Control of Domain Wall Motion in Ferromagnetic Nanowire with Perpendicular Magnetic Anisotorpy

Abstract

As mechanical limitation of present electric memories being mentioned, magnetic devices with nano-structure have been expected to overcome their limitation. To make next generation magnetic memories, we should know in physical theory how magnetic domain walls (DWs) in nanowire geometry can be controlled. Especially, ferromagnetic nanowire in perpendicular magnetic anisotropy (PMA) has more much benefits than in in-plain anisotropy; like small domain size, simple configurations and narrow width of DWs. But there have been fewer studies in PMA materials because of difficulty making PMA samples and domination of disorders for DW motion in PMA. In this paper, we observed DW motion in well-made PMA ferromagnetic sample with a scanning magneto-optical Kerr-effect (MOKE) microscope, changing applied magnetic field and current. Using the microscope, we made DWs in specific directions with thermal energy of laser or Oersted field of writing electrode. Also, DWs were detected with scanning magnetization in 50-nm spatial radiation, and investigated arrival times at any positions with 1-ms time-resolved detection in 500-nm spatial radiation. At first, we measured field-driven DW depinning motions in Pt/Co90Fe10/PT nanowire having notches with several different widths, to know efficiency of the notch for the motions in PMA material. The sample film showed clear circular expansion with field, which means having only few disorders, and strong PMA hysteresis loops. In Néel-Brown interpretation, we found out that depinning field and activation volume has linear dependence with inverse of notch width. This result shows good efficiency of notch geometry to control the DW depinning. Secondly, we tested current-induced DW motion with making and moving several DWs simultaneously in Pt/Co/Pt nanowire, PMA material. We repeated Oersted field writing and propagated DWs with injecting current pulses through the nanowire, which could make any four domains in the direction specified on 15-μm nanowire. When we propagated whole DWs with low current pulses 9.8×〖10〗^6 A/cm^2, they moved in the positive current direction and same velocities, independently with DW direction. From these results, we could realize 4-bit shift register. Next, we found out a universal class between field and current dependence on DW velocity in same Pt/Co/Pt nanowire. From contour map of velocity as function of field and current density, effective field had nonlinear dependence with current density. Especially, the quadratic term of current density, which means adiabatic spin-transfer-torque (STT), was not negligible in contrast to previous results in magnetic semiconductor (Ga,Mn)As. It means we found out another universality class for field and current, which could help understanding how current and field effect on DW motion. At last, we investigated how field and current effect on DW depinning as extension of former study. MgO/Co/Pt sample which we used had some strong defects, which could suppose as single energy barrier problem. The energy barrier expected from depinning characteristic time could be expressed as contour map with field and current. The map was similar to DW velocity map, showing quadratic dependence of current density. Comparing to previous studies which denied adiabatic STT effect, the result had much smaller nonadiabatic coefficient. If ignoring adiabatic effect, the coefficient in our result took similar value with others, which means adiabatic effect in current-induced DW depinning is not negligible opposite to previous studies. Our finding should provide physical understanding of DW dynamics in thermally activated regimes and DW based devices.