Magnetic characterization of (001) and (111) Ni films epitaxially grown on MgO
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Magnetic characterization of (001) and (111) Ni films epitaxially grown on MgO R. A. Lukaszew1, V. Stoica and R. Clarke Randall Laboratory of Physics, University of Michigan, Ann Arbor, MI 48109 1 Department of Physics and Astronomy, University of Toledo, Toledo, OH 43606 ABSTRACT One interesting application of epitaxial magnetic thin films is to use them as one of the electrodes in a spin-dependent tunneling junction, in order to use the magnetocrystalline anisotropy to define the required two states of the magnetization. [1] In our preliminary work, we prepared epitaxial magnetic films on copper buffer layers grown on silicon substrates. [2] The single crystalline quality of the films was particularly evident in the magnetization hysteresis loops, showing a sharp reversal at fairly high fields (120 Oe), when the samples were magnetized along the crystallographic easy axis. One technological disadvantage in this type of samples is the chemical interaction between the metallic layers and the silicon substrate. In order to explore the possibility of epitaxial magnetic films on less reactive substrates, we studied the growth on MgO substrates. We have shown that it is possible to obtain epitaxial (001) and (111) Ni films grown on MgO substrates. [3] In particular we observed that the crystalline quality of the films improved considerably after 10 nm of film growth. We will now present our studies on the magnetic properties of these films, particularly the azimuthal dependence of the magnetization reversal using MOKRE, correlating our finding with the structural characterization obtained with RHEED, STM and XRD. INTRODUCTION In general, the magnetic properties, particularly the anisotropy, of epitaxial thin films are dominated by the crystallographic structure of the metal/substrate interface as well as the surface quality. In addition, for spin-dependent tunneling devices, the roughness at the surface must be very small in order to ensure the integrity of the subsequent deposition of ultra-thin, pinhole-free insulating layers. Thus, we have considered the growth of magnetic films on MgO substrates, which can be prepared with very smooth surfaces. [3] Here we report on the magnetic switching behavior of these thin films. Our studies reveal the existence of a second-order transition from a single-domain state to a symmetry-broken multidomain state, which leads to singularities in the switching field along specific in-plane crystallographic directions, similarly to what we have observed for epitaxial Co/Cu multilayers and Co thin films grown on silicon. In our previous studies on Co/Cu multilayers when considering the azimuthal dependence of the coercive field, we observed the presence of singular cusps along the crystallographic hard axes, with large coercive fields values, similar to the ones observed along the easy axes. We also observed fourfold symmetry for the (001) oriented samples and uniaxial symmetry for the (111) oriented ones. In order to understand these results we developed a phenomenological model starting
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