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内容記述 |
Manipulation of magnetization is one of the most central topics in the field of spintronics because of its essentiality from both fundamental and technological viewpoints. Among the various ways of magnetization manipulation, all-optical switching (AOS) which involves manipulating magnetization using laser pulse with the absence of magnetic field is attracting an increasing interest, due to its ultrafast switching process down to picosecond scale as well as low energy consumption which are desirable to future logical and memory devices. In this study, a new type of artificial ferrimagnetic multilayers contains a CoFeB layer which is popularly used in MTJs was developed for the purpose of AOS study. The samples investigated in this study have a stacking structure of Ta(3 nm)/Pt(3 nm)/Co(0.8 nm)/Gd(x nm)/CoFeB(y nm)/MgO(2 nm)/Ta(5 nm), which were deposited on thermally oxidized Si substrate at room temperature by magnetron sputtering. The ferrimagnetic coupling in Co/Gd/CoFeB tirlayer is confirmed by element-specified hysteresis loop measurement using XMCD. Hysteresis loops suggest a well-defined PMA is established in our sample, and the entirely overlapping feature of the hysteresis loops suggest Co, Gd and CoFeB layers are antiferromagnetically exchange-coupled to each other. The AOS experiment was performed by using a home-made femtosecond laser integrated magneto-optic Kerr effect (MOKE) microscope. A linearly polarized laser pulse (wavelength: 1033 nm; width: 253 fs) was irradiated to the sample surface at normal incidence. MOKE images were taken before and after the laser irradiation, and then the differential images were processed to check the laser irradiation induced magnetization change. For pulse energy density around 5.1 mJ/cm2~7.8 mJ/cm2, deterministic AOS was observed. The reversal area increases with the pulse energy which can be interpreted by the spatial Gaussian shape of the laser pulse. Below this energy range, magnetization reversal was found to be not sufficient. Above this energy range, complex AOS behaviors involve deterministic AOS at the out-edge and random multidomain formation at the center region was observed. Through a systematic AOS study on a double-thickness-wedge sample, we found rather than the total magnetization, but the individual magnetization of Gd and CoFeB layers play a critical role in determining the AOS in Co/Gd/CoFeB, which can be well interpreted in terms of a purely thermal toggle process. This newly developed CoFeB-based artificial ferrimagnet will provide a new building block for developing MTJs with both ultrafast optically switching and large magnetoresistance. |
