DE3851809T2 - Superconductor with high critical current. - Google Patents

Description

The present invention relates to superconducting materials that exhibit high critical currents, and in particular to improved superconducting materials and techniques for achieving high critical currents in Type II superconductors.

Superconductivity is a phenomenon that occurs in many materials and was first observed in 1911. In general, it is characterized by the disappearance of electrical resistance in a material at a well-defined temperature, as well as by the ability of a material to emit a magnetic field in a superconducting state. Superconductivity occurs in many materials, including about a quarter of the elements in the periodic table, and in over 1000 alloys and other multicomponent systems. In general, superconductivity is believed to be a property of the metallic state of matter, since all known superconductors are metallic under conditions that make them superconducting. For example, some normally non-metallic materials become superconducting under very high pressures, where the pressure transforms them into metals before they exhibit their superconducting behavior.

As mentioned above, many types of superconducting materials are known. The advantages of different materials compared to each other often depend on their superconducting transition temperature Tc, which is also often called the critical temperature. This is the temperature above which there is no superconductivity. Until recently, the material that had the highest superconducting transition temperatures was Nb3Ge, which had a Tc of about 23 K at normal atmospheric pressure.

Recently, the remarkable discovery of J.G. Bednorz and K.A. Müller, reported in Z. Phys. B. - Condensed Matter 64 (1986), p. 189, and Europhysics Letters 3, 379 (1987), has completely changed the direction and importance of this technology. The discovery by Bednorz and Müller was that certain metal oxides can have transition temperatures to superconductivity considerably higher than 23 K. These materials are often referred to as "high-Tc superconductors".

Since these first discoveries by Bednorz and Müller, a great deal of research and development has been invested worldwide to investigate these types of superconducting materials in more detail, with the aim of extending the temperature range over which they are superconducting, as well as to understand the basic mechanisms for superconductivity in this class of materials.

First, Bednorz and Müller demonstrated superconductivity in copper oxide mixtures, including typical rare earth and/or rare earth-like elements and alkaline earth elements, e.g. La, Ba, Sr, etc., with a perovskite-like structure. After the first reports by Bednorz and Müller, other researchers showed that the application of pressure can increase the transition temperature and that, in addition, transition temperatures of up to 100 K could be achieved in the Y-Ba-Cu-O systems (see, e.g., M.K. Wu et al. Phys. Rev. Lett., 58 (1987) 908). Many papers have appeared in technical journals all over the world, addressing various aspects of superconductivity in these materials.

As pointed out in an article by P. Chaudhari et al., published in Physical Review Letters 58, p. 2684, June 1987, entitled "Critical Current Measurements in Epitaxial Films of YBa₂Cu₃O7-x Compounds", one of the disadvantages is often cited for this class of superconducting materials is their small critical current. Prior to the work of P. Chaudhari et al., reported in this Physical Review Letters article, the highest critical current value found in this new class of materials was very small compared to that in the more familiar metallic alloys. The highest value reported to date at 77 K was about 10³ A/cm². Since the range of potential use of these materials depends on the critical current, this problem remained the major obstacle to the use of these materials prior to the above-mentioned contribution of Chaudhari et al. In their Physical Review Letters article, they described epitaxial superconducting films made from these materials in which the critical current at 77 K was above 10⁵ A/cm². Thus, their work showed for the first time that these materials could exhibit critical currents high enough to make them valuable for many applications.

It is known that the members of this class of superconducting metal oxides belong to the "Type II superconductors" in which the occurrence of a superconducting current creates a trapped magnetic flux and circulating currents in the superconductor, which lead to the occurrence of magnetic vortices. These vortices are known to hinder the flow of the superconducting current through a Type II superconductor because they limit the strongest superconducting current, called the critical current, that can be passed through the material without it losing its superconducting state.

To maximize this critical current, these vortices must be fixed in place rather than being allowed to move freely in the superconductor. This can be done by introducing particles and defects into the superconductor that attract the vortices and fix them in certain places. For example, such defects can include structural arrangements such as displacements or defects that tend to to fix the vortices. Also, impurity atoms can be introduced into the superconductor to create localized sites where superconductivity is lost or at least persists over a smaller temperature range. Such impurity sites, while affecting the temperature dependence of superconductivity, can fix the vortices to allow higher critical currents. As is known in the art, the use of these impurities and the effect on the superconducting properties of the material must be balanced against any increase in the value of the critical current through the superconducting material. Such design considerations have been exploited in the past to build, for example, high-field superconducting magnets. Impurities are introduced into the magnet wire material, such as NbTe, to create defects in the superconducting material that fix the vortices, although it is clear that these impurities also tend to destroy the superconductivity. Since these materials operate at liquid helium temperatures and the critical temperatures of these alloys are close to about 20 K, the loss of some superconductivity can be tolerated if the critical currents flowing in the wires are increased.

In the practice of the present invention, an improved technique has been developed to fix the magnetic vortices which limit the critical currents in this class of superconductors. Furthermore, the invention can be applied not only to high critical temperature superconductors of the type first described by Bednorz and Müller, but also to Type II superconductors. In particular, the teachings of the invention can be applied to any superconductor in which critical currents are restricted by vortices, the superconductor being characterized by a coherence length (i.e., the distance in the superconductor over which superconductivity extends) which is sufficiently short. In the practice of the invention, the Coherence length in the superconductor is on the order of atomic dimensions of the superconductor.

Generally speaking, the technique and materials of the present invention exploit the magnetic fixation of the vortices in these superconductors. The most direct way to achieve magnetic fixation is to introduce atoms into the superconductor that have magnetic moments that interact with the magnetic fields of the vortices to create an energy minimum for the vortices and thus fix the vortices at the location occupied by the atoms introduced into the superconductor. As an example of atoms that can be introduced to effect fixation, magnetic rare earth atoms are particularly suitable for use in metal oxide superconductors of the type first described by Bednorz and Müller.

It is known that magnetic elements have been introduced into high-Tc superconducting oxides. For example, see P.H. Hor et al., Phys. Rev. Lett. 58, p. 1891 (1987), who reported on the superconductor GdBa₂Gu₃Ox. In this material, however, the critical current Ic is approximately the same as the critical current in the superconductor YBa₂Cu₃Ox. Thus, the presence of the magnetic Gd atoms in place of all Y atoms will not cause any significant fixation of the magnetic flux vortices and therefore does not increase the critical current.

The article by PVPSS Sastry et al., "Enhancement of transport critical current density by Gd substitution in YBa₂Cu₃O₇", Appl. Phys. Lett. 52, No. 17, pp. 1447/8 (1988) discusses the replacement of all Y by Gd with a current density Jc of only 90 A/cm², and a partial substitution in amounts of less than 0.5%, which resulted in an increase in the current density Jc to just 400 A/cm². However, no effort is reported to optimize the Gd replacement so that that one arrives at Jc values that would be really important, because the role of gadolinium was not understood as a means of fixing the flow vortices, but it was interpreted as a "rinsing agent of the insulating phases".

Today, it is believed that no one has ever demonstrated magnetic pinning in these high-Tc superconducting oxides, nor has anyone observed magnetic pinning in other Type II superconductors. In the course of the applicant's work, it was discovered how magnetic pinning can be exploited to achieve high critical currents in these high-Tc metal oxide superconductors, as well as in previously known Type II non-oxide superconductors, if these non-oxide superconductors have sufficiently small coherence lengths.

Accordingly, it is a primary object of the present invention to provide superconducting material in which magnetic pinning is applied to increase the critical current in these materials.

Another object of the present invention is to provide improved Type II superconductors having small coherence lengths.

Another object of the invention is to provide a technique for increasing the critical current available in all three Type II superconductors.

Another object of the present invention is to change the composition of high-Tc metal oxide superconductors so as to increase their electrical conductivity without reducing the temperature range in which they are superconducting.

A further object of the invention is to increase the electrical conductivity of metal oxide superconductors of the type characterized by critical temperatures above 23 K, and in particular to provide such materials with critical currents above 10⁴ A/cm² at 77 K.

The invention more broadly relates to superconducting materials with increased critical current values, where the currents are high because magnetic field vortices in the superconductors are effectively fixed by a magnetic fixing technique. This is achieved by incorporating atoms into the superconducting material, the atoms having associated magnetic moments which serve to fix the magnetic flux vortices in the superconductor. However, the invention goes further than just incorporating atoms with associated magnetic moments; instead, the invention is directed to the specific application of these atoms to achieve the fixing of many magnetic flux vortices. Further, and of critical importance, is the fact that the atoms which effect the magnetic fixing must occur in a compositionally random distribution in the superconductor.

If the magnetic atoms are present in compositional order in the superconductor, the flux vortices will not be fixed so as to give high critical currents. This will become clearer in the detailed description of the preferred embodiments, but as an example it is sufficient to note that an optimum amount of fixing atoms is at about 50% replacement for the rare earth or near rare earth elements in the well-known high-Tc metal oxide superconductors. Therefore, if all the Y atoms of the compound YBa₂Cu₃Ox are completely replaced by atoms with a magnetic moment, no significant vortex fixing will occur and the critical current will not be significantly increased. In this latter situation, the magnetic atoms are arranged in the superconductor lattice so that it becomes predictable which atom at all Y-sites in the lattice, and therefore there is no random distribution in the composition.

The atoms that can be included in the superconducting composition to effect magnetic fixation of the flux vortices are all atoms with which a magnetic moment is associated. However, it has been found that atoms with a large magnetic moment generally lead to superconductors with increased critical flux behavior, even when the temperature in the material is increased. In the case of high-Tc metal oxide superconductors of this type, a rare earth or rare earth-like element, an alkaline earth element, copper and oxygen are usually included; suitable replacement atoms provide a magnetic moment for fixing the flux and are magnetic rare earth elements such as Yb, Er, Gd, Dy, Tb etc. Other magnetic atoms are Cu²&spplus; and Ti²⁺, as well as Fe, Co and Ni, although these latter three elements are less favorable because they tend to hinder superconductivity in these high-Tc metal oxides.

For the high-Tc metal oxide superconductors, the location of the atom with the magnetic moment is not critical, as long as the superconducting properties are not adversely affected for the application for which the superconductor is to be used. Thus, these magnetic atoms can be located on the rare earth or alkaline earth sites, as well as on some of the copper sites, as long as a large number of the copper sites responsible for the superconductivity are not adversely affected. Expressions for the number of magnetic elements required to exert a significant flux fixing effect and to ensure a suitable random distribution of these magnetic atoms in the composition will be given.

Although the invention is particularly suitable for use in high-Tc metal oxide superconductors, it can also be applied to any Type II superconductor that has a sufficiently small coherence length. In such materials, broad, localized magnetic fields are generated in the vortices and the present invention can be used to fix these vortices. Therefore, this prescription can be applied to Type II superconductors that have coherence lengths on the order of the unit cell in the superconductor, which in most superconductors is about 0.3 to 3 nm.

Details of embodiments of the invention will become apparent from the following more detailed descriptions with reference to the accompanying drawings which illustrate a superconductive material, which may be a bulk material or a foil material, having means for generating a superconductive current therein and having magnetic flux vortices fixed by random distribution in the composition of the magnetic atoms in the superconductor in accordance with the principles of the present invention.

The invention uses magnetic confinement of flux vortices in superconductors to increase the critical current capabilities of the superconductors. In a particular embodiment, it can be applied to high-Tc oxide superconductors. It is known that atoms with a magnetic moment can be present in these materials without adversely affecting the superconducting properties of the material. In particular, these atoms can be located in locations that are not essential to the superconducting properties of the material and therefore do not destroy the superconductivity. In the practice of the present invention, magnetic atoms are confined in these high-Tc superconducting materials in such a way that a substantial number of magnetic flux vortices are confined so that the critical current of the material is increased. As will be seen, there are certain critical considerations regarding the confinement of these magnetic atoms; for example, if they are not in sufficient quantities, or in such a large quantity that 100% of the foreign atoms in the superconducting material are replaced, suitable flux fixation for high critical currents cannot be achieved. Furthermore, the locations where the magnetic atoms are located must be randomly distributed in their composition, ie it must not be possible to predict whether a magnetic atom is located at a certain location. Thus, there are critical conditions that must be met for the sufficient fixation of the flux vortices in order to increase the critical current.

It is generally known that magnetic flux vortices in Type II superconductors can move within these materials and that they generally move perpendicular to the supercurrent through the materials. When they are fixed by confining magnetic atoms according to the present invention, there is a mutual interference between the magnetic moment of the atom and the magnetic field associated with the vortex. When these magnetic fields are aligned, the vortex settles into an energy minimum and is thus fixed. Fixing these vortices then allows maximum supercurrents to flow in the material. Magnetic atoms fix the vortices where the vortices can be closely packed, such as at intervals of about 100 nm (or less).

It has been found that with the help of magnetic atoms, it is possible to fix the vortices to a significant extent without adversely affecting the superconductivity of the high-Tc material. The magnetic atoms exert very strong fixation forces on the vortices, but the atoms must be randomly distributed in the composition of the superconductor in order to cause an increase in the critical current. They must not be built into the superconductor in such a way that they are arranged in a regularly distributed order in the lattice of the superconductor.

In the case of a high-Tc oxide superconductor including a rare earth element, an alkaline earth element, copper and oxygen, the magnetic atoms can be located at the rare earth sites, the alkaline earth sites or even the copper sites. However, some of the copper sites cause the high-Tc superconductivity in these materials and if sufficient amounts of the magnetic atoms are located at the copper sites causing the superconductivity, the superconductivity of the material is adversely affected. However, if the magnetic atoms are located at the rare earth sites, the alkaline earth sites or the copper sites not causing the superconductivity, considerable flux pinning can occur without adversely affecting the superconductivity properties of the material.

The incorporation of these magnetic atoms to increase the critical current can be applied to any of these high-Tc superconductors, whether the superconductor is in bulk form or in foil form, and regardless of the geometry of the superconductor (wire, plate, etc.). Furthermore, the superconductors can be single crystal or multi-crystal material. The magnetic atoms are generally incorporated during the fabrication process of the superconductor, in a manner described in detail below. Such techniques are well known in the art.

In high-Tc oxide superconductors, it has been found that the incorporation of magnetic atoms in sufficient quantities leads to a compositionally approximately random distribution of these magnetic atoms, which in turn cause a significant flux fixation (ie enough fixation to increase the Ic). This happens whether the magnetic atoms are located at the rare earth sites, the alkaline earth sites, or even at the copper sites of this material. The location of the magnetic atom in turn determines the valence of the magnetic atom in the material. For example, if Ti is used as a fixation atom When Ti is incorporated into a copper site or an alkaline earth site, it has a 2+ valence when it is incorporated into a copper site or an alkaline earth site, but it has a 3+ valence when it is incorporated into a rare earth site. Other representative atoms that have a magnetic moment and are suitable for inclusion in these high-Tc superconductors to fix the vortices are the magnetic rare earths such as Gd, Tb, Yb, Dy, Er, La, Eu, Lu, etc. For some of these elements, the site they are to occupy is critical. For example, for Eu, Lu and La, these atoms acquire a 3+ valence when they replace an element such as Y in the rare earth site. When they have a 3+ valence, the magnetic moment associated with these three elements becomes zero. That is, when they are incorporated into the rare earth sites, they are not suitable for fixing magnetic flux vortices. Even if rare earth magnetic atoms or other magnetic atoms have their moments aligned by single crystal fields rather than by the vortex magnetic field, they are not effective in pinning vortices.

As noted earlier, magnetic elements such as Fe, Co and Ni can be used, although these are generally less suitable atoms for this purpose. In particular, the atoms of these three elements generally tend to occupy the copper sites in these high-Tc superconductors and therefore tend to directly affect the superconductivity of the material. Since it is generally difficult to target the location where these three elements are to adhere, since they tend to occupy the copper sites, their presence will disturb the superconductivity of the material more. Thus, they must generally be present only in small amounts and, when present, do not determine the degree of vortex fixation required to achieve the highest possible critical currents.

It is also possible to use Cu²⁺ to achieve magnetic fixation in these materials. In these high-Tc oxide superconductors, generally about 1 atom per two unit cells is Cu³⁺ and the rest is Cu²⁺. The Cu²⁺ and Cu³⁺ are randomly distributed over the Cu sites, and Cu²⁺ can be amplified to provide some flux fixing. However, other atoms offer broader magnetic moments than Cu²⁺ and are generally more effective at flux fixing.

As stated earlier, the locations of the magnetic fixation atoms must be randomly distributed throughout the superconductor lattice. If they are ordered, the vortices will not be effectively fixed, and if they are absolutely ordered, none will be fixed at all. In addition, it has been found that atoms contributing a large magnetic moment, in addition to providing stronger fixation forces, also favorably affect the temperature dependence of the critical current in the superconductor. For example, if the critical current is plotted against temperature in a high-Tc oxide superconductor with a random distribution of the magnetic fixation atoms in the composition, it is found that the increase in the critical current extends over a larger temperature range when the magnetic atoms have large magnetic moments. Thus, the incorporation of an element such as Gd, which has a larger magnetic moment per atom than, for example, B. Cu², cause an increase in the critical current over a wider temperature range than Cu²+.

Although the invention is designed for a particular application in high-Tc oxide superconductors, it can in principle be applied to any type II superconductor having a small coherence length. The coherence length is a measure of the range over which superconductivity extends in a material and is usually large in most non-oxide type II superconductors. However, if the coherence length of the superconductor is small, the local magnetic The field strength in the vortex is very high, and the flux fixation via magnetic atoms can be used to generate highly critical currents. In order for the magnetic fixation in the type II superconductors to be effective, the coherence length must be of the order of magnitude of the atoms of the superconductor. This means that it must be approximately the length of the unit cell on the superconductor, which means that it should generally be about 0.3-3 nm.

The incorporation of magnetic pinning atoms in high-Tc superconductors can also introduce disorder in the planes that carry the supercurrents, thus reducing the coherence length and increasing the magnetic field in the vortices. This contributes to further pinning the vortices.

The figure shows a representative superconducting material in which the fixation of magnetic currents is used to increase the critical current. In contact with the superconductor 10 are electrodes 12A and 12B which in turn are connected to a variable resistor R and to a current source 14. The locations of magnetic flux fixation are indicated by the randomly distributed points 16. When the superconductor 10 is cooled to a temperature at which superconductivity occurs and a current is applied, the critical current Ic increases due to the presence of a random distribution of atoms with magnetic moments in the composition, the number of these magnetic atoms being sufficient to fix a significant number of flux vortices in the superconductor 10. Although the figure is a schematic representation, those skilled in the art will understand that the superconductor may be in bulk or foil form, a single crystal material or a multi-crystal material. The means for cooling the superconductor 10 are not shown in the figure, but are well known in the art. For a high-Tc oxide superconductor, such as YBa₂Cu₃Ox, liquid nitrogen is suitable, while for a type II superconductor with a critical transition temperature below 77 K cooling with liquid helium is applied.

The magnetic atoms suitable for fixing the vortices can be easily incorporated into the base material or sheet material. For example, if a high-Tc bulk superconductor containing a rare earth or rare earth element, an alkaline earth element, copper or oxygen is to be produced, a powder containing the magnetic atoms is mixed into the powders that are mixed to form the base material superconductor and later heated. In the present example, where a Y-Ba-Cu-O oxide superconductor is desired, the starting additives can be a powder of a magnetic element, such as Gd oxide in powder form. The Gd atoms penetrate to the Y sites in the material and produce Gd-substituted Y-Ba-Cu-O oxide superconductors. In general, magnetic rare earth atoms tend to migrate to the rare earth sites when the magnetic atoms preferentially have a 3+ valence, and preferentially migrate to the alkaline earth sites where they replace the Ba when the magnetic rare earths preferentially have a 2+ valence. Techniques for incorporating magnetic atoms into these high-Tc bulk superconductors are well known to those skilled in the art.

The incorporation of a magnetic atom into a high-Tc oxide material in foil form is also easy to achieve. For example, a YBa₂Cu₃Ox thin film epitaxially deposited on a SrTiO₃ substrate can be treated to incorporate magnetic atoms into the YBa₂Cu₃Ox superconducting foil. To do this, the foil/substrate combination is heated so that part of the substrate dissolves into the superconducting foil. This introduces Ti atoms that penetrate either the Y sites or the Ba sites as well as the Cu²⁺ sites. The Ti atoms are randomly distributed in the superconducting foil and if enough of these Ti atoms are present, considerable flux pinning occurs and causes the superconducting foil to exhibit a high critical current, even at temperatures close to the superconducting transition temperature.

As an example of a fabrication process to achieve magnetic fixation in a YBa2Cu3Ox film epitaxially deposited on SrTiO3, one can use a post-deposition flash anneal for 1 to 2 seconds at about 950ºC. This is followed by an anneal for a few minutes at about 925ºC to equilibrate the structure. Then it is cooled in the furnace to absorb the right amount of oxygen to achieve the high-temperature superconductivity transition.

Instead of incorporating atoms with a magnetic moment from a substrate, it is of course also possible to incorporate atoms during the deposition process. For example, when depositing a film of YBa₂Cu₃Ox, one can use the method described by R.B. Laibowitz in Phys. Rev. B 35 (1987) p. 8821, in which an evaporation source consisting of magnetic atoms is incorporated. For example, in this technique for producing superconducting films, the various metals that the superconducting film to be deposited contains are sputtered or evaporated onto a substrate in an oxygen atmosphere. An additional source would then be added for the magnetic atoms. Depending on the electron beam energy supplied to these sources and the time required to evaporate the individual metals, the magnetic atoms are then incorporated directly into the layered superconducting foil.

Now the mechanisms of magnetic vortex fixation and the methods for determining the number of magnetic ions required to generate a significant vortex fixation and thus an increased critical current are to be discussed. In particular, the calculations are carried out with respect to high-Tc oxide superconductors and in particular to the well-known material Y₁Ba₂Cu₃Ox. However, the principles evaluated here also apply to other type II superconductors having very small coherence lengths of the sizes given above.

If the fixation source is arranged randomly, ie not every vortex is fixed, then the effective fixation force per vortex is reduced. This is shown here in a simplified manner for the case of an arrangement in which the vortices influence each other so that Fij = -Fij, where Fij is the force on the i-th vortex originating from the j-th vortex. If the stream exerts a force Fi and the Kth vortex is fixed with a force Fp,k, then

Summing over i gives

if the current distribution is uniform. If only a fraction of the vortices are pinned, the resulting pinning of the array is reduced by that fraction. To achieve the full effect of pinning, it would be desirable to pin each vortex with the greatest possible force. The lattice parameter of an Abrikson lattice is approximately equal to 1.075 (Φ/B)1/2, where Φ is equal to the flux quantum and B is the magnetic field within the superconductor. If B is set equal to Hc1, the average distance between the vortices is of the order of 30 nm and decreases with increasing field.

One of the remarkable properties of high-Tc superconductors is their short coherence length. The strength of the magnetic field within the core of these superconductors is therefore large and is comparable to - but smaller than - that associated with a molecular field in a ferromagnet. When magnetic ions are present in the core, a mutual influence occurs, reducing the energy of the core by an amount (ΦNgJuB(x))/2π, where N is the number of magnetic ions per unit volume, gJ is the moment per ion, uB is the Bohr magneton, and B(x) is the Brillouin function of the argument x = (2gJuBHc₂/kT), where k is the Boltzmann constant and T is the temperature. This energy is usually a few percent of the normal nuclear energy if a few magnetic impurities per unit cell with a moment gJ equal to one are assumed. Clearly this can also become a significant proportion if enough magnetic ions have a large moment. This indicates that the Hc of these materials can be lowered by adding magnetic ions.

Suppose that a vortex is flexible and is to be fixed by fluctuations in the number of magnetic atoms with which it intersects. These fluctuations arise from a random distribution of the atoms. If the magnetic moment were distributed at atomic sites in an ordered arrangement, then there would be no fixation, regardless of the magnitude of the moment per site. The appropriate scale over which the fluctuations are important is determined by the coherence length. The smaller this quantity is, the larger the value of the fluctuation becomes. Below is an approximate solution for the critical current using this argument.

The number of magnetic atoms per volume V of the unit cell is given by N, and each atom has a moment gJ. For the sake of simplicity, it is assumed that only one type moments contribute to the fixation. The number Np of magnetically active atoms in a volume defined by the coherence length ξ is α4πξ³/3 V. If the deviation from the average value Np is only small, it can be represented by a normal distribution, and the volume V of the material in which the probability of finding a fluctuation is one is then given by

V∼(π/e⊃min;¹/π) (α4πα³/3 V) (4πα³/3) (3)

The average distance l between these fluctuations is V1/3. If we assume that the fluctuation in the number of atoms in the volume defined by the coherence length is Np1/2, the fixation energy is (2Hc₂gJuβB(x)) Np and the average force required to fix the vortex is this value divided by l. Equating this value to the force on the vortex exerted by a critical current Jc gives the following equation:

JcΦol = (2Hc₂gJuβB(x))-⊃min;¹Np1/2 (4)

or

Jc(A/cm²) = 3.37·10⊃min;²¹(α/V)⊃min;1/6(gJB(x))ξ⊃min;9/2 (5)

In the Y₁Ba₂Cu₃Ox compounds (called the 1-2-3 compounds), the magnetic moment depends on the Cu²⁺ ions. A random distribution results from the presence of some Cu³⁺ ions. Therefore, the following values are set: gJ = 1 and α = 1/2. This latter value corresponds to the number of carriers generally assumed to be associated with the Cu³⁺ state. The coherence length at low temperatures is about 3 nm. Inserting these values into equation (5) gives a value for Jc between 2·10⁵ and 10⁺ A/cm² for �xi = 2 to 3 nm. The calculated critical current agrees qualitatively with the experimental observations.

The temperature dependence of the critical current is based not only on the dependence of the coherence length on the temperature, but also on the Brillouin function. Near the transition temperature Tc, the argument of the Brillouin function is less than 1, and using the "rough calculation" approximation for the temperature dependence of the coherence length, one obtains the following expression for Jc:

Jc(A/cm²)= 1.13·10⊃min;²⊃0;(gJB(x))h⊃min;1ξ-3 (6)

for very thin films, where h is the film thickness.

However, for most of the temperature range, the Brillouin function cannot be calculated approximately. Thus, the full expression is used and using well-known parameters, the temperature of the critical current can be calculated. This value was compared with the individual single crystal data and the agreement is quite good.

Let us now consider superconducting epitaxial foils of YBa₂Cu₃Ox on SrTiO₃. During annealing of the superconducting compound, the SrTiO₃ dissolves in the 1-2-3 compound. Both Sr and Ti atoms migrate into the superconductor. Although the exact locations of these atoms in the superconductor are not known, it is reasonable to assume that the titanium appears either as Ti²⁺ or as Ti³⁺. In both cases it carries a magnetic moment that is at least twice as large as the Cu²⁺ moment. The Brillouin function approaches 1 at much higher temperatures, so the temperature dependence is not quite as strong. A comparison with experimental data on an epitaxial foil shows that the agreement is again good. This qualitative agreement supports the assumption that vortices fix by Randomly distributed magnet atoms can be used to increase the critical currents in these high-temperature superconductors.

As already noted, the proper incorporation of atoms with a magnetic moment can be used to successfully pin flux vortices in the superconducting material, with the temperature range over which significant vortex pinning occurs depending on the magnitude of the magnetic moment per unit atom. The number of magnetic atoms that must be incorporated into the superconducting material must be sufficiently large to achieve significant vortex pinning (i.e., a number that increases Ic), and such that the magnetic pinning atoms are randomly distributed in the composition. This means that not 100% of the rare earth atoms or the alkaline earth atoms in, for example, the superconducting material will be present. B. High-Tc oxide superconductors of the types well known in the art, nor must the number of vortex-fixing atoms be so small that they are randomly distributed but the number of fixed vortices is too small to increase Ic. As has been seen, about 50% of the atoms located at the rare earth or alkaline earth sites are replaced by atoms with a magnetic moment, a significant number of the vortices are fixed and the increase in the critical current is achieved. For a more complete analysis of the number of magnetic atoms causing random distribution and thus vortex fixation, reference is made to the above equations, and in particular to equation 5.

In the practice of the invention, type II superconductors with small coherence lengths on the order of the unit cell in the superconductor were modified by including atoms that cause magnetic fixation sites for the flux vortices. These magnetic fixation sites must be randomly distributed in the superconductor in order to create a randomly composed material. They must also be present in sufficient quantity to cause the required random distribution and at the same time be present in sufficient quantity to allow substantial vertebral fixation. Of course, any amount of vertebral fixation will help to enhance Ic, but the greater the number of vertebrae fixed, the higher the magnitude of Ic.

The pinning of the vortices must be achieved without seriously reducing the superconductivity of the material. For example, if the superconductor is degraded, the temperature range over which superconductivity is achieved will be affected and even if high critical currents are indicated, the material will not be particularly suitable if the temperature range of superconductivity cannot be readily achieved and maintained. The skilled person will take into account these design considerations, which may vary depending on the intended application of the superconducting material.

The invention has been described above with reference to specific embodiments, but it will be understood by those skilled in the art that various changes may be made without departing from the scope and spirit of the invention. In this regard, more than one type of atom having a magnetic moment may be incorporated into the superconductor, and the magnitude of the magnetic moment of these atoms may vary from small to large amounts. Of course, more effective pinning over a wider temperature range may be achieved if the atoms have large magnetic moments. On the other hand, atoms having very large magnetic moments occupying superconductivity-causing sites may cause a loss of superconductivity in the material.

Furthermore, special techniques for incorporating atoms with magnetic moment to achieve vortex fixation were demonstrated, However, the person skilled in the art will be aware that the incorporation of the magnetic atoms for vortex fixation in the base material or in the foil material can be carried out in different ways. The general concept is to provide the magnetic fixation forces in a random distribution in order to produce an increased Ic. Furthermore, the person skilled in the art will understand that this invention relates to superconductors which can be polycrystalline or single crystals and which can be produced in any geometric shape (plate, wire, etc.).

In particular, this technique can be applied to produce high-Tc oxide superconductors and devices, especially copper oxide superconductors with a Tc higher than 77 K, which have critical currents above 10⁴ A/cm² at 77 K.

Claims (13)

1. Type II superconductor with a short coherence length in the order of magnitude of the atomic dimensions in this superconductor, in which vortices occur which, when freely mobile, limit the maximum critical current through the superconductor, this superconductor having fixing points for magnetically fixing these vortices in order to prevent their mobility in the superconductor, these fixing points being regular lattice points which are randomly distributed as a result of partial substitution by atoms with magnetic moments associated with them of sufficient magnitude for the purpose of fixing these vortices, characterized in that this coherence length is in the approximate range of 0.3 to 3 nm, that about 50% of the lattice points of a component of this superconductor are occupied by atoms with magnetic moments which are larger than those of Cu²⁺ atoms, so that the critical current Ic of the superconductor is increased and a value greater than 10⁴ A/cm² at 77 K. 2. Superconductor according to claim 1, characterized in that the superconductor is a bulk material superconductor. 3. Superconductor according to claim 1, characterized in that the superconductor is a foil. 4. A superconductor according to claim 1, characterized in that the superconductor is a high-Tc superconductor which exhibits superconductivity at temperatures above 26 K, wherein the superconductor includes a rare earth and/or a rare earth element, an alkaline earth element, a transition metal element and oxygen. 5. Superconductor according to claim 4, characterized in that these atoms with the magnetic moments at the Places of the rare earths or of the rare earth-like elements in the lattice of the superconductor. 6. Superconductor according to claim 4, characterized in that these atoms with the magnetic moments are located at the locations of the alkaline earths in the lattice of the superconductor. 7. Superconductor according to claim 4, characterized in that these atoms with the magnetic moments are located at the transition metal sites in the lattice of the superconductor. 8. Superconductor according to claim 1, characterized in that these atoms with the magnetic moments are rare earth atoms. 9. Superconductor according to claim 1, characterized in that these atoms with the magnetic moments are transition metal atoms. 10. Superconductor according to claim 4, characterized in that it has a rhombic structure. 11. Superconductor according to claim 4, characterized in that it has a layered crystalline structure. 12. Superconductor according to claim 4, characterized in that the superconductor is a Y-Ba-Cu-O compound. 13. Superconductor according to claim 12, characterized in that the superconductor has a nominal composition YBa₂Cu₃Ox.