${\mathrm{YCo}}_2$: Intrinsic Magnetic Surface of a Paramagnetic Bulk Material

Here we report on results of a spin-resolved photoelectron spectroscopic (SRPES) study of ${\mathrm{YCo}}_2$ thin films (150 Å-thick) grown on a W(110) substrate. The films were prepared by co-deposition of stoichiometric amounts of Y and Co onto a clean W surface followed by thermal annealing leading to ($2\times 2$) overstructure with respect to W(110) in the low-energy electron diffraction pattern indicated formation of a structurally ordered ${\mathrm{YCo}}_2(111)$ surface. While no clear spin asymmetry was observed for bulk-sensitive SRPES data taken at $h\nu =1253.6\mathrm{eV}$, the more surface-sensitive SRPES data obtained at $h\nu =21.2\mathrm{eV}$ photon energy revealed a clear spin-asymmetry probing the validity of the recent theoretical prediction.

Figure 1

Crystallographic structure of ${\mathrm{YCo}}_2(111)$ surface. (a) The ${\mathrm{YCo}}_2$ bulk unit cell. Large and small spheres are Y and Co atoms, respectively. Shaded plane means (111) surface. (b) Growth model of the ${\mathrm{YCo}}_2(111)$ epitaxial film on top of the W(110) single crystal. Inset shows the low-energy electron diffraction spots obtained on clean W(110) surface (upper LEED image) and 150 Å-thick ${\mathrm{YCo}}_2(111)$ film (bottom LEED image). Observed LEED image is clear from the following consideration of the crystallographic structure of the ${\mathrm{YCo}}_2(111)/\mathrm{W}(110)$ system. The distorted hexagon in the right lower corner of Fig. 1b is formed on the basis of unit cell of the W(110) surface (the unit vectors of this cell are marked by dark arrows: ${\overrightarrow{a}}_1$ and ${\overrightarrow{b}}_1$). The bright vectors (${\overrightarrow{a}}_2$ and ${\overrightarrow{b}}_2$), obtained by transformation from W(110)-unit cell’s vectors, can be transformed to the unit vectors of the hexagonal cell of ${\mathrm{YCo}}_2(111)$ surface by slight distortion [mainly in $⟨1\overline{1}0⟩$ direction of the W(110) surface]. The resulted structure of the ${\mathrm{YCo}}_2(111)$ surface together with hexagonal unit cell and unit vectors (bright hexagon and arrows) is shown in the right upper corner. The presented crystallographic structure was also confirmed by LEED simulation performed by LEEDpat2 package [24].

Figure 2

Manifestation of the surface magnetism of ${\mathrm{YCo}}_2$. (a) Bulk-sensitive spin-resolved photoelectron spectrum and corresponding spin polarization (filled symbols) of ${\mathrm{YCo}}_2$ obtained at $h\nu =1253.6\mathrm{eV}$ of photon energy. (b) Surface-sensitive spin-resolved spectrum and corresponding spin polarization (filled circles) of the ${\mathrm{YCo}}_2(111)$ surface obtained at $h\nu =21.2\mathrm{eV}$ demonstrates clear spin asymmetry. The spin-polarization curves for Co(0001) thin films (open symbols) are presented here for comparison by open symbols. In case of the $h\nu =21.2\mathrm{eV}$ photon energy the corresponding spin-polarization values for Co film were reduced by factor of 2. (c) Left: results of spin-polarized slab calculations show spin-resolved density of states of central-slab Co atom by dashed area, and ones of Co surface layer for relaxed Co- (thin lines) as well as Y-terminated (thick lines) of ${\mathrm{YCo}}_2(111)$ surface. Right: corresponding spin polarization for Co- (thin lines) and Y-terminated (thick lines) surfaces: dashed lines—only for topmost Co atoms, solid lines—for ${\mathrm{YCo}}_2(111)$ slab after taking into account the photoelectron escape depth.