A. Rajanikanth

Large magnetoresistance in current-perpendicular-to-plane pseudospin valve using a Co2Fe"Ge0.5Ga0.5… Heusler alloy

Using a newly developed highly spin-polarized Heusler alloy, Co2Fe(Ga0.5Ge0.5) (CFGG), as ferromagnetic layers, we have fabricated a current-perpendicular-to-plane pseudospin valve with large resistance change-area product (ΔRA) of 9.5 mΩ μm2 and magnetoresistance (MR) ratio (100×ΔR/R) of 41.7% at 300 K. These values were further enhanced to ΔRA=26.4 mΩ μm2 and MR=129.1% at 10 K. The large MR values are attributed to the high spin polarization of the CFGG alloy confirmed by point contact Andreev reflection measurements.

Enhanced Spin Polarization of Co2MnGe Heusler Alloy by Substitution of Ga for Ge

Point contact Andreev reflection (PCAR) measurements indicated that the conductive spin polarization of the Co2MnGe Heusler alloy was enhanced substantially by the substitution of Ga for Ge. While the spin polarization of the conductive electrons of Co2MnGe and Co2MnGa was estimated to be around 0.6, that of the Co2MnGe0.75Ga0.25 quaternary alloy was 0.74, the highest value measured by PCAR for Heusler alloys. Moreover, the alloy was found to be an L21 intermetallic compound having a high Curie temperature of 895 K.

The effect of iron addition on the spin polarization and magnetic properties of Co2CrGa Heusler alloy

We report the effect of the substitution of Co with Fe in Co2−xFexCrGa Heusler alloys (x = 0–0.3) on their magnetic property, electronic band structure and spin polarization, both experimentally and theoretically. The intrinsic spin polarization of Co2CrGa measured by the point-contact Andreev reflection method was P = 0.61 ± 0.02, whereas a small substitution with Fe for Co of x = 0.1 enhanced the spin polarization to P = 0.67 ± 0.03 regardless of a weak L21 order. Ab initio calculations for both the perfectly L21 ordered and partially disordered structures showed that the small amount of Fe induced a robustness of the spin polarization against the structural disorder. These results demonstrate the effect of the Fe addition in Co2CrGa is to improve the spin polarization even with the presence of chemical disorder.

Antiferromagnetism and Spin Polarization in Double Perovskite SrLaVMoO6

The magnetic and transport properties of bulk polycrystalline samples of double perovskite SrLaVMoO6 have been investigated. Structural analyses have shown that the samples are almost in single phase with an ordered double perovskite structure. Magnetization versus temperature measurements have indicated that the SrLaVMoO6 compound exhibits a clear cusp at 125 K, which can be attributed to an antiferromagnetic transition. Electrical resistivity ρ for the compound showed metallic temperature dependence from 10 to 300 K, and the resistivity ρ value at 10 K was as low as 450 µΩcm. A spin polarization P value as high as 0.50 at 4.2 K was measured using the point-contact Andreev reflection (PCAR) technique. These results were discussed in terms of possible half metallic antiferromagnetism.

Low-temperature grown quaternary Heusler-compound Co2Mn1−xFexSi films on Ge(111)

Highly ordered quaternary Co2 Mn1-x FexSi films on Ge(111) are explored for spintronic device applications on Si-large-scale integrated circuit (LSI) platform. By using low-temperature molecular beam epitaxy techniques, relatively large magnetic moments are demonstrated for x between 0.50 and 1.0 despite extremely low temperature growth of 130 oC. Also, L21-ordered crystal structures can be realized even on a group-IV semiconductor substrate, Ge, compatible with Si-LSI technologies. By the point contact Andreev reflection method, the spin polarization of Co2Mn0.5 Fe0.5Si films is estimated to be P = 0.58 ± 0.02. We believe that this study will be a first step for integration of high-performance spintronic applications with next ultra LSI.

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.

Spin polarization of Co–Fe alloys estimated by point contact Andreev reflection and tunneling magnetoresistance

The compositional dependence on spin polarization of Co100−xFex alloys has been studied by point contact Andreev reflection (PCAR) and tunneling magnetoresistance (TMR) measurements. The intrinsic spin polarization for bcc Co75Fe25 alloy is P=0.58±0.03 at 4.2K contrary to the pure Fe (P=0.46±0.03) and Co (P=0.45±0.03). The tunneling spin polarization values of Co75Fe25 (110) textured polycrystalline electrode and (001) epitaxially grown electrode was estimated to be PT=0.5±0.01 and PT=0.57±0.01 at 8K from the TMR ratios using Julliere’s model for the MTJs prepared on oxidized Si and MgO (001) substrates. The spin polarization obtained from the tunneling junctions and PCAR experiments have been discussed.

The enhancement of the spin polarization of Co2MnSn by Fe doping

We have investigated the effect of Fe doping on the spin polarization of Co2Mn1−xFexSn (x=0.0, 0.05, 0.1, and 0.2) Heusler alloys by using the point contact Andreev reflection method. The spin polarization increased from 0.6 for x=0.0 to 0.65 for x=0.05. This is attributed to the increase in the spin up states near the Fermi level since Fe is a high valent element compared to Mn. However, with further increase in Fe content, the spin polarization decreased to 0.63 and 0.58 for x=0.1 and 0.2, respectively. Hence, the alloy Co2Mn0.95Fe0.05Sn with low Fe doping (1.25 at. % Fe) could be a potential material for spintronics applications.

Coexistence of antiferromagnetism and spin polarization in double perovskite SrLaVMoO6

The magnetic and transport properties of SrLaVMoO6 bulk samples with an ordered double perovskite structure have been investigated. Magnetization measurements have indicated that the SrLaVMoO6 compound exhibits a cusp at 125 K, which is attributable to an antiferromagnetic transition. Electrical resistivity ρ for the compound showed metallic temperature dependence from 10 to 300 K, and a spin polarization P value was measured to be 0.50 using the point-contact Andreev reflection (PCAR) technique. It has been found from X-ray photoemission spectroscopy (XPS) study that SrLaVMoO6 closely resembles the half-metallic Sr2FeMoO6 in the electronic state of the Mo.

Suppression of magnon excitations in Co2MnSi Heusler alloy by Nd doping

We have investigated the effect of Nd doping on the transport and magnetic properties of Co2MnSi. The substitution of 1.25 at. % Nd for Co results in a single L21 phase with a thin layer of Nd enriched phase grain boundaries. However, from resistivity measurements at low temperatures, the suppression of electron-magnon scattering was observed in the alloy with Nd enriched phase. The magnetic moment of Co2MnSi was 5.01μB, whereas that of Co1.95Nd0.05MnSi is 5.35μB, which indicates that the Nd rich phase couples ferromagnetically with the Co1.95Nd0.05MnSi phase. Moreover, point contact Andreev reflection measurements at 4.2 K have shown almost the same spin polarization with (P=0.59) and without Nd doping (P=0.58), indicating that Nd enrichment in the grain boundary does not hamper the spin polarization, but prevents the temperature dependent decay of transport properties.