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(a) Temporal MOKE profiles obtained for two different external fields H ext = 1000 and 250 Oe with pump fluencies of I 0, 3 I 0, and 4 I 0 (I 0 = 12 μJ/cm2). (b) A plot of precession amplitude at the first oscillation θ MOKE 1st under 250 and 1000 Oe, representing the angle between H eff and M at t ∼ 0 ps, vs. pump fluence I pump. The inset shows the temporal MOKE profile obtained for H ext = 250 Oe with 6 μJ/cm2. (c) Schematic diagram of laser-induced magnetization precession: (c-1) In equilibrium, magnetization, M , is parallel to the effective field H eff. (c-2) A laser pulse tips off H eff and M starts precessing about a new effective field, H eff’. (c-3) M keeps precessing about a slowly relaxing H eff’ with natural damping. H eff’ is assumed to relax back to its initial state H eff within the laser pulse interval of 13 nsec. (d) Plots of precession frequency Ω vs. I pump with various H ext.
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(a) Plots of precession frequency Ω as a function of external field H ext for two different pump fluences I pump = 12 (gray circles) and 36 (black circles) μJ/cm2. Solid lines are theoretical fits to the experimental data obtained with I pump = 12 (gray lines) and 36 (black lines) μJ/cm2. (b) A plot of dΩ/dI pump as function of H ext. Solid lines are theoretical fits to experimental data. Inset shows temperature dependence of H ani extracted from magnetization curves measured along a hard axis.
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Temporal MOKE profiles (closed symbols) obtained for two, sequential pump pulses, P1 and P2, under two different external magnetic fields, H ext = 1000 and 2000 Oe. Arrows represent graphically the time of arrival of the second pulse P2, whereas Δt is the time interval between P1 and P2. Profiles obtained for one pump pulse (P1 only, open symbols) are also presented for comparison. Inset shows MOKE hysteresis loop at T = 300 K measured with H ext to the surface.
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We have investigated thermal effects on the dynamics of laser-induced precession of magnetization in ferrimagnetic GdFe thin films under low-excitation conditions (6-60 μJ/cm2). An increase in quasi-equilibrium temperature by laser heating causes a shift in precession frequency, which is explained analytically by the alteration of the magnetic anisotropy field by 2.2 [Oe] at a pulse fluence of 1 μJ/cm2. We have also demonstrated coherent control of the precession amplitude using a sequence of two laser pulses, each with a fluence of 18 μJ/cm2, and point out the importance of low-power excitation for precise control of the dynamic states.
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