[Show abstract][Hide abstract] ABSTRACT: Here we review the literature concerning measurement of the Andreev
reflection between a superconductor (S) and ferromagnet (F), with particular
attention to the case where the ferromagnet is a transition metal oxide. We
discuss the practicality of utilisation of the current models for determination
of the transport current spin polarisation and examine the evidence for Andreev
[Show abstract][Hide abstract] ABSTRACT: Here we study the magnetotransport properties of the ferropnictide crystals BaFe2As2 and BaFe1.985Co0.015As2 . These materials exhibit a high field linear magnetoresistance that has been attributed to the quantum linear magnetoresistance model. In this model, the linear magnetoresistance is dependent on the concentration of scattering centers in the material. By using proton-beam irradiation to change the defect scattering density, we find that the dependence of the magnitude of the linear magnetoresistance on scattering quite clearly contravenes this prediction. A number of other scaling trends in the magnetoresistance and high field Hall data are observed and discussed.
Physical Review B 01/2015; 91(5). DOI:10.1103/PhysRevB.91.054512 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Here we analyse the spectroscopic information gathered at a number of
single CrO2/Pb interfaces. We examine thin films requiring
additional interfacial layers to generate long-range spin triplet
proximity effect superconductivity (CrO2/TiO2) or
not (CrO2/Al2O3). We analyse the data
using two theoretical models and explore the use of a parameter-free
method to determine the agreement between the models and experimental
observations, showing the necessary temperature range that would be
required to make a definitive statement. The use of the excess current
as a further tool to distinguish between models is also examined. The
analysis of the spectra demonstrates that the temperature dependence of
the normalised zero-bias conductance is independent of the substrate
onto which the films are grown. This result has important implications
for the engineering of interfaces required for the long-range spin
triplet proximity effect.
[Show abstract][Hide abstract] ABSTRACT: Thick FeSe films (1–2μm) were grown from high temperature solution with SeSn as the flux. Electron backscatter diffraction confirmed the films of tetragonal β phase with high crystallinity. Superconducting transition was observed by magnetic measurements, with the onset Tc of 6.1K for the as-grown films and rising to 6.9K after post-growth annealing at 400°C, which was still 1.5K lower than the sintered powder samples. X-ray photoelectron spectroscopy showed that the Fe 2p3/2 binding energy in the FeSe compound was composed of two peaks at 707.8eV and 706.6eV, respectively. The former was close to the value of Fe in polarized ionic bonds, while the later had the typical value in metallic bondings. The ratio of the two bondings was 1.56 and 1.94 for the films and sintered powders, respectively. The critical temperature may have some correlation with the ratio of the two bondings. A lower average Fe valence was probably the cause for the lower Tc observed in thick films.
Journal of Alloys and Compounds 06/2011; 509(22):6350-6353. DOI:10.1016/j.jallcom.2011.03.054 · 3.00 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In a number of recent experiments, holmium has been shown to promote spin-triplet pairing when in proximity to a spin-singlet superconductor. The condition for the support of spin-triplet pairing is that the ferromagnet should have an inhomogeneous magnetic state at the interface with the superconductor. Here we use Andreev reflection spectroscopy to study the properties of single ferromagnet/superconductor interfaces formed of holmium and niobium, as a function of the contact resistance of the junction between them. We find that both single-crystal and c-axis-oriented thin-film holmium show unusual behavior for low junction contact resistance, characteristic of spin-mixing-type properties, which are thought necessary to underpin spin-triplet formation. We also explore whether this signature is observed when the junction is formed of Ni0.19Pd0.81 and niobium.
[Show abstract][Hide abstract] ABSTRACT: We report on isofield magnetic relaxation data on a single crystal of Ba1−xKxFe2As2 with superconducting transition temperature Tc=32.7 K which exhibit the so-called fish-tail effect. A surface map of the superconducting transition temperature shows that the superconducting properties are close to homogeneous across the sample. Magnetic relaxation data, M(t), was used to obtain the activation energy U(M) in order to study different vortex-dynamics regimes. Results of this analysis along with time-dependent measurements as a function of field and temperature extended to the reversible region of some M(H) curves demonstrate that the irreversibility as well the second magnetization peak position, Hp(T), are time dependent and controlled by plastic motion of the vortex state. In the region delimited by a characteristic field Hon (well below Hp), and Hp, the vortex dynamics is controlled by collective pinning. For fields below Hon the activation energy, U0, increases with field as expected for collective pinning, but the pinning mechanism is likely to be in the single vortex limit.
[Show abstract][Hide abstract] ABSTRACT: FeSe1−x samples have been prepared by a solid-state reaction in the Ar+3% H2 gas flow. The samples sintered at 410°C were of the tetragonal PbO structure (β-phase), showing little trace of δ-phase (hexagonal). However, the samples sintered at higher temperatures contained a fair amount of δ-phase, which was unable to be eliminated completely by subsequent annealing at ≤450°C for a prolonged period. Both M–T and R–T measurements showed that sintered samples of β-phase were superconducting with an onset Tc above 8.1 K. Thick films of FeSe1−x were grown on LaAlO3 substrates by liquid phase processing with SeSn as the flux. In contrast to the sintering, the films grown around 910°C from the Sn-containing liquid had a pure β-phase. Although the phase diagram showed some solid solubility of Sn in FeSe, energy-dispersive X-ray spectroscopy failed to find Sn trace in the crystallized films, which showed a steeper superconducting transition at lower Tc of 6.0 K. To clarify whether the possible Sn contamination caused the drop of Tc, FeSe1−x:5% Sn samples were sintered, which showed little change of Tc from the pure β-FeSe1−x. The cause for the Tc reduction of thick films remains unclear.
Journal of the American Ceramic Society 07/2010; 93(10):3195 - 3200. DOI:10.1111/j.1551-2916.2010.03912.x · 2.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Point contact Andreev reflection spectra have been taken as a function of temperature and magnetic field on the polycrystalline form of the newly discovered iron-based superconductor Sr2ScFePO3. A zero bias conductance peak which disappears at the superconducting transition temperature dominates all of the spectra. Data taken in high magnetic fields show that this feature survives until 7 T at 2 K and a flattening of the feature is observed in some contacts. Here we inspect whether these observations can be interpreted within a d-wave, or nodal order parameter, framework which would be consistent with the recent theoretical model where the height of the P in the Fe–P–Fe plane is key to the symmetry of the superconductivity. However, in polycrystalline samples care must be taken when examining Andreev spectra to eliminate or take into account artefacts associated with the possible effects of Josephson junctions and random alignment of grains.
[Show abstract][Hide abstract] ABSTRACT: We use point contact Andreev reflection measurements to determine the spin polarization of the transport current in pulse laser deposited thin films of ZnO with 1% Al and with and without 2%Mn. Only films with Mn are ferromagnetic and show spin polarization of the transport current of up to 55 $\pm$ 0.5% at 4.2 K, in sharp contrast to measurements of the nonmagnetic films without Mn where the polarization is consistent with zero. Our results imply strongly that ferromagnetism in these Al doped ZnO films requires the presence of Mn. Comment: Published version
[Show abstract][Hide abstract] ABSTRACT: The scaling exponent of 1.6 between anomalous Hall and longitudinal conductivity, characteristic of the universal Hall mechanism in dirty-metal ferromagnets, emerges from a series of CrO2 films as we systematically increase structural disorder. Magnetic disorder in CrO2 increases with temperature and this drives a separate topological Hall mechanism. We find that these terms are controlled discretely by structural and magnetic defect populations, and their coexistence leads to apparent divergence from exponent 1.6, suggesting that the universal term is more prevalent than previously realized.
[Show abstract][Hide abstract] ABSTRACT: Oxypnictide superconductor NdFeAsO0.85 sample was irradiated with 2 GeV Ta ions at a fluence of 5x10^10 ions/cm2. High resolution transmission electron microscopy study revealed that the irradiation produced columnar-like defects. The effect of these defects on the irreversible magnetisation in polycrystalline randomly oriented fragments was studied as a function of field angle and field sweep rate. We find that the critical current density is enhanced at fields below the matching field (~1 Tesla) but only marginally. The pinning enhancement is anisotropic and maximum along the defect direction at high temperatures but the pinning then becomes more isotropic at low temperatures. The creep rate is suppressed at high temperatures and at fields below the matching field, indicating the columnar defects are efficient pinning sites at these H and T conditions. Comment: 10 pages, 4 figures
[Show abstract][Hide abstract] ABSTRACT: Bulk samples of TbFeAsO0.9 F0.1 (Tcon = 50 K) were measured by point contact Andreev reflection spectroscopy. The spectra show unambiguous evidence for multiple gap-like features plus the presence of high-bias shoulders. By measuring the spectra as a function of temperature with both gold and superconducting niobium tips, we establish that the gap-like features are associated with the superconducting order parameter in this material. We discuss whether the well-defined zero-bias conductance peak that we observe infrequently is associated with a nodal superconducting order parameter.
New Journal of Physics 02/2009; 11(2):025015. DOI:10.1088/1367-2630/11/2/025015 · 3.56 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The newly discovered oxypnictide family of superconductors show very high critical temperatures of up to 55 K. Whilst there is growing evidence that suggests a nodal order parameter, point contact Andreev reflection spectroscopy can provide crucial information such as the gap value and possibly the number of energy gaps involved. For the oxygen deficient NdFeAsO0.85 with a Tc of 45.5 K, we show that there is clearly a gap value at 4.2 K that is of the order of 7 meV, consistent with previous studies on oxypnictides with lower Tc. In addition, taking the spectra as a function of gold tip contact pressure reveals important changes in the spectra which may be indicative of more complex physics underlying this structure.
[Show abstract][Hide abstract] ABSTRACT: Here we report the use of global and local magnetometry and Hall probe imaging to investigate the electromagnetic connectivity of the superconducting current path in the oxygen-deficient fluorine-free Nd-based oxypnictides. High resolution transmission electron microscopy and scanning electron microscopy show strongly layered crystallites, evidence for a ~5 nm amorphous oxide around individual particles, and second phase neodymium oxide which may contribute to the large paramagnetic background at high field and at high temperatures. From global magnetometry and electrical transport measurements it is clear that there is a small supercurrent flowing on macroscopic sample dimensions (mm), with a lower bound for the average (over this length scale) critical current density of the order of 103 A cm-2. From magnetometry of powder samples and local Hall probe imaging of a single large conglomerate particle ~120 µm it is clear that, on smaller scales, there is better current connectivity with a critical current density of the order of 5 × 104 A cm-2. We find enhanced flux creep around the second peak anomaly in the magnetization curve and an irreversibility line significantly below Hc2(T) as determined by ac calorimetry.
[Show abstract][Hide abstract] ABSTRACT: Here we use global and local magnetometry and Hall probe imaging to investigate the electromagnetic connectivity of the superconducting current path in the oxygen-deficient fluorine-free Nd-based oxypnictides. High resolution transmission electron microscopy and scanning electron microscopy show strongly-layered crystallites, evidence for a ~ 5nm amorphous oxide around individual particles, and second phase neodymium oxide which may be responsible for the large paramagnetic background at high field and at high temperatures. From global magnetometry and electrical transport measurements it is clear that there is a small supercurrent flowing on macroscopic sample dimensions (mm), with a lower bound for the average (over this length scale) critical current density of the order of 103 A/cm2. From magnetometry of powder samples and local Hall probe imaging of a single large conglomerate particle ~120 microns it is clear that on smaller scales, there is better current connectivity with a critical current density of the order of 5 x 104 A/cm2. We find enhanced flux creep around the second peak anomaly in the magnetisation curve and an irreversibility line significantly below Hc2(T) as determined by ac calorimetry. Comment: 11 pages, 4 figures
Superconductor Science and Technology 06/2008; · 2.33 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The diffusivity ratio η, which measures the relative intraband scattering in the π and σ bands in MgB2 has been determined by fitting the Hc2 (T) at T ~ Tcand by Point Contact Andreev Reflection. We find a satisfactory agreement between the values for η obtained by both methods for c-axis orientated MgB2 thin films. Point contact Andreev Reflection was then applied to bulk MgB2 containing Mg vacancies. Spectra obtained in zero field indicate a distribution of the two gaps Δσ,π but no merging of the values with increased magnesium deficiency. Spectra fitted as a function of field are consistent with an increase in π intraband scattering with increasing magnesium deficiency. Measurement of the point contact Andreev reflection spectra as a function of temperature revealed features not immediately expected from current theoretical models.
Journal of Physics Conference Series 03/2008; 97(1):012213. DOI:10.1088/1742-6596/97/1/012213
[Show abstract][Hide abstract] ABSTRACT: For Andreev spectroscopy to be a useful tool to detect spin accumulation in semiconductors, we show by simulation that there is a maximum value for the interface scattering parameter that can be tolerated. Three different fabrication routes for Pb/InAs planar junctions are explored and we find that the "etch-back" processing strategy is the most promising. Using the parameters extracted from the spectroscopic analysis, we find that the interface properties fall into four different regimes of behavior.
[Show abstract][Hide abstract] ABSTRACT: By applying a combination of characterisation tools, changes in structural and superconducting properties with nominal Mg non-stoichiometry in Mg xB2 are found. The non-stoichiometry produces enhanced in-field critical current densities (Jc's) and upper critical field / irreversibility field (Hc2/Hirr(T)) values. Upper critical fields of ∼ 21 T (4.2 K) were obtained in nominal Mg-deficient samples compared to ∼ 17 T (4.2 K) for near-stoichiometric samples.
[Show abstract][Hide abstract] ABSTRACT: Here, we use Andreev reflection spectroscopy to study the spin polarization of high quality CrO2 films. We study the spin polarization as a function of growth temperature, resulting in grain size and electrical resistivity. In these films low temperature growth appears to be a necessary but not sufficient condition to guarantee the observation of high spin polarization, and this is only observed in conjunction with suppressed superconducting gap values and anomalously low interface properties. We suggest that this combination of observations is a manifestation of the long range spin triplet proximity effect.