Ultrafast demagnetization of ferromagnetic transition metals: The role of the Coulomb interaction

Abstract

The Elliott-Yafet ͑EY͒ mechanism is arguably the most promising candidate to explain the light-induced ultrafast demagnetization dynamics in ferromagnetic transition metals on time scales on the order of 100 fs. So far, only electron-phonon ͑or impurity͒ scattering has been analyzed as the scattering process needed to account for the demagnetization. We show that an EY-like mechanism based on electron-electron scattering has the potential to explain time-resolved magneto-optical Kerr effect measurements on thin magnetic Co and Ni films, without reference to a "phononic spin bath." Current research in femtosecond magnetism is concerned with elucidating the fundamental mechanisms of lightinduced spin dynamics as well as searching for potential applications in data processing. 1-3 Despite important experimental studies employing various time-resolved techniques, no consensus on a microscopic understanding of ultrafast magnetization dynamics in ferromagnets has emerged. Rather, demagnetization dynamics is typically described in the framework of the phenomenological three-temperature model. In this model, temperatures are assigned to the electron, lattice, and spin "subsystems," and the exchange of energy ͑and spin͒ is driven by the temperature differences between the respective subsystems. Although the threetemperature model provides an intuitive picture of demagnetization, its relation to the microscopic dynamics behind the demagnetization is still an active field of research.The most popular candidate 4 for the microscopic process behind light-induced ultrafast demagnetization is a mechanism of the Elliott-Yafet ͑EY͒ type. 5 In the EY mechanism, the demagnetization arises because, in the presence of the spin-orbit ͑SO͒ interaction, spin is not a good quantum number, so that any momentum-dependent scattering mechanism changes the spin admixture when an electron is scattered from state ͉k ជ ͘ to ͉k ជ + q ជ͘. So far, the scattering processes responsible for the EY mechanism have been assumed to be ͑quasi͒elastic electron-phonon and electron-defect scattering in several theoretical and experimental studies. 4,6-9 Unlike these papers, we analyze the ultrafast demagnetization in ferromagnetic metals due to an EY-like mechanism based exclusively on electron-electron Coulomb scattering. This scattering mechanism is not ͑quasi͒elastic, so that the available phase space for transitions from minority to majority bands is much larger than for electron-phonon scattering, which can only cause transitions near points in the Brillouin zone where the bands are energetically close. As a proof of principle for the importance of electron-electron scattering for the demagnetization, we demonstrate quantitative agreement for the demagnetization time and magnetization quenching between time-resolved magneto-optical Kerr effect ͑TR-MOKE͒ measurements on Co and Ni, and numerical results based on the EY mechanism due to electron-electron scattering.To resolve the electronic demagnetization dynamics on ultrafast time scales, we calculate the...

Ultrafast magnetization enhancement in metallic multilayers driven by superdiffusive spin current

“…Several competing processes have been proposed for magnetization dynamics on ultrafast timescales [5][6][7][8][9][10][11][12][13] . For the trilayer systems explored here, we find that the observed magnetization dynamics in the Ni and buried Fe layer is consistent with superdiffusion of Fig.…”

“…A number of distinct models have been proposed to describe how laser excitation can couple so quickly to the spins, given that the light itself does not directly exert a significant torque on the electron spins. Most of these models are based on phonon-, electronand magnon-mediated spin-flip processes [5][6][7][8][9] , direct laser-induced spin-flips 10 or relativistic spin-light interaction 11 . Most recently, a new model based on superdiffusive spin transport has been proposed 12,13 .…”

Efficiency of ultrafast laser-induced demagnetization in GdxFe100−x−yCoMedapalli1, Razdolski2, Savoini3 et al. 2012Phys. Rev. B323265View full textAdd to dashboardCite

“…[8][9][10][11][12][13][14] Although in most cases the demagnetization is not complete, being often at the level of only a few percent, the majority of this research has been focused on understanding the time scale of this demagnetization and on novel channels of angular momentum transfer from the spin system. 1 At the same time, however, it has been noted that in the case of incomplete demagnetization the rate at which the magnetization drops might have nothing to do with the characteristic interaction time of the spins with other reservoirs of energy and angular momentum (electrons and phonons).…”

Superdiffusive Spin Transport as a Mechanism of Ultrafast Demagnetization

“…However, in spite of the technological importance the mechanism underlying the femtosecond demagnetization remains highly controversial. A common belief is that there should exist an ultrafast channel for the dissipation of spin angular momentum [4][5][6][7][8]. Several such mechanisms through which an excited electron can undergo a spinflip in a ferromagnetic metal are currently being debated.…”