S. X. Huang

Spin polarization of amorphous CoFeB determined by point-contact Andreev reflection

Point-contact Andreev reflection measurements reveal that amorphous CoxFe80−xB20 (x=20, 40, and 60) alloys possess spin polarization of as much as 65%, much higher than the values of 43%–45% for Co and Fe. This accounts for the high magnetoresistance values in magnetic tunnel junctions incorporating amorphous CoFeB as the ferromagnetic electrodes. The crystallization of the amorphous alloys substantially reduces the spin polarization.

Control of Tetrahedral Coordination and Superconductivity in FeSe0.5Te0.5 Thin Films

We demonstrate a close relationship between superconductivity and the dimensions of the Fe-Se(Te) tetrahedron in FeSe0.5Te0.5. This is done by exploiting thin film epitaxy, which provides controlled biaxial stress, both compressive and tensile, to distort the tetrahedron. The Se/Te height within the tetrahedron is found to be of crucial importance to superconductivity, in agreement with the scenario that (π, π) spin fluctuations promote superconductivity in Fe superconductors.

Pressure effects on strained FeSe0.5Te0.5 thin films

The pressure effect on the resistivity and superconducting Tc of prestrained thin films of the iron chalcogenide superconductor FeSe0.5Te0.5 is studied. Films with different anion heights above the Fe layer showing different values of ambient pressure Tc’s are compressed up to a pressure of 1.7 GPa. All films exhibit a significant increase of Tc with pressure. The results cannot solely be explained by a pressure-induced decrease of the anion height but other parameters have to be considered to explain the data for all films.

Magnetotransport properties of polycrystalline and epitaxial chromium dioxide nanowires

Temperature dependent magnetotransport measurements were performed on polycrystalline and epitaxial chromium dioxide (CrO2) nanowires fabricated using the selective-area growth technique. Polycrystalline nanowires showed a negative temperature coefficient of resistivity at low temperatures because of strong grain boundary scattering. The magnetoresistance (MR) value exhibited a width dependence, reaching a maximum of 20% for a 150nm wide wire. In contrast, the MR response of single crystal CrO2 wires was mainly determined by magnetocrystalline and shape anisotropy.

Unusual magnetoresistance in cubic B20 Fe0.85Co0.15Si chiral magnets

The B20 chiral magnets with broken inversion symmetry and C4 rotation symmetry have attracted much attention. The broken inversion symmetry leads to the Dzyaloshinskii–Moriya that gives rise to the helical and Skyrmion states.Wereport the unusual magnetoresistance (MR) of B20 chiral magnet Fe0.85Co0.15Si that directly reveals the broken C4 rotation symmetry and shows the anisotropic scattering by Skyrmions with respect to the current directions. The intimacy between unusual MR and broken symmetry is well confirmed by theoretically studying an effective Hamiltonian with spin–orbit coupling. In conclusion, the unusual MR serves as a transport signature for the Skyrmion phase.

Spin Transfer Torques by Point-Contact Spin injection

Spin-transfer torques (STT) provides a new mechanism to alter the magnetic configurations in magnetic heterostructures, a feat previously only achieved by an external magnetic field. A current flowing perpendicular through a magnetic noncollinear spin structure can induce torques on the magnetization, depending on the polarity of the current. This is because an electron carries angular momentum, or spin, part of which can be transferred to the magnetic layer as a torque. A spin-polarized current of a substantial current density (e.g., 108 A/cm2) is required to observe the effect of the spin transfer torques. Consequently, switching by spin-polarized currents is often realized in small structures with sub-micron cross sections made by nanolithography. Here we demonstrate spin transfer torque effects using point-contact spin injection involving no lithography. In a continuous Co/Cu/Co trilayer, we have observed hysteretic reversal of sub-100 nm magnetic elements by spin injection through a metal tip both at low temperature and at room temperature. A small magnetic domain underneath the tip in the top Co layer can be manipulated to align parallel or anti-parallel to the bottom Co layer with a unique bias voltage. In an exchange-biased single ferromagnetic layer, we have observed a new form of STT effect which is the inverse effect of domain wall magnetoresistance effect rather than giant magnetoresistance effect. We further show that in granular solids, the STT effect that can be exploited to induce a large spin disorder when combined with a large magnetic field. As a result, we have obtained a spectacular MR effect in excess of 400%, the largest ever reported in any metallic systems.