Simulation of spin-polarized scanning tunneling microscopy on complex magnetic surfaces: Case of a Cr monolayer on Ag(111)

Abstract

We propose an atom-superposition-based method for simulating spin-polarized scanning tunneling microscopy (SP-STM) from first principles. Our approach provides bias-dependent STM images in high spatial resolution, with the capability of using either constant current or constant height modes of STM. In addition, topographic and magnetic contributions can clearly be distinguished, which are directly comparable to results of SP-STM experiments in the differential magnetic mode. Advantages of the proposed method are that it is computationally cheap, it is easy to parallelize, and it can employ the results of any ab initio electronic structure code. Its capabilities are illustrated for the prototype frustrated hexagonal antiferromagnetic system, Cr monolayer on Ag(111) in a noncollinear magnetic 120 • Néel state. We show evidence that the magnetic contrast is sensitive to the tip electronic structure, and this contrast can be reversed depending on the bias voltage.

Three-dimensional Wentzel-Kramers-Brillouin approach for the simulation of scanning tunneling microscopy and spectroscopy

“…Note that the total current contains both topographic and magnetic contributions, therefore z(x, y) will be the simulated SP-STM image [56,90]. In the presented model its bias and temperature dependence is considered as well.…”

“…It seems to be straightforward that a constant TOPO current contour is closer to this constant tunneling transmission criterion than a constant TOTAL current contour due to the appearance of spin dependent effects in the latter one. On the other hand, the calculation of any current contour is simple within the presented 3D WKB approach [56]. Since the I TOPO = const experimental method is not routinely available at the moment, we restrict ourselves to consider the I TOTAL = const contours when calculating the 2D differential conductance maps, and examples will be shown in Section 3.8.…”

“…Based on the work of Heinze [90], a hybrid model has been proposed [56], that uses essentially the TersoffHamann formalism but the tip electron density of states (DOS) is not restricted to be constant in energy. This means that different tip models [76] and their effect on the tunneling properties can be investigated.…”

“…Evaluation of the simulated tunneling properties from a data set on a 3D fine grid above the studied surface is presented thereafter. This includes the extraction of SP-STM images [56], magnetic contrasts [57], differential conductance and effective spin polarization maps [10], as well as magnetic asymmetries [10].…”

“…A very few works focused on the investigation of the bias dependent magnetic contrast so far. Among those, a magnetic contrast reversal was reported in two different magnetic systems [55,56], and the effect was related to the surface electronic structure rather than to the effect of the tip. In another study, such contrast reversals were observed during the scanning with the STM tip at fixed bias, and this was identified to be due to the magnetic switching of the tip [46].…”