Revealing the 120° Antiferromagnetic Néel Structure in Real Space: One Monolayer Mn on Ag(111)

With spin-polarized scanning tunneling microscopy operating in the constant current mode, the 120° antiferromagnetic Néel structure was found for a monolayer Mn on Ag(111) in both fcc and hcp stacking. The existence of structurally equivalent, but magnetically distinguished, Mn islands was observed. While both the fcc and hcp Mn islands display the frustrated spin structure, the orientations of their magnetic moments differ by 30° possibly due to the spin-orbit coupling.

Figure 1

120° Néel antiferromagnetic structure in a hexagonal lattice (a) and the projection of the magnetic moments on the axis (b) parallel and (c) perpendicular to one atomic moment. The magnetic unit cell is indicated by the gray diamond.

Figure 2

(a) Topography of 0.6 ML Mn on Ag(111) together with a section profile. $U_{\mathrm{bias}}=0.23\mathrm{V}$; $I_t=20\mathrm{nA}$. The arrows indicate Ag substrate step edges. While most Mn islands point upwards, a few of them point downwards as marked by the circles. (b) Four possible types of Mn ML triangular islands on Ag(111). Red represents the Mn atoms, green and blue the atoms of the first and second Ag layer, respectively. The two fcc (hcp) islands are distinguished by their step edge orientations and the lateral displacements between the Mn atomic rows and the Ag atom rows measuring towards the base of the triangular from Mn to Ag. (c) The atomically resolved topographic images on both the Mn (red) island and the Ag (green) substrate for islands pointing up and pointing down. $U_{\mathrm{bias}}=15\mathrm{mV}$; $I_t=20\mathrm{nA}$. (d) The atomically resolved topographic image close to the Ag step where the growth of Mn along the Ag step is clearly visible. No dislocation is present between Ag and Mn as shown by the lattice grid. $U_{\mathrm{bias}}=2\mathrm{mV}$; $I_t=10\mathrm{nA}$.

Figure 3

(a) Four possible configurations of the 120° Néel structure (modulo a 120° rotation of all spins in the same sense). (b) and (c) are two groups of constant current images taken with magnetic tips. The size of the images is about $2\times 2.5{\mathrm{nm}}^2$. $U_{\mathrm{bias}}=20\mathrm{mV}$; $I_t=20\mathrm{nA}$. The magnetic unit cells are marked in the images together with the projection of the sample spin polarization onto the tip spin polarization direction. By assuming a magnetic orientation of the tip, the projections of the four domains in (a) on the tip magnetization direction result in different magnetic images. All of the images were taken on the up islands.

Figure 4

(a) Spin image of the rectangular area in (b). $U_{\mathrm{bias}}=20\mathrm{mV}$; $I_t=20\mathrm{nA}$. The right part is the hcp region, and the left part is the fcc region. In the middle, the image appears darker which is considered as the smooth structural transition region from hcp to fcc. With the assumed tip magnetization orientation, one possible configuration of the 120° Néel structure is given. (b) Topography of submonolayer Mn. Islands are connected together.