Andreev Reflection Technique Research Articles

Direct observation of hopping induced spin polarization current in oxygen deficient Co-doped ZnO by Andreev reflection technique

Oxygen vacancy induced ferromagnetic coupling in diluted magnetic oxide (DMO) semiconductors have been reported in several studies, but technologically more crucial spin-polarized current (SPC) is still under-developed in DMOs. Few studies have claimed that VRH mechanism can originate the SPC, but, how VRH mechanism associated with percolation path, is not clearly understood. We used Point-contact Andreev reflection (PCAR) technique to probe the SPC in Co-doped ZnO (CZO) films. Since the high resistance samples cause broadening in conductance(G)-voltage(V) curves, which may result in an unreliable evaluation of spin polarization, we include two extra parameters, (i) effective temperature and (ii) spreading resistance, for the simulation to avoid the uncertainty in extracting spin polarization. The effective G-V curves and higher spin polarization can be obtained above a certain oxygen vacancy concentration. The number of completed and fragmentary percolation paths is proportional to the concentration of oxygen vacancies. For low oxygen vacancy samples, the Pb-tip has a higher probability of covering fragmentary percolation paths than the complete ones, due to its small contact size. The completed paths may remain independent of one another and get polarized in different directions, resulting in lower spin-polarization value. High oxygen vacancy samples provide a high density of completed path, most of them link to one another by crossing over, and gives rise to high spin-polarization value.

Spin polarization in Cu2MnSn Heusler alloy produced by melt-spinning

We report on magnetism, transport and spin polarization characteristics of the melt-spun Cu2MnSn alloy prepared by the rapid quenching technique. The as- cast ribbons showed a relatively well ordered chemical composition (Cu = 50.4%, Mn = 27.1%, Sn = 22.5%). The structural characterization by using X-ray diffraction shows L21/B2 crystalline structure with the lattice parameter a = 6.196 Å. The magnetic and transport measurements show a metallic behavior with the Curie temperature of 530 K and reveal anisotropic character with the easy magnetization plane parallel with respect to the ribbon plane. As-cast Cu2MnSn ribbons show the spin polarization measured by using Andreev reflection technique within the range 68–75%.

Magnetization and Spin Polarization of Heusler Alloys Co $_{2}$TiSn and Co$_{2}$ TiGa$_{0.5}$Sn $_{0.5}$

The substitution effect on half-metallicity for Co-based Heusler alloys Co 2 TiGa 1-x Sn x was investigated by experimental measurements and first-principles band calculations. The synthesized alloys crystalized in the ordered L2 1 crystal structure. The spontaneous magnetization obeyed the Slater-Pauling rule, indicating the possibility that Co 2 TiSn and Co 2 TiGa 0.5 Sn 0.5 are half-metals. The first-principles band calculations revealed that Co 2 TiSn is half-metal and Co 2 TiGa 0.5 Sn 0.5 is nearly half-metal. The spin polarization P was determined by the Andreev reflection technique. The intrinsic P values were 58.9% for Co 2 TiSn and 58.2% for Co 2 TiGa 0.5 Sn 0.5 , respectively. The P values of Co 2 TiGa 1-x Sn x were relatively higher than other Co-based Heusler alloys. Thus, Co-based Heusler alloys Co 2 TiGa 1-x Sn x are possible candidates for half-metallic electrode materials in spintronic devices.

Effect of Co substitution for Mn on spin polarization and magnetic properties of ferrimagnetic Mn2VAl

Bulk (Mn1-xCox)2VAl (x = 0, 0.25, 0.50) alloys in highly ordered L21 structure with a very small amount of B2 disorder have been prepared and their magnetic properties have been measured. The value of saturation magnetizations of the alloys with x = 0, 0.25 and 0.50 are 1.88 μB, 0.84 μB and 0.07 μB, respectively, being consistent with 2.00 μB, 1.00 μB and 0 μB, respectively, predicted by the Slater–Pauling rule. This indicates that the stoichiometric MnCoVAl alloy is a fully compensated ferrimagnet (FCF). Spin polarization measurements using point contact Andreev reflection technique showed that the quaternary alloys exhibited higher intrinsic spin polarization than the parent ternary composition. The spin polarization of 0.60 deduced for the MnCoVAl alloy suggests that spin-polarized current can be extracted from FCF. Curie temperature (TC) and the effective anisotropy constant of (Mn1-xCox)2VAl alloys decrease with increase in Co content.

Spin Polarization Measurements of Heusler Alloy ${\hbox{Ru}}_{0.2}{\hbox{Fe}}_{1.8}{\hbox{CrSi}}$ by Andreev Reflection Technique

We report spin polarization P of Fe-rich Heusler alloy Ru0.2Fe1.8CrSi, which was determined by the Andreev reflection technique. Heusler alloy Ru2-xFexCrSi was predicted to be half-metal by the first-principle band calculations. Fe-rich Ru0.2Fe1.8CrSi compounds are ferromagnetic and the saturation magnetization M0 almost coincides with that expected from the Slater-Pauling rule. Two theoretical approaches in the determination of spin polarization by Andreev reflection are discussed and compared from an experimental perspective. The spin polarization more than P=0.60 is obtained for Ru0.2Fe1.8CrSi specimens and it is highest value in all Ru2-xFexCrSi compounds. The difference in values of spin polarization as determined by two models is less than 0.02.

Magnetization and spin polarization of Co2−xFexMnSi Heusler alloys

We report magnetization and spin polarization of Co2-xFexMnSi Heusler alloys. Co2-xFexMnSi Heusler alloys are theoretically predicted to be half-metals (HM's) and generally possess high Curie temperature. The fact that the saturation magnetization of Co2-xFexMnSi for 0 ≤ x ≤ 1.5 obeys the Slater-Pauling rule reveals that those compounds are experimentally expected to be HM's. Therefore, the spin polarization of Co2-xFexMnSi Heusler alloys was determined by the Andreev reflection technique using Co2-xFexMnSi/Pb planar-type junctions. The obtained differential conductance was able to be fitted very well by the modified Blonder-Tinkham-Klapwijk theory. From analysis of spin polarization measurements, we have found the spin polarization of P = 0.50 for x = 0.5 and P = 0.52 for x = 1.25, respectively. Consequently, the spin polarization was less dependent of different concentrations of x.

Spin polarization of Fe-rich ferromagnetic compounds in Ru2−xFexCrSi Heusler alloys

We report spin polarization P of Ru2−xFexCrSi Heusler alloys by the Andreev reflection technique. Ru2−xFexCrSi with L21-type structure and saturation magnetic moment of 2μB per formula unit is theoretically predicted to be half-metals in the wide range of the composition x. We had clarified that the experimental results of saturation magnetic moment in Fe-rich compounds had coincided with the theoretical prediction. Therefore, we have measured the differential conductance of Ru2−xFexCrSi/Pb planar-type junctions. The P value of Ru2−xFexCrSi was determined by fitting the differential conductance with the modified Blonder–Tinkham–Klapwijk theory. We have found that the behavior of P for Ru2−xFexCrSi was independent of the composition x in the Fe-rich region; P=0.53 for both of x=1.5 and 1.7. The spin polarization is the similar value to Co-based Heusler alloys.

Spin polarization of currents in Co/Pt multilayer and Co–Pt alloy thin films

The point contact Andreev reflection technique was employed to estimate the spin polarization of currents in Co/Pt multilayered thin film and Co–Pt alloy film with perpendicular anisotropy. The spin polarization of currents in the Co–Pt alloy film was estimated to be 0.48, while that in Co/Pt multilayered film was 0.56. These results suggest that the spin-polarization of currents in multilayered film is enhanced compared to that in alloy film.

Measurements of spin polarization of [formula omitted] Heusler alloys by Andreev reflection

We report spin polarization P of Ru2-xFexCrSi Heusler alloys by Andreev reflection technique. Ru2-xFexCrSi with the L21-type structure is theoretically predicted to be half-metals in the wide range of the composition x. The experimental results of saturation moment in Fe-rich compounds measurements also support the theoretical prediction. We have measured the differential conductance of Ru2-xFexCrSi/Pb planar-type junctions. The P value of Ru2-xFexCrSi was determined by fitting the differential conductance with the modified Blonder–Tinkham–Klapwijk theory. The behavior of P was independent of the composition x in the Fe-rich region; P=0.53 for x=1.5 and P=0.52 for x=1.7, respectively. We have found that the spin polarization of Ru2-xFexCrSi in Fe-rich compounds was the similar values to Co-based Heusler alloys.

Disorder effects in half-metallic Sr2FeMoO6 single crystals

Double perovskites such as Sr2FeMoO6 (SFMO) have been predicted to be half-metallic (100% spin polarized). However, this conclusion is reached under the assumption that SFMO has a perfect crystalline structure. We measure the values of spin polarization P for SFMO single crystals with 11%, 15%, and 16% of B-site disorder using the point contact Andreev reflection technique. The highest spin polarization of P≈70% was determined. Using local spin density approximation, we compute P≈53% for the degree of B-site disorder of 12.5% in good agreement with the experimental results. While our results show that imperfect SFMO is not a half-metal, the observed correlation between the degree of antisite disorder, the magnetic properties, and the values of spin polarization implies that a perfectly ordered SFMO crystal might be half-metallic.

Inelastic scattering and spin polarization in dilute magnetic semiconductor (Ga,Mn)Sb

The point contact Andreev reflection technique has already been used to measure the spin polarization of some of the dilute magnetic semiconductors, such as narrow-band (In,Mn)Sb, as well as wider gap (Ga,Mn)As. While conventional Andreev reflection has been demonstrated in (In,Mn)Sb, quasiparticle density of states broadening has been observed in (Ga,Mn)As, possibly due to inelastic scattering effects. Here, we investigate the spin polarization, magnetic, and transport properties of epitaxially grown (Ga,Mn)Sb films with the Curie temperature of ∼10K. The spin polarization of 57±5% was measured. Spectrum broadening in (Ga,Mn)Sb has also been observed.

High spin polarization in a two phase quaternary Heusler alloy Co2MnAl1−XSnX

We have investigated the structure and the spin polarization of Co2MnAl, Co2MnSn, and Co2MnAl0.5Sn0.5 alloys. The structures of the Co2MnAl and Co2MnSn alloys that were annealed for four days at 450°C were B2 and L21, respectively. Co2MnAl0.5Sn0.5 had a dendrite microstructure that was formed by the phase separation into Al-enriched B2 and Sn-enriched L21 phases during solidification. The saturation magnetization values of Co2MnAl, Co2MnAl0.5Sn0.5, and Co2MnSn were 3.66μB, 4.3μB, and 4.9μB, respectively, which are slightly lower than the theoretical values of 4μB, 4.5μB, and 5μB due to the low degree of L21 ordering. The spin polarization P measured by the point contact Andreev reflection technique were 0.56 for B2-Co2MnAl, 0.6 for L21-Co2MnSn, and interestingly, a higher value of 0.63 for the two phase Co2MnAl0.5Sn0.5 alloy.

Spin polarization of Co2MnGe and Co2MnSi thin films with A2 and L21 structures

Spin polarizations (P) of polycrystalline Co2MnGe and Co2MnSi films that were sputter deposited on thermally oxidized Si substrates at various substrate temperatures (Ts) were measured by the point contact Andreev reflection technique. While continuous films were grown at Ts=500°C with the L21 structure, Mn-deficit islands were formed at Ts>700°C. P showed strong dependence on the state of order of the alloys; P=0.58 for L21 ordered Co2MnGe, P=0.35 for A2 Co2MnGe, P=0.54 for L21 ordered Co2MnSi, and P=0.52 for A2 Co2MnSi. The experimentally determined P values of the L21 ordered films are lower than the theoretical predictions, which is attributed to the imperfect L21 ordering.

Origin of high transport spin polarization inLa0.7Sr0.3MnO3:Direct evidence for minority spin states

Using the point-contact Andreev reflection technique, we have carried out a systematic study of the spin polarization in the colossal magnetoresistive manganite, ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_{3}$ (LSMO). Surprisingly, we observed a significant increase in the current spin polarization with the residual resistivity. This counterintuitive trend can be understood as a transition from ballistic to diffusive transport in the contact. Our results strongly suggest that LSMO does have minority spin states at the Fermi level. However, since its current spin polarization is much higher than that of the density of states, this material can mimic the behavior of a true half-metal in transport experiments. Based on our results we call this material a transport half-metal.

Highly spin-polarized chromium dioxide thin films prepared by chemical vapor deposition from chromyl chloride

Highly spin-polarized chromium dioxide (CrO2) thin films were deposited on (100) TiO2 substrates by chemical vapor deposition using chromyl chloride as a precursor. The spin polarization, as measured by the point contact Andreev reflection technique, was 81±3%. X-ray diffraction θ/2θ scans indicated the films grew completely (100) oriented, in registry with the (100) oriented TiO2 substrate. X-ray diffraction φ scans on the CrO2 (110) reflection indicated the expected twofold symmetry, with no evidence of misaligned material. The resistivity at room temperature was 240 μΩ cm and decreased to 10 μΩ cm at 5 K, consistent with metallic behavior. The films were ferromagnetic with a Curie temperature of 395 K and a coercivity of ∼100 Oe at 298 K. The use of chromyl chloride as a precursor resulted in efficient and controlled CrO2 film growth.

Transport spin polarization ofNixFe1−x:Electronic kinematics and band structure

We present measurements of the transport spin polarization of Ni_xFe_{1-x} (0<x<1) using the recently-developed Point Contact Andreev Reflection technique, and compare them with our first principles calculations of the spin polarization for this system. Surpisingly, the measured spin polarization is almost composition-independent. The results clearly demonstrate that the sign of the transport spin polarization does not coincide with that of the difference of the densities of states at the Fermi level. Calculations indicate that the independence of the spin polarization of the composition is due to compensation of density of states and Fermi velocity in the s- and d- bands.