Cheong-Gu Cho

Control of domain wall pinning in ferromagnetic nanowires by magnetic stray fields

We have found that the depinning field of domain walls (DWs) in permalloy (Ni(81)Fe(19)) nanowires can be experimentally controlled by interactions between magnetic stray fields and artificial constrictions. A pinning geometry that consists of a notch and a nanobar is considered, where a DW traveling in the nanowire is pinned by the notch with a nanobar vertical to it. We have found that the direction of magnetization of the nanobar affects the shape and local energy minimum of the potential landscape experienced by the DW; therefore, the pinning strength strongly depends on the interaction of the magnetic stray field from the nanobar with the external pinning force of the notch. The mechanism of this pinning behavior is applied for the instant and flexible control of the pinning strength with respect to various DW motions in DW-mediated magnetic memory devices.

Maximizing domain-wall speed via magnetic anisotropy adjustment in Pt/Co/Pt films

We report an experimental observation that indicates that a direct relation exists between the speed of the magnetic domain-wall (DW) motion and the magnitude of the perpendicular magnetic anisotropy (PMA) in Pt/Co/Pt films. It is found that by changing the thicknesses of the nonmagnetic Pt layers, the PMA magnitude can be varied significantly and the field-driven DW speed can also be modified by a factor of up to 50 under the same magnetic field. Interestingly, the DW speed exhibits a clear scaling behavior with respect to the PMA magnitude. A theory based on the DW creep criticality successfully explains the observed scaling exponent between the DW speed and the PMA magnitude. The presented results offer a method of maximizing the DW speed in DW-mediated nanodevices without altering the thickness of the magnetic Co layer.

Simultaneous excitation of two different spinwave modes by optical ultrafast demagnetization

We report here an experimental technique to generate spinwaves using femtosecond laser pulses. The femtosecond laser pulses induce ultrafast demagnetization, which causes the propagation of the spinwaves from the laser spot. The observed spinwaves exhibit an anisotropic behavior by showing both the forward and backward modes depending on the propagation direction with respect to the in-plane magnetization direction, as confirmed by micromagnetic simulations. The forward mode is found to propagate over a few micrometers with small dissipation, providing a possible spinwave source.

Domain Patterns and Magnetization Reversal Behaviors in Oxide/Co/Pt Films

The domain patterns and magnetization reversal behaviors in oxide/Co/Pt films are examined with varying the thickness of Co layer from 0.7 nm to 2.0 nm. The films are grown on Al2O3, MgO and SiO2 layers. All the films exhibit strong perpendicular magnetic anisotropy, but the anisotropy decreases with increasing Co layer thickness for all the oxide layers. The decrement rate is different for different oxide layers. The domain patterns show the transition between the wall-motion and dendrite-growth dominant behaviors, and the critical thickness is estimated to be about 1.0 nm irrespective of the oxide layers. We summarize the magnetic properties of oxide/Co/Pt films with respect to the Co-thickness and various oxide layers.