Topmost layer magnetization of ultrathin Cr films on Fe(100) from proton-induced spin-polarized electron emission

Abstract

The magnetic ordering of the topmost surface layer of ultrathin Cr films grown on Fe͑100͒ is studied via spin-polarized electron emission, excited by fast protons grazingly scattered from the film surface. We find that most electrons originate from the topmost layer. Based on simple assumptions we are able to deduce the layer-dependent magnetic moments from the observed spin polarization of electrons. We demonstrate that our method has a clearly smaller probing depth than conventional spin-polarized electron spectroscopies.

Layer-dependent magnetization at the surface of a band ferromagnet

“…The probing depth in secondary-electron emission can be considerably reduced by using energetic ions instead of electrons as primary particles [21,22]. Grazingly incident ions are reflected from the top surface layer and do not penetrate into the bulk ("surface channeling").…”

“…In practice, structural imperfections like surface steps mediate penetration of some projectiles, leading to a contribution of excited electrons from layers beneath the surface. From computer simulations emulating ion trajectories [23] and an overlayer experiment [21], we infer for scattering of 25 keV protons from our Fe(100) surface a probing depth of λ = (0.5 ± 0.2) ML for electrons of 10 − 20 eV kinetic energy. We note that λ seems to increase for lower electron energies owing to cascade multiplication governed by electron-electron scattering [21].…”

“…From computer simulations emulating ion trajectories [23] and an overlayer experiment [21], we infer for scattering of 25 keV protons from our Fe(100) surface a probing depth of λ = (0.5 ± 0.2) ML for electrons of 10 − 20 eV kinetic energy. We note that λ seems to increase for lower electron energies owing to cascade multiplication governed by electron-electron scattering [21]. Hence, energy resolution is mandatory if maximum surface sensitivity is aspired.…”

“…Spin analysis is performed for electrons with 10 − 20 eV kinetic energy in a subsequent LEED spin polarization detector [21]. Each polarization spectrum is obtained from two identical measurements with reversed magnetizations to eliminate instrumental asymmetries.…”

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“…The scenario of floating of atoms during the sample growth and the model for calculation of magnetic interface profile can be used for the interpretation of experiments with thin overlayers of Cr/Fe [49] and with Fe films embedded in Cr [50,51]. Kubik et al [50,51] have measured Mössbauer spectra for 57 Fe films of different thicknesses (from 1 to 14 ML) sandwiched between Cr(100) layers.…”