GB2468924A

GB2468924A - High temperature superconductors

Info

Publication number: GB2468924A
Application number: GB0905360A
Authority: GB
Inventors: Stuart Wimbush; Judith Driscoll
Original assignee: Cambridge Enterprise Ltd
Current assignee: Cambridge Enterprise Ltd
Priority date: 2009-03-27
Filing date: 2009-03-27
Publication date: 2010-09-29
Also published as: GB0905360D0

Abstract

A high temperature superconductor composition includes a matrix of a high temperature superconductive oxide. Non-superconductive particles are distributed in the matrix. At least some of the non-superconductive particles comprise either a rare earth transition metal oxide or a barium or strontium transition metal oxide. The non-superconductive particles are ferromagnetic, superparamagnetic, ferrimagnetic or antiferromagnetic at a temperature of 77K. Suitable compositions include a superconductive matrix of YBCO with YFe03 non-superconductive particles distributed in the matrix. The composition allows practical magnetic pinning to be achieved. A composition comprising Y, Nd, Sm, Eu, Gd, Ho or a mixture thereof; Ba; Cu; a rare earth, Sr or Ba; a transition metal; and oxygen in various proportions is also disclosed, as is a method of manufacturing an electrical conductor comprising deposition of a layer of material on a substrate which includes a matrix formed of high temperature superconductive oxide or its precursor.

Description

HIGH TEMPERATURE SUPERCONDUCTORS

BACKGROUND TO THE INVENTION

Field of the invention

The present invention relates to high temperature superconductor compositions, electrical conductors produced using such compositions and to methods of producing such compositions and conductors.

Related art High temperature superconducting oxides include yttrium-barium-copper oxides and related materials. For example, the composition YBa2Cu3O7o (referred to herein and in the academic literature as YBOO) is a superconductor at temperatures below a critical temperature T, which varies with the value of ö.

Improvement of flux pinning and thus the amount of current that can be carried in YECO (in self-field and/or in applied field) is important for achieving widespread applications of this technologically important material. Practical pinning enhancement methods developed within the last 5 years such as incorporating nanoinclusions in the film or on the substrate surface, disorder effects from rare earth (RE) modifications, and microstructural modification have all been successful to some extent in specific field and temperature regimes. Improvements in the critical current density of YECO by flux pinning have been demonstrated approaching an order of magnitude [Reference 1].

Typically, the most successful pinning mechanisms to date have relied on creating non-superconductive regions within the superconductive matrix, the length scales of the non-superconductive regions being tailored to the vortex width associated with flux lines through the superconductive matrix. For example, US 2006/0025310 discloses the formation of BaZrO3 (BZO) particles in YBCO films grown on strontium titanate (either as a single crystal substrate, as a buffer layer on MgO single crystal, or as a buffer layer on MgO formed on nickel-based alloy) . 5 mol% BaZrO3 was used. Compared with similar YBCO films formed without BaZrO3, the YBCO films incorporating BaZrO3 provided significantly improved J in magnetic field strengths (poH) upto7Tat75.5K.

It is also of interest to consider magnetic pinning in both low and high temperature superconductors. The basic concept for magnetic pinning is to enhance pinning forces due to the magnetic interaction of the pinning centres and the vortex lattice [Reference 21. Common approaches to magnetic pinning include base [Reference 3) or surface [Reference 4] layer arrays of lithographically patterned magnetic dots, unpatterned bilayer systems exploiting the domain structure of a magnetic layer [Reference 5] or substrate [Reference 6], and nanoscale multilayer ferro-magnetic/superconducting thin film aggregates [Reference 7]. However, to date no practical approach to the creation of magnetic pinning centres in the high-temperature superconductive materials has been demonstrated, although recent attempts have been made using Ni, Fe203, BaFe12O, and La067Ca0*33MnO3 [Reference 8], resulting in suppression of Tc.

SUMMARY OF THE INVENTIOM

The present inventors have realised that it may be possible to obtain practical magnetic pinning in commercially interesting superconductors. The present inventors have found that this is possible by forming ferromagnetic or superparamagnetic pinning centres of a rare earth transition metal oxide or a barium transition metal oxide. This constitutes a general aspect of the invention.

In a first preferred aspect, the present invention provides a composition including a matrix of a high temperature superconductive oxide, with non-superconductive particles distributed in the matrix, at least some of the non-superconductive particles comprising either a rare earth transition metal oxide, or a barium or strontium transition metal oxide, and wherein said non-superconductive particles are ferromagnetic, superparamagnetic, ferrimagnetic or antiferromagnetic at a temperature of 77K.

In general, it is preferred that the non-superconductive particles are not paramagrietic and not diamagnetic.

In a second preferred aspect, the present invention provides a composition comprising: (i) at least one of Y, Nd, Sm, Eu, Gd, Ho or a mixture thereof, in a combined amount in the range 6-9 atomic percent; (ii) Ba in an amount in the range 13-17 atomic percent; (iii) Cu in an amount in the range 19-26 atomic percent; (iv) at least one rare earth element, optionally additional to (i), or Sr, or Ba, optionally additional to (ii), in a combined amount in the range 0.01-10 atomic percent; (v) at least one transition metal in a combined amount in the range 0.01-10 atomic percent; (vi) incidental and/or trace impurities; and (vii) balance oxygen.

In a third preferred aspect, the present invention provides an electrical conductor including a layer of the composition according to the first aspect.

In a fourth preferred aspect, the present invention provides a method of manufacturing an electrical conductor, including depositing a layer of material on a substrate, the material including a matrix formed of high temperature superconductive oxide or the precursor thereof, the material further including, in addition to that stoichiometrically required to form the high temperature superconductive oxide, a rare earth element, or barium, and a transition metal.

Preferred and/or optional features are set out below.

Similarly, further aspects of the invention are set out below. The preferred and/or optional features are combinable singly or in any combination with any aspect of the invention, unless the context demands otherwise. Where lists of options are given, it is specifically intended that any shorter list is also envisaged, formed by deletion of one or more members of the list.

Preferably, the rare earth element of the non-superconductive particles is selected from one or more of scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tu), ytterbium (Yb) and lutetium (Lu) . Yttrium is most preferred. This is particularly the case where the superconductive oxide matrix includes yttrium. A further preferred rare earth element is europium. This is particularly the case where the superconductive oxide matrix includes europium. A further preferred rare earth element is samarium. This is particularly the case where the superconductive oxide matrix includes samarium. A further preferred rare earth element is gadolinium. This is particularly the case where the superconductive oxide matrix includes gadolinium, e.g. Gd-Ba-Cu-O. A further preferred rare earth element is dysprosium. This is particularly the case where the superconductive oxide matrix includes dysprosium, e.g. Dy-Ba-Cu-O.

Preferably, the transition metal is selected from one or more of scandium (Sc), titanium (Ti), vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), yttrium (Y), zirconium (Zr), niobium (Nb), molybdenum (Mo), technetium (Tc), ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), cadmium (Cd), lanthanum (La), hafnium (Hf), tantalum (Ta), tungsten (W), rhenium (Re), osmium (Os), iridium (Ir), platinum (Pt), gold (Au) and mercury (Hg), with the proviso that the transition metal in the rare earth transition metal oxide is not identical to the rare earth element. Iron is most preferred.

In particular, those transition metals which are themselves ferromagnetic at 77K are preferred. These include Fe, Co, and Ni. Further, those transition metals that form aritiferromagnetic oxides are preferred. These include Cr and Mn. (Note, however, that it is expected that such compositions would not be orthoferrites.) Furthermore, although not contained in the list above, Sr-containing or Ba-containing material is of interest, in particular Sr ferrite or Ba ferrite.

Preferably, the non-superconductive particles are formed of a material having a magnetization at 77K of at least 0.001 Bohr magnetons per formula unit at lT applied field.

Preferably, the non-superconductive particles are formed of a material having a magnetization at 77K of at least 0.01 Bohr magnetons per formula unit at iT applied field.

Typically, at iT, the non-superconductive particles with have reached their saturation magnetization. For example, in the specific case where the non-superconductive particles are formed of YFeO3, the formula unit is the YFeO3 unit. It is specifically of interest for the composition to be used at liquid nitrogen temperatures (77K at atmospheric pressure, but variable to a known extent with changes in pressure) It is for this reason that the saturation magnetization is defined at 77K. However, for many materials, the saturation magnetization varies only moderately between room temperature and 77K.

For example, where the non-superconductive particles are formed of RFeO3 (R = rare earth), then the magnetization at 77K at iT is typically in the range 0.03-0.05 Bohr magnetoris per formula unit.

Diamagnetic materials (from which the non-superconductive particles are preferably not formed) typically have a negative susceptibility between 0 and -1, more typically of the order of _106 at 77K. Paramagnetic materials (from which the non-superconductive particles are also preferably not formed) typically have a positive susceptibility, more typically of the order of i05 at 77K. Ferromagnetic materials (from which the non-superconductive particles may be formed) have a positive susceptibility typically of the order of i03 at 77K. Ferrimagnetic materials (from which the non-superconductive particles may be formed) tend to have a slightly lower susceptibility than ferromagnetic materials.

Some weak ferromagnetic materials have a significantly lower susceptibility, e.g. of about i04 at 77K.

Thus, additionally or alternatively to the magnetization behaviour set out above, the non-superconductive particles may be formed of a material having a magnetic susceptibility of at least i0 at 77K. More preferably, the non-superconductive particles are formed of a material having a magnetic susceptibility of at least at 77K, at least 10 2 at 77K, at least 10' at 77K, at least 1 at 77K, at least at 77K, at least 100 at 77K, or at least iü at 77K.

It is of particular interest to use the conductor at liquid nitrogen temperatures. At atmospheric pressure, liquid nitrogen boils at 77K. However, when pumped down to a lower pressure, it is possible to reduce the temperature of boiling nitrogen down to 65K. Therefore the present invention may be used preferentially in the temperature range 65-77K.

Furthermore, other operating temperatures are contemplated.

Conventional "low temperature" superconductors are typically operated at 4.2K (liquid helium temperatures) . Thus, the present invention may be operated at temperatures below 30K, e.g. in the range 4-20K.

Still further, other operating temperatures are contemplated.

For example, mechanical cryocoolers allow cooling to temperatures of about 20K and above, allowing operation in the range 20-65K. Mechanical cryocoolers may also allow operation at different temperature ranges, e.g. at temperatures below 20K.

The non-superconductive particles may be formed of a ferromagnetically hard material. Of particular interest here are materials having significant hysteresis in their magnetisation behaviour. Ferromagnetically hard materials may provide behaviour which is advantageous in some circumstances, in that the pinning force can be tuned to specific magnetic field strengths. However, preferably, the non-superconductive particles are formed of ferromagnetically soft materials. Such materials typically have only small or zero hysteresis in their magnetisation behaviour. Such materials can provide a more linear response of pinning force with varying magnetic field strength.

Preferably, the rare earth element, or barium or strontium, is provided at a concentration of at least 0.001 atomic percent relative to the total composition. Where the superconductive matrix contains a rare earth element, or barium, preferably this lower limit is for an additional amount, compared to that stoichiometrically required in order to form the superconductive matrix.

Preferably, the transition metal is provided at a concentration of at least 0.001 atomic percent relative to the total composition.

Preferably, the non-superconductive particles conform to the phase AaBbOz, where A is Ba or a rare earth element, B is a transition metal and 0 is oxygen, wherein: (I) a is 1 or about 1, b is 1 or about 1 and z is 3 or about 3, or (II) a is 3 or about 3, b is 5 or about 5 and z is 12 or about 12.

The phase AaBbOz may have an orthorhombic crystal structure, such as an orthoferrite crystal structure. Alternatively, phase AaBbOz may have a garnet crystal structure.

Alternatively, phase AaBbOz may have a hexaferrite crystal structure. The hexaferrites are materials existing in the compositional field BaO-MO-Fe203, where M is a divalent metal, or mixture. Ba can be replaced (in whole or in part) by other group II metals, most notably Sr. There are considered to be four classes of hexaferrites, labelled M, W, Y and Z corresponding to (BaO + MO) / Fe203 ratios of 1/6, 3/8, 4/6 and 5/12, respectively. Explicitly, these are then BaFe12O1g, BaM2Fe16O27, Ba2M2Fe12O22, Ba3M2Fe24O41. The reader is referred to Reference 13 (Smit & Wijn, 1959), for suitable hexaferrite materials of interest in the present invention, the content of which is hereby incorporated by reference in its entirety.

However, it is preferred that the non-superconductive particles are not formed of BaFe12O. Typically, the non-superconductive particles are not formed of Lao67Cao.33MnO3.

The non-superconductive particles may be formed of any of the compositions disclosed in Reference 11 (Treves, 1965) In particular, the non-superconductive particles may be formed of one or more orthoferrites, such as LaFeO3, PrFeO3, NdFeO3, SmFeO3, EuFeO3, GdFeO3, TbFeO3, DyFeO3, YFeO3, HoFeO3, ErFeO3, TmFeO3, YbFeO3, LuFeO3. YFeO3 is particularly preferred. It is possible that some or all of the Fe in such compositions could be substituted by one or more alternate transition metals, as listed above.

Preferably, the non-superconductive particles have an average particle size of 100 nm or less. This average is preferably a nunther average and may be determined, for example, by TEN analysis of individual non-superconductive particles in the superconductive material matrix. More preferably, the non-superconductive particles have an average particle size of 90 nm or less, 80 nm or less, 70 nm or less, 60 nm or less, 50 nm or less, 40 nm or less, 30 nm or less or 20 nm or less. Preferably, the non-superconductive particles have an average particle size of 1 nm or more, more preferably 2 nm, 3 nm, 4 nm or 5 nm or more. An average particle size for the non-superconductive particles of about 10 nm has been found to be satisfactory.

It is possible for the non-superconductive particles to have a particle size that is larger (in some cases only slightly larger) than the size of a single ferromagnetic domain of the same material.

It is also possible, and in some cases preferable, for the non-superconductive particles to have a particle size that corresponds to a size substantially the same as the ferromagnetic domain size for bulk material of the same composition as the non-superconductive particles. It is considered that the magnetic response of such material would be ferromagnetic.

It is further possible, and in some cases preferable, for the non-superconductive particles to have a particle size that is smaller than the ferromagnetic domain size for bulk material of the same composition as the non-superconductive particles. In such a regime, a ferromagnetic material may demonstrate superparamagnetic behaviour. Thus, a method of achieving superparamagnetic behaviour may include decreasing the particle size of the non-superconductive particles to achieve a superparamagnetic response rather than a ferromagnetic response.

Preferably, the high temperature superconductive oxide is a barium copper oxide. For example, the high temperature superconductive oxide may be an yttrium barium copper oxide such as YBa2Cu3O7-5. In this case, the non-superconductive particles most preferably contain Y as the rare earth element or as one of the rare earth elements. In the method of manufacturing such a composition, it is strongly preferred to use an amount of Y in excess of that required stoichiometrically to form YBa2Cu3O7-.

Preferably, in the conductor of the third aspect, layer is aligned so that the c-axis of at least one crystal grain of the high temperature superconductive oxide is aligned substantially parallel to the thickness direction of the layer. Such c-axis alignment typically demonstrates much higher transport current properties than alternative alignments. Preferably, the superconductive oxide layer is a substantially epitaxial layer on a substrate or on a buffer layer or buffer layers formed on the substrate.

The non-superconductive particles may be substantially preferentially oriented with respect to the superconductive oxide and/or the substrate and/or a buffer layer, if present.

It is envisaged that further nanoscale particles may be provided in the superconductive matrix, said further nanoscale particles preferably being non-magnetic. Such particles may provide correlated pinning enhancement, e.g. as set out in co-pending US provisional application 61/086841, filed 7 August 2008 (corresponding to GB0814598.9, filed 8 August 2008) Preferably, the thickness of the layer of superconductive oxide is at least 100 nm. The layer of superconductive oxide may be greater than this, in order to provide high engineering critical current densities. For example, the thickness of the layer of superconductive oxide may be at least 0.2 pm, at least 0.3 pm, at least 0.4 pm, at least 0.5 pm, at least 0.6 pm, at least 0.7 pm, at least 0.8 pm, at least 0.9 pm or at least 1 pm.

In a further aspect, the present invention provides a target for a film deposition process, the target having a composition according to the first or second aspect, or a composition corresponding to the proportion of elements required to form a composition according to the first or second aspect.

In a still further aspect, the present invention provides a combination of precursor solutions or suspensions for a film deposition process, the combination having a composition according to any one of claims 1 to 15 or a composition corresponding to the proportion of elements required to form a composition according to any one of claims 1 to 15, further optionally including additional carrier materials.

Suitable carrier materials (e.g. solvents, binders, etc.) will be well known to the skilled person.

Preferably, in the method of manufacturing the electrical conductor, there is included the step of forming non-superconductive particles, distributed in the matrix, said particles comprising either a rare earth transition metal oxide, or a barium transition metal oxide, and wherein said non-superconductive particles are ferromagnetic or superparamagnetic at a temperature of 77K.

Preferably, in the method of manufacturing the electrical conductor, there is included at least one heat treatment, which may be at the same time as the deposition step and/or subsequent thereto. Typically, such a heat treatment allows suitable adjustment of the oxygen concentration of the superconductor matrix.

The superconductive oxide layer may be deposited by, for example, pulsed laser deposition. However, other deposition methods are contemplated, such as evaporation (including coevaporation, e-beam evaporation and activated reactive evaporation), sputtering (including magnetron sputtering, ion beam sputtering and ion assisted sputtering), cathodic arc deposition, chemical vapor deposition, organometallic chemical vapor deposition, plasma enhanced chemical vapor deposition, molecular beam epitaxy, a sol-gel process, liquid phase epitaxy, hybrid liquid phase epitaxy, polymer assisted deposition, atomic layer deposition, chemical solution deposition and metallorganic deposition, e.g. the trifluoroacetic acid (TFA) method and the like.

Further preferred and/or optional features will be apparent from the detailed description of the preferred embodiments set out below.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below, with reference to the accompanying drawings, in which: Fig. 1 shows an AFM image of the surface of a pure YBCO film (control) formed on a (100) SrTiO3 substrate.

Fig. 2 shows an AFM image of the surface of an YBCO film according to an embodiment of the invention, formed on a (100) SrTiO3 substrate.

Fig. 3 shows an AFM image of the sample shown in Fig. 2, except at higher magnification.

Fig. 4 shows a high resolution TEM image of a nanoparticle inclusion within the YBCO matrix of a 0.1 im thick sample.

The sample has the same composition as the sample shown in Figs. 2 and 3.

Fig. 5 shows a TE!1 image of a 0.5 im thick, 3% doped YECO film.

Fig. 6 shows a TEM image of the same sample as Fig. 5, but at a higher magnification and resolution.

Fig. 7 shows X-ray diffraction e-2e scans obtained from a series of 0.5 pm thick samples of differing dopant concentrations (upwards from bottom: pure, 1 at.%, 3 at.%, 5 at.%) Fig. 8 shows a further X-ray diffraction e-2e scan for a pure YBCO film on (100) SrTiO3.

Fig. 9 shows further X-ray diffraction e-2e scans for a series of doped YBCO films on (100) SrTiO3.

Fig. 10 shows a further X-ray diffraction e-2e scan for a pure YFeO3 film on (100) SrTiO3.

Fig. 11 shows magnetic hysteresis loops measured in-plane (H parallel to ab) and out-of-plane (H parallel to C) on a 0.5 pm thick 1 at.% doped sample at 100 K (above Tc for the sample) Fig. 12 shows a magnetic hysteresis loop measured out-of-plane (H parallel to c) on a 0.5 pm thick 1 at.% doped sample at 77 K (below above Tc for the sample) Fig. 13 shows a magnetic hysteresis loop measured in-plane (H parallel to ab) on a 0.5 pm thick 1 at.% doped sample at 77 K (below Tc for the sample) Fig. 14 shows a similar magnetic hysteresis loop as Fig. 12, except for a 5 at.% doped sample.

Fig. 15 shows magnetic hysteresis loops for a 1 at.% dopes sample at 300 K, 100 K and 77 K (temperatures indicated on the respective graphs) Fig. 16 shows resistance vs temperature measurements for a series of 1 pm thick samples of different dopant concentrations.

Fig. 17 also shows resistance vs temperature measurements for various 0.5 pm thick YBCO films (pure YECO, and 1 at.%, 3 at.% and 5 at.% doping) Fig. 18 shows the field dependence (with H parallel to c) of the critical current density at 77K for a series of 0.5 pm thick samples of differing dopant concentrations (pure YBCO, and 1 at.%, 3 at.% and 5 at.% doping) Fig. 19 shows the angular dependent of the critical current density at 77 K (1 T applied field), of a series of 0.5 pm thick samples of differing dopant concentrations (pure YBCO, and 1 at.%, 3 at.% and 5 at.% doping) DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS, INCLUDING FURTHER PREFERRED NW/OR OPTIONAL FEATURES In a preferred embodiment of the invention, nanoscale ferromagnetic pinning centres of YFeO3 are incorporated within YBa2Cu3O7o (YBCO) thin films. YFeO3 is an orthoferrite material combining four distorted perovskite units into an orthorhombic unit cell [Reference 9] Orthoferrites of this type in bulk form, including YFeO3, typically exhibit a weak ferromagnetism resulting from a slight canting of the antiferromagnetically coupled Y moments due to an asymmetric exchange interaction [Reference 10].

As described in detail below, at sufficiently low dopant concentrations, the suppression of transition temperature due to poisoning of the YBCO is overcome. Even at the low dopant levels used in the preferred embodiments, the resultant films exhibit a coexistence of superconductivity and ferromagnetism below the superconducting transition temperature Tc, and a consequent absolute enhancement of the critical current density Jc by up to a factor two under all applied fields and field orientations, including self field.

Below are set out experimental details relevant to the preferred embodiments.

Thin films of nominal thickness 0.1 pin, 0.5 pm and 1 pm were deposited onto SrTiO3 (100) single crystal substrates by pulsed laser deposition (KrF 248 nm, 10 Hz, about 2 Jcm2) from composite targets prepared by standard solid state ceramic processing techniques (mixing, pressing, sintering) Starting materials for the targets were 99.99% YBa2Cu3O powder (SCI Engineered Materials) and 99.9% FeO powder (Aldrich) . In alternative embodiments, the starting materials for the targets were 99.99% YBa2Cu3O powder (SCI Engineered Materials) and 99.8% Co304 powder (Aldrich) . In further alternative embodiments, the starting materials for the targets were 99.99% YBa2Cu3O powder (SCI Engineered Materials) and 99.9% FeO powder (Aldrich) and 99.8% Co304 powder (Aldrich) The substrates were held at 765°C in an atmosphere of 30 Pa flowing oxygen gas. The films were annealed in situ for one hour post-deposition at 520°C in 0.5 bar static oxygen to achieve optimal oxygenation.

Initial trials centred on YCoO3 and various other additions.

The present inventors discovered that it was possible to form both YFeO3 and CoFeO3 precipitates within the resultant films. There was consequently a Y deficiency within the YBCO. It was surprising to the present inventors to find that YFeO3 could form within YBCO.

Further experiments were conducted in which targets were prepared to form films with differing concentrations (1 at.%, 3 at.%, 5 at.%) of YFeO3 dopant, in addition to pure control YBCO and YFeO3 samples.

The films were structurally analysed by x-ray diffraction, transmission electron microscopy (TEN) and atomic force microscopy (AFM) . Their magnetic properties were measured in a vibrating sample magnetometer (VSM) . Their superconducting properties were determined by electrical transport measurements after photolithographic patterning and ion beam etching into bridge structures under varying temperatures and applied magnetic fields in an 8T cryogenic superconducting magnet. Typical bridge dimensions were pm wide and 1 mm long.

Fig. 1 shows an AFM image of the surface of a pure YBCO film (control) of thickness 0.1 pm formed on a (100) SrTiO3 substrate.

Fig. 2 shows an AFN image of the surface of an YBCO film according to an embodiment of the invention, of thickness 0.1 pm, with nominal composition YBCO + lat% CoFe2O4, formed on a (100) SrTiO3 substrate.

Fig. 3 shows an AFN image of the sample shown in Fig. 2, except at higher magnification. Small precipitates are seen to be dispersed uniformly across a wide area. The observed particle size is typically in the range of 10-20 nm, although a few larger particles up to 100 nm in size are also seen. The nanoparticles in Fig. 3 are shown to be distributed across the entire area of the sample. Single unit cell steps forming the growth spirals of the YBCO are also visible.

Additional investigations by TEN confirmed the presence of these nanoparticles dispersed throughout the film volume.

Fig. 4 shows a high resolution TEN image of a nanoparticle inclusion within the YBCO matrix of a 0.1 pm thick sample, the sample having the same composition as the sample shown in Figs. 2 and 3. These observations also revealed a typical particle size of about 10 rim and a clean, sharp interface with the YBCO matrix.

Fig. 5 shows a TEN image of a 0.5 pm thick, 3at% YFeO3 doped YBCO film. This image reveals widespread nanoprecipitate formation throughout the film (see arrowheads in Fig. 5) The precipitates are seen to be very small, typically around nm in diameter, explaining their lack of presence in the x-ray measurements (see below) . Due to their size, the particles are more evident in the thinner regions of the sample, although they existed throughout the sample.

Fig. 6 shows a TEN image of the same sample as Fig. 5, but at a higher magnification and resolution. A larger particle is pictured in the YBCO matrix, in order to obtain a selected area diffraction pattern from the particle to assist in determining its structure and orientation with respect to the YBCO matrix. The crystal structure observed for the particle is in accordance with that expected for YFeO3. The present inventors consider that the YFeO3 (YFO) particles and the YBCO matrix have a preferred alignment (namely c-axis aligned and 45° in-plane rotation) Fig. 7 shows X-ray diffraction 8-20 scans obtained from a series of 0.5 pm thick samples of differing dopant concentrations (upwards from bottom: pure, 1 at.%, 3 at.%, 5 at.%) . The intensity scale is logarithmic and the patterns are offset vertically for clarity. Fig. 7 shows strong epitaxial YSCO (001) peaks, but little evidence of additional phases. This supports the microscopy observations of small, well-dispersed particles. It is considered that a 10 nm particle size would give rise to a peak width ranging from 0.9 degrees at 28=10 degrees to 1.8 degrees at 2Ol20 degrees. Such a broad peak combined with the small dopant level in a thin film sample would result in a very small diffracted intensity arising from the inclusions.

Fig. 8 shows a further X-ray diffraction 8-28 scan for a pure YBCO film on (100) SrTiO3, the same sample as used in Fig. 7. Fig. 9 shows further X-ray diffraction -28 scans for a series of doped YBCO films on (100) SrTiO3, film thickness 1 pm. Fig. 10 shows a further X-ray diffraction 8-20 scan for a pure YFeO3 film on (100) SrTiO3, film thickness 0.5 pm. In the results for each of the pure samples, the STO substrate peaks are labelled, as well as K peaks arising from them. In each of the pure samples, the peak corresponding to the relevant phase are indexed.

Dashes highlight small impurity phases present in both pure samples. With reference to Fig. 9, there is only scant evidence of a secondary phase, partly due to the close overlap of the YFeO3 peaks with those of the substrate.

However, the pure YFeO3 sample (Fig. 10) reveals a (100) oriented growth with respect to the substrate, which can be expected to persist in the doped films. The lack of clear peaks relating to YFeO3 in the doped samples (Fig. 9) is attributed to the small volume fraction of YFeO3 present, and its likely presence in the form of nanoparticles, resulting in weak, broad peaks.

Fig. 11 shows magnetic hysteresis loops measured in-plane (H parallel to ab) and out-of-plane (H parallel to c) on a 0.5 pm thick 1 at.% doped sample at 100 K (above Tc for the sample) . At room temperature, and down to 100 K, the samples exhibit rather soft ferromagnetic hysteresis loops with coercivity values around 20-3D mT. A slight anisotropy in the coercivity values for fields applied in the plane of the sample and out-of-plane suggests that the magnetic particles may be oriented with respect to the epitaxial YBCO matrix.

Fig. 12 shows a magnetic hysteresis loop measured out-of-plane (H parallel to c) on a 0.5 pm thick 1 at.% doped sample at 77 K (below above Tc for the sample) . Thus, below the superconducting transition temperature, the magnetic response is dominated by the superconductivity, which produces a characteristically diamond-shaped loop. It is not yet clear if the anisotropic form of this loop is an effect due to the magnetic inclusions.

Fig. 13 shows a magnetic hysteresis loop measured in-plane (H parallel to ab) on a 0.5 pm thick 1 at.% YFeO3 doped YBCO sample at 77 K (below Tc for the sample) . The in-plane response, with the magnetic field applied parallel to the film surface, barely shows the superconductivity (only a slight remnant is seen due to sample mismounting) and a ferromagnetic response is observed instead. Thus, the ferromagnetic response persists even below Tc, coexisting with the superconductivity.

Fig. 14 shows a similar magnetic hysteresis loop as Fig. 12, except for a 0.5 jim thick 5 at.% YFeO3 doped sample. The hysteresis loop is strongly asymmetric, indicating that flux penetrates the sample more easily than it is removed. Such behaviour can form the basis for a "vortex ratchet" device.

The lighter shade lines shown on the graph are reflections in order to highlight the asymmetry of the response.

Figs. 15A, 15B, 15C show magnetic hysteresis loops for a 1 at.% doped sample at 300 K, 100 K and 77 K (temperatures indicated on the respective graphs) . At 300 K and 100 K, measurements are taken both in-plane (IP) and out-of-plane (COP) . Only an out-of-plane measurement is taken for 77 K. It is to be noted here that the magnetic properties of the samples confirm the presence of YFeO3, with a clear Curie transition expected above 680K.

The superconducting transition temperature, Tc, of the composite samples, determined from resistance vs temperature measurements, is shown in Fig. 16 for a series of 1 jim thick samples of different dopant concentrations. Tc is found to decrease gradually as the dopant concentration is increased.

For low dopant concentrations around 1%, however, the transition temperature is indistinguishable from that of pure YBCO (91 K -see Fig. 17) . This indicates good phase separation in the composite, and a limited "poisoning" of the YSCO by Fe. This is unexpected, since conventionally Fe has been considered to have a Tc-suppressing effect on YBCO.

Fig. 17 shows a similar set of results to Fig. 16, except for films of thickness 0.5 pm and including a pure YBCO control sample.

The resistive measurements of the superconducting transition of the samples shown in Figs. 16 and 17 reveal a typical behaviour for YBCO, with the normal state resistivity increasing with dopant level due to increased scattering by the YFeO3 precipitates. The Tc of the samples decreases from 91 K for the pure and 1% doped samples to 85 K for the 5% doped sample, a slight but significant reduction indicative of some degree of poisoning' of the YBCO by the ferromagnetic additives. This Tc reduction has a knock-on effect via a reduced irreversibility line on the Jc of the more highly doped samples, which nonetheless provide a useful exposition of the magnetically-induced properties.

Fig. 18 shows the field dependence (with H parallel to c) of the critical current density at 77K for a series of 0.5 pm thick samples of differing dopant concentrations (pure YBCO, and 1 at.%, 3 at.% and 5 at.% doping) . Fig. 18 shows that all of the composite samples outperform the pure YBCO sample prepared under the same conditions throughout the entire field range, with the exception of the most highly doped (5%) sample, the Jc of which drops off more rapidly at high fields (i0H > 2 T) due to its suppressed Tc (87 K) and consequently reduced irreversibility field at 77 K. The fact that the self-field Jc of this sample nonetheless retains its enhanced value signifies a greatly increased degree of flux pinning also in this case.

In general, the sample with the lowest doping level (1%) exhibits the best performance, with a self-field Jc in excess of 5 MAcm2, compared to 3 MAcm2 for the pure sample.

At higher fields, the enhancement is even more dramatic, reaching almost an order of magnitude for a0H > 5 T. However, in an intermediate field range between about 0.1 T and 0.5 T, the more highly doped 5% sample performs slightly better.

Fig. 19 shows the angular dependence of the critical current density at 77 K (1 T applied field), of a series of 0.5 pm thick samples of differing dopant concentrations (pure YBCO, and 1 at.%, 3 at.% and 5 at.% doping) . Fig. 19 shows that there is enhancement (to a similar degree shown in Fig. 18) at all field angles, indicative of a random pinning process.

This is also a useful result, since most pinning enhancement approaches are focused on enhancing the Jc in the c direction, while here the ab pinning of importance to certain applications is also strongly enhanced, indicative of a new pinning mechanism in these samples. A slight c-axis peak evident in the composite samples does not increase in magnitude with the dopant concentration, but rather is strongest in the most weakly doped (1%) sample.

The present inventors have therefore successfully introduced nanoparticulate ferromagnetic pinning centres into YBCO thin films via a simple doping process. Samples doped in this manner exhibit repeatable world-leading Jc values of up to 2.4 MAcm2 (77 K, self field) in a 1 pm thick film, or 5.4 MAcm2 in a 0.5 pm thick film (77 K, self-field) . The samples yield a ferromagnetic response from room temperature down to Tc, below which ferromagnetism is expected to persist, but where the signal is swamped by the diamagnetic superconducting response. Poisoning of the YSCO by the ferromagnetic constituent is limited, and at sufficiently low dopant concentrations, the Tc of the composite film is indistinguishable from pure YECO. Compared to pure YBCO films prepared via a similar process, the 77 K self field Jc is enhanced by a factor of two. To the knowledge of the inventors, this is the first time such a drastic enhancement in self field Jc has been achieved, and the first time such a low dopant level has been shown to have such a large effect. The Jc enhancement is universal, spanning all applied fields and field directions (supporting the inventors' consideration that the nanoparticulate pinning sites are enhanced by utilizing soft ferromagnetic material), and thereby lending itself to combination with existing methods of correlated pinning enhancement.

Although not wishing to be bound by theory, one possible mechanism for strong Jc enhancement by magnetic pinning is explained in Reference 12, based on the mechanism of Lorentz force reduction by which a strong Jc enhancement might be expected due to magnetic inclusions even under zero applied

field.

The above embodiments have been described by way of example.

On reading this disclosure, modifications of these

embodiments, further embodiments and modifications thereof will be apparent to the skilled person. In particular, the disclosure here may be applied to the formation of longer lengths of conductor, e.g. to superconductive oxides formed on buffered biaxially aligned metallic substrates formed from nickel or nickel alloys. Such longer lengths of conductor may be formed into power lines or wound to form solenoids.

REFERENCES

The following documents, referred to in the description of the related art and/or the description of the invention, and their content are hereby incorporated by reference in their entirety: El] S. R. Foltyn, L. Civale, J. L. MacManus-Driscoll, Q. X. Jia, B. Maiorov, H. Wang, and M. Maley, "Materials science challenges for high-temperature superconducting wire," Nature Mater., vol. 6, pp. 631-642, Sep. 2007.

[2] D. 8. Jan, J. Y. Coulter, M. E. Hawley, L. N. Bulaevskii, M. P. Maley, Q. X. Jia, B. B. Maranville, F. Heilman, and X. Q. Pan, "Flux pinning enhancement in ferromagnetic and superconducting thin-film multilayers," Appi. Phys. Lett., vol. 82, pp. 778-780, Feb. 2003.

[3] J. I. Martin, M. Vélez, J. Nogués, and I. K. Schuller, "Flux pinning in a superconductor by an array of submicrometer magnetic dots," Phys. Rev. Lett., vol. 79, pp. 1929-1932, Sep. 1997; David J. Morgan and J. B. Ketterson, "Asymmetric flux pinning in a regular array of magnetic dipoles," Phys. Rev. Lett., vol. 80, pp. 3614-3617, Apr. 1998.

[4] 0. Geoffroy, 0. Givord, Y. Otani, B. Pannetier, and F. Ossart, "Magnetic and transport properties of ferromagnetic particulate arrays fabricated on superconducting thin films," J. Magn. Magn. Mater., vol. 121, pp. 223-226, Mar.

1993; Y. Otani, B. Pannetier, J. P. Nozières, and D. Givord, Magnetostatic interactions between magnetic arrays and superconducting thin films," J. Magn. Magn. Mater., vol. 126, pp. 622-625, Sep. 1993.

[5] M. Lange, M. J. Van Bael, V. V. Moshchalkov, and Y. Bruynseraede, "Magnetic-domain-controlled vortex pinning in a superconductor! ferromagnet bilayer," Appi. Phys. LeLt., vol. 81, pp. 322-324, Jul. 2002.

[6] Z. Yang, M. Lange, A. Volodin, R. Szymczak, and V. V. Moshchalkov, "Domain-wall superconductivity in superconductor-ferromagnet hybrids," Nature Mater., vol. 3, pp. 793-798, No. 2004.

[7] L. N. Bulaevskii, E. M. Chudnovsky, and M. P. Maley, "Magnetic pinning in superconductor-ferromagnet multilayers," Appi. Phys. Lett., vol. 76, pp. 2594-2596, May 2000; 0. B. Jan, J. Y. Coulter, N. E. Hawley, L. N. Bulaevskii, N. P. Maley, Q. X. Jia, B. B. Maranville, F. Heliman, and X. Q. Pan, "Flux pinning enhancement in ferromagnetic and superconducting thin-film multilayers," Appi. Phys. Lett., vol. 82, pp. 778-780, Feb. 2003.

[8] J. Wang, J. Yoon, H. Wang, and D. G. Naugle, "Microstructural and pinning properties of YBa2Cu3O7-6 thin turns doped with magnetic nanoparticles," Applied Superconductivity Conference 2008, IEEE Trans. Appi.

Supercond. to appear; T. J. Haugan, N. A. PIerce, J. Reichart, M. Mullins, F. J. Baca, I. Maartense, and P. N. Barnes, "Flux pinning enhancements of YBCO with nanosize magnetic additions," Applied Superconductivity Conference 2008, IEEE Trans. Appi. Supercond. to appear, copy of paper not available at the time of writing.

[91 P. Coppens and M Eibschütz, "Determination of the crystal structure of yttrium orthoferrite and refinement of gadolinium orthoferrite," Acta Cryst., vol. 19, pp. 524-531, 1965. PDF 73-1345.

[10] D. Treves, "Magnetic studies of some orthoferrites," Phys. Rev., vol. 125, pp. 1843-1853, Mar. 1962.

[11] D. Treves, "Studies on orthoferrites at the Weizmann Institute of Science", Journal of Applied Physics, Vol. 36, No. 3, pp. 1033-1039, March 1965.

[12] M. G. Blamire, R. B. Dinner, S. C. Wimbush, and J. L. MacManus-Driscoll, "Critical current enhancement by Lorentz force reduction in superconductor/ferrornagnet nanocomposites," Supercond. Sci. Technol., 22 (2009) 025017.

[13] J. Smit and H. P. J. Wijn, Ferrites, John Wiley, New York, 1959, p.177.

Claims

CLAIMS1. A composition including a matrix of a high temperature superconductive oxide, with non-superconductive particles distributed in the matrix, at least some of the non-superconductive particles comprising either a rare earth transition metal oxide, or a barium or strontium transition metal oxide, and wherein said non-superconductive particles are ferromagnetic, superparamagnetic, ferrimagnetic or antiferromagnetic at a temperature of 77K.
2. A composition according to claim 1 wherein the rare earth element of the non-superconductive particles is selected from one or more of scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tu), ytterbium (Yb) and lutetium (Lu)
3. A composition according to claim 1 or claim 2 wherein the transition metal is selected from one or more of scandium (Sc), titanium (Ti), vanadium (V)1 chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), yttrium (Y), zirconium (Zr), niobium (Nb), molybdenum (Mo), technetium (Tc), ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), cadmium (Cd), lanthanum (La), hafnium (Hf), tantalum (Ta), tungsten (W), rhenium (Re), osmium (Os), iridium (Ir), platinum (Pt), gold (Au) and mercury (Hg), with the proviso that the transition metal in the rare earth transition metal oxide is not identical to the rare earth element.
4. A composition according to any one of claims 1 to 3 wherein the non-superconductive particles are formed of a material having a magnetization at 77K of at least 0.001 Bohr magnetons per formula unit at iT applied field.
5. A composition according to any one of claims 1 to 4 wherein the non-superconductive particles are formed of a material having a magnetic susceptibility of at least i04 at 77K.
6. A composition according to any one of claims 1 to 6 wherein the rare earth element, or barium or strontium, is provided at a concentration of at least 0.001 atomic percent relative to the total composition.
7. A composition according to any one of claims 1 to 6 wherein the transition metal is provided at a concentration of at least 0.001 atomic percent relative to the total composition.
8. A composition according to any one of claims 1 to 7 wherein the non-superconductive particles conform to the phase AaBbOz, where A is Ba or a rare earth element, B is a transition metal and 0 is oxygen, wherein: (I) a is 1 or about 1, b is 1 or about 1 and z is 3 or about 3, or (II) a is 3 or about 3, b is 5 or about 5 and z is 12 or about 12.
9. A composition according to claim 8 wherein the phase AaBbOz has an orthorhombic crystal structure.
10. A composition according to claim 8 wherein the phase AaBbOz has an orthoferrite crystal structure.
11. A composition according to any one of claims 1 to 10 wherein the non-superconductive particles have an average particle size of 100 nm or less.
12. A composition according to any one of claims 1 to 11 wherein the non-superconductive particles have a particle size that corresponds to a size smaller than the ferromagnetic domain size for bulk material of the same composition as the non-superconductive particles.
13. A composition according to any one of claims 1 to 12 wherein the high temperature superconductive oxide is a barium copper oxide.
14. A composition according to any one of claims 1 to 13 wherein the high temperature superconductive oxide is an yttrium barium copper oxide.
15. A composition comprising: (i) at least one of Y, Nd, Sm, Eu, Gd, Ho, or a mixture thereof, in a combined amount in the range 6-9 atomic percent; (ii) Ba in an amount in the range 13-17 atomic percent; (iii) Cu in an amount in the range 19-26 atomic percent; (iv) at least one rare earth element, optionally additional to (i), or Sr, or Ba, optionally additional to (ii), in a combined amount in the range 0.01-10 atomic percent; (v) at least one transition metal in a combined amount in the range 0.01-10 atomic percent; (vi) incidental and/or trace impurities; and (vii) balance oxygen.
16. An electrical conductor including a layer of the composition according to any one of claims 1 to 15.
17. A conductor according to claim 16 wherein the layer is aligned so that the c-axis of at least one crystal grain of the high temperature superconductive oxide is aligned substantially parallel to the thickness direction of the layer.
18. A conductor according to claim 16 or claim 17 wherein the superconductive oxide layer is a substantially epitaxial layer on a substrate or on a buffer layer or buffer layers formed on the substrate.
19. A conductor according to any one of claims 16 to 18 wherein at least one crystalline axis of the non-superconductive particles is substantially preferentially oriented with respect to a corresponding crystalline axis of the superconductive oxide and/or the substrate and/or a buffer layer, if present.
20. A conductor according to any one of claims 16 to 19 wherein the thickness of the layer of superconductive oxide is at least 100 nm.
21. A target for a film deposition process, the target having a composition according to any one of claims 1 to 15 or a composition corresponding to the proportion of elements required to form a composition according to any one of claims 1 to 15.
22. A combination of precursor solutions or suspensions for a film deposition process, the combination having a composition according to any one of claims 1 to 15 or a composition corresponding to the proportion of elements required to form a composition according to any one of claims 1 to 15, further optionally including additional carrier materials.
23. A method of manufacturing an electrical conductor, including depositing a layer of material on a substrate, the material including a matrix formed of high temperature superconductive oxide or the precursor thereof, the material further including, in addition to that stoichiometrically required to form the high temperature superconductive oxide, a rare earth element, or barium or strontium, and a transition metal.
24. A method according to claim 23 further including the step of forming non-superconductive particles, distributed in the matrix, said particles comprising either a rare earth transition metal oxide, or a barium or strontium transition metal oxide, and wherein said non-superconductive particles are ferromagnetic, superparamagnetic, ferrimagnetic or antiferromagnetic at a temperature of 77K.