Magnetism of ordered $\mathrm{Sm}∕\mathrm{Co}\left(0001\right)$ surface structures

The epitaxial system $\mathrm{Sm}∕\mathrm{Co}\left(0001\right)$ was studied for Sm coverages up to 1 monolayer (ML) on top of ultrathin $\mathrm{Co}∕\mathrm{W}\left(110\right)$ epitaxial films. Two ordered phases were found for $1∕3$ and 1 ML Sm, respectively. The valence state of Sm was determined by means of photoemission and magnetic properties were measured by magneto-optical Kerr effect. We find that 1 ML Sm causes a strong increase of the coercivity with respect to that of the underlying Co film. Element-specific hysteresis loops, measured by using resonant soft x-ray reflectivity, show the same magnetic behavior for the two elements.

Figure 1

Visible-light MOKE hysteresis curves of $\mathrm{Sm}(1∕3\mathrm{ML})∕\mathrm{Co}\left(8\mathrm{ML}\right)∕\left(0001\right)$ measured at (a) $273\mathrm{K}$ and (b) $80\mathrm{K}$. The LEED pattern for an electron energy of $150\mathrm{eV}$, shown in (c) with inverted contrast, corresponds to a $(\sqrt{3}\times \sqrt{3})\mathrm{R}30°$ superstructure. In (d), the proposed atomic structure of this phase (top view) is shown schematically; large and small circles represent Sm and Co atoms, respectively.

Figure 2

(a) Visible-light MOKE hysteresis curve of $\mathrm{Sm}\left(1\mathrm{ML}\right)∕\mathrm{Co}\left(8\mathrm{ML}\right)∕\mathrm{W}\left(110\right)$ measured at $80\mathrm{K}$. In (b), the corresponding curve measured under the same conditions for the 8 ML $\mathrm{Co}∕\mathrm{W}\left(110\right)$ film prior to Sm deposition is shown. LEED patterns ($150\mathrm{eV}$, inverted contrast) of the two surfaces are displayed in (c) and (d), respectively.

Figure 3

Normal-emission photoemission spectra of (a) $\mathrm{Sm}(1∕3\mathrm{ML})∕\mathrm{Co}∕\mathrm{W}\left(110\right)$ (showing the $(\sqrt{3}\times \sqrt{3})\mathrm{R}30°$ superstructure) and (b) $\mathrm{Sm}\left(1\mathrm{ML}\right)∕\mathrm{Co}∕\mathrm{W}\left(110\right)$, both recorded with $141\mathrm{eV}$ photons. The shifts of the $4f^5$ multiplet lines are indicated by vertical bars.

Figure 4

Dichroic x-ray reflectivity spectra recorded across (a) the Sm $M_{4,5}$ and (b) the Co $L_{2,3}$ resonances of $\mathrm{Sm}(1∕3\mathrm{ML})∕\mathrm{Co}\left(10\mathrm{ML}\right)∕\mathrm{W}\left(110\right)$. Closed and open circles correspond to nearly parallel and antiparallel orientations of photon spin and sample magnetization, respectively.

Figure 5

Dichroic x-ray reflectivity spectra recorded across (a) the Sm $M_{4,5}$ and (b) the Co $L_{2,3}$ resonances of $\mathrm{Sm}\left(1\mathrm{ML}\right)∕\mathrm{Co}\left(10\mathrm{ML}\right)∕\mathrm{W}\left(110\right)$. Closed and open circles correspond to nearly parallel and antiparallel orientations of photon spin and sample magnetization, respectively. The insert shows element-specific hysteresis loops measured at the Sm $M_5$ and Co $L_3$ maxima with an x-ray incidence angle of 10°.