The discovery of superconductivity at 39 K in magnesium diboride offers the possibility of a new class of low-cost, high-performance superconducting materials for magnets and electronic applications. This compound has twice the transition temperature of Nb3Sn and four times that of Nb-Ti alloy, and the vital prerequisite of strongly linked current flow has already been demonstrated. One possible drawback, however, is that the magnetic field at which superconductivity is destroyed is modest. Furthermore, the field which limits the range of practical applications-the irreversibility field H*(T)-is approximately 7 T at liquid helium temperature (4.2 K), significantly lower than about 10 T for Nb-Ti (ref. 6) and approximately 20 T for Nb3Sn (ref. 7). Here we show that MgB2 thin films that are alloyed with oxygen can exhibit a much steeper temperature dependence of H*(T) than is observed in bulk materials, yielding an H* value at 4.2 K greater than 14 T. In addition, very high critical current densities at 4.2 K are achieved: 1 MA cm-2 at 1 T and 105 A cm-2 at 10 T. These results demonstrate that MgB2 has potential for high-field superconducting applications.

We report a significant enhancement of the upper critical field H c2 of different MgB 2 samples alloyed with nonmagnetic impurities. By studying films and bulk polycrystals with different resistivities ρ, we show a clear trend of H c2 increase as ρ increases. One particular high resistivity film had zero-temperature H c2 (0) well above the H c2 values of competing non-cuprate superconductors such as Nb 3 Sn and Nb-Ti. Our high-field transport measurements give record values H c2 ⊥ (0) ≈ 34T and H c2 || (0) ≈ 49 T for high resistivity films and H c2 (0) ≈ 29 T for untextured bulk polycrystals. The highest H c2 film also exhibits a significant upward curvature of H c2 (T), and temperature dependence of the anisotropy parameter γ(T) = H c2 || / H c2⊥ opposite to that of single crystals: γ(T) decreases as the temperature decreases, from γ(T c ) ≈ 2 to γ(0) ≈ 1.5. This remarkable H c2 enhancement and its anomalous temperature dependence are a consequence of the two-gap superconductivity in MgB 2 , which offers special opportunities for further H c2 increase by tuning of the impurity scattering by selective alloying on Mg and B sites. Our experimental results can be explained by a theory of two-gap superconductivity in the dirty limit. The very high values of H c2 (T) observed suggest that MgB 2 can be made into a versatile, competitive high-field superconductor.

Epitaxial La 0.67 Sr 0.33 MnO 3 ͑LSMO͒ / SrRuO 3 ͑SRO͒ ferromagnetic bilayers have been grown on ͑001͒SrTiO 3 ͑STO͒ substrates by pulsed laser deposition with atomic layer control. We observe a shift in the magnetic hysteresis loop of the LSMO layer in the same direction as the applied biasing field (positive exchange bias). The effect is not present above the Curie temperature of the SRO layer ͑T c SRO ͒, and its magnitude increases rapidly as the temperature is lowered below T c SRO . The direction of the shift is consistent with an antiferromagnetic exchange coupling between the ferromagnetic LSMO layer and the ferromagnetic SRO layer. We propose that atomic layer charge transfer modifies the electronic state at the interface, resulting in the observed antiferromagnetic interfacial exchange coupling. © 2004 American Institute Of Physics. [DOI: 10.1063 Interactions at magnetic interfaces are central to the operation of virtually all magnetic heterostructures. When the interface is between two magnetic materials, the exchange interaction between spins at the interface is dominant, and can dramatically change the magnetic response of the overall heterostructure. Interfacial interactions in bilayers of two ferromagnets can couple the layers so strongly that both switch as a single unit. 1 In a bilayer of two ferromagnets with very different coercive fields, an "exchange spring" effect 2 can result, where the magnetization of the soft layer can reversibly "twist" with respect to the hard layer. The interfacial interaction can also shift the magnetic hysteresis loop so that it is no longer symmetric about zero applied field. This exchange bias effect, most commonly implemented by a ferromagnet/antiferromagnet interface, has been extensively used to pin the magnetization in one of the layers of a magnetic spin valve or tunnel junction.Magnetization loop shifts of a ferromagnetic film arising from interfacial exchange interactions with a "pinning layer" have been observed in many systems, including ones in which the pinning layer is ferromagnetic 3 or ferrimagnetic, 4 as well as antiferromagnetic. 5 Although the details of the interfacial spin arrangements are in some cases believed to be quite complicated, in all cases a preferred direction of the pinning layer is induced by the application of a "bias" magnetic field. In the case of an antiferromagnetic ͑AF͒ pinning layer, the dipole interaction with the applied field is usually small, and the preferred direction is determined by interaction of the AF layer with the ferromagnetic layer. An applied field can directly manipulate the magnetic orientation of ferro-and ferrimagnetic pinning layers through the dipole interaction.These interactions are characterized by the exchange field H E , the shift from zero of the magnetization loop center along the field axis. In almost all cases, this shift of the magnetization loop is opposite to the direction of applied bias field. This case is sometimes referred to as negative exchange bias. This amount of shift is the exchange ...

SrRuO 3 (SRO) is widely used as an electrode in oxide electronic device applications due to its excellent material properties such as metallic conductivity, chemical stability, good lattice match with multifunctional oxides, and atomically smooth and welldefined surfaces. [1][2][3][4][5][6] Especially in the fabrication of epitaxial thin-film heterostructures, the crystal symmetry and domain structure of the overlayer thin film are strongly dependent on those of the bottom electrode. Thus, it is critical to investigate the crystal symmetry and domain structure of the bottom electrode at the growth temperature and during cooling of the epitaxial heterostructures to room temperature if the bottom electrode undergoes structural transitions.In ABO 3 perovskite materials, the ideal cubic symmetry can be distorted by several mechanisms such as distortions of the octahedra, cation displacements within the octahedra, and tilting of the octahedra. The first two mechanisms are driven by electronic instabilities of the octahedral metal ion as exemplified by the Jahn-Teller distortion in KCuF 3 or the ferroelectric displacement of titanium in BaTiO 3 . [7,8] The third and most common mechanism, octahedral tilting, can be realized by tilting essentially rigid BO 6 octahedra while maintaining their corner-sharing connectivity. This type of distortion is typically observed when the A cation is too small for the cubic BO 3 corner-sharing octahedral network.At room temperature bulk SRO exhibits orthorhombic symmetry (Pbnm).[9] Figure 1 shows the sequence of phase transitions in unstrained bulk SRO from orthorhombic to tetragonal and then cubic symmetry with increasing temperature.[10] According to the Glazer notation, octahedral tilting in orthorhombic SRO is described by a À a À c þ , implying that RuO 6 octahedra are rotated in opposite directions by equivalent magnitude along [100] and [010] and in the same direction about [001]. [11,12] Tetragonal SRO is a one-tilt system, where RuO 6 octahedra are rotated only about the [001] direction (a 0 a 0 c À ). The tetragonal phase of SRO is stable within the very narrow temperature range from 547 to 677 8C and, finally, high-symmetry cubic perovskite (Pm3m) becomes stable above 677 8C.[10]Enormous strains exist in thin films when one material is deposited onto a substrate due to differences in crystal symmetry, lattice parameters, and thermal expansion coefficients between the film and the underlying substrate. [2,14] As a result, the properties of thin films can be differ widely from the intrinsic properties of the unstrained bulk counterparts. For example, recent experiments have shown strain-induced ferroelectricity in SrTiO 3 (STO) films at room temperature [15] and huge changes in the ferroelectric transition temperature in both BaTiO 3 . [16] Several groups have reported on the structural phase transition of SRO in thin-film form. The structural transition temperature of an epitaxial SRO thin film on (001) STO substrates, investigated using in situ transmission electron micro...

An important predicted, but so far uncharacterized, property of the new superconductor MgB 2 is electronic anisotropy arising from its layered crystal structure. Here we report on three caxis oriented thin films, showing that the upper critical field anisotropy ratio H c2 || /H c2 ⊥ is 1.8 to 2.0, the ratio increasing with higher resistivity. Measurements of the magnetic field-temperature phase diagram show that flux pinning disappears at H* ≈ 0.8H c2is strongly enhanced by alloying to 39 T for the highest resistivity film, more than twice that seen in bulk samples.The discovery of superconductivity at almost 40 K in MgB 2 has reawakened the search for high critical temperature T c in compounds with light elements [1]. In spite of the high T c of bulk MgB 2 samples, the upper critical field H c2 (T) at which bulk superconductivity is destroyed and the irreversibility field H*(T) at which bulk supercurrent densities disappear are both comparatively low. The maximum extrapolations of µ 0 H c2 (0) give 16-18 T, while H*(0) is about 0.5H c2 (0) [2-8]. µ 0 H*(4.2 K) is thus 7 T, well below the 10.5 T irreversibility field of Nb47wt.%Ti, for which T c is 9 K and µ 0 H c2 (4.2 K) is ~12 T [9]. At present it is not known whether the low irreversibility field of MgB 2 is related to its electronic anisotropy, a problem that is well known in the strongly anisotropic, high-temperature copperoxide superconductors [10]. Since MgB 2 consists of alternating