Point Contact Andreev Reflection Research Articles

Half-metallic Ni2MnSn Heusler alloy prepared by rapid quenching

We have employed melt-spinning method to produce Ni2MnSn-based half-metallic Heusler alloy. It allows fast and simple production of large amount of materials in a single production step avoiding high temperature post-production annealing. Microstructural, magnetic and spin polarization study of Ni2MnSn ribbon is used for characterization. SEM analysis reveals the polycrystalline structure with the columnar crystals grown perpendicularly to the ribbon plane. A single-phase austenite with L21 structure was confirmed by X-ray. Magnetic measurements shows the ordinary ferromagnetic behavior with Curie temperature 344K and magnetic moment 4.08µB/f.u. Particular crystal structure leads to the well defined anisotropy having an easy plane in the ribbon's plane. Finally, the spin polarization parameter P0 estimated by Point-Contact Andreev-reflection Spectroscopy is varying in the range 40–70% for Ni2MnSn which is comparable with other values reported earlier for other Heusler alloys.

Spin gapless semiconducting behavior in equiatomic quaternary CoFeMnSi Heusler alloy

Spin gapless semiconductors (SGS) form a new class of magnetic semiconductors, which has a band gap for one spin sub band and zero band gap for the other, and thus are useful for tunable spin transport based applications. In this paper, we report the first experimental evidence for spin gapless semiconducting behavior in CoFeMnSi Heusler alloy. Such a behavior is also confirmed by first principles band structure calculations. The most stable configuration obtained by the theoretical calculation is verified by experiment. The alloy is found to crystallize in the cubic Heusler structure (LiMgPdSn type) with some amount of disorder and has a saturation magnetization of 3.7 Bohr's magneton/f.u.. and Curie temperature of 620 K. The saturation magnetization is found to follow the Slater-Pauling behavior, one of the prerequisites for SGS. Nearly temperature-independent carrier concentration and electrical conductivity is observed from 5 to 300 K. An anomalous Hall coefficient of 162 S/cm is obtained at 5 K. Point contact Andreev reflection data has yielded the current spin polarization value of 0.64, which is found to be robust against the structural disorder. All these properties are quite promising for the spintronic applications such as spin injection and can bridge a gap between the contrasting behavior of half-metallic ferromagnets and semiconductors.

Investigation of the quaternary Fe2 − xCoxMnSi (0 ≤ x ≤ 0.6) alloys by structural, magnetic, resistivity and spin polarization measurements

A detailed study of the Fe2 − xCoxMnSi (0 ≤ x ≤ 0.6) alloys has been carried out by investigating the samples by x-ray diffraction, Mössbauer spectroscopy, magnetization, transport and current spin polarization measurements. A perfectly ordered L21 phase is found to exist for x = 0.4. Competing magnetic interactions between ferromagnetic and antiferromagnetic (AFM) phases are seen in the alloys with x < 0.2, whereas the AFM phase is completely absent for x ≥ 0.2 as revealed by the magnetization and resistivity data. The Curie temperature and saturation magnetization values increase with increase in the Co concentration. Anomalous, non metallic-like behaviour is observed for x = 0.4. Current spin polarization values of 0.61 ± 0.01 and 0.66 ± 0.01 were deduced for x = 0.2 and x = 0.4 respectively, using the point contact Andreev reflection spectroscopy . Considering the high spin polarization and Curie temperature, materials with x ≥ 0.2 appear to be very promising for spintronic devices.

Dependence of Andreev reflection and Schottky barriers on GaMnAs/Nb interface treatment

We studied the interfacial contact between GaMnAs and superconducting Nb micro-structures both with and without removing the native GaMnAs surface oxide. Our results show that a strong Schottky barrier forms at the interface when the oxide layer is left between Nb and GaMnAs. This barrier can be confused for Andreev Reflection and erroneously used to extract spin polarization. A simple acid etch is shown to remove the oxide film, thus decreasing the interface resistance, removing the Schottky barrier, and causing a clear Andreev reflection effect. One key recommendation for point contact Andreev reflection studies is to push the tip hard enough into contact and verify that the total resistance is not too high.

Minimal model of point contact Andreev reflection spectroscopy of multiband superconductors

We formulate a minimal model of point contact Andreev reflection spectroscopy of a normal- metal/multiband superconductor interface. The theory generalizes the Blonder-Tinkham-Klapwijk (BTK) formulation to a multiband superconductor and it is based on the quantum waveguides theory. The proposed approach allows an analytic evaluation of the Andreev and normal reflection coefficients and thus is suitable for a data fitting of point contact experiments. The obtained differential conductance curves present distinctive features similar to the ones measured in the experiments on multiband systems, like the iron-based pnictides and the MgB2.

A new simple method for point contact Andreev reflection (PCAR) using a self-aligned atomic filament in transition-metal oxides.

Point contact Andreev reflection (PCAR) has become a standard method for measuring the spin polarization (P) of spintronic materials due to its unique simplicity and the firm physical ground, but it is still challenging to achieve a clean point contact between a superconductor (SC) and a metal (N) for implementing PCAR. In this work, we suggest a much simpler method for PCAR measurement, where a point contact between SC and N is provided by a metallic filament in a transition-metal oxide generated by electrical bias. This method has been successfully demonstrated using a structure composed of Nb/NiO/Pt, where P of the Ni filament was estimated to be about 40%, consistent with the known value of the bulk Ni. In addition, we investigated the dependence of the conductance spectrum on the measurement temperature and the magnetic field. We found that the superconductivity is not fully suppressed until 9 T far above the critical field of Nb, which is associated with the nm-sized constriction of our SC/N junction, much smaller than the coherence length of the SC.

Isotropic multi-gap superconductivity in BaFe1.9Pt0.1As2 from thermal transport and spectroscopic measurements

Thermal conductivity, point contact spectroscopy, angle-resolved photoemission and Raman spectroscopy measurements were performed on BaFe1.9Pt0.1As2 single crystals obtained from the same synthesis batch in order to investigate the superconducting energy gap structure using multiple techniques. Low temperature thermal conductivity was measured in the superconducting state as a function of temperature and magnetic field, revealing an absence of quasiparticle excitations in the limit up to 15 T applied magnetic fields. Point-contact Andreev reflection spectroscopy measurements were performed as a function of temperature using the needle-anvil technique, yielding features in the conductance spectra at both 2.5 meV and 7.0 meV scales consistent with a multi-gap scenario. Angle-resolved photoemission spectroscopy probed the electronic band structure above and below the superconducting transition temperature of Tc = 23 K, revealing an isotropic gap of magnitude meV on both electron and hole pockets. Finally, Raman spectroscopy was used to probe quasiparticle excitations in multiple channels, showing a threshold energy scale of 3 meV below Tc. Overall, we find strong evidence for an isotropic gap structure with no nodes or deep minima in this system, with a 3 meV magnitude gap consistently observed and a second, larger gap suggested by point-contact spectroscopy measurements. We discuss the implications that the combination of these results reveal about the superconducting order parameter in the BaFe2−xPtxAs2 doping system and how this relates to similar substituted iron pnictides.

High spin polarization in CoFeMnGe equiatomic quaternary Heusler alloy

We report the structure, magnetic property, and spin polarization of CoFeMnGe equiatomic quaternary Heusler alloy. The alloy was found to crystallize in the cubic Heusler structure (prototype LiMgPdSn) with considerable amount of DO3 disorder. Thermal analysis result indicated the Curie temperature is about 750 K without any other phase transformation up to melting temperature. The magnetization value was close to that predicted by the Slater-Pauling curve. Current spin polarization of P = 0.70 ± 0.01 was deduced using point contact andreev reflection measurements. The temperature dependence of electrical resistivity has been fitted in the temperature range of 5–300 K in order to check for the half metallic behavior. Considering the high spin polarization and Curie temperature, this material appears to be promising for spintronic applications.

Point-contact Andreev-reflection spectroscopy in Fe(Te,Se) films: multiband superconductivity and electron-boson coupling

We report on a study of the superconducting order parameter in thin films (with different Se contents: ) by means of point-contact Andreev-reflection spectroscopy (PCARS). The PCARS spectra show reproducible evidence of multiple structures, namely two clear conductance maxima associated to a superconducting gap of amplitude and additional shoulders at higher energy that, as we show, are the signature of the strong interaction of charge carriers with a bosonic mode whose characteristic energy coincides with the spin-resonance energy. The details of some PCARS spectra at low energy suggest the presence of a smaller and not easily discernible gap of amplitude . The existence of this gap and its amplitude are confirmed by PCARS measurements in single crystals. The values of the two gaps and , once plotted as a function of the local critical temperature , turn out to be in perfect agreement with the results obtained by various experimental techniques reported in literature.

Direct Measurement of Spin Polarization in Ferromagnetic-C&lt;sub&gt;60&lt;/sub&gt; Interfaces Using Point-Contact Andreev Reflection

An important step has been developed combining the potential of spintronics with organic electronics to reveal the promising field of molecular spintronics, which can offer more flexibility, higher recyclability, and low-production costs compared with inorganic devices. Room temperature magnetoresistance (MR) of 5% has been obtained from C 60 spin valves with the structure Co(20 nm)/Al 2 O 3 (1.2 nm)/C 60 (5-60 nm)/Py(20 nm)/Al(1.5 nm). We observe an asymmetric dependence at low temperatures of the MR with voltage, and surprisingly with the magnetic field as well. This behavior has been attributed to the organic interface formed at the junction, which results in a change of the ferromagnet's spin polarization. The spin polarization at the organic-ferromagnetic interface is extracted by measuring the bias dependence of the conductance of a metallic-superconducting point contact and analyzed the spectra with the modified Blonder-Tinkham-Klapwijk theory. Point-contact Andreev reflection measurements reveal that the Co-C 60 possess spin polarization of 30% ± 1%, compared with 40% ± 1% for our Co films without C 60 . This could account for the asymmetry of the MR in spin valves incorporating C 60 and the need for an alumina spacer to maximize the MR.

Andreev-spectroscopy study of unconventional superconductivity in MgB2:(La,Sr)MnO3 nanocomposite

Unconventional high-temperature superconductivity in MgB2:La0:65Sr0:35MnO3 (MgB:LSMO) nanocomposite has been found recently [V. N. Krivoruchko and V. Yu. Tarenkov, Phys. Rev. B 86, 10502 (2012)]. In this report, the symmetry of the nanocomposite superconducting order parameter and plausible pairing mechanisms have been studied by the point-contact Andreev-reflection (PCAR) spectroscopy. To clarify the experimental results obtained, we consider a model of a ferromagnetic superconductor, which assumes a coexistence of itinerant ferromagnetism and mixed-parity superconductivity. The Balian–Werthamer state, with quasiparticle gap topology of the same form as that of the ordinary s- wave state, fits the experimental data reasonably well. Utilizing the extended Eliashberg formalizm, we calculated the contribution of MgB2 in the total composite's conductivity and estimated the magnitude of the electron–phonon effects originated from MgB2 in I–V characteristics of the composite at above-gap energies. It was found that distinctive features observed in the PC spectra of the MgB:LSMO samples and conventionally attributed to the electron–phonon interaction cannot be related to the MgB2 phonons. It is argued that the detected singularities may be a manifestation of the electron-spectrum renormalizations due to strong magnetoelastic (magnon–phonon) interaction in LSMO.

Large magnetoresistance of MnBi/Bi/MnBi spin valve

A transport magetoresistance (MR) of MnBi/Bi/MnBi spin valve device was calculated using density functional theory coupled with nonequilibrium Green׳s function method. The calculated transmission MR of the MnBi/Bi/MnBi spin valve device is around 750%, Obtained MR is very large compared with MR observed experimentally in MnBi junctions at room temperature (MR~70%). Large MR is consistent with a large transport spin polarization was demonstrated in MnBi films by the point contact Andreev reflection spectroscopy. MR of experimental point contacts is observed to be low is probably due to the rough interfaces that increased scattering and contact resistance. Consequently, a spin-valve MnBi/Bi/MnBi device could potentially have large MR that could be controlled by varying the thickness of the Bi spacer. Thus, MnBi is a promising candidate for high MR devices with tunable spacer properties.

High field magnetotransport and point contact Andreev reflection measurements on CuCr2Se4 and CuCr2Se3Br—Degenerate magnetic semiconductor single crystals

Single crystals of the metallically degenerate fully magnetic semiconductors CuCr2Se4 and CuCr2Se3Br have been prepared by the Chemical Vapour Transport method, using either Se or Br as transport agents. The high-quality, millimetre-sized, octahedrally faceted, needle- and platelet-shaped crystals are characterised by means of high field magnetotransport (μ0H≤ 14 T) and Point Contact Andreev Reflection. The relatively high spin polarisation observed |P|&amp;gt;0.56, together with the relatively low minority carrier effective mass of 0.25 me, and long scattering time 10−13 s, could poise these materials for integration in low- and close-to-room temperature minority injection bipolar heterojunction transistor demonstrations.

Ferromagnetism and spin polarization in indium nitride, indium oxynitride, and Cr substituted indium oxynitride films

We have investigated the structural, electrical, and magnetic properties of both sputter deposited indium oxynitride and Cr substituted indium oxynitride films as well as InN films grown by molecular beam epitaxy. The degenerate oxynitride films exhibit n-type carrier concentrations in excess of 1020cm−3 and remain conducting to low temperatures, while the InN samples have much lower carrier concentrations and are insulating at low temperatures. At the same time all of these films show a room temperature ferromagnetic signal, with saturation magnetization ranging from 0.05emucm−3 for the indium oxynitride and InN films to 0.30emucm−3 for the Cr substituted film. Low temperature point contact Andreev reflection measurements find a spin polarizations from 46±2% for indium oxynitride to 50±2% for the Cr substituted films. These results highlight the potential of nitrides for spintronic applications.

Doping and critical-temperature dependence of the energy gaps in Ba(Fe1−xCox)2As2thin films

The dependence of the superconducting gaps in epitaxial Ba(Fe_{1-x}Co_{x})_2As_2 thin films on the nominal doping x (0.04 \leq x \leq 0.15) was studied by means of point-contact Andreev-reflection spectroscopy. The normalized conductance curves were well fitted by using the 2D Blonder-Tinkham-Klapwijk model with two nodeless, isotropic gaps -- although the possible presence of gap anisotropies cannot be completely excluded. The amplitudes of the two gaps \Delta_{S} and \Delta_{L} show similar monotonic trends as a function of the local critical temperature T_{c}^{A} (measured in the same point contacts) from 25 K down to 8 K. The dependence of the gaps on x is well correlated to the trend of the critical temperature, i.e. to the shape of the superconducting region in the phase diagram. When analyzed within a simple three-band Eliashberg model, this trend turns out to be compatible with a mechanism of superconducting coupling mediated by spin fluctuations, whose characteristic energy scales with T_{c} according to the empirical law \Omega_{0}= 4.65*k_{B}*T_{c}, and with a total electron-boson coupling strength \lambda_{tot}= 2.22 for x \leq 0.10 (i.e. up to optimal doping) that slightly decreases to \lambda_{tot}= 1.82 in the overdoped samples (x = 0.15).

On the half-metallicity of Co2FeSi Heusler alloy: Point-contact Andreev reflection spectroscopy andab initiostudy

Co${}_{2}$FeSi, a Heusler alloy with the highest magnetic moment per unit cell and the highest Curie temperature, has largely been described theoretically as a half-metal. This conclusion, however, disagrees with point contact Andreev reflection (PCAR) spectroscopy measurements, which give much lower values of spin polarization, $P$. Here, we present the spin polarization measurements of Co${}_{2}$FeSi by the PCAR technique, along with a thorough computational exploration, within the DFT and a GGA+$U$ approach, of the Coulomb exchange $U$ parameters for Co and Fe atoms, taking into account spin-orbit coupling. We find that the orbital contribution (${m}_{o}$) to the total magnetic moment (${m}_{T}$) is significant, since it is at least 3 times greater than the experimental uncertainty of ${m}_{T}$. The account of ${m}_{o}$ radically affects the acceptable values of $U$. Specifically, we find no values of $U$ that would simultaneously satisfy the experimental values of the magnetic moment and result in the half-metallicity of Co${}_{2}$FeSi. On the other hand, the ranges of $U$ that we report as acceptable are compatible with spin polarization measurements (ours and the ones found in the literature), which all are within approximately the 40--60 $%$ range. Thus, based on reconciling experimental and computational results, we conclude that (a) spin-orbit coupling cannot be neglected in calculating Co${}_{2}$FeSi magnetic properties, and (b) Co${}_{2}$FeSi Heusler alloy is not half-metallic. We believe that our approach can be applied to other Heusler alloys such as Co${}_{2}$FeAl.

Measurement of the transport spin polarization of FeV using point-contact Andreev reflection

The Fe1−xVx alloy system exhibits the lowest known Gilbert relaxation rate of any ferromagnetic metal or binary alloy with G = 35 MHz at x = 27% V. Low relaxation rates are of particular interest in modern spin electronic applications involving spin torque. The transport spin polarization of a series of sputtered epitaxial Fe1−xVx samples was measured using point contact Andreev reflection. Values of the transport spin polarization agree well with those measured for pure Fe and are independent of composition. The results indicate that the substitution of up to 50% of V for Fe does not reduce the spin polarization in the alloy.

Point contact Andreev reflection from semimetallic bismuth—The roles of the minority carriers and the large spin-orbit coupling

The Point Contact Andreev Reflection (PCAR) technique has been traditionally and predominantly applied to either the characterisation of superconductors or of spin-polarised metals. The sufficiently large and slowly varying, on the scale of tens of meV around the Fermi level (EF), Density of States (DOS) in metals and highly degenerate semiconductors significantly simplifies the interpretation and quantitative analysis of the measured Differential Conductance Spectra (DCS). Semimetals in general, and particularly the heavy semimetals, because of the presence of several carrier bands and the larger Spin-Orbit coupling, exhibit significantly higher variations in the DOS, close to EF, which can be observed and evaluated with PCAR. Here, DCS of bismuth/niobium point contacts are examined to reveal the relative energy offsets [0.12(3), 2.32(5), and 5.7(2)] meV and the approximate relative DOS contributions of the three main carrier bands (16:5:4). Apart from the bulk, some surface states are also expected to contribute to the observed DOS and be potentially spin-split. The spin polarisation due to all conducting states is evaluated at P≤ 0.09(7).

Point-contact Andreev-reflection spectroscopy of doped manganites: Charge carrier spin-polarization and proximity effects (Review Article)

Materials with spin-polarized charge carriers are the most demanded in the spin-electronics. Particularly requested are the so-called half-metals which have the maximum attainable value of carrier spin polarization. Doped manganites are in the list of compounds with, potentially, half-metallic properties. The point-contact (PC) Andreev-reflection (AR) spectroscopy is a robust and direct method to measure the degree of current spin polarization. In this report, advances in PCAR spectroscopy of ferromagnetic manganites are reviewed. The experimental results obtained on “classic” s-wave superconductor—ferromagnetic manganites PCs, as well as related theoretical models applied to deduce the actual value of charge carrier spin-polarization, are discussed. Data obtained on “proximity affected” contacts is also outlined. Systematic and repeatable nature of a number of principal experimental facts detected in the AR spectrum of proximity affected contacts suggests that some new physical phenomena have been documented here. Different models of current flow through a superconductor–half-metal ferromagnet interface, as well as possibility of unconventional superconducting proximity effect, have been discussed.

Point-contact Andreev-reflection spectroscopy in anisotropic superconductors: The importance of directionality (Review Article)

Point-contact Andreev-reflection spectroscopy (PCARS) has demonstrated to be one of the most effective experimental tools for the investigation of fundamental properties of superconductors such as the superconducting gap and the electron–phonon (or, more generally, electron–boson) coupling. By reviewing relevant examples reported in literature and presenting new results, in this paper we show that when the direction of the interface with respect to the crystallographic axes can be controlled (as in single crystals and epitaxial films) PCARS can provide invaluable information about the anisotropy of the pairing wavefunction or—in the case of multiband superconductors—on the number, amplitude and symmetry of the energy gaps. Moreover, the analysis of PCARS results within a suitable 3D generalization of the BTK model allows obtaining qualitative information about the topology of the Fermi surface.