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The structure, electrical transport, and magnetic properties have been systematically studied in superconducting Ti1−xMgxO (x = 0, 0.003, 0.02, 0.08, and 0.27) films. It is found that, both the zero resistance and onset superconducting transition temperatures of Ti1−xMgxO first increase with increasing the Mg content from x = 0 to x = 0.02, and then decrease with the further increase of x. More interestingly, the Mg-doping simultaneously introduces a ferromagnetic order into this superconducting system, and the ferromagnetism gradually increases with increasing Mg content. According to the first-principles calculations, the non-monotonous Mg doping-dependent superconductivity, carrier density, and disorder strength may be explained as that the Mg atoms first fill the Ti vacancy sites and then substitute the Ti atoms. The calculations also reveal that the ferromagnetism is highly related to the spin polarization of 3d states of Ti atoms surrounding Mg atoms. The discovery of enhanced superconductivity with coexisting ferromagnetism in Ti1−xMgxO films provides an excellent platform to investigate the interaction between superconductivity and ferromagnetism.

We studied the origin of the enhancement of current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) effect by inserting a thin amorphous CoFeBTa (CFBT) underlayer below a Co2(Mn0.6Fe0.4)Ge (CMFG) Heusler alloy ferromagnetic (FM) layer. Large magnetoresistance ratio of ∼25% and resistance change-area product of ∼7.5 mΩ μm2 were obtained at room temperature by inserting a CFBT (1.2 nm) underlayer. X-ray diffraction (XRD) and transmission electron microscope analyses confirmed that the CMFG FM layer deposited on the CFBT underlayer was amorphous in the as-deposited state and crystallized to a B2-ordered polycrystalline film by annealing at 300 °C. The degree of B2 order (SB2) of the CMFG films was estimated by anomalous XRD using x-ray energies around the Co K-absorption edge. SB2 of the CMFG film deposited on the amorphous CFBT (1.2 nm) underlayer was ∼0.76, much larger than that of the CMFG film deposited on a crystalline CoFe underlayer (SB2 ∼0.47). First-principles calculations indicated that the spin polarization of the sp-conduction electrons in CMFG increases with increasing SB2, which accounts for the enhanced CPP-GMR effect in the pseudo spin-valve by inserting an amorphous CFBT underlayer.

Magnetoresistance (MR) devices fabricated with half-metallic Co-based Heusler alloys (Co2YZ) have a large MR ratio but are subject to substantial temperature degradation. Reduction of the spin polarization in the bulk electrode at finite temperatures is one possible reason for the reduction in the MR ratio. In this study, we investigated the temperature dependence of the spin polarization of Co2Fe(Ga0.5Ge0.5) (CFGG) using density-functional theory and the disordered local-moment method. We found that the reduction in the spin polarization at finite temperatures is smaller in CFGG compared to the well known Co2MnSi (CMS) half-metal, which is attributed to the higher Curie temperature of CFGG than CMS. On the other hand, it was also found that modulation of the Fermi level position in the half-metallic gap of CFGG within the rigid band model by electron and hole doping can improve the temperature dependence of the spin polarization. However, off-stoichiometric Co-rich and Fe-rich CFGG corresponding to electron and hole doping show a reduction in spin polarization compared to that of stoichiometric CFGG at 0 K. Therefore, we propose that modulation of the Fermi level position through varying the Y and Z site compositions of Co2YZ will be necessary to improve the temperature dependence of the spin polarization.

The spin polarization ( P ) of ferromagnetic Heusler alloys, Co 2 Fe(Ga x Ge 1− 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 Co 2 FeGe and Co 2 FeGa alloys are 0.58 and 0.59, respectively, the corresponding value for Co 2 Fe(Ge 0.5 Ga 0.25 ) is as high as 0.69. Co 2 Fe(Ge 0.5 Ga 0.5 ) alloy shows a strong tendency for L2 1 ordering and a high Curie temperature of 807 °C. Ab initio calculations indicate that the band structures of the Co 2 FeGe and Co 2 Fe(Ge 0.5 Ga 0.5 ) alloys with L2 1 or B2 structures are half-metallic. Thin films of the quaternary Co 2 Fe(Ge 0.5 Ga 0.5 ) alloy grown on MgO(0 0 1) substrates order to the L2 1 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 Co 2 Fe(Ge 0.5 Ga 0.5 ) alloy is promising as a spin polarized current source for spintronics devices.

Point contact Andreev reflection measurements of Co2MnSn1−xGax alloys showed that the spin polarization of L21 ordered quaternary Co2MnSn0.5Ga0.5 compound increased to 0.72 compared to 0.6 for Co2MnSn and Co2MnGa ternary alloys. The L21 Co2MnSn0.5Ga0.5 phase was found to be an intermetallic compound, in which the L21 structure was stable up to the melting temperature. The increase in spin polarization was attributed to the high degree of L21 order as well as an increase in the spin-up density of states near the Fermi level.

We show experimentally the effect of the crystal structure of a ferromagnetic (FM) electrode on tunneling magnetoresistance (TMR) by changing only the crystal structure of the bottom FM electrode in a magnetic tunnel junction (MTJ) and observing a significant TMR difference. Co 87 Fe 13 was selected as the bottom FM electrode because the difference in stability between its face-centered cubic (fcc) and body-centered cubic (bcc) structures is very small. This enables us to compare the TMR of MTJs comprising ferromagnetic layers at the same composition but with different crystal structures. We find a significant increase in the TMR when changing from an fcc-Co 87 Fe 13 to bcc-Co 87 Fe 13 bottom FM electrode. The structurally induced TMR enhancement is attributed to a higher s-electron spin polarization for the bcc structure, which was confirmed for bulk Co and Co 87.5 Fe 12.5 by ab initio calculations. These results unambiguously demonstrate the role of crystal structure and the associated electronic structure of FM electrodes in spin-dependent tunneling.

The longitudinal current in Si-doped gallium arsenide was spin-polarized using circularly polarized light. The spin current was detected by the extraordinary Hall effect. An enhancement of Hall conductivity with increasing moderately Si-doping was found, indicating that the introduction of dopants increases the electronic spin polarization. This finding may provide an opportunity for controlling and manipulating nonmagnetic semiconductors via electron spin for operating device applications. Band energy calculations using pseudopotentials confirm the influence of Si content and electron-phonon interaction on the behaviour of the spin current and hence on the spin-dependent Hall voltage.

The microstructures, magnetic properties and spin polarization of quaternary Co 2 Cr 1− x V x Al, Co 2 V 1− x Fe x Al and Co 2 Cr 1− x Fe x Al alloys were investigated. Phase separation into A2 and B2/L2 1 structure occurs in Co 2 CrAl and Co 2 Cr 0.6 Fe 0.4 Al, whereas Co 2 FeAl exhibits a single-phase B2 structure. The ordered L2 1 structure becomes more stable with increasing vanadium concentration ( x ⩾ 0.35). The saturation magnetization measured at 5 K for Co 2 Cr 1− x V x Al alloy changes from 1.4 to 2.0 μ B when x increases from 0.0 to 0.5 and then becomes 1.4 μ B for x = 1.0. This behavior can be attributed to the variation in the local magnetic moment of Co atoms. The saturation magnetization of Co 2 V 1− x Fe x Al and Co 2 Cr 1− x Fe x Al alloys increases with increasing Fe concentration. The spin polarization decreases from 0.62 to 0.56 with increasing x for Co 2 Cr 1− x Fe x Al alloy. Also, the spin polarization decreases with increasing x for Co 2 Fe 1− x V x Al and Co 2 Cr 1− x V x Al alloys. Possible reasons for the reduced spin polarization in these alloys are discussed.

The electronic structure and magnetic properties of Co/Al2O3/Co with O-terminated and Al-terminated interface models of different thicknesses are investigated by first-principles discrete variational method with the local-spin-density approximation. Our calculations results show that the magnetic moment of interface Co layer is enhanced for Al-terminated and weakened for Oterminated interface compared with that of bulk Co. For O-terminated interface models, spin polarization at Fermi energy of Co layer at interface exhibits negative and becomes positive for O layer at interface. In contrast, both Co and Al layers at interface possess negative SP for the Al-terminated interface models. We have also found that TMR ratio of Al-terminated interface models is much larger than that of O-terminated interface. In addition, the change of SP with the thickness of insulating layer is in a similar way as that of magnetic moment. 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.