Ultrafast Spin Dynamics of Ferromagnetic Thin Films Observed by fs Spin-Resolved Two-Photon Photoemission

Abstract

Using a "pump and probe" technique, the time scale of spin relaxation effects in ferromagnetic thin films is investigated. Samples are excited by a 3 eV "pump" laser pulse ͑t ഠ 170 fs͒, and electrons are photoemitted by a 6 eV "probe" pulse, after delays ranging up to 1 ns. The spin polarization of the photoelectrons is measured using a Mott detector. The time dependence of the spin polarization of thin Ni films on Ag is explained in a local magnetic moment picture with two excitation processes. Stoner excitations are responsible for the reduction of the observed spin polarization on a time scale of 1 ps and phonon-magnon scattering leads to the loss of long range magnetic order on a time scale of 500 ps. [S0031-9007(97)04890-4] The interactions between quasiparticles like electronelectron, electron-phonon, and electron-magnon scattering determine the macroscopic properties of matter at finite temperatures, and the dynamical response to external excitations; also properties like superconductivity and giant magnetoresistance are governed by them. Experimentally these interactions are reflected by the lifetime broadening of spectroscopic features. If these interactions are specifically included in the interpretation of experimental results, then they are usually treated as instantaneous, meaning that the system under consideration is always in thermodynamical equilibrium, whereby the true dynamics of these processes is neglected.The magnetism of transition metals at finite temperatures is one of the most challenging topics in modern physics and much work has been done to calculate thermodynamic properties like the magnetization M, the susceptibility x, and the Curie temperature T C from first principles. An itinerant ferromagnet shows two types of magnetic excitations. Magnons which follow an approximately quadratic dispersion law are responsible for the low and medium temperature properties, whereas single particle excitations, the Stoner excitations, cost much more energy. Early models either neglected spin fluctuations (Stoner model) and thus predicted much too high Curie temperatures, or neglected the itinerant character leading to a Heisenberg-like description. Modern theories combine both pictures. A Stoner-type intinerant part leads to the formation of local moments and a Heisenberg-like part describes their interaction [1,2]. We will show in this Letter that both kinds of magnetic excitations are important to understand the dynamics of transition metal ferromagnets.An experimental approach to investigate these interactions within a real-time experiment is very difficult, because a time resolution on the time scale of the electronelectron scattering (ϳfs) has to be achieved [3][4][5][6][7]. In the last few years these time scales became accessible by the development of commercial laser systems generating ultrashort pulses. These laser systems are most prominently applied in investigating electron scattering dynamics in semiconductors due to the industrial interest in high frequency devices [8][9][10][11]....

Ultrafast spin dynamics in nickel

“…[1]. The SLR time in Ni is about 304 ps as calculated from a formalism similar to that applied to Gd before [6], which can be compared with the experimental value of 400 ps in Ni [16]. From our calculation it is suggested that the high-speed limit of spin dynamics is about tens of femtoseconds, which is not yet exhausted by experiments.…”

Influence of photon angular momentum on ultrafast demagnetization in nickel

“…Since the observation by Beaurepaire et al that excitation by femtosecond laser pulses can induce a demagnetization in a nickel thin film on a subpicosecond time scale, 1 laserinduced magnetization dynamics received a growing attention. [2][3][4][5][6][7][8] The possibility of optically manipulating spins on such an ultrafast time scale offers, indeed, many potential applications in technology. Beside the technological relevance, research in this field is motivated by scientific interest, the microscopic mechanisms that lead to ultrafast magnetization response being not yet fully understood.…”

Ultrafast Spin Dynamics and Critical Behavior in Half-Metallic Ferromagnet:Sr2FeMoO6

“…However, as it has been discussed in recent papers on Ni [6][7][8][9], it is premature to directly connect the MOKE signal with demagnetization in such ultrafast time scale.…”

“…Typeset using REVT E X 1 Control and manipulation of spins by ultrafast optical excitation, which gives rise to photo-induced magnetization change and magnetic phase transitions in dilute magnetic semiconductor quantum structures [1][2][3], doped semiconductors [4] and ferromagnetic metals [6][7][8][9][10], have attracted considerable attention. Recent study on the magnetization dynamics in the photo-excited Ni films with nonlinear optical techniques has revealed ultrafast spin process within 50 fs [9].…”