Interface formation for a ferromagnetic/antiferromagnetic bilayer system studied by scanning tunneling microscopy and first-principles theory

The initial stages of interface formation for a real-world ferromagnet/antiferromagnet bi-layer system (iron/manganese nitride) are investigated down to the atomic scale using a combination of molecular beam epitaxy, in situ scanning tunneling microscopy, and first-principles theoretical calculations. Submonolayer deposition of iron onto manganese nitride nanopyramid surfaces results in an unexpected yet well-ordered structural and magnetic arrangement. It is shown that although the island structures seen in scanning tunneling microscopy images are of single monolayer height, their chemical composition, based on Auger electron spectroscopy, conductance map imaging, and theoretical models, does not consist of iron. It is found theoretically that models that consider iron on the surface of manganese nitride are highly unfavorable. Instead, models with iron atoms incorporated into specific subsurface layers are most stable, in excellent agreement with Auger spectroscopy measurements. Calculations also reveal the magnetic alignment of iron with the manganese nitride layers.

Figure 1

(a) STM topograph and (b) corresponding $dI/dV$ map of the ${\mathrm{Mn}}_3{\mathrm{N}}_2$(001) substrate ($V_s=-0.3$ V; $I_t=0.1$ nA). (c) and (d) represent zoom-in views corresponding to the rectangular boxed areas shown in (a) and (b), respectively. For particular terraces, both B and A1/C terminations are present on the same terrace (with boundaries indicated by dashed lines).

Figure 2

STM topographs for two selected island coverages and corresponding substrate temperatures during growth ($V_s=-0.7$ V; $I_t=0.1$ nA).

Figure 3

(a) STM topograph and (b) corresponding $dI/dV$ map for the 0.15 ML case ($V_s=-0.3$ V; $I_t=0.1$ nA). The inset in (a) is a line profile taken along path A. (c) Contrast map from (b) showing the electronic contrast differences between islands and corresponding substrate terraces.

Figure 4

(a) Atomic model of ${\mathrm{Mn}}_3{\mathrm{N}}_2$(001) nanopyramids in which three different types of terraces can be observed: A1, B, and C; the inset represents a three-dimensional rendered STM image of part of one pyramid to show the correspondence with the actual model. (b) Atomic models for A1, B, and C terraces with Fe atoms (in red) at different locations. In addition, the panel for model C shows the magnetization directions of different layers. Differently colored arrows on the Fe and Mn atoms indicate the different magnetic moments.

Figure 5

Surface formation energy plots versus chemical potential for the models shown in Fig. 4.