Sang-Jun Yun

Electric Control of Multiple Domain Walls in Pt/Co/Pt Nanotracks with Perpendicular Magnetic Anisotropy

The electric control of the motion of multiple domain walls (DWs) is demonstrated by using the Pt/Co/Pt nanotracks with perpendicular magnetic anisotropy. Because of the weak microstructural disorders with a small DW propagation field (approximately a few mT), a purely current-driven DW motion is achieved in the creep regime at current densities less than 107 A/cm2 at room temperature. It is confirmed by using a scanning magneto-optical Kerr effect microscope that several DWs are simultaneously and identically displaced by the same distance in the same direction. Utilizing such DW motion, we succeeded in realizing the writing and transferring of random bits in a device prototype of 4-bit shift registers.

Optical Measurement of Magnetic Anisotropy Field in Nanostructured ferromagnetic Thin Films

(Received 15 January 2015, Received in final form 10 February 2015, Accepted 11 February 2015)The magnetic anisotropy field plays an important role in spin-orbit-torque-induced magnetization dynamicswith electric current injection. Here, we propose a magnetometric technique to measure the magneticanisotropy field in nanostructured ferromagnetic thin films. This technique utilizes a magneto-optical Kerreffect microscope equipped with two-axis electromagnets. By measuring the out-of-plane hysteresis loops andthen analyzing their saturated magnetization with respect to the in-plane magnetic field, the magneticanisotropy field is uniquely quantified within the context of the Stoner-Wohlfarth theory. The present techniquecan be applied to small nanostructures, enabling in-situ determination of the magnetic anisotropy field ofnanodevices.Keywords : perpendicular magnetic anisotropy field, magneto optical Kerr effect, hysteresis loop, Stoner-Wohlfarthmodel

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.

Novel Method for Reducing Operation Current in Magnetic Domain-Wall Motion by the Use of Curved Wedge Nanowire

We propose a method to reduce the operation current of domain-wall (DW) motion in ferromagnetic nanowires with perpendicular magnetic anisotropy. It is based on the phenomena that a DW placed in wedged nanowire geometry moves spontaneously without any external magnetic field by consumption of the DW energy. Based on energy consideration, we found that there exists a geometry-induced effective field, which depends solely on the radius of circular arc intersecting normal to both the nanowire edges. Geometry for a constant effective field is analytically given with a curved wedge shape, which keeps the radius unchanged irrespective of the DW position. The validity of the analytic prediction is confirmed by a micromagnetic simulation.

Optical measurement and analysis technique for spin-orbit torques

Department of Physics and Astronomy, Seoul National University, Seoul 151-747, Korea(Received 15 January 2015, Received in final form 10 February 2015, Accepted 11 February 2015)We demonstrate here an optical measurement technique to quantify the spin-orbit torques. The magnetization dynamics induced bythe spin-orbit torques with a sinusoidal current injection is measured by use of polar magneto-optical Kerr effect. The measured signalis then analyzed based on the Landau-Lifhshitz-Gilbert equation with consideration of the spin-orbit torques. The present measurementtechnique is applied to Pd/Co/Pt films and then, the longitudinal and transverse components of the spin-orbit torques are successfullyquantified. The present optical technique provides an alternative way to quantify the spin-orbit torques.Keywords : spin-orbit torque, optical measurement, magneto-optical Kerr effect

Precessional Motion of Ferromagnetic Pt/Co/Pt Thin Film with Perpendicular Magnetic Anisotropy

We developed a time-resolved magneto-optical Kerr effect microscope system to investigate ultrafast magnetization dynamics. Based on the pump-probe method, 0.1-ps time resolution was achieved by use of a fs Ti:Sapphire laser. The magnetization dynamics was then measured on Pt/Co/Pt thin films with various Co thicknesses. All the samples exhibited ultrafast demagnetization within a few ps by direct heating of pump laser. Some thicker samples showed precessional motion of magnetization, from which the Gilbert damping constant was determined based on the Landau-Lifshitz-Gilbert equation.