Magnetic anisotropy engineering: single crystalline Fe films on ripple surfaces

Nanostructuring by means of ion erosion has proven its versatility with respect to the surface morphology control. By varying different ion irradiation parameters, e.g. ion energy, fluence, incident angle, and sample temperature, the assembly of self-organized periodically ordered arrays of nano-dots [1] and ripples [2] is possible. This has been proven for semiconductors as well as for metals [3,4]. Especially, nanopatterning of magnetic materials is intriguing due to the fact that not only the surface morphology is affected, but the overall magnetic properties are accordingly modified. For example, ion bombardment of single crystal Fe films enables the manipulation of the in-plane uniaxial magnetic anisotropy (UMA) that is associated with the ripple morphology [5].

Here we present a novel bottom-up method of magnetic film patterning, where a highly ordered periodic MgO ripple surface with a wavelength on the nanometer scale is coated by a magnetic Fe layer. The modulations can be induced along any arbitrary in-plane orientation and outstandingly, the surface stays crystalline upon ion irradiation. Thus, due to the low lattice mismatch single crystalline Fe can be grown onto such templates. Despite the intrinsic magnetic cubic anisotropy (CA) of bcc Fe additional UMA is introduced.

As a reference a model cubic system, i.e. Fe on flat MgO(100), has been grown. 15 nm of Fe (tFe) was deposited onto the MgO(100) substrate at room temperature by means of molecular beam epitaxy (MBE). In-plane magneto-optic Kerr effect (MOKE) and ferromagnetic resonance (FMR) angular measurements were performed revealing clear evidence of a four-fold symmetry (CA; K4||/M=186.3 Oe) superimposed with a very small two-fold symmetry contribution (UMA; K2||/M=2 Oe). The magnetic reversal curves (MRCs) shown in Fig. 1a (top panel) correspond to three distinguished sample orientations with respect to the angle φH of the external magnetic field. The curves represent the hard (HA), intermediate, and easy (EA) magnetization orientations of the cubic system. The MRC at φH=18° exhibits a two-jump magnetization process that is a consequence of nucleation and propagation of 90° domain walls (DWs), which is confirmed by magnetic domain observations performed by magneto-optical Kerr microscopy.

In a second step, ripple MgO substrates of a wavelength λrip≈20 nm were prepared (for the AFM micrograph refer to Fig. 1a, top panel) and coated with Fe. Several Fe thicknesses were evaporated in the range of 5-30 nm onto templates with a few distinct in-plane ripple orientations with respect to the MgO [100] direction (0°≤φrip≤60°). Thus, due to the combination of the periodic morphology and the intrinsic magnetic properties of Fe, an ensemble of two-fold and four-fold magnetic symmetry is created. This is confirmed by FMR and MOKE measurements. In general, the UMA originates from dipolar and step-edge effects. The orientation and strength of the UMA depends on the angle of ripple ridges elongation φrip with respect to the MgO [100] direction and Fe film thickness, respectively. First evidence of strong UMA emerges already from MRCs analysis as shown for a sample with φrip≈25° (Fig. 1a, bottom panel), where for in-plane orientations between HA and EA directions a three-jump magnetization process is found. From Kerr microscopy investigations, the magnetization reversal process can be described as a subsequent nucleation and propagation of 90°, 180°, and then again 90° DWs. FMR analysis, i.e. fit of the in-plane angular resonance frequency fres dependence, reveals anisotropy fields and orientations of both anisotropy contributions (Fig. 1b). E.g., for the ripple sample with tFe=15 nm the CA and UMA fields are K4||/M=306 Oe and K2||/M=20 Oe, respectively. Both values are strongly increased relative to flat Fe films. Moreover, the orientation of the UMA coincides with the elongation direction of the ripple ridges for all the samples. In addition, we find that the UMA decreases as a function of Fe thickness, whereas the CA behaves the opposite way. The origin and interaction of the magnetic anisotropy contributions will be discussed in detail. References: [1] S. Facsko et al., Science 285, 1551 (1999)
[2] E. Chason et al., Phys. Rev. Lett. 72, 3040 (1994)
[3] R. Moroni et al., Phys. Rev. Lett. 91, 167207 (2003)
[4] F. Bisio et al., Phys. Rev. Lett. 96, 057204 (2006)
[5] Q. F. Zhan et al., Phys. Rev. B 80, 094416 (2009)