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Magnetization manipulation induced by spin current and ultrafast laser

Abstract : Magnetization manipulation is one of the most actively researched topics in the field of spintronics. Different ways of manipulation can trigger magnetization dynamics on different time scales. Among these dynamics, magnetization precession and ultrafast demagnetization have attracted substantial interests. The frequency of magnetization precession is normally in the GHz range corresponding to a period of hundreds of ps, which is the basic mechanism of spin torque nano-oscillators (STNO), a new type of microwave devices which show advantages over conventional voltage-controlled oscillator (VCO) in terms of size, energy consumption and tunable frequency. Ultrafast demagnetization was first observed in Ni which takes places in hundreds of femtoseconds. Triggered by this, All-Optical Switching (AOS) was then demonstrated which is much faster than any torque induced switching, promising for application in the high-speed magnetic memory. Although many studies on these two phenomena have been reported, several issues need to be addressed before they move toward application. STNOs are supposed to be used for amplitude shift keying (ASK) or frequency shift keying (FSK), but the optimal conditions for these two types of microwave modulation are still not well explored. As for AOS, the influence of the laser parameters such as fluence and pulse duration and the material properties such as the composition and the thickness has not been systematically investigated. In this thesis, these two types of magnetization manipulation are studied in detail. Concerning magnetization precession, we demonstrate that a stronger magnetic field allows a wider frequency tuning range while a smaller magnetic field results in a wider amplitude tuning range. Thus, these two scenarios are applicable to FSK and ASK, respectively, providing guidelines for STNO in microwave modulation. In the second study, we demonstrate that AOS depends strongly on pulse characteristic. This was shown by building a magnetization state diagram for GdFeCo and Co/Pt which are two typical materials showing All-Optical Helicity-Independent Switching (AO-HIS) and All-Optical Helicity-Dependent Switching (AO-HDS), respectively. These results allow a better understanding of the fundamental mechanism behind laser-induced magnetization dynamics.
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Submitted on : Thursday, October 14, 2021 - 4:43:07 PM
Last modification on : Saturday, October 16, 2021 - 11:20:13 AM


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Jiaqi Wei. Magnetization manipulation induced by spin current and ultrafast laser. Physics [physics]. Université de Lorraine; Beihang university (Pékin), 2021. English. ⟨NNT : 2021LORR0121⟩. ⟨tel-03378777⟩



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