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()Ultrafast magnetization dynamics of nickel has been studied for different degrees of electronic excitation, using pump-probe second-harmonic generation with 150 fs/800 nm laser pulses of various fluences. Information about the electronic and magnetic response to laser irradiation is obtained from sums and differences of the SHG intensity for opposite magnetization directions. The classical M(T)-curve can be reproduced for delay times larger than the electron thermalization time of about 280 fs, even when electrons and lattice have not reached thermal equilibrium. Further we show that the transient magnetization reaches its minimum ≈ 50 fs before electron thermalization is completed.PACS numbers: 42.65. Ky , 75.40.Gb , 78.47.+p Ultrafast spin dynamics in ferromagnets is of great interest from both theoretical and experimental points of view. In particular, the short-time dynamics of magnetism in transition metals, with many excited electrons not at equilibrium with the lattice, is a new area of physics. Such studies are important for developing a theory of transient magnetization behavior in the subpicosecond range. It seems that the only experimental data which can guide theoretical analysis are the ones reported by Beaurepaire et al.[1] on time-resolved demagnetization of Ni induced by femtosecond laser pulses of 620 nm at one specific fluence. The authors utilized the magneto-optical Kerr effect to detect hysteresis loops for different time delays between pump and probe pulses. By comparing the time-dependent remanence with the equilibrium temperature dependence of magnetization, M (T ), they derived the time evolution of the spin temperature within the framework of the phenomenological three-temperature model [2]. Clearly, it is of great importance to confirm whether or not M (T ) can be used to describe the transient magnetic response to electron excitations in itinerant ferromagnets and whether there is a time delay between electron thermalization and magnetization changes.In this Letter we present time-resolved data on the transient magnetization measured by pump-probe second-harmonic generation (SHG). The great advantage of this technique is that it allows to simultaneously follow electron-temperature relaxation and transient magnetization, without further need for additional calibration measurements. This is a consequence of the even and odd contributions to the nonlinear susceptibility [3]. The measurements were carried out for a large variety of pump fluences leading to different initial electron temperatures. After equilibration of the electron bath, we find the transient magnetization to be governed by the electron temperature T e via the classical M (T )-curve [4]. However, we observe a strong deviation of the data 1

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Coherent spin dynamics in the THz domain coupled to a coherent phonon is observed in the time-resolved second harmonic response of the Gd(0001) ferromagnetic metal surface. An LO phonon of 2.9 THz is excited by a transient charge displacement at the surface caused by resonant absorption of a fs laser pulse in the exchange-split surface state. This lattice vibration modulates the interlayer distance inducing a coherent variation of the exchange interaction between spins in adjacent layers. The resulting magnetization dynamics is considered as optical magnon wave packets coupled to the phonon.

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