M. G. Blamire

Greatly reduced leakage current and conduction mechanism in aliovalent-ion-doped BiFeO3

Transport properties of aliovalent-ion-doped BiFeO3 (BFO) thin films have been studied in order to identify the cause of high leakage currents. Doping of 2at.% Ti4+ ions increased the dc resistivity by more than three orders of magnitude. In contrast, doping of 2+ ions such as Ni2+ reduced the dc resistivity by two orders of magnitude. Current–voltage (I–V) characteristics indicated that the main conduction mechanism for pure and Ni2+ doped BFO was space charge limited, which was associated with the free-carriers trapped by the oxygen vacancies, whereas in the Ti4+ doped BFO, field-assisted ionic conduction was dominant.

Controlled Injection of Spin-Triplet Supercurrents into a Strong Ferromagnet

Maintaining the Supercurrent When a superconductor is placed in contact with a ferromagnet, the antiparallel spin pairs that form the supercurrent are expected to be broken almost immediately upon entering the ferromagnet, which tends to orient spins parallel to each other. If the supercurrent survives for more than a few nanometers, it is assumed that a change of pairing symmetry has taken place, with the spin-singlet pairs having been converted into spin-triplets. Magnetic inhomogeneity at the superconductor-ferromagnet interface is thought to account for this change. Robinson et al. (p. 59, published online 10 June) have now been able to observe long-ranged supercurrents in a symmetric junction consisting of a superconductor, a conical magnet, and a ferromagnet. The conical magnet layer provided the required inhomogeneity, and varying its thickness enabled control over the magnitude of the current. Unusual magnetic ordering in a rare earth metal is used to create superconducting currents with aligned spins. The superconductor-ferromagnet proximity effect describes the fast decay of a spin-singlet supercurrent originating from the superconductor upon entering the neighboring ferromagnet. After placing a conical magnet (holmium) at the interface between the two, we detected a long-ranged supercurrent in the ferromagnetic layer. The long-range effect required particular thicknesses of the spiral magnetically ordered holmium, consistent with spin-triplet proximity theory. This enabled control of the electron pairing symmetry by tuning the degree of magnetic inhomogeneity through the thicknesses of the holmium injectors.

Magnetoresistance of artificial La0.7Sr0.3MnO3 grain boundaries as a function of misorientation angle

The resistance of polycrystalline doped LaMnO3 materials has been shown to be highly sensitive to low magnetic fields. To enable direct study of the properties of isolated grain boundaries we have grown epitaxial La0.7Sr0.3MnO3 films on a series of bicrystal substrates with different misorientation angles and patterned the films into a Wheatstone bridge geometry. We show that the grain boundary resistance and its magnetic field dependence vary strongly with the misorientation angle. The temperature dependence of the grain boundary resistance is also presented. We have obtained resistance changes of over 3% in fields of 2 mT at 300 K.

High-resolution x-ray diffraction and transmission electron microscopy of multiferroic BiFeO3 films

High-resolution x-ray diffraction and transmission electron microscopy (TEM) have been used to study BiFeO3 thin films grown on the bare and SrRuO3 buffered (001) SrTiO3 substrates. Reciprocal space mapping (RSM) around (002) and (103) reflections revealed that BFO films with a thickness of about 200 nm were almost fully relaxed and had a rhombohedral structure. Cross-sectional, high-resolution TEM showed that the films started to relax at a very early stage of growth, which was consistent with the RSM results. A thin intermediate layer of about 2 nm was observed at the interface, which had a smaller lattice than the overgrown film. Twist distortions about the c axis to release the shear strain introduced by the growth of rhombic (001) BiFeO3 on cubic (001) SrTiO3 were also observed. The results indicate that a strained, coherent BiFeO3 film on (001) SrTiO3 is very difficult to maintain and (111) STO substrates are preferable.

High Curie temperatures in ferromagnetic Cr-doped AlN thin films

Al1−xCrxN thin films with 0.02⩽x⩽0.1 were deposited by reactive co-sputtering onto c-plane (001) sapphire. Room-temperature ferromagnetism with a coercive field of 85 Oe was observed in samples with chromium contents as low as x=0.027 (2.7%). With increasing Cr content the mean magnetic moment is strongly suppressed, with a maximum saturation moment of 0.62 and 0.71 μB per Cr atom at 300 and 50 K, respectively. We show that the Curie temperature of Al1−xCrxN for x=0.027 is greater than 900 K.

Room temperature ferromagnetism in bulk Mn-Doped Cu2O

Bulk Mn-doped Cu2O samples were produced by reacting Cu2O and Mn2O3 powders in Ar gas at 650 and 800°C to give a nominal composition of 1.7at.% Mn-doped Cu2O. From x-ray energy dispersive spectrum analysis, the actual doping level was lower at 0.3–0.5at.% Mn. Room temperature ferromagnetism with a coercive field of 50Oe was found in the 650°C samples. The Curie temperature (TC) of samples sintered at 650°C was above 300K, whereas for 800°C samples it was 215±5K. Using the nominal doping level, the magnetization saturation value was calculated to be ∼0.4μB∕Mn at 10K.

Critical current oscillations in strong ferromagnetic pi junctions

We report magnetic and electrical measurements of Nb Josephson junctions with strongly ferromagnetic barriers of Co, Ni, and Ni80Fe20 (Py). All these materials show multiple oscillations of critical current with a barrier thickness implying repeated 0-pi phase transitions in the superconducting order parameter. We show, in particular, that the Co barrier devices can be accurately modeled using existing clean limit theories and that, despite the high exchange energy (309 meV), the large IcR(N) value in the pi state means Co barriers are ideally suited to the practical development of superconducting pi-shift devices.

Spin-filter Josephson junctions

Josephson junctions with ferromagnetic barriers have been intensively investigated in recent years. Of particular interest has been the realization of so called π-junctions with a built-in phase difference, and induced triplet pairing. Such experiments have so far been limited to systems containing metallic ferromagnets. Although junctions incorporating a ferromagnetic insulator (I(F)) have been predicted to show a range of unique properties including π-shifts with intrinsically low dissipation and an unconventional temperature dependence of the critical current I(c), difficulties with the few known I(F) materials have prevented experimental tests. Here we report supercurrents through magnetic GdN barriers and show that the field and temperature dependence of I(c)is strongly modified by the I(F). In particular we show that the strong suppression of Cooper pair tunnelling by the spin filtering of the I(F) barrier can be modified by magnetic inhomogeneity in the barrier.

Planar superconductor-normal-superconductor Josephson junctions in MgB2

We report the successful creation of planar MgB2 junctions by localized ion damage in thin (100 nm) films of MgB2 on sapphire by milling a 50 nm trench with a focused-ion beam across tracks of widths between 1 and 5 μm. When the depth of the trench is between 70% and 80% of the film thickness, devices show critical currents (IC) for temperatures below 25 K. The IC of these devices is strongly modulated by applied microwave radiation and magnetic field. The product of the critical current and normal state resistance (ICRN) is remarkably high, implying a potential for very-high-frequency applications.

Strong influence of boron precursor powder on the critical current density of MgB2

The influence of the nature of the boron precursor on the superconducting properties of polycrystalline MgB2 was studied. Critical current densities (Jc) for MgB2 made from high purity amorphous boron are at least a factor of three higher than typical values measured for standard MgB2 samples made from amorphous precursors. Two possible mechanisms are proposed to account for this difference. Samples made from crystalline boron powders have around an order of magnitude lower Jc compared to those made from amorphous precursors. X-ray, Tc and resistivity studies indicate that this is as a result of reduced current cross-section due to the formation of (Mg)B–O phases. The samples made from amorphous B contain far fewer Mg(B)–O phases than crystalline B despite the fact that the amorphous B contains more B2O3. The different reactivity rates of the precursor powders accounts for this anomaly.