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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

Since the invention of the first magnetic memory disk in 1954, much effort has been put into enhancing the speed, bit density and reliability of magnetic memory devices. In the case of magnetic random access memory (MRAM) devices, fast coherent magnetization rotation by precession of the entire memory cell is desired, because reversal by domain-wall motion is much too slow. In principle, the fundamental limit of the switching speed via precession is given by half of the precession period. However, under-critically damped systems exhibit severe ringing and simulations show that, as a consequence, undesired back-switching of magnetic elements of an MRAM can easily be initiated by subsequent write pulses, threatening data integrity. We present a method to reverse the magnetization in under-critically damped systems by coherent rotation of the magnetization while avoiding any ringing. This is achieved by applying specifically shaped magnetic field pulses that match the intrinsic properties of the magnetic elements. We demonstrate, by probing all three magnetization components, that reliable precessional reversal in lithographically structured micrometre-sized elliptical permalloy elements is possible at switching times of about 200 ps, which is ten times faster than the natural damping time constant.

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