Point Contact Andreev Reflection Technique Research Articles

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.

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.

Spin polarization and Gilbert damping of Co2Fe(GaxGe1−x) Heusler alloys

The spin polarization (P) of ferromagnetic Heusler alloys, Co2Fe(GaxGe1−x) (x=0,0.25,0.5,0.75, and 1), is investigated by point contact Andreev reflection (PCAR) measurements. While the P values of the ternary Co2FeGe and Co2FeGa alloys are 0.58 and 0.59, respectively, the corresponding value for Co2Fe(Ge0.5Ga0.25) is as high as 0.69. Co2Fe(Ge0.5Ga0.5) alloy shows a strong tendency for L21 ordering and a high Curie temperature of 807°C. Ab initio calculations indicate that the band structures of the Co2FeGe and Co2Fe(Ge0.5Ga0.5) alloys with L21 or B2 structures are half-metallic. Thin films of the quaternary Co2Fe(Ge0.5Ga0.5) alloy grown on MgO(001) substrates order to the L21 structure upon annealing at 500°C, giving rise to a high P of 0.75. This is the highest P value reported for Heusler alloy thin films using the PCAR technique. Ferromagnetic resonance measurements show that the Gilbert damping constant of the film is ∼0.008. All these indicate that the Co2Fe(Ge0.5Ga0.5) alloy is promising as a spin polarized current source for spintronics devices.

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.

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.

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 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.

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.

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.