Surface Morphology and Magnetic Anisotropy

Understanding the correlation between film structure and its ferromagnetic properties is very important for applications. Despite significant lattice mismatch epitaxial (001) fcc Ni films can be grown on MgO substrates using sputtering or molecular beam epitaxy (MBE). For both types of films it is observed that the average magnetization switching field is very similar but its azimuthal dependence is not. Structural characterization indicates very similar structure for both types of films where subtle differences are responsible for the striking difference in the anisotropy of the magnetic properties. STUDIES ON EPITAXIAL (001) NI FILMS We have shown that epitaxial single-crystal magnetic thin films may be used in magnetic devices such as spin-dependent tunneling applications [1]. In general, the physical properties, particularly the anisotropy [2], of epitaxial thin films are dominated by the crystallographic structure of the metal/substrate interface as well as the surface quality. In addition, for many technological applications, the roughness at the surface must be very small. To that end, we have considered the growth of Ni films on MgO substrates, which can be prepared with very smooth surfaces [3]. Theoretical studies have indicated that for Ni films grown on MgO substrates, Ni is expected to strongly interact with MgO [4]. Various researchers have studied the orientation of Ni films on MgO substrates under various growth conditions [5], and some reports indicate that Ni may form an epitaxial relationship with Ni[001]//MgO[001] and Ni(010)//MgO(010) for films deposited using dc sputtering on MgO substrates held at 100C [6]. In the following, we present our studies on the molecular beam epitaxy (MBE) growth/annealing and surface, structural and magnetic characterization of single domain Ni films grown on (001) oriented MgO substrates. We compare the films with similar ones grown using DC sputtering. Ni films were grown in an MBE VG 80 M system with a background pressure < 5 x 10 torr. Ni was evaporated from a 99.999% pure source. The deposition rate was 0.5Å /sec. The substrates used in the experiment were 0.5 mm thick, 1 x 1 cm prepolished MgO (001) oriented single crystals, which were heat-treated in UHV at 800C for 1 hr. The combination of flat polished substrates and the UHV heating cycle to allow the surface layers to regain crystalline order has been proven to permit growth of single crystal metal films as well as exhibiting sharp reflection high-energy electron diffraction (RHEED) from the MgO surface. Ex-situ atomic force microscopy (AFM) characterization of the annealed surfaces indicated smooth surfaces with a root mean square (rms) surface roughness of 0.2nm for (001) oriented MgO substrates. The substrate temperature was then lowered to the appropriate deposition temperature for metal growth (i.e. 150C). Heat transfer was by direct radiation between the heater and MgO substrate. Similar growth and annealing conditions were used to grow epitaxial films using DC sputtering in a different UHV chamber. For the present study the thickness of both types of films was in the 25-50 nm range. RHEED patterns were recorded continuously during MBE deposition and during subsequent annealing of the films. The RHEED pattern of heat-treated (001) MgO substrates showed long streaks characteristic of a smooth, single-domain surface. Sharp Kikuchi lines indicated long-range lateral coherence. The Ni RHEED pattern evolved from wide and diffuse streaks at the beginning of the growth into sharper and spotty streaks indicating three-dimensional growth [Fig. 1(a)]. The RHEED pattern for a 30 nm thick film indicated single crystal structure for all azimuthal orientations. The surface morphology of the as-deposited and annealed films was also determined in-situ with scanning tunneling microscopy. The mounded quality of the surface was corroborated with in-situ STM. The rms surface roughness of the asgrown films was 0.5 nm. In order to further smoothen the surface, the films were annealed in UHV at 573K (~1/3 of the Ni melting temperature) for several hours. Sharpening of the RHEED pattern during annealing indicated a better crystalline quality as well as smoothening of the surface. It also showed the presence of halforder streaks [Fig. 1(b)] typical of a (2x1) surface reconstruction.