Ultrafast Magnon Generation in an Fe Film on Cu(100)

A. B. Schmidt, M. Pickel, M. Donath, P. Buczek, A. Ernst, V. P. Zhukov, P. M. Echenique, L. M. Sandratskii, E. V. Chulkov, M. Weinelt
2010 Physical Review Letters  
We report on a combined experimental and theoretical study of the spin-dependent relaxation processes in the electron system of an iron film on Cu(100). Spin-, time-, energy-and angle-resolved two-photon photoemission shows a strong characteristic dependence of the lifetime of photoexcited electrons on their spin and energy. Ab initio calculations as well as a many-body treatment corroborate that the observed properties are determined by relaxation processes involving magnon emission. Thereby
more » ... emission. Thereby we demonstrate that magnon emission by hot electrons occurs on the femtosecond time scale and thus provides a significant source of ultrafast spin-flip processes. Furthermore, engineering of the magnon spectrum paves the way for tuning the dynamic properties of magnetic materials. Magnons are the fundamental collective excitations of the electron system of magnetic materials. In collinear ferromagnets each magnon lowers the magnetization by 2 B . Understanding the generation of such spin waves away from thermal equilibrium is necessary to develop a microscopic picture of electron relaxation processes [1,2]. The signature of magnon generation by hot electrons has been found in spin-polarized electron energy loss spectroscopy [3], high-resolution photoemission spectra [4] , and inelastic tunneling spectroscopy [5] . However, these experiments do not address the fundamental problem of the time required to generate a magnon. This is, in particular, important since magnon emission by excited electrons is commonly viewed as a slow process, occurring within picoseconds [6] . In contrast, laser-induced magnetic phase transitions are reported to take place in the femtosecond range [7-9]. Though parameterized models now describe major properties of laser-induced demagnetization dynamics, a microscopic understanding of these processes is still under debate [10] [11] [12] . In this Letter we present a combined experimental and theoretical approach to study ultrafast electronic dynamics in magnetic systems. The system we consider is a 3 monolayer iron film on Cu(001), since theory predicts spin-wave effects to be strong in Fe [13, 14] . A precise two-photon photoemission (2PPE) experiment with spin, time, energy, and angular resolution allows us to detect characteristic properties in the spin, energy, and momentum dependence of the lifetime of excited states. Based on ab initio calculations of the ground state and the magnetic excitation of the system as well as the evaluation of the electron lifetimes within a many-body treatment of the electron self-energy, we show that the observed properties are determined by electron-magnon interaction. Our most fundamental result is that the emission of magnons by hot electrons takes place on a femtosecond time scale. This implies that magnon emission must be considered an important contribution to femtomagnetic properties of 3d ferromagnets. In our 2PPE experiment we probe the spin-dependent decay mechanisms of photoexcited electrons occupying image-potential states [15] . This class of surface states is localized several angstroms in front of the surface and they are established as the model system of choice for the study of electron dynamics [16] . As sketched in Fig. 1(a) , a first femtosecond laser pulse @! a excites an electron from bulk states below the Fermi level E F into the unoccupied imagepotential-state band and from there a second pulse @! b raises the excited electron above the vacuum level E vac . The photoelectrons are then selected by energy and their spin polarization is determined (for further details see Ref. [17]). Measuring the energy as a function of the momentum parallel to the surface Eðk k Þ yields the first and second band (n ¼ 1, 2) with a two-dimensional parabolic dispersion [18] as depicted in Fig. 1(a) . Their exchange splitting is a signature of the ferromagnetic order in the iron film at the measurement temperature of 90 K (T=T C ¼ 0:24) [19, 20] . The lifetime of electrons photoexcited to the n ¼ 1 band is accessed by shifting the time delay between pump and probe pulse. Figure 1(b) features six of such traces recorded at fixed kinetic energy, parallel momentum, and PRL 105, 197401 (2010)
doi:10.1103/physrevlett.105.197401 pmid:21231194 fatcat:au65acuk5bdvrgzy2bmbyeumfy