Yi Ji

Spin injection, diffusion, and detection in lateral spin-valves

We demonstrate the injection of spin-polarized electrons into paramagnetic Au nanowires by driving an electric current from a ferromagnetic permalloy (Py) electrode. The nonequilibrium spin accumulation in Au results in a difference between the chemical potentials for spin-up and spin-down electrons that is detected as a field-dependent voltage signal using a second Py electrode. The magnitude of the voltage contrast (>10%) and its coincidence with the magnetic switching of the Py electrodes attest to the spin-sensitive origin of the signals. By increasing the separation of the Py injector and detector, we observe an exponential decay of the spin signals. The measurements yield a spin-diffusion length of 63±15nm and an injected spin polarization of 3% in Au at 10 K.

Current-induced spin-wave excitations in a single ferromagnetic layer.

Evidence for a current-induced spin-transfer torque effect has been investigated in a series of point contacts to single ferromagnetic layers. At specific current densities, abrupt resistance changes, similar to those attributed to current-induced spin-wave excitations in multilayers, have been observed for one current polarity. The critical current for these resistance changes depends linearly on the external field applied perpendicular to the layer. The observed effect is interpreted as a current-driven heterogeneous instability in an otherwise uniform ferromagnetic layer.

Enhanced spin injection polarization in Co∕Cu∕Co nonlocal lateral spin valves

Nonlocal spin injection is demonstrated in Co∕Cu∕Co lateral spin valves. Measurement of a series of structures with varying Cu length between the Co electrodes permits a direct determination of both the Cu spin diffusion length λs and the spin injection polarization P at the Co∕Cu interface. The value of λs is 200±20nm at 10K and ⩾110nm at 300K, which is shorter than previously reported, due to the possible presence of impurities in the Cu. However, the value of P is >7%, which is improved compared with previous reports of 2%, which is attributed to a higher interfacial quality.

High spin injection polarization at an elevated dc bias in tunnel-junction-based lateral spin valves

Submicron metallic lateral spin valves are fabricated with AlOx tunnel junctions as spin injection and detection barriers. The spin polarization is estimated to be ∼20%, determined by both Hanle effect and variations of device dimensions. The polarization is maintained at a large dc injection current density >2×106 A/cm2. Both the spin polarization and spin diffusion length are weakly temperature dependent.

Switching by point-contact spin injection in a continuous film

Spin-polarized currents injected through a point-contact into a continuous Co/Cu/Co trilayer film can reversibly switch the magnetization of small magnetic bits in the top Co layer. The magnetic states written depend on the polarity of the injection currents, and remain stable at room temperature. The reversible writing can be achieved for a wide range of contact resistances with a well-defined voltage for the reversal.

Non-local spin injection in lateral spin valves

Injection and detection of non-local spin accumulation have been demonstrated in a series of nanoscale metallic lateral spin valves, fabricated by electron beam lithography. A distinct feature of non-local spin valves is that a pure spin current without charge flow can be generated. The measured spin signal is only sensitive to the relative orientations of the detector spin and the spin accumulation, and exhibits little charge background voltage. The injection polarizations of the interfaces and the spin diffusion lengths of the non-magnetic components have been determined by measuring the spin signals from a set of similar spin valves with varying injector and detector separations. In Py/Au/Py lateral spin valves, where Py is permalloy, the injection polarization across the Py/Au interface is 3%, and the spin diffusion length of Au is 63 ± 15 nm at 10 K. In Co/Cu/Co lateral spin valves, the injection polarization across the Co/Cu interface is increased to over 7%, due to better interfacial quality compared with Py/Au. The spin diffusion length of Cu is 200 ± 20 nm at 10 K, and > 110 nm at 300 K.

The origin of high surface spin-flip rate in metallic nonlocal spin valves

The origin of high surface spin-flip rate in metallic nonlocal spin valves has been explored by tracking the evolution of the spin signals during the natural oxidation of the devices in air. The temperature dependence of spin signals evolves from a non-monotonic dependence with a maximum at ∼60 K for as-fabricated devices to a monotonic increase with decreasing temperatures for fully oxidized devices. The experimental results suggest that the high surface spin-flip rate originates from magnetic impurities near the side surfaces of the nonmagnetic channel and can be reduced by oxidation.

Properties of epitaxial chromium dioxide films grown by chemical vapor deposition using a liquid precursor

Epitaxial chromium dioxide (CrO2) films have been grown using chemical vapor deposition on (100) TiO2 substrate with chromyl chloride (CrO2Cl2) as a liquid precursor. The films are extremely smooth (rms roughness less than 4.6 A for a 1000-A-thick film) and have the largest spin polarization (P=98.4%) yet observed, as determined by point contact Andreev reflection. Magnetization switching properties of the films are close to those of a single-domain particle. Preliminary results on the in situ growth of exchange-biased CrO2/Cr2O3 multilayers are also reported. Although a bias field is observed, it is much smaller in comparison with the coercivity of the CrO2 film.

Anisotropy in magnetic and mechanical properties in textured Hiperco® FeCoV alloys

Hiperco® FeCoV alloys are soft magnetic materials suitable for applications at high temperatures and under high stresses. We have studied the evolution of the rolling texture and its effects on the mechanical and magnetic properties after annealing the Hiperco® sheets at high temperatures. X-ray pole-figure measurements reveal that the main texture of the as-rolled HA50HS and HA50 sheets is {001}〈110〉. This rolling texture causes the mechanical and magnetic properties to exhibit a uniaxial anisotropy, which is reduced by annealing.

Reduction of spin-flip scattering in metallic nonlocal spin valves

Cobalt-copper nonlocal spin valves are fabricated by shadow evaporation through nanoscale masks. The thickness of Co electrodes is varied and the spin diffusion length of the Cu channel is determined. Short spin diffusion lengths are found in devices with thick (>20 nm) Co layers. Co impurities are introduced into the Cu channel in the shadow evaporation process during the fabrication, and the impurities cause spin-flip scattering. The amount of Co impurities can be reduced by decreasing the thicknesses of Co electrodes. Spin diffusion lengths of ∼400 nm at 295 K and ∼800 nm at 4.2 K are measured in devices with thin (<10 nm) Co layers.