Thin Mn germanide films studied by XPS, STM, and XMCD

Thin Mn germanide films with nanoscale thicknesses on Ge(111) have been studied by low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), core-level spectroscopy (CLS), and x-ray magnetic circular dichroism. The 260 ${}^{\circ }$C annealing of 16 monolayers of Mn deposited on Ge(111)$c$($2\times 8$) resulted in a uniform film with intense threefold split $\sqrt{3}\times \sqrt{3}$ LEED spots and Moiré patterns in the STM images. This ultrathin film shows a clear ferromagnetism with a Curie temperature of $\sim$250 K. High-resolution Ge 3$d$ CLS spectra were recorded with photon energies between 50 and 90 eV at normal and 60${}^{\circ }$ emission angle. To achieve a consistent fit over the energy and angular range three components were used in the line-shape analysis. The low temperature (260 ${}^{\circ }$C) annealed film shows significant differences in terms of electronic structure and magnetism in contrast to the high temperature (330 ${}^{\circ }$C or above) annealed ones. Our results indicate that the annealing temperature and the Mn coverage play important roles in the formation of a thin magnetic Mn germanide film.

Figure 1

LEED pattern from the Mn germanide film with a Mn content of 16 ML, annealed at 260 ${}^{\circ }$C. The primary energy of the electrons was 42 eV.

Figure 2

Topographic STM images from the Mn germanide surface. (a) The layered structure of the Mn germanide film, recorded at $V_s=1.0$ V, $I_t=50$ pA, $200\times 200$ nm. (b) A Moiré pattern with a periodicity of $\sim$10.9 nm, $V_s=0.15$ V, $I_t=50$ pA, $33\times 33$ nm${}^2$.

Figure 3

Topographic STM images from the Mn germanide surface. (a)–(d) Filled-state images recorded at $V_s=-0.1$ V to $-0.25$ V, $I_t=25$ pA. (e)–(h) Empty-state images from the same area, $V_s=+0.1$ V to $+$0.25 V, $I_t=25$ pA. The size of the images is $2.2\times 1.8$ nm${}^2$. The black rhombus indicates the unit cell.

Figure 4

Ge $3d$ CLS spectra recorded at 100 K. (a)–(c) recorded at normal emission and (d) at 60${}^{\circ }$. All the incident angles were 50${}^{\circ }$. The spectra were recorded at photon energies of 50, 70, and 90 eV. Circles are the raw spectra and the solid lines are the total contribution from the components used to fit the spectra. Underneath each spectrum the residual line from the fitting procedure is presented.

Figure 5

XAS spectra recorded at 120 K with an x-ray angle of 45${}^{\circ }$ towards the sample. A clear dichroic signal is seen in the $L_3$ and $L_2$ lines as the sample magnetization is reversed. The inset shows a hysteresis curve recorded.

Figure 6

XMCD signals from different samples recorded at 120 K with an x-ray angle of 45${}^{\circ }$ towards the sample. Note that a clear dichroic signal was found for the 8 ML sample in the $\Delta$$L_3$ and $\Delta$$L_2$ lines after 330 ${}^{\circ }$C annealing, but no signal after 260 ${}^{\circ }$C annealing. The 32 ML sample was chosen as a reference.