sheet layer is selected

from ZrO2-10% Y203 (YSZ), Ce02 and BaZrO3. In a preferred embodiment of the invention, the insulating or barrier- sheet layer has been of YSZ, the magnetic material being BFO (BaFe120l9) and the superconducting material YBCO (YBa2CU307).

Thus, the appropriate magnetic pinning of the superconducting material's vortexes depends on the physical properties of the magnetic material and on the structural crystalline characteristics of both the substrate and the barrier sheet.

With the purpose of attaining the second object of the present invention, the improved process for the pinning of the magnetic flux lines associated to a vortex comprises obtaining a composite structure that exhibits a superconducting-current density higher than 101° A/m2, such process comprising : a) the epitaxial growth of: -a first layer of magnetic material, -a second layer of a barrier sheet, and -a third layer of superconducting material, these layers being, in the described order, on a base substrate; b) the heating of this substrate to a temperature ranging from 650°C and 900°C before carrying out the epitaxial growth of each one of the three layers of step a), with the purpose of assuring the atoms'recombination of each one of the deposited materials; and c) the application of an oxygen partial pressure so as to compensate the loss thereof during the growth of each layer.

The epitaxial growth of step a) can be carried out by means of different techniques provided that they allow

the atomic growth on the base substrate. Among the most suitable techniques, molecular beam epitaxy, cathode sputtering, deposition from metalloorganic decomposition or pulsed-laser deposition may be mentioned, the latter being the one that has allowed to obtain the best results because of its particular congruent emission properties, i. e. the deposited material preserves the stoichiometry of the starting material, and of high energy of the pinned material.

Advantageously, the oxygen partial pressure to use with the pulsed-laser deposition technique during the growth of the magnetic material layer ranges from 0.05 mbar to 0.15 mbar, during the growth of the barrier-sheet layer ranges from 2*10-4 mbar to 4*10-4 mbar, and during the growth of the superconducting material layer ranges from 0.2 mbar to 0.4 mbar.

With the process of the invention it is carried out the epitaxial growth of: -a first layer of a magnetic material selected from BaFe120l9 (BFO), Tb-Fe, Co-Pt and CoCrioTa4 ; -a second layer of a barrier sheet selected from Zr02-10% Y203 (YSZ), Ce02 and BaZr03 ; and -a third layer of a superconducting material selected from YBa2Cu307 (YBCO), Bi2Sr2CaCu208, Tl2Ba2CaCu208, Bi2Sr2Ca2Cu301o, Tl2Ba2Ca2Cu301o and other high-temperature superconducting cuprates.

The pulsed-laser deposition technique is preferable because, on one hand, it allows carrying out the sequential growth of the three layers of step a) with the same substrate temperature and, on the other hand, the whole growth is performed without breaking the vacuum in

the deposition system. Accordingly, with this technique the sequential-deposition process of the various materials is accelerated because the substrate temperature has not to be modified during the growth of each layer.

With the purpose of obtaining the desired superconducting properties, i. e. a current density higher than 101° A/m2 and preferably higher than 1011 A/m2, in the so obtained composite structure according to the process of the invention, the epitaxial growth of step a) is carried out until obtaining a layer thickness ranging from: -50 to 150 nm for said first layer of magnetic material; -10 to 20 nm for said second barrier-sheet layer; and -50 to 150 nm for said third layer of superconducting material.

Description of the drawings Figure 1 shows a schematic representation of the composite structure of the invention. In said figure the arrangement and order of the layers can be observed: base substrate (SB), magnetic material layer (M), barrier-sheet or insulating layer (I) and superconducting-material layer (SC). The thicknesses of said layers are shortened dsB, DMKT DI and dSC, respectively.

Figure 2 shows the comparative curves ZFC (Zero Field Cooled)-and FC (Field Cooled)-of magnetic moment MYYBY and MYBCO, at different scales due to the difference of the obtained values, expressed in emu (1 emu/cm3 = 103 A/m) vs. temperature (K).

Said comparative curves have been obtained for a superconductor at 0.05 T and for a composite structure according to the invention at 0.05 T and 0.1 T. The first curve, represented by black circles, corresponds to a 100 nm-thick pure superconductor of YBCO (YBa2Cu307) on a base substrate of LaAlO3 to which a magnetic field of 0.05 T has been applied. The second curve, represented by open circles, corresponds to a composite structure of YYBY 100/100, shortened YBCO (100 nm thick) (SC), YSZ (I), BFO (100 nm thick) (M) and YSZ (SB), to which a magnetic field of 0.05 T has been applied. The third curve, represented by black squares, corresponds to the same composite structure of YYBY 100/100, but in this case, a magnetic field of 0.1 T has been applied.

The curves represented in figure 2 have been obtained as follows. First, the material is cooled down in the absence of a magnetic field until the minimum temperature sought. Next, a magnetic field is applied and the magnetization is measured as a function of the increasing temperature. This determines what is known as ZFC curve.

Subsequently, without withdrawing the magnetic field, the material is cooled down again, and measurements are repeated while increasing temperature. This defines the FC curve. Below the transition temperature from the normal to the superconductor state, as temperature diminishes both curves coincide, until reaching a value starting from which they begin to separate.

In said figure 2, it can be observed that for each magnetic field applied to the pure superconductor and to the composite structure of the invention a temperature exists at which the corresponding curves ZFC and FC join together. This temperature determines the corresponding point in the irreversibility line (LI) for the phase

diagram magnetic field-temperature.

Figure 3 shows the irreversibility line (LI) extracted from the measurements taken in the graph of figure 2 as a function of the magnetic field expressed in kOe (1 kOe = 0. 1 T) vs. the temperature ratio T/Tc where Tc is the critical temperature. In said figure 3, the results for a pure superconductor of YBCO and for two composite structures of YYBY with different thickness in the layer of magnetic material are shown. One of these composite structures is 100 nm thick and the other 80 nm thick and YYBY 100/100 and YYBY 100/80 represent them in figure 3.

When the temperature at which the different curves join is represented as a function of the magnetic field, a downward-curve, called irreversibility line, appears. This line separates the vortexes'behavior in two regions: for magnetic field-temperature coordinates below the irreversibility line, the vortexes are pinned and they can only be moved by means of thermal activation processes (irreversible processes); above the line, the pinning centers stop to be effective, the vortexes are released and they can move without limitations through the material (reversible processes).

As it can be appreciated in figure 3, the irreversibility lines of the composite structure of the invention move up with regard to the irreversibility line of the pure sample of YBCO, which indicates that the pinning is more intense in the composite structure than in the pure superconductor.

The position of the irreversibility line in the phase diagram magnetic field-temperature directly agrees with

the pinning intensity of the vortexes. For a certain magnetic field, the higher is the temperature necessary for crossing the line, the more intense will be the energy associated to the pinning centers and, therefore, pinning will be more effective. The irreversibility line corresponding to the samples of the composite structure of the invention is located over the line corresponding to the pure superconductor. Thus, for example, for a reduced temperature of 0. 9, the magnetic field necessary to release the vortexes in the composite structure is two- fold the required one in the pure material. That is to say, the pinning is twice as effective and, consequently, the superconducting currents circulating through the composite structure are higher than those in the pure material.

Results In the composite structure of the invention, the effective magnetoinduction B in the YBCO layer is the sum of the applied magnetic field H and the demagnetization field 4aM in the BFO layer: B = H + 4sM. The magnetic domains form a strip-shaped definite structure if the applied field is smaller than the coercive force, which is of the order of 0.5 T in the BFO layer. However, while this layer is not saturated, i. e. below 1 T, this structure, which is responsible for the magnetic pinning of the vortexes of the superconducting layer, should be preserved.

The domain structure modulates the effective magnetic field in the YBCO layer and generates the pinning potential of the vortexes. The intensity of said potential depends on the absolute value of the magnetization, which proves to be constant at low temperature. In this case,

the potential should not change too much below the superconducting transition temperature, if the applied magnetic field is not overly strong. In fact, this is what is observed in the composite structure. The LI of the composite structure is above the LI of the pure YBCO sample up to 1T, the maximum field applied in the measurements carried out. For each temperature, the effectiveness of the pinning increase associated to the magnetic domains can be reckoned as the quotient of the irreversibility field values obtained for the composite structure and for the pure YBCO sample.

The increase obtained in the experiments carried out has been of a factor of two, although higher factors can be obtained in the composite structure of the invention with the combination of materials that comply with the requirements described for each layer.

The behavior of the curves ZFC-FC shown in figure 2 confirms that the magnetization values corresponding to the composite structure are up to three orders of magnitude lower than those corresponding to the pure superconducting sample are. This indicates that, after the application of the magnetic field necessary for measuring the ZFC curve, the pinning that takes place in the composite structure of the invention is much more effective and it prevents the massive egression of vortexes that occurs in the pure superconductor before measuring the magnetization.

In the case of the pure YBCO sample, the pinning potential is due to defects characteristic of the material. The energy barrier typical for a unit cell, due to the defects, is the condensation energy of the Cooper's couples in the core volume Ucp and it is of the order of 1.000 K (1K = 8.6*10-5 eV), being Up- (0/81,) , where (Do

is the magnetic flux quantum and BB is the depth of magnetic penetration. In the case of the pinning potential created in the YBCO layer by the magnetic domains present in the BFO layer, its maximum value can be reckoned starting from Ump-OoModscy where Mo is the saturation magnetization of the BFO layer, and dsc is the thickness of the YBCO layer. The values of these parameters in the analyzed composite structures give a constant value for the barrier of 10.000 K in the whole temperature range.

This way, in principle, the critical current in the composite structure could increase in a factor 10 with regard to the pure superconducting sample.

Examples Composite structure Superconductor (SC) (YBa2Cu307 (YBCO))/insulator (I) (ZrO2- 10% Y203 (YSZ))/magnetic material (M) (BaFe120l9 (BFO)) on an YSZ base substrate (SB) according to the direction (100).

The thickness values of each one of these layers were: 100 nm of YBCO, 10 nm of YSZ and 100 nm of BFO.

Preparation process of said composite structure In a preferred embodiment of the invention, the pulsed-laser deposition technique was used for the preparation of the composite structure. With the process of the invention, a composite structure of the following composition was obtained: Superconductor (SC) (YBa2Cu307 (YBCO))/insulator (I) (Zr02-10% Y203 (YSZ))/magnetic material (M) (BaFe120l9 (BFO)) on a YSZ base substrate (SB) according to the direction (100).

The growth of this structure was conditioned by the different crystalline structures of these materials and by

the strong chemical interactions that take place among them during the deposition of each layer at high temperatures. This problem was solved by using a very thin fine layer of ZrO2-10% Y203 (10% yttrium oxide-stabilized zirconium oxide, YSZ) as barrier sheet or insulator.

The 0.5 mm-thick YBCO/YSZ/BFO layers were deposited on YSZ substrates according to the direction (100) by means of the pulsed-laser deposition by using a KrF excimer laser. The composite structure was prepared in a single step by sequentially directing the laser beam, with an approximate energy density of 2*104 J/m2, on stoichiometric targets of BaFe120l9, ZrO2 (doped with 9 molar percent Y203) and YBa2Cu307. The 100 nm-thick fine BFO layers were prepared under optimal conditions of 800°C for the substrate temperature and an oxygen pressure of 0.1 mbar.

Subsequently, and before the growth of YBCO, a barrier sheet was deposited. With such an aim, a sheet of YSZ was chosen because it presents a high structural compatibility with the lower BFO layer and the upper YBCO layer. YSZ also has a low chemical reactivity with both materials, just as it was confirmed from the preparation of the composite structure with YSZ barrier sheets having thicknesses so thin as 10 nm.

The optimal processing parameters (800°C, 3x10-4 mbar oxygen) for the YSZ layers coincided with those used for their direct growth on Si substrates according to the direction (100). The upper layer of the composite structure, YBCO, needed a higher oxygen pressure (0.3 mbar) during the growth, which produced optimal superconducting properties at the same growth temperature

used for the other layers (800°C), therefore allowing a quick and reproducible deposition.

This way, two YYBY composite structures were prepared, with relative thicknesses of YBCO and BFO of 100/100 and 100/80 nm, respectively.

The thickness of the layers was measured by profilometry. X-ray diffraction measurements in a four- dial diffractometer with CuKa radiation revealed that the YBCO and BFO layers had a unique orientation, according to the direction (100) outside of the plane. The analysis inside the plane showed that the layers were epitaxial with a (001) YBCO [100]// (0001) BFO [11 20]//(OOl) YSZ [010] ratio, where the direction of growth is specified in brackets and the planes parallel to each other during the growth are specified in square brackets.

The electric transfer properties as a function of temperature were measured between 30 and 300 K by using a four-point method. With these measurements, superconducting transition-temperatures of 90,88, and 87.5 K were obtained for the YBCO samples YYBY 100/100 and YYBY 100/80, respectively.