Magnetic x-ray dichroism in core-level photoemission from ferromagnets

Abstract

We report on the novel effect of magnetic x-ray dichroism in core-level photoemission from ferromagnets with circularly polarized radiation. Depending on the relative orientation of photon spin and sample magnetization (parallel or antiparallel), a single emission line may be resolved into two lines, due to exchange splitting of the core level. The mechanism is explained in terms of spin-selective dipole transitions in the presence of spin-orbit coupling. Possible applications of magnetic x-ray dichroism in photoemission from magnetic samples are pointed out.PACS numbers: 79.60. Cn, 75.50.Bb, 78.20.Ls The transmission or reflection of circularly polarized light interacting with magnetic matter may depend on its helicity, i.e., the sense of rotation of the electric vector of the electromagnetic wave. A familiar example for this "magnetic dichroism" is the magneto-optic Kerr effect. The coupling between the aligned electron spins (s = y ) and the photon spin (s = 1) is in this case, as well as in all others to be discussed below, provided by the spinorbit interaction of the electronic states-either in the initial or in the final state, or in both. A counterpart to the optical polar Kerr effect was recently reported in the x-ray region. 1,2 It was found that the absorption cross section for x-ray photons at core levels of ferromagnets depends on the relative orientation of the photon spin and the sample magnetization. This was successfully interpreted in terms of the spin-split density of states at and above the Fermi level. 3 As shown earlier by van der Laan et at., 4 magnetic x-ray dichroism may in special cases even be observed with linearly polarized radiation. A general theoretical treatment of linear and circular dichroism in absorption was very recently given by Carra and Altarelli. 5 The new phenomenon we report on here is distinct from all the above in that it occurs in emission of electrons from core levels into free-electron states, rather than absorption of photons at core-level edges. The basic observation is that a single core-level emission peak from a ferromagnet may be resolved into two energetically separated lines, if excited by circularly polarized x rays with the photon spin aligned with the sample polarization. The relative intensity of these lines reverses when the magnetization of the sample or the helicity of the photons is reversed. Since the final energy of the electrons is far above the Fermi level, we basically probe the spin-split electronic structure of the core levels, rather than the spin-split empty density of states in the valence band. "Magnetic x-ray dichroism in photoemission" may find wide application in the study of the elementspecific local magnetic structure on the atomic scale in ferromagnets, ferrimagnets, and antiferromagnets in the near-surface region. Our observations also shed new light on a long-standing issue of the interpretation of exchange splittings in the x-ray photoelectron spectrum of iron.A number of proposals for the production of circularly polarized lig...

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