Ultrafast Spin Dynamics in Ferromagnetic Nickel

Abstract

The relaxation processes of electrons and spins systems following the absorption of femtosecond optical pulses in ferromagnetic nickel have been studied using optical and magneto-optical pump-probe techniques. The magnetization of the film drops rapidly during the first picosecond, but different electron and spin dynamics are observed for delays in the range 0-5 ps. The experimental results are adequately described by a model including three interacting reservoirs (electron, spin, and lattice).[S0031-9007(96)00167-6] PACS numbers: 75.40. Gb, 78.20.Ls, 78.47.+p Ultrafast optical spectroscopy is an ideal technique to investigate the electronic relaxation processes in metallic materials [1][2][3]. A femtosecond optical pulse can induce a nascent nonequilibrium electron gas which subsequently thermalizes to a Fermi distribution. This thermalization, which takes place within about 500 fs as measured for instance in noble metals, is due to electron-electron interactions [4][5][6]. In the next step, the hot electron gas relaxes its energy to the lattice due to electron-phonon interactions, a process which occurs within 1-10 ps depending on the incident laser intensity. During this time, the temperature exchange between the electron bath (temperature T e ) and the lattice (temperature T l ) can be investigated. From such measurements, one can deduce the characteristic times of the microscopic interactions which govern the basic metallic properties like electron transport and superconductivity [7].In spite of the large amount of work performed in this field, little attention has been paid to magnetic effects occurring in the femtosecond time scale. An important challenge is to know whether the initial hot electron distribution can induce a spin dynamics associated with a spin temperature T s different from T e and T l . This would lead to an ultrafast demagnetization, a relevant effect in regards to applications in magneto-optic devices. From a conceptual point of view, such a fast magnetization change is not trivial since the optical transitions preserve the electronic spin. However, one can expect that, during the transient hot electron regime, spin populations are modified due to spin dependent electron scattering. The characteristic time of this process has not been investigated with a femtosecond temporal resolution. Previous experiments performed with picosecond pulses on Ni [8] or Fe [9] have shown no demagnetization effect up to the melting point of the samples. The authors concluded that the spin-lattice relaxation times of these metals are larger than 30 ps. A more sophisticated pump-probe experiment, using 10 ns pump and 60 ps probe pulses, made on Gd films could deduce a spinlattice relaxation time of 100 6 80 ps [10,11]. All these experiments were performed on a time scale where electrons and lattice temperatures are in equilibrium and therefore could not resolve separately the effects of electron-spin and spin-lattice relaxation mechanisms on the demagnetization process.The aim of this paper is to study b...

Ultrafast magnetization enhancement in metallic multilayers driven by superdiffusive spin current

“…This first observation of an ultrafast magnetization enhancement in the Fe layer can be explained by superdiffusive spin transport 12,13 (see also Supplementary Discussion), taking place on timescales comparable to the demagnetization processes explored in earlier works [1][2][3]5 . The mechanism we propose for enhancement of the magnetization is based on filling of majority spin states above the Fermi energy in the Fe layer by majority spins coming from Ni.…”

“…Spin dynamics is in-itself of fundamental interest for understanding spin-photon-charge-phonon interactions and their complex interplay on femtosecond to picosecond timescales. The potential to rapidly manipulate spins was first demonstrated in a pioneering experiment 1 on Ni that observed that the magnetization of 3d ferromagnets can be optically quenched within 50-300 fs after excitation by a femtosecond laser pulse 2,3 . This ultrafast demagnetization is then followed by a slower magnetization recovery on picosecond timescales.…”

Laser-induced ultrafast demagnetization in the presence of a nanoscale magnetic domain network

“…2c, radial intensity profiles are plotted for different pump-probe time delays. The relation between real space distances, r x y = + 2 2 , and wave vector transfer, q, is given by equation (1). A comparison of these curves reveals that peak position and shape remain unchanged.…”

“…It reproduces the general behaviour previously observed for transition metal-based ferromagnetic materials 2 , namely, a very fast initial demagnetization on the timescale of a few hundred femtoseconds followed by a slow partial recovery. This behaviour is described by the phenomenological three temperature model proposed for uniformly magnetized films 1 . The electronic system is heated by the optical excitation almost instantaneously.…”

“…T he phenomenon of ultrafast demagnetization 1 , first discovered in 1996, resulting from excitation by an intense femtosecond infrared laser pulse, has been extensively investigated, yet its underlying mechanism remains poorly understood 2 . Additionally, it has attracted considerable interest as a possible path to magnetization control on a femtosecond timescale 3 .…”