Domain Wall Orientation in Magnetic Nanowires E.Y. Vedmedenko, 1,2 A. Kubetzka, 2 K. von Bergmann, 2 O. Pietzsch, 2 M. Bode, 2 J. Kirschner, 1 H. P. Oepen, 2 and R. Wiesendanger 2 1 Max-Planck-Institut fu ¨r Mikrostrukturphysik, Weinberg 2, D-06120 Halle, Germany 2 Institute of Applied Physics, University of Hamburg, Jungiusstrasse 11, D-20355 Hamburg, Germany (Received 31 July 2003; published 20 February 2004) Scanning tunneling microscopy reveals that domain walls in ultrathin Fe nanowires are oriented along a certain crystallographic direction, regardless of the orientation of the wires. Monte Carlo simulations on a discrete lattice are in accordance with the experiment if the film relaxation is taken into account. We demonstrate that the wall orientation is determined by the atomic lattice and the resulting strength of an effective exchange interaction. The magnetic anisotropy and the magnetostatic energy play a minor role for the wall orientation in that system. DOI: 10.1103/PhysRevLett.92.077207 PACS numbers: 75.60.Ch, 07.79.–v, 75.70.Ak Magnetism of systems with reduced dimensions poses a number of topical questions, one intriguing issue being the orientation of domain walls. It has been shown ex- perimentally that the mesoscopic pathway of domain walls in ultrathin films can either be arbitrary, as in Co=Au111 [1], or follow certain crystallographic direc- tions, as in Fe=W110 [2]. Although the knowledge of domain patterns and, in particular, the domain wall ori- entation on the nanoscale is of great importance for the fundamental physics of magnetism, as well as for tech- nical applications, the orientation of domain walls on a local, microscopic scale has not yet been studied. One experimentally accessible and, for future applica- tions, very perspective geometrical shape is a so-called nanowire—a quasi-one-dimensional structure of infinite length and lateral dimensions on the nanometer scale. The nanowire geometry is particularly advantageous for the investigation of the domain wall orientation as the latter can be governed by a minimization of the total wall length. On the other hand, it has been demonstrated that in ultrathin nanostructures the discreteness of the crys- talline lattice can also change the magnetization configu- ration [3]. The role of the lattice for the domain wall orientation has not been analyzed systematically. For many experimental systems, e.g., Fe=Cu100, the shortest wall path coincides with one of the crystallo- graphic axes which makes it impossible to distinguish between the role of the lattice for the domain formation and other effects. Only if the shortest distance is different from any principal axes of a lattice the mechanism under- lying the orientation of the domain walls can be revealed. A suitable and experimentally well-studied model system is the double layer (DL) Fe nanowires on stepped W(110) [2,4–8] being characterized by perpendicularly magne- tized domains separated by domain walls. Experimental and ab initio electronic structure calculations [9] led to a comprehensive understanding of the electronic and the magnetic properties. The relationship between the orien- tation of domain walls and of the DL Fe stripes, however, has not yet been investigated. This study is devoted to the analysis of the influence of the discrete nature of an atomic lattice on the orientation of domain walls in nanostructures. Scanning tunneling microscopy on areas with different local miscut orienta- tions reveals that the domain walls are oriented along the 1  10 and less often along the 3  31 direction, regardless of the orientation of the nanowires. Employing Monte Carlo simulations (MCS) we demonstrate that the wall orientation is determined by the underlying crystalline lattice and the exchange interactions. The magnetic an- isotropy and the magnetostatic energy, which can align walls along certain crystallographic directions in bulk material, play a minor role for the wall orientation. We regard these results to be valid for a large class of low symmetry ultrathin ferromagnetic films. The experiments have been performed in a commercial variable temperature STM attached to a five-chamber UHV system. The instrument is equipped with an x-y sample positioning facility which allows one to access different areas on the same sample. We used etched tung- sten tips for the measurements. Fe was deposited onto the W(110) substrate by molecular beam epitaxy at a pressure p  1  10 10 mbar. To achieve step flow growth the crystal was held at T  500 K during thin film deposi- tion. Simultaneously to constant current images, maps of the differential conductance dI=dU were recorded by means of the lock-in technique. Figure 1 shows the topography (a) and maps of dif- ferential conductance (b)–(d) of 1.7 ML (monolayer) Fe=W110. While the dI=dU map of Fig. 1(b) has been measured simultaneously with and at the same position as the topographic image, the dI=dU maps of Figs. 1(c) and 1(d) show other areas of the same sample which exhibit different local miscut orientations. In any case the Fe DL nanowires can be distinguished from sample locations which are covered by a single Fe layer (SL) due to their different electronic properties resulting in a dI=dU signal that is lower for the SL than for the DL. The DL nano- wires shown in Figs. 1(a) and 1(b) extend approximately along [001], the ones in Fig. 1(c) along 1  10, while in PHYSICAL REVIEW LETTERS week ending 20 FEBRUARY 2004 VOLUME 92, NUMBER 7 077207-1 0031-9007= 04=92(7)=077207(4)$22.50  2004 The American Physical Society 077207-1