In recent years, Racetrack memory¹ has been actively studied as a candidate of the future magnetic memory. Many papers on magnetic wire memories using ferromagnetic materials with large magnetization have been reported. Recently, it has been found that this current density can be reduced by using a ferrimagnetic material with small magnetization. However, the current density reduction of current-induced domain wall (DW) motion and improvement of the DW velocity remain as big issues for practical use. In this study, laser annealing (laser irradiation) effect on the ferrimagnetic wire is investigated to reduce further the current density.
A magnetic wire of 1 μm width and 70 μm in length is prepared by using electron beam lithography and magnetron sputtering. The sample structure is Pt (5 nm) / Tb₂₇Co₇₃ (6 nm) / SiN (10 nm) on a Si substrate with a thermal oxide film (350 nm). It is placed in the field of view of polarizing microscope with an objective lens NA of 0.5, and a blue laser is irradiated to the TbCo wire through the objective lens. The spot size is approximately 1 μm in diameter which is almost same as the wire width. Laser annealing is scanned along the TbCo wire and the speed is 7 μm/s. Laser annealing can slightly change the DW energy of the TbCo wire.
Fig. 1 shows DW velocity versus current density when the DW in the magnetic TbCo wire is driven by applying 3 ns pulse currents to the wire. Black squares and red circles are the symbols before and after laser annealing with the laser power of 3.1 mW. As shown in this figure, It was found that by laser annealing to the TbCo wire, the current density required for driving the DW was reduced by 20 %, and the DW velocity was doubled. Fig. 2 shows the mobility of the domain wall before and after laser annealing. Mobility is DW velocity divided by current density. After laser annealing, mobility is about 1.8 times higher than before. These may be due to the slight decrease in the DW energy caused by laser annealing.
This work is partially supported by Grant-in-Aid for Scientific Research (20H02185 and 21K18735).
References
1 S. S. P. Parkin, M. Hayashi and L. Thomas, Science, Vol. 320, p.190-194 (2008).
2 K. S. Ryu, L. Thomas and S.H. Yang, Nat. Nanotechnol., Vol. 8, p.527-533 (2013).
3 S. H. Yang, K. S. Ryu and S. Parkin, Nat. Nanotechnol., Vol. 10, p.221-226 (2015).

DW velocity versus current density in TbCo wire.

Comparison of the DW mobility before and after laser annealing with other reports².