Real-Space Imaging of Two-Dimensional Antiferromagnetism on the Atomic Scale

Abstract

A two-dimensional antiferromagnetic structure within a pseudomorphic monolayer film of chemically identical manganese atoms on tungsten(110) was observed with atomic resolution by spin-polarized scanning tunneling microscopy at 16 kelvin. A magnetic superstructure changes the translational symmetry of the surface lattice with respect to the chemical unit cell. It is shown, with the aid of first-principles calculations, that as a result of this, spin-polarized tunneling electrons give rise to an image corresponding to the magnetic superstructure and not to the chemical unit cell. These investigations demonstrate a powerful technique for the understanding of complicated magnetic configurations of nanomagnets and thin films engineered from ferromagnetic and antiferromagnetic materials used for magnetoelectronics.

Tip effects in scanning tunneling microscopy of atomic-scale magnetic structures

“…The program computes the integral numerically by summing up all states within a given energy interval. The interval chosen in our simulations was 50meV, it is larger than the thermal broadening of states at the temperature of measurements, which Heinze et al give as 16K [1]. The reason for this increase is that we sought to avoid an unrealistically selective evaluation, which might be distorted by numerical limitations.…”

“…In this range neighboring atoms with different magnetic properties can act as nanomagnets, their magnetic orientation driven by competing exchange interactions between the electrons. The field gained a considerable boost last year by the first demonstration of magnetism on the atomic scale [1]. It is expected that the packing density of information on ultrathin magnetic films can ultimately be brought down to the very level of single atoms.…”

“…However, our calculation did not include spin-orbit coupling. Spin-orbit coupling on this surface favors AFM ordering [1] and thus shifts the balance slightly towards the AFM configuration.…”

“…In this calculation we used the same number of k-points (16), and the same GGA exchange correlation potential (PW91). Heinze et al [1] obtained the AFM surface as the magnetic groundstate. In our calculation the energy difference between FM and AFM ordering of -5 meV/Mn is below the level of precision even of full-potential methods.…”

“…To this end we simulate STM measurements from first principles, employing magnetic and and non-magnetic model tips and the STM code recently introduced (bSCAN [3]). In particular we analyze, whether (i) the corrugation is really substantially enhanced if a tungsten terminated tip is used [1]; and (ii) whether contamination of the tip with surface atoms might change the images. The first point seems important because extensive simulations of STM scans on an iron surface brought no evidence for an actual enhancement of the corrugation [4].…”