PHYSICAL REVIEW B 84, 054456 (2011) Magnetic domain patterns in Co 2 MnGe Heusler nanostripes K. Gross, P. Szary, O. Petracic, F. Br¨ ussing, K. Westerholt, and H. Zabel Institut f ¨ ur Experimentalphysik /Festk¨ orperphysik, Ruhr-Universit¨ at Bochum, D-44780 Bochum, Germany (Received 2 March 2011; revised manuscript received 20 June 2011; published 15 August 2011) We have prepared thin films of the ferromagnetic Heusler alloy Co 2 MnGe on a-plane Al 2 O 3 substrates exhibiting growth-induced, superimposed fourfold and uniaxial magnetic anisotropies, the symmetry being determined by the single-crystalline Al 2 O 3 substrate. The magnitude of the uniaxial anisotropy compared to the cubic anisotropy can be tuned over a wide range by the film thickness and the growth conditions. On submicrometer-wide stripes of Co 2 MnGe prepared by electron beam lithography we studied magnetic domain patterns by magnetic force microscopy. For stripes with a sufficiently large uniaxial anisotropy and with the easy axis oriented perpendicular to the stripe axis, we find perfectly regular domain patterns with the magnetization direction perpendicular to the stripe axis and alternating from domain to domain. The highly regular and controllable domain patterns in Co 2 MnGe nanostripes could be useful for magnetic storage devices and applications related to spin transfer torque. DOI: 10.1103/PhysRevB.84.054456 PACS number(s): 75.75.Cd, 75.60.Ch, 75.70.Ak I. INTRODUCTION For current-driven domain wall processing on nanosized magnetic stripes, a domain structure with highly symmetric domain patterns and with the magnetization direction perpen- dicular to the stripe axis would be favorable. This type of domain structure can be realized in ferromagnetic films with a uniaxial magnetic anisotropy axis perpendicular to the stripe axis. 1 Model systems for this situation in the literature are single-crystalline Co nanostripes with the magnetic easy axis perpendicular to the stripe axis 2–4 and Fe nanostripes grown epitaxially on GaAs. 5–7 However, the uniaxial anisotropy constant K u of hexagonal Co (K u ≈ 5 × 10 5 J/m 3 ) (Ref. 2) as well as for epitaxially grown Fe stripes (K u ≈ 5 × 10 4 J/m 3 ) (Ref. 5) is relatively large, and since the critical current density for current-driven domain wall motion scales with the anisotropy energy, 8 domain wall motion by spin transfer torque would require very high current densities. The ferromagnetic Heusler half metals such as Co 2 MnGe have a cubic crystal structure and in the bulk exhibit a weak cubic magnetic anisotropy. 9 When grown as thin films they often develop a uniaxial magnetic anisotropy superimposed on an anisotropy with cubic symmetry. Examples in the literature are Co 2 MnGe grown on GaAs(100) (Refs. 10–12) and Co 2 MnGe grown on Al 2 O 3 (11 ¯ 20). 13–15 Here we show that in nanosized stripes of Co 2 MnGe grown on Al 2 O 3 , a growth- induced uniaxial anisotropy with a value of K u approximately two orders of magnitude smaller than in single-crystalline Co is sufficient to induce the formation of similar highly symmetric perpendicular domain patterns. In addition to the aforementioned magnetic anisotropy, ferromagnetic half metallic Heusler alloys, such as the title compound Co 2 MnGe, have recently attracted much interest due to their large potential as spintronic materials. 16,17 One of the exceptional features of the half metallic Heusler com- pounds is the full spin polarization at the Fermi level, predicted theoretically for chemically perfectly ordered alloys. 18 Al- though, to the best of our knowledge, this full spin polarization has not been achieved yet in real devices, the very high tunnel magnetoresistance in magnetic tunneling junctions with Heusler electrodes demonstrate their technological potential. 19 Heusler alloys are also discussed as the material of choice for giant magnetoresistance (GMR)-based read heads in the next generation of hard disk drives, 20 and they possess a combination of properties, 21 making them ideally suited for applications using the spin torque transfer mechanism 22 such as the spin transfer torque oscillator, 23 or domain wall logic. 24 Recently it has been shown that the critical current density needed for current-driven magnetization reversal in a Co 2 MnAl 0.5 Si 0.5 Heusler nanocontact 25 was smaller than for similar Permalloy nanocontacts. 26 II. PREPARATIONAND EXPERIMENTAL Thin films of Co 2 MnGe were prepared by rf sputtering from a Heusler alloy target with a stoichiometric composition on (11 ¯ 20) Al 2 O 3 substrates at a substrate temperature of 300 ◦ C. Prior to the Co 2 MnGe film growth, a 4-nm-thick V seed layer was deposited to induce a high-quality (110) textured growth of the Heusler layer (for details, see Ref. 27). A 5-nm-thick Au cap layer protects the film against oxidation. No further postgrowth treatment of the samples was applied. The crystallographic structure of the film was studied by an in-house x-ray diffractometer using Cu Kα radiation. For in-plane rocking scans of selected samples we employed synchrotron radiation at the beam line W1.1 of the HaSyLab (DESY, Hamburg) on a six-circle diffractometer using a photon energy of 10 510 eV. The film thicknesses were checked by x-ray reflectivity measurements. The x-ray out-of-plane Bragg scan of a 100-nm-thick Co 2 MnGe film grown on Al 2 O 3 -(11 ¯ 20) is depicted in Fig. 1. Besides the substrate Bragg reflections, only the Heusler Bragg reflections (220) and (440) appear, thus confirming the expected (110) out-of-plane texture of the film. The in-plane rocking scan of the (022) Bragg reflection of the same sample is shown in Fig. 2. For pure epitaxial growth one would expect a twofold symmetry of the (022) reflection. The film, however, exhibits a 2 × 6 = 12-fold symmetry, i.e., six domains with twofold symmetry. The neighboring Bragg peaks enclose exactly 30 ◦ between each other. The c axis of the Al 2 O 3 substrate points in the direction of one of the 054456-1 1098-0121/2011/84(5)/054456(8) ©2011 American Physical Society