Ultrafast hot-electron induced quenching of Tb 4f magnetic order

Abstract

Abstract:We have investigated ultrafast quenching of the Tb4f magnetic order in Co 74 Tb 26 alloys, induced by femtosecond hot-electrons pulses. The hot-electron pulses were produced in specific non-magnetic capping layers by infrared fs laser pulses. Our experimental results show that sub-picosecond dynamics of Tb4f magnetic moments can be induced by nonthermal and thermal hot-electrons. We further demonstrate that the demagnetization efficiencies of non-thermal and thermal hot electrons are similar. However, the characteristic demagnetization times show values of 0.35 ps for non-thermal hot-electrons excitations and 1.2 ps for thermal hot-electrons excitations. We explain this temporal elongation by the propagation time of thermal hot-electrons through the 15 nm thick CoTb film. 2 A. Introduction:Determining the ultimate speed for deterministic control of magnetization manipulation is a hot-topic in modern magnetism, mainly due to technological application in data manipulation [1]. In this context, the discovery in 1996 [2] of ultrafast magnetization quenching upon femtosecond laser excitation has driven intensive experimental and theoretical works [3]. In the frame of the thermodynamic 3 temperatures model (3TM), the laser pulse injects its energy in the electronic system that relaxes due to the coupling with the lattice (electronphonon coupling) and with the spins (electrons-spin coupling) [2]. At the sub-picosecond time scale, the actual phenomenological model as proposed by Beaurepaire et al. does neither address the conservation of the angular momentum nor the microscopic mechanisms driving the dissipation of the angular momentum [2]. However, both aspects are heavily debated in many experimental and theoretical works. In 2009, Koopmans et al. proposed a microscopic 3TM, in which spin-flips resulting from the collision between laser-excited electrons with phonons and/or impurities have been introduced to account for the angular momentum conservation [4]. Even though this model accurately reproduced some trends of the demagnetization in transition metal [5,6], rare-earth elements [4] and in many magnetic alloys [7][8][9], doubts have been casted upon the efficiency of spin-flip scattering in ultrafast demagnetization [10][11]. Alternatively, the transport of spin-polarized hot-electrons has been proposed to explain ultrafast demagnetization [12,13,14]. In metals, the pump IR pulses create a strongly out-of-equilibrium electronic distribution called the "non-thermal hotelectrons". These non-thermal hot-electrons can propagate at the Fermi velocity over tens of nanometers. Due to electron-electron collisions, the non-thermal hot-electrons reach an internal equilibrium characterized by a Fermi-Dirac distribution, called "thermal hotelectrons". Malinowski et al. [12] proposed a ballistic transport of spin-polarized hot-electrons as an efficient channel for angular momentum dissipation during the ultrafast demagnetization. Within this model the different amplitudes and time scales which were experimentally obse...

Nonequilibrium heat transport in Pt and Ru probed by an ultrathin Co thermometer

“…Transport of hot electrons in non-magnetic metals is an important driver for ultrafast demagnetization of an adjacent ferromagnetic material. Although prior work emphasizes the role of ballistic transport of hot electrons in the non-magnetic metallic layer for inducing indirect excitation of the adjacent ferromagnetic layer,[25,[32][33][34] we argue that based on the results of this work, diffusive transport of electrons can be as effective as ballistic transport in terms of heat flux when lep is substantially longer than the optical absorption depth. Our work also gives a different interpretation of the results of Ferté et al[33]: the authors showed the demagnetization of CoTb occurs more slowly when CoTb is heated by a Cu/Pt/Cu trilayer than by a single Cu layer of similar thicknesses.…”

Laser-induced ultrafast demagnetization and perpendicular magnetic anisotropy reduction in a Co88Tb12 thin film with stripe domains

“…In the present paper, we describe TR-XRMS experiments on amorphous Co-Tb thin films with magnetic stripe domains conducted at the free-electron laser FERMI. Although several femtomagnetism studies have already been performed on ferrimagnetic Co-Tb [25][26][27][28], a material system of great technological relevance for future all optical magnetic data storage [29][30][31], experiments describing the evolution of magnetic domain structures in rare earth-transition metal (RE-TM) alloys following an ultrashort optical pulse are still scarce. Recently, Fan et al performed a TR-XRMS study on Co 88 Tb 12 samples using a tabletop HHG source but their analysis remained limited to the first magnetic diffraction order at the N edge of Tb (155 eV) [32].…”

Laser-induced ultrafast demagnetization and perpendicular magnetic anisotropy reduction in a Co$_{88}$Tb$_{12}$ thin film with stripe domains

“…In the present work, we describe tr-XRMS experiments on amorphous Co-Tb thin films with magnetic stripe domains conducted at the free-electron laser FERMI. Although several femtomagnetism studies have already been performed on ferrimagnetic Co-Tb [25][26][27][28] , a material system of great technological relevance for future all optical magnetic data storage [29][30][31] , data describing the evolution of magnetic domain structures in rare earth -transition metal (RE-TM) alloys following an ultrashort optical pulse are still scarce. Recently, Fan et al performed a first tr-XRMS study on Co 88 Tb 12 samples using a tabletop HHG source but their analysis remained limited to the first magnetic diffraction order at the N-edge of Tb (155 eV) 32 .…”