Colossal magnetoresistance—a huge decrease in resistance in response to a magnetic field—has recently been observed in manganese oxides with perovskite structure. This effect is attracting considerable interest from both fundamental and practical points of view1. In the context of using this effect in practical devices, a noteworthy feature of these materials is the high degree of spin polarization of the charge carriers, caused by the half-metallic nature of these materials20,21; this in principle allows spin-dependent carrier scattering processes, and hence the resistance, to be strongly influenced by low magnetic fields. This type of field control has been demonstrated for charge-carrier scattering at tunnelling junctions2,3 and at crystal-twin or ceramic grain boundaries4,5, although the operating temperature of such structures is still too low (⩽150 K) for most applications. Here we report a material—Sr2FeMoO6, an ordered double perovskite6—exhibiting intrinsic tunnelling-type magnetoresistance at room temperature. We explain the origin of this behaviour with electronic-structure calculations that indicate the material to be half-metallic. Our results show promise for the development of ordered perovskite magnetoresistive devices that are operable at room temperature.

Room-temperature magnetoresistance in an oxide material with an ordered double-perovskite structure

We review recent experimental work falling under the broad classification of colossal magnetoresistance (CMR), which is magnetoresistance associated with a ferromagnetic-toparamagnetic phase transition. The prototypical CMR compound is derived from the parent compound, perovskite LaMnO 3. When hole doped at a concentration of 20–40% holes/Mn ion, for instance by Ca or Sr substitution for La, the material displays a transition from a high-temperature paramagnetic insulator to a low-temperature ferromagnetic metal. Near the phase transition temperature, which can exceed room temperature in some compositions, large magnetoresistance is observed and its possible application in magnetic recording has revived interest in these materials. In addition, unusual magneto-elastic effects and charge ordering have focused attention on strong electron–phonon coupling. This coupling, which is a type of dynamic extended-system version of the Jahn–Teller effect, in conjunction with the double-exchange interaction, is also viewed as essential for a microscopic description of CMR in the manganite perovskites. Large magnetoresistance is also seen in other systems, namely Tl 2Mn2O7 and some Cr chalcogenide spinels, compounds which differ greatly from the manganite perovskites. We describe the relevant points of contrast between the various CMR materials.

REVIEW ARTICLE: Colossal magnetoresistance

Large-scale superconducting electric devices for power industry depend critically on wires with high critical current densities at temperatures where cryogenic losses are tolerable This restricts choice to two high-temperature cuprate superconductors, (Bi,Pb)2Sr2Ca2Cu3Ox and YBa2Cu3Ox, and possibly to MgB2, recently discovered to superconduct at 39 K Crystal structure and material anisotropy place fundamental restrictions on their properties, especially in polycrystalline form So far, power applications have followed a largely empirical, twin-track approach of conductor development and construction of prototype devices The feasibility of superconducting power cables, magnetic energy-storage devices, transformers, fault current limiters and motors, largely using (Bi,Pb)2Sr2Ca2Cu3Ox conductor, is proven Widespread applications now depend significantly on cost-effective resolution of fundamental materials and fabrication issues, which control the production of low-cost, high-performance conductors of these remarkable compounds

High-Tc superconducting materials for electric power applications.

This review provides a theoretical basis for understanding the current-phase relation (CPhiR) for the stationary (dc) Josephson effect in various types of superconducting junctions The authors summarize recent theoretical developments with an emphasis on the fundamental physical mechanisms of the deviations of the CPhiR from the standard sinusoidal form A new experimental tool for measuring the CPhiR is described and its practical applications are discussed The method allows one to measure the electrical currents in Josephson junctions with a small coupling energy as compared to the thermal energy A number of examples illustrate the importance of the CPhiR measurements for both fundamental physics and applications

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The current-phase relation in Josephson junctions

The elastic properties, elastic models and elastic perspectives of metallic glasses

A new class of `powder-in-tube' Mg-B superconducting conductors has been prepared using two different methods: an in situ technique where an Mg + 2B mixture was used as a central conductor core and reacted in situ to form MgB2, and an ex situ technique where fully reacted MgB2 powder was used to fill the metal tube. Conductors were prepared using silver, copper and bimetallic silver/stainless steel tubes. Wires manufactured by the in situ technique, diffusing Mg to B particles experienced ~25.5% decrease in density from the initial value after cold deformation, due to the phase transformation from Mg + 2(β-B)→MgB2 all with hexagonal structure. A comparative study of the intergranular current and grain connectivity in wires was conducted by AC susceptibility measurements and direct four point transport measurements. Using a SQUID magnetometer, magnetization versus magnetic field (M-H) curves of the round wires before and after sintering and reactive diffusion were measured at 5 K and in magnetic fields up to 5 T to define the Jcmag. The direct current measurements were performed in self field at 4.2 K. A comparison between zero-field-cooled (ZFC) and field-cooled (FC) susceptibility measurements for sintered Ag/MgB2, and reacted Cu/Mg + 2B conductors revealed systematic differences in the flux pinning in the wires which is in very good agreement with direct high transport current measurements.

https://iopscience.iop.org/article/10.1088/0953-2048/14/4/304/pdf

Superconductivity of powder-in-tube MgB2 wires

A theoretical model for relaxation in glassy materials, in particular metallic glasses, based on a spectrum of available processes distributed in activation energy is presented. The model is used to discuss “In t” kinetics, “reversibility” and “crossover” effects which have been observed experimentally. Where direct comparison is possible between the theory and experiment the agreement is good over all these various observed phenomena.

Activation energy spectra and relaxation in amorphous materials

A number of different compounds, such as those derived from LaMnO3, have recently been shown to exhibit very large changes (up to 106%) in electrical resistance when a magnetic field is applied1–4—a phenomenon known as colossal magnetoresistance (CMR). But magnetic fields of several tesla are typically required to obtain such a large magnetoresistive effect, thus limiting the potential for applications. Nevertheless the complex and intimate link between magnetic structure, crystallographic structure and electrical resistivity in CMR materials, in addition to being of fundamental scientific interest, appears to provide some scope for engineering a more sensitive magnetoresistive response. Here we elucidate the effect of specific structural defects on the CMR behaviour of the compound La0.7Ca0.3MnO3. We have made thin film devices that isolate the contribution of a single grain boundary that was introduced into an epitaxial film of the material by growing it on a bicrystal substrate. These devices display sharp resistance switching in magnetic fields orders of magnitude less than those normally associated with CMR. These results both provide insight into the role of grain boundaries, and demonstrate the potential for developing sub-micrometre magnetic field sensors based on the CMR effect.

Large Low-Field Magnetoresistance in La0.7Ca0.3Mno3 Induced by Artificial Grain-Boundaries

Relation of critical current irreversibility to trapped flux and microstructure in polycrystalline YBa2Cu3O7

The dependence of the critical current density ${j}_{c}$ on the orientation of the magnetic field $\mathbf{H}$ has been measured for a ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ bicrystal film. When the field is rotated in the plane of the grain boundary, ${j}_{c}$ displays a maximum for $\mathbf{H}$ close to the $c$ axis in fields up to 7 tesla. The same experiment performed in the interior of the grain shows no maximum. The pinning force at the maximum varies as ${H}^{0.5}$, consistent with vortex pinning by a dense planar distribution of line pins. These results are a clear demonstration of flux pinning by dislocations in low-angle grain boundaries.

J.E. Evetts

Papers

Superconductivity of powder-in-tube MgB2 wires

Activation energy spectra and relaxation in amorphous materials

Large Low-Field Magnetoresistance in La0.7Ca0.3Mno3 Induced by Artificial Grain-Boundaries

Relation of critical current irreversibility to trapped flux and microstructure in polycrystalline YBa2Cu3O7

Evidence for Vortex Pinning by Dislocations in YBa 2 Cu 3 O 7 − δ Low-Angle Grain Boundaries