Point Contact Andreev Reflection Research Articles

Pairing state in the ruthenocuprate superconductorRuSr2GdCu2O8: A point-contact Andreev reflection spectroscopy study

The results of point contact Andreev reflection spectroscopy on polycrystalline $\mathrm{Ru}{\mathrm{Sr}}_{2}\mathrm{Gd}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8}$ pellets are presented. The wide variety of the measured spectra are all explained in terms of a modified Blonder--Tinkham--Klapwijk model considering a $d$-wave symmetry of the superconducting order parameter. Remarkably low values of the energy gap $\ensuremath{\Delta}=(2.8\ifmmode\pm\else\textpm\fi{}0.2)\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ and of the $2\ensuremath{\Delta}∕{k}_{B}{T}_{c}\ensuremath{\simeq}2$ ratio are inferred. From the temperature evolution of the $dI∕dV$ vs $V$ characteristics we extract a sublinear temperature dependence of the superconducting energy gap. The magnetic field dependence of the conductance spectra at low temperatures is also reported. From the $\ensuremath{\Delta}$ vs $H$ evolution, a critical magnetic field ${H}_{{c}_{2}}\ensuremath{\simeq}30\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ is inferred. To properly explain the curves showing gap-like features at higher voltages, we consider the formation of a Josephson junction in series with the point contact junction, as a consequence of the granularity of the sample.

Ce Mn Ni 4 : A soft ferromagnet with a high degree of transport spin polarization

In this letter we introduce a soft ferromagnetic compound, i.e., CeMnNi4, which exhibits a large moment (∼4.95μB∕Mn) and high degree of spin polarization. The system has a ferromagnetic transition temperature of 140K. Isothermal magnetization measurements at 5K reveal that the material is a soft ferromagnet with a magnetization saturating at about 1500Oe and a coercive field of <5Oe. We determine the transport spin polarization of this material from point contact Andreev reflection measurements to be 66% thereby making this material potentially important for spintronic applications.

High spin polarization in the ferromagnetic filled skutteruditesKFe4Sb12andNaFe4Sb12

The spin polarization of ferromagnetic alkali-metal iron antimonides $\mathrm{K}{\mathrm{Fe}}_{4}{\mathrm{Sb}}_{12}$ and $\mathrm{Na}{\mathrm{Fe}}_{4}{\mathrm{Sb}}_{12}$ is studied by point-contact Andreev reflection using superconducting Nb and Pb tips. From these measurements an intrinsic transport spin polarization ${P}_{t}$ of 67% and 60% for the K and Na compound, respectively, is inferred, which establishes these materials as a class of highly spin-polarized ferromagnets. The results are in accord with band structure calculations within the local spin density approximation that predict nearly 100% spin polarization in the density of states. We discuss the impact of calculated Fermi velocities and spin fluctuations on ${P}_{t}$.

Andreev reflection spectroscopy of niobium point contacts in a magnetic field

The Blonder-Tinkham-Klapwijk model of the point-contact Andreev reflection (PCAR) spectroscopy does not provide an adequate description of experiments in the presence of a magnetic field. We demonstrate this using a junction between a niobium tip and a copper film. We modify the theory to explicitly take into account the contribution to conductance that stems from the normal vortex cores in the superconductor. These results have important implications for the interpretation of transport spin polarization measurements using the PCAR technique. We demonstrate that stray magnetic fields can be responsible for the experimentally observed dependence of the spin polarization on the strength of the interface barrier and potential misassignment of the inferred spin polarization.

Spin injection between epitaxial Co2.4Mn1.6Ga and an InGaAs quantum well

Electrical spin injection in a narrow [100] In0.2Ga0.8As quantum well in a GaAs p-i-n optical device is reported. The quantum well is located 300nm from an AlGaAs Schottky barrier and this system is used to compare the efficiencies and temperature dependences of spin injection from Fe and the Heusler alloy Co2.4Mn1.6Ga grown by molecular-beam epitaxy. At 5K, the injected electron spin polarizations for Fe and Co2.4Mn1.6Ga injectors are 31% and 13%, respectively. Optical detection is carried out in the oblique Hanle geometry. A dynamic nuclear polarization effect below 10K enhances the magnetic field seen by the injected spins in both devices. The Co2.4Mn1.6Ga thin films are found to have a transport spin polarization of ∼50% by point contact Andreev reflection conductivity measurements.

Spin polarization and electronic structure of ferromagnetic Mn5Ge3 epilayers

AbstractGermanium‐based alloys hold great promise for future spintronics applications, due to their potential for integration with conventional Si‐based electronics. High‐quality single phase Mn5Ge3(0001) films, grown by solid‐phase epitaxy on Ge(111) and GaAs(111), exhibit strong ferromagnetism up to the Curie temperature TC ∼ 296 K. Point Contact Andreev Reflection (PCAR) measurements on Mn5Ge3 epilayers reveal a spin‐polarization P = 42 ± 5% for both substrates. We also calculate the spin polarization of bulk Mn5Ge3 in the diffusive and ballistic regimes using density‐functional theory (DFT). The measured spin polarization exceeds the theoretical estimates of PDFT = 35 ± 5% and 10 ± 5% for the diffusive and ballistic limits, respectively. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spin polarization measurements on polycrystalline strontium ruthenates using point-contact Andreev reflection

Point-contact Andreev reflection (PCAR) measurements were made on bulk polycrystalline SrRu0.8Ti0.2O3 and SrRu0.92O3 samples to determine the transport spin polarization. The parent compound SrRuO3 undergoes ferromagnetic ordering at TC∼160K with relatively high spin polarization (∼60%). For the SrRu0.8Ti0.2O3 and SrRu0.92O3 samples, a reduction in TC occurs. Moreover, our measurements indicate that the SrRu0.8Ti0.2O3 system retains a high degree of spin polarization (P∼0.6), similar to the parent compound. However, due to the insulating nature of SrRu0.92O3, the PCAR technique did not yield information on P. Instead, the data displayed nonlinear I–V consistent with the tunnelinglike spectra.

Possibilities and limitations of point-contact spectroscopy for measurements of spin polarization

Major issues that limit the applicability of point-contact Andreev reflection (PCAR) spectroscopy for measurements of transport spin polarization $(P)$ are investigated. We show that the analysis of the PCAR spectra on the basis of the generalized Blonder-Tinkham-Klapwijk model may lead to degenerate solutions; i.e., the straightforward determination of $P$ from the multiparameter fitting is ambiguous. We have developed a refined method of analysis that reduces the ambiguity. This method is applied to several test cases (copper, NiMnSb, ${\mathrm{Co}}_{2}\mathrm{Mn}\mathrm{Si}$, ${\mathrm{Sr}}_{2}\mathrm{Fe}\mathrm{Mo}{\mathrm{O}}_{6}$), and it is found to work in materials with low and high $P$.

Structural, magnetic, electronic, and spin transport properties of epitaxialFe3Si∕GaAs(001)

We report experimental results on the structural, magnetic, electronic, and spin transport properties of a $21\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ ${\mathrm{Fe}}_{3}\mathrm{Si}∕\mathrm{GaAs}(001)$ heterostructure epitaxially grown by coevaporation. High-resolution x-ray diffraction shows an almost stoichiometric film, which is lattice matched in-plane to the $\mathrm{GaAs}$ substrate and therefore slightly tetragonal distorted. Polarized neutron reflectometry measurements yield a magnetic moment of $(1.107\ifmmode\pm\else\textpm\fi{}0.014){\ensuremath{\mu}}_{\mathrm{B}}$ per atom at room temperature (RT), while superconducting quantum interference device magnetometry yields a magnetic moment of $(0.9\ifmmode\pm\else\textpm\fi{}0.1){\ensuremath{\mu}}_{\mathrm{B}}$ per atom at RT, both close to the bulk value. Magneto-optic Kerr effect measurements show that this system has in-plane cubic anisotropy with easy axes along the $⟨100⟩$ directions and a cubic anisotropy constant ${K}_{1}=(3.1\ifmmode\pm\else\textpm\fi{}0.6)\ifmmode\times\else\texttimes\fi{}{10}^{4}\phantom{\rule{0.3em}{0ex}}\mathrm{erg}∕{\mathrm{cm}}^{3}$ at RT. A resistivity of $(4.1\ifmmode\pm\else\textpm\fi{}0.4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}\phantom{\rule{0.3em}{0ex}}\ensuremath{\Omega}\phantom{\rule{0.3em}{0ex}}\mathrm{m}$ in the ${\mathrm{Fe}}_{3}\mathrm{Si}$ film was measured, which is close to the bulk value. Optical spin orientation in the $\mathrm{GaAs}$ was used for spin transport measurements and spin detection is demonstrated at RT for this system. Point contact Andreev reflection spectroscopy was used to determine the spin polarization of the transport current, yielding $P=(45\ifmmode\pm\else\textpm\fi{}5)%$.

Anomalous structures in point contact Andreev reflection spectrum

We report the systematic study of the anomalous structures viz. dips and large zero bias enhancement in point contact Andreev reflection spectrum obtained between a normal metal and a superconductor. These are not explained in Blonder–Tinkham–Klapwizk formalism, which is conventionally used for analyzing normal metal–superconductor ballistic point contacts. We demonstrate that the anomalous structures are due to the contribution from the critical current of the point contact to the contact resistance when the contact is not in the pure ballistic regime.

Electronic properties of thin Ni 2MnIn Heusler films

The half-metallic Heusler alloy Ni 2MnIn is of high interest for use in spin electronics since at the Ni 2MnIn/InAs interface a spin polarization of 100% is predicted. We prepare high-quality thin films of 20–60 nm thickness by co-evaporation and DC magnetron sputtering. Point-contact Andreev reflection spectroscopy yields a spin polarization of up to 54%. By spectral generalized magneto-optical ellipsometry, the dielectric and magneto-optical properties are determined and ferromagnetic behavior below the Curie temperature T C = 318 K is proved.

Analysis of point-contact Andreev reflection spectra in spin polarization measurements

We present a systematic analysis of point-contact Andreev reflection (PCAR) spectra for ferromagnetic materials, using both modeling and experimental data. We emphasize the importance of consistent data analysis to avoid possible misinterpretation of the data. We consider the relationship between ballistic and diffusive transport, the effect of different transport regimes on spin polarization measurements, and the importance of unambiguous identification of the type of transport regime. We find that in a realistic parameter range, the analysis of PCAR spectra of purely diffusive character by a ballistic model yield approximately the same (within $\ensuremath{\sim}3%$) values of the spin polarization and the barrier strength $Z$ larger by $\ensuremath{\sim}0.5--0.6$. We also consider the dependence of polarization values on $Z$, and have shown by simple modeling that letting the superconducting gap vary as an adjustable parameter can result in a spurious dependence of the spin-polarization ${P}_{c}$ on $Z$. At the same time we analyzed the effects of finite $Z$ on the apparent value of ${P}_{c}$ measured by the PCAR technique, using a large number of examples from both our own measurements and from the literature. We conclude that there is a system-dependent variation in ${P}_{c}\phantom{\rule{0.2em}{0ex}}(Z)$, presumably due to spin-flip scattering at the interface. However, the exact type of this dependence is hard to determine with any statistical certainty.

The magnetic and transport properties of the Co2FeGa Heusler alloy

The magnetic and transport properties of the Co2FeGa Heusler alloy have been investigated. The results show that the temperature dependence of the magnetization follows the spin-wave behaviour at low temperature. The electrical resistivity behaves according to a ∼T2 power law, which may be mainly attributed to electron–electron scattering, and the contribution of electron–phonon scattering to the resistivity seems to be small. We have not observed remarkable magnetoresistance in our measurements. Point contact Andreev reflection measurements of the spin-polarization yield a polarization of 59%, which is consistent with the theoretical prediction by a first-principles calculation.

Spin polarization of the transport current at the free surface of bulkNiMnSb

The point contact Andreev reflection employing niobium tips was used to determine the degree of transport current spin polarization ( Pt ) at the free surface of bulk NiMnSb at 4.2 K . The data was analyzed within the framework of a modified version of the Blonder, Tinkham, and Klapwijk formulism taking into account the two spin polarized channels in the ferromagnet and treating the interface as a planar delta function barrier of height Z between free electron materials. We find that the measured spin polarization is rather insensitive to different surface preparations and magnetic domain structure, and the maximal value of the Pt at Z=0 is 44% , consistent with recent calculations of the surface reconstruction of NiMnSb . Saturation magnetization of the samples was found to be 3.6 iB per formula unit indicative of a small amount of atomic disorder.

Synthesis and physical properties of arc melted NiMnSb

Several polycrystalline samples of the half-Heusler alloy NiMnSb were grown by arc melting of stoichiometric and nonstoichiometric amounts of high-purity constituent elements. The structure and the phase-purity of the prepared samples were examined systematically by powder x-ray diffraction. The transport properties of the best sample, with saturation magnetization Ms(5 K)=4 μB/formula unit, were studied by measuring electrical resistivity, thermal conductivity, and thermopower. Features in both magnetic and transport data are consistent with NiMnSb being in a half-metallic state at low temperatures, i.e., the conduction electrons are fully spin polarized. However, point-contact Andreev reflection measurements on the same sample at 4.2 K demonstrate only ∼45% spin polarization.

Measurement of spin polarization by Andreev reflection in ferromagnetic In1−xMnxSb epilayers

We carried out point contact Andreev reflection (PCAR) spin spectroscopy measurements on epitaxially grown ferromagnetic In1−xMnxSb epilayers with a Curie temperature of ∼9 K. The spin sensitivity of PCAR in this material was demonstrated by parallel control studies on its nonmagnetic analog, In1−yBeySb. We found the conductance curves of the Sn point contacts with In1−yBeySb to be fairly conventional, with the possible presence of proximity-induced superconductivity effects at the lowest temperatures. These measurements provided control data for subsequent PCAR measurements on ferromagnetic In1−xMnxSb, which indicated spin polarization in In1−xMnxSb to be 52±3%.

Structural and transport studies of stoichiometric and off-stoichiometric thin films of the full Heusler alloy Co2MnSi

Co 2 MnSi Heusler alloy thin films have been grown by cosputtering from three elemental targets. Changes in growth temperature and stoichiometry affect the film texture and the temperature dependence of the resistivity. However, the spin polarization of the transport current is insensitive to these changes, being of the order of 54% as determined by point contact Andreev reflection spectroscopy. Stoichiometric films are single phase, have a strong (110) texture, and a saturation magnetization of 1007 emu/cc (4.95 μB/formula unit) at 10 K.

Role of critical current on the point-contact Andreev reflection spectra between a normal metal and a superconductor

The point contact spectrum between a normal metal and a superconductor often shows unexpected sharp dips in the conductance at voltage values larger than the superconducting energy gap. These dips are not predicted in the Blonder-Tinkham-Klapwizk (BTK) theory, commonly used to analyse these contacts. We present here a systematic study of these dips in a variety of contacts between different combinations of a superconductor and a normal metal. From the correlation between the characteristics of these dips with the contact area, we can surmise that such dips are caused by the contact not being in the ballistic limit. An analysis of the possible errors introduced while analysing such a spectrum with the standard BTK model is also presented.

Structural, magnetic, and transport properties of thin films of the Heusler alloy Co2MnSi

Thin films of Co2MnSi have been grown on a-plane sapphire substrates from three elemental targets by dc magnetron cosputtering. These films are single phase, have a strong (110) texture, and a saturation magnetization of 4.95μB/formula unit at 10 K. Films grown at the highest substrate temperature of 715 K showed the lowest resistivity (47 μΩ cm at 4.2 K) and the lowest coercivity (18 Oe). The spin polarization of the transport current was found to be of the order of 54% as determined by point contact Andreev reflection spectroscopy. A decrease in saturation magnetization with a decrease in film thickness and different transport behavior in thinner films indicate graded disorder in these films grown on nonlattice matched substrates.

Spin polarisation and anomalous Hall effect in NiMnSb films

Abstract NiMnSb has attracted a great deal of interest as a spin injector/detector in spintronic devices because it has a Curie temperature of 728 K and is predicted to be half-metallic (100% spin polarized). NiMnSb has been reported to have greatly reduced surface polarization, and to lose its half metallicity above 80 K. Here we report the investigation of the surface polarisation and electronic structure of NiMnSb by measurement of the transport spin polarization using point contact Andreev reflection spectroscopy, and anomalous Hall effect in thin films on Si(0 0 1). A comparison to bulk properties is made.