We report direct observation of current-driven magnetic domain wall (DW) displacement by using a well-defined single DW in a microfabricated magnetic wire with submicron width. Magnetic force microscopy visualizes that a single DW introduced in a wire is displaced back and forth by positive and negative pulsed current, respectively. The direct observation gives quantitative information on the DW displacement as a function of the intensity and the duration of the pulsed current. The result is discussed in terms of the spin-transfer mechanism.

It was found that high current density needed for the current-driven domain wall motion results in the Joule heating of the sample. The sample temperature, when the current-driven domain wall motion occurred, was estimated by measuring the sample resistance during the application of a pulsed current. The sample temperature was 750 K for the threshold current density of $6.7\times {10}^{11}\mathrm{A}∕{\mathrm{m}}^2$ in a 10-nm-thick ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ wire with a width of 240 nm on thermally oxidized silicon substrate. The temperature was raised to 830 K for the current density of $7.5\times {10}^{11}\mathrm{A}∕{\mathrm{m}}^2$ , which is very close to the Curie temperature of bulk ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$ . When the current density exceeded $7.5\times {10}^{11}\mathrm{A}∕{\mathrm{m}}^2$ , an appearance of a multidomain structure in the wire was observed by magnetic force microscopy, suggesting that the sample temperature exceeded the Curie temperature.

The authors report the rectification of a constant wave radio frequency (rf) current by using a single-layer magnetic nanowire; a direct-current voltage is resonantly generated when the rf current flows through the nanowire. The mechanism of the rectification is discussed in terms of the spin-torque diode effect reported for magnetic tunnel junction devices, and the rectification is shown to be directly attributable to resonant spin wave excitation by the rf current.

The broadband ferromagnetic resonance (FMR) measurement has been carried out by using a rectifying effect in two sets of ${\text{Ni}}_{81}{\text{Fe}}_{19}$ wires. One wire is deposited on the middle strip line of the coplanar waveguide (CPW) and another is deposited between two strip lines of the CPW, measuring the FMR induced by in-plane and out-of-plane magnetic fields, respectively. The FMR frequency is defined by detecting the magnetoresistance oscillation due to the magnetization dynamics induced by a radio frequency (rf) field. The magnetic-field dependence of the resonance frequency and the rectification spectrum are analytically interpreted based on our uniform magnetization precession model. These findings reveal that two distinct rectifying signals are anticipated by a rf field and a rf current, which can easily be controlled by engineering the ferromagnetic wire shape and the external field orientation. These fundamental understandings are crucial for future rf device applications in spintronics.

We investigated the aspect ratio (thickness/width) dependence of the threshold current density required for the current-driven domain wall (DW) motion for the Ni81Fe19 nanowires. It has been shown theoretically that the threshold current density is proportional to the product of the hard-axis magnetic anisotropy Kperp and the DW width lamda. (Phys. Rev. Lett. 92, 086601 (2004).) We show experimentally that Kperp can be controlled by the magnetic shape anisotropy in the case of the Ni81Fe19 nanowires, and that the threshold current density increases with an increase of Kperp*l. We succeeded to reduce the threshold current density by half by the shape control.Comment: 16 pages, 4 figure

Spin-polarized radio frequency (RF) currents and RF-Oersted fields resonantly excite a magnetic vortex core confined in a micron-scale soft magnetic disk. In this study, we measured the rectifying voltage spectra caused by the anisotropic magnetoresistance oscillation due to the gyration of the vortex with different polarity and chirality. The measured spectra are presented such that we can determine the vortex properties and strength of the spin torques and Oersted field accurately and directly through analytical calculation.