Mapping the spin-dependent electron reflectivity of Fe and Co ferromagnetic thin films

Spin polarized low energy electron microscopy is used as a spin-dependent spectroscopic probe to study the spin-dependent specular reflection of a polarized electron beam from two different magnetic thin film systems: Fe/W(110) and Co/W(110). The reflectivity and spin-dependent exchange-scattering asymmetry are studied as a function of electron kinetic energy and film thickness, as well as the time dependence. The largest value of the figure of merit for spin polarimetry is observed for a five monolayer thick film of Co/W(110) at an electron kinetic energy of 2 eV. This value is 2 orders of magnitude higher than previously obtained with state of the art mini-Mott polarimeter. We discuss implications of our results for the development of an electron-spin-polarimeter using the exchange-interaction at low energy.

Figure 1

(Color online) Panels (a) and (b) show SPLEEM images of Co/W(110) and Fe/W(110) films, respectively. For both films the magnetic easy axis points along the horizontal direction in the images. Both images were recorded with electron spin polarization aligned to the right (white arrow). The magnetic domains are shown with blue and red contrast; the magnetization direction of the domains is indicated by black arrows. Black contours are the regions of interest for average reflectivity measurements. (c),(d) Color-scale plots of the measured reflectivity as a function of the electron kinetic energy (horizontal axis) and film thickness (vertical axis) for both thin-film systems. (e),(f) Color-scale plots of the spin asymmetry of the reflectivity.

Figure 2

(Color online) (a) Energy dependence of the reflectivity for spin parallel and antiparallel to the Co/W(110) film magnetic axis for different film thickness. The energy scans are incrementally shifted by 0.05. (b) Expanded view of the spin dependence of the reflectivity for the 4 ML thick film. (c) Spin asymmetry for a 4 ML thick Co/W(110) film obtained from Fig 1b. (d) Band structure calculation for a 4 ML thick Co/W(110) film (Ref. 16).

Figure 3

(Color online) (a),(b) Color-scale plots of the “figure of merit” for Co/W(110) and Fe/W(110) thin films, respectively, as a function of the electron kinetic energy (horizontal axis) and film thickness (vertical axis). (c),(d) The respective figure of merit, plotted as a function of energy for Co and Fe at different thicknesses. For clarity, the energy scans are incrementally shifted by ${10}^{-2}$. The location of the scans is shown by color horizontal lines in panels (a) and (b).

Figure 4

(Color online) Aging effects and regeneration for a 5 ML thick cobalt film: (a) Evolution of the reflectivity of the film with time in UHV conditions. Red (blue) energy scans represent incident electrons with spin polarization parallel (antiparallel) to the magnetization. For clarity, each curve is shifted by 0.01. (b) The spin asymmetry calculated from the reflectivity distribution curves in panel (a). The effect of annealing on the reflectivity and the corresponding spin asymmetry are shown in (c) and (d), respectively, after 56 h of room-temperature aging in UHV.