Surface morphology and magnetic anisotropy of Fe/MgO(001) films deposited at oblique incidence

Abstract

We have studied surface morphology and magnetic properties of Fe/MgO(001) films deposited at an angle varying between 0 o and 60 o with respect to the surface normal and with azimuth along the Fe [010] or the Fe[110] direction. Due to shadowing, elongated grains appear on the film surface for deposition at sufficiently large angle. X-ray reflectivity reveals that, depending on the azimuthal direction, films become either rougher or smoother for oblique deposition. For deposition along Fe[010] the pronounced uniaxial magnetic anisotropy (UMA) results in the occurrence of "reversed" two-step and of three-step hysteresis loops. For deposition along Fe[110] the growth-induced UMA is much weaker, causing a small rotation of the easy axes.Magnetic anisotropy of epitaxial films and its relationship to surface morphology have attracted much attention in recent years [1]. Both film properties are intimately related to the molecular beam epitaxy (MBE) deposition process. Oblique incidence deposition results via a self-shadowing effect [2,3] in the formation of grains in the plane of the film that are elongated perpendicular to the incident flux direction and with aspect ratio increasing at larger deposition angle with respect to the surface normal [4]. Consequently, an in-plane uniaxial magnetic anisotropy (UMA) with easy axis perpendicular to the incident flux direction is induced during growth of the magnetic films [4,5,6]. UMA was found to play an important role in determining the magnetization reversal in thin films of cubic systems [7,8]. Depending on the strength and orientation of the UMA, hysteresis curves with one, two and three steps are observed in various films at different field orientation. The appearance of the steps can be explained in terms of nucleation and propagation of domain walls (DWs) [8,9]. Consequently, understanding the influence of oblique incidence growth is very important because of its ability to control both magnetic anisotropy and surface morphology [10,11,12].Although shadowing effects have been studied in many magnetic systems, including Fe on MgO(001) [13] and Co on Cu(001) [14], the plane of oblique incidence was always kept parallel to the in-plane cubic easy axes of the magnetic layers. Here, we report on the influence of oblique deposition on the surface morphology and magnetic properties of Fe/MgO(001) films for deposition azimuth both along the Fe Fig. 1(c)). The elongated grains are believed to result from a redistribution of the incident flux due long-range attractive forces [15,16]. During evaporation the incident Fe atoms arrive preferentially on top of already formed grains rather than behind these grains. This shadowing effect is also observed for film deposition at 60 o with azimuth along [110]. The Fe grains on the surface are rhombic in shape with typical length of 15 nm and typical width of 6 nm (see Fig. 1(d)). Based on the STM images we conclude that the self-shadowing effect can be neglected for a deposition angle below 30 o . Moreover, the self-shadowing effect is...

Modulation of magnetic anisotropy in Fe/Si(111) thin films through interface property

“…The surface was composed of nano-sized grains and there was no observable geometrical anisotropy in the surface morphology and microstructure. Oblique deposition induced 1-d nanostructures and uniaxial magnetic anisotropy have been reported as a convincing and universal experimental conclusion [3][4][5][6][7][8]. This study, especially the MOKE measurement, indeed reveals this idea works well on a sputtered semiconductor substrate even with a capping layer.…”

“…1 exhibits the basic ideas of this experiment. When the evaporated Fe atoms were deposited obliquely to the Si(111) substrate, the shadowing effect will result in anisotropic nucleation, leading to the formation of elongated nano-grains or quasi-1-dimensional nanostructures [3][4][5][6][7][8]. The different oblique angle and the deposit coverage may result in difference consequence of the quasi-1-d nanostructures on the Fe surface.…”

“…As shown in Fig. 1(a), because of the shadowing effect, the oblique deposition of magnetic atoms on suitable substrates can lead to highly elongated grains or ripples on nanometer scale [3][4][5][6][7][8]. These elongated nanograins or ripples can induce a uniaxial MAE.…”

“…This method of manipulating MAE is easy to apply to magnetic thin films and nano-devices. Beside of the conventional metal(001) substrates, such as Co/Cu(001) and Fe/Ag(001) [3,4], recent studies have applied the oblique-incidence epitaxial growth to the heterostructures of metal/semicondutor and metal/insulator, for example Fe/MgO(001) and Co/Si(111) [5][6][7][8]. Our previous report also demonstrated that in Pd/obliquely deposited Fe/Al 2 O 3 (0001), despite of a protective Pd overlayer and the insulating Al 2 O 3 (0001) substrate, the obliquely deposited Fe thin films still revealed a significant uniaxial MAE [1].…”

Fe on W(001) from continuous films to nanoparticles: Growth and magnetic domain structure

“…This oblique incidence could lead with increasing deposition time to the formation of ripples perpendicular the direction of the vapor beam that would introduce some shape anisotropy with the easy axis along the ripple direction [38,39]. However, we can rule out such oblique incidence growth-induced ripple morphology here as the origin of the [110]-oriented magnetization in thick films ( 8 ML) because domains with an easy axis along the [110] axis, orthogonal to the easy axis shown in Figs.…”