PHYSICAL REVIEW B 86, 064432 (2012) Uniaxial magnetic anisotropy of cobalt films deposited on sputtered MgO(001) substrates Kai Chen, 1,3,* Robert Fr¨ omter, 1 Stefan R¨ ossler, 1 Nikolai Mikuszeit, 2 and Hans Peter Oepen 1 1 Institut f ¨ ur Angewandte Physik, Universit¨ at Hamburg, Jungiusstr. 11, 20355 Hamburg, Germany 2 Instituto Madrile ˜ no de Estudios Avanzados en Nanociencia, IMDEA-Nanociencia, Campus Universidad Aut´ onoma de Madrid, 28049 Madrid, Spain 3 Institute for Materials Research, Helmholtz-Zentrum Geesthacht, 21502 Geesthacht, Germany (Received 26 April 2012; revised manuscript received 4 July 2012; published 22 August 2012; publisher error corrected 24 August 2012) We present a systematic investigation of the in-plane uniaxial magnetic anisotropy induced by the morphology due to ion erosion of MgO(001). Ion milling at oblique incidence forms a ripple structure on the MgO surface the grooves run along the ion beam direction. Ultrathin cobalt films grown on such templates show a dominant uniaxial magnetic anisotropy with the easy axis along the ion beam direction. Both the strength of anisotropy and its symmetry can be controlled via the milling conditions, allowing one to fine-tailor the anisotropy in magnetic films. A uniaxial, volumelike anisotropy contribution is found, which is explained by the modulation of the magnetization perpendicular to the ripples that causes an increase of exchange energy. DOI: 10.1103/PhysRevB.86.064432 PACS number(s): 75.70.−i, 75.30.Gw, 75.60.Ej, 81.16.−c I. INTRODUCTION The artificial tuning of magnetic properties of ultrathin films by means of the surface and interface structure is a fascinating issue from both fundamental and technological points of view. 1–3 In particular, manipulating the magnetic anisotropy is one of the most effective ways to optimize the performance of thin-film based devices. 4–7 Zhan et al. demonstrated the capability of tuning of the in-plane uniaxial anisotropy of Fe/MgO(001) films via ion milling. 8 Bisio et al. investigated the possibility of isolating the step-induced in-plane uniaxial magnetic anisotropy in order to rule out some ambiguities of vicinal surfaces using an ion sculpted Ag substrate. 9 With respect to applications, it is most advantageous to tailor both the magnitude and symmetry of the magnetic anisotropy. Changing the morphology of surfaces by oblique-incidence ion beam irradiation has been successfully performed with metals, semiconductors, and insulators. 10–14 Via oblique- incidence ion beam milling, a self-assembled formation of nanometer-scale surface ripples has been observed, which were aligned either parallel or perpendicular to the direction of the ion beam. 15–18 Magnetic films that are deposited on substrates with such microstructure reveal a uniaxial magnetic anisotropy with the easy axis in general along the direction of the ripples. 19–22 Furthermore, the magnitude of the uniaxial magnetic anisotropy can be controlled via the surface morphol- ogy, which depends sensitively on the sputtering conditions. Even when a magnetic film is ion milled after deposition, a uniaxial magnetic anisotropy was found due to the formation of ripple structures in the film itself. 7,9,23,24 In this paper, we report on the manipulation of the uniaxial magnetic anisotropy of ultrathin Co films deposited on MgO(001) that are sputtered by Ar + ions prior to the Co deposition. Changing the sputter geometry from normal to oblique incidence (angle of 60 ◦ with respect to the surface normal) a transformation from biaxial to uniaxial anisotropy is found for the Co films. It is shown that the anisotropy can be tuned by changing the ion dose  c . On variation of the film thickness we find a constant anisotropy contribution K v u , which we attribute to the volume, and a surface contribution of K s u /t , which is inversely proportional to the film thickness. II. EXPERIMENT The preparation and characterization of the Co/MgO(001) system is performed in an ultrahigh-vacuum chamber with a base pressure in the low 10 −7 Pa range, which increases to 1 × 10 −6 Pa during Co deposition. The in situ magnetic characterization is performed by means of magneto-optic Kerr effect (MOKE). The single-crystal MgO(001) substrate is sputtered with Ar + ions at an energy of 1400 eV at room temperature. The sputtering is either performed at normal incidence or at a fixed angle of 60 ◦ with respect to the surface normal. After sputtering the substrate is annealed at 800 K for 10 min. This procedure creates a ripple structure on the MgO(001) surface. An atomic force microscopy (AFM) image is shown in Fig. 1(a). A period of ∼100 nm and a height modulation of ∼20 nm (peak-to-peak) is found here for an ion dose of 2.28 × 10 17 ions/cm 2 , which corresponds to 200 MLE (fcc monolayer equivalent). The ripple structure on the MgO(001) surface is confirmed with in situ low- energy electron diffraction (LEED). For sputtering at normal incidence the LEED pattern exhibits a fourfold symmetry [Fig. 1(c)], and it reveals that the fourfold symmetry is not broken upon normal incidence sputtering. Sputtering under grazing incidence, however, creates spots that present an elliptical shape [Figs. 1(d) and 1(e)]. This change of the diffraction pattern originates from a ripple structure with twofold symmetry. In the grazing-incidence geometry the azimuthal orientation of the sample has been varied. In case the in-plane orientation of the beam is changed from parallel to MgO[100] [Fig. 1(d)] to MgO[010] [Fig. 2(e)] the orientation of the long axis of the elliptical spots switches by 90 ◦ . The ripples are oriented parallel to the in-plane projection of the ion beam. AFM images were also taken after depositing Co films. For a 10 nm Co film on a rippled MgO surface the AFM image [Fig. 1(f)] reveals that the morphology is preserved in the film. A period of ∼100 nm and a modulation of ∼12 nm (peak-to-peak) is observed. Co films up to a thickness of 15 monolayers (ML) are deposited by electron-beam evaporation at room temperature. The deposition rate is kept constant at about 2 ML per minute. The film thickness is calibrated using Auger Electron 064432-1 1098-0121/2012/86(6)/064432(7) ©2012 American Physical Society