Structure and magnetism of Tm atoms and monolayers on W(110)

We investigated the growth and magnetic properties of Tm atoms and monolayers deposited on a W(110) surface using scanning tunneling microscopy and x-ray magnetic circular and linear dichroism. The equilibrium structure of Tm monolayer films is found to be a strongly distorted hexagonal lattice with a Moiré pattern due to the overlap with the rectangular W(110) substrate. Monolayer as well as isolated Tm adatoms on W present a trivalent ground-state electronic configuration, contrary to divalent gas phase Tm and weakly coordinated atoms in quench-condensed Tm films. Ligand field multiplet simulations of the x-ray absorption spectra further show that Tm has a $|J=6,J_z=\pm 5\rangle$ electronic ground state separated by a few meV from the next lowest substates $|J=6,J_z=\pm 4\rangle$ and $|J=6,J_z=\pm 6\rangle$. Accordingly, both the Tm atoms and monolayer films exhibit large spin and orbital moments with out-of-plane uniaxial magnetic anisotropy. X-ray magnetic dichroism measurements as a function of temperature show that the Tm monolayers develop antiferromagnetic correlations at about 50 K. The triangular structure of the Tm lattice suggests the presence of significant magnetic frustration in this system, which may lead to either a noncollinear staggered spin structure or intrinsic disorder.

Figure 1

(a) LEED image taken on the W(110) surface after cleaning using the procedure described in the text, (b) STM images of W(110) showing terraces, (inset) detail of residual carbon impurities remaining after the cleaning procedure, (c) STM image of Tm adatoms (coverage $\sim 2$%) on the W(110) surface. The larger objects correspond to Tm dimers and trimers.

Figure 2

(a) LEED image taken on a Tm deposit showing a characteristic surface reconstruction of rare earth on the (110) W surface, (b) LEED image taken on compact Tm hcp islands coexisting with areas with bare (110) W surface, (c) STM image taken on a terrace covered with Tm; the Moiré pattern can be observed (short and long arrows are the [001] and [$1\overline{1}0$] W crystal directions, respectively).

Figure 3

(a) Experimental linearly polarized XAS of Tm adatoms in a 50-mT field; (b) experimental linearly polarized XAS of Tm monolayer in a 5-mT field. (c), (d) Simulated linearly polarized XAS for noninteracting Tm atoms in a 50-mT field applied parallel to the x-ray beam direction and in a 175-T field along the surface normal, respectively. All data are measured/simulated at $T=8$ K with the substrate normal tilted at ${70}^{\circ }$ with respect to the x-ray beam direction.

Figure 4

XAS and XMCD spectra of Tm adatoms and monolayers on W(110) measured in a 5-T magnetic field applied parallel to the x-ray direction at $T=8$ K. The XMCD spectra are calculated as the difference between $I{}^{-}$ and $I{}^{+}$.

Figure 5

Field dependence of the Tm XMCD intensity measured at the $M{}_5$ edge for the adatom and the monolayer samples at 8 K.

Figure 6

Tm monolayer XMCD as a function of temperature (dots) compared to the paramagnetic behavior expected for noninteracting Tm atoms (lines). The experimental data were taken in a 5-T magnetic applied parallel to the x-ray beam, at $\theta =0^{\circ }$, ${55}^{\circ }$, and ${70}^{\circ }$. The paramagnetic curves were calculated using a spin Hamiltonian model and crystal-field parameters obtained from the multiplet simulations (solid line) and by assuming a Brillouin function with the ${\mathrm{Tm}}^{3+}$ parameters $J=6$ and $g=\frac{7}{6}$ (dashed line). The calculated curves are normalized to the experimental data at 300 K.

Figure 7

(a)–(c) Linearly polarized XAS spectra measured on the Tm monolayer at three different temperatures. All spectra were taken with the substrate normal tilted at $\theta ={70}^{\circ }$ with respect to the x-ray beam direction. A small (5-mT) magnetic field was applied to enhance the electron yield of the sample without significantly affecting its magnetic configuration. The first and second peaks in the $h$-polarized spectra are labeled A and B, respectively. (d) Temperature dependence of the difference between A and B peaks.