Thickness Dependent Perpendicular Magnetic Domain Patterns in Sputtered Epitaxial FePt (001) L1 0 Films
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ABSTRACT The present paper discusses the magnetic anisotropy and magnetic domain structure of highly ordered epitaxial FePt(001) films grown on Pt seeded MgO(001) substrates. These films were grown by dc-magnetron sputtering from a Fe 50Pt 50 alloy target at a substrate temperature of 550 'C during deposition. Thicknesses were varied between 15 and 170 nm. The presence of the highly anisotropic face centered tetragonal Li 0 crystal structure with a maximum long range chemical ordering of 95% and a low degree of misorientations was confirmed by specular and grazing incidence X-ray diffraction measurements. For film thicknesses > 50 nm in-plane and out-of-plane hysteresis measurements indicate large perpendicular magnetic anisotropy and at the same time low remanent magnetisation. Magnetic force microscopy reveals highly interconnected perpendicular stripe domain patterns. From their characteristic width, which is strongly dependent on the film thickness, a value of the dipolar length, Do, of 50 ±5 nm is derived. Assuming an exchange constant of 106 erg/cm, this value is consistent with an anisotropy constant Ku - 1.101 erg/cc. INTRODUCTION Among the thin film and superlattice systems exhibiting perpendicular magnetic anisotropy, the face centered tetragonal (fct) phase of binary alloy systems like CoPt, FePd and FePt, also referred to as CuAu(I) or L10 phase, has in recent years attracted great interest. This phase consists of a monatomic, chemically modulated superlattice of the two elements. When grown with the monatomic layers parallel to the film plane, i. e. with the c-axis of the fct unit cell in the film normal direction, this structure results in perpendicular magnetic anisotropy. Large polar magneto-optical Kerr effects of up to 0.8' at 2 eV photon energy in FePt [1], make these materials attractive candidates for media applications in magneto-optical recording. Furthermore, it is generally assumed that high anisotropy is a prerequisite to support large coercivities and thermal stability at ever smaller physical grain sizes in high density magnetic recording media [2]. The magneto-crystalline anisotropies of FePt and CoPt are among the highest reported in the literature [3], making them attractive base materials for future high density magnetic recording media [4]. Epitaxial growth of chemically ordered FePt thin films with the c-axis perpendicular to the film plane has previously been demonstrated by molecular beam epitaxy (MBE) [1] and magnetron sputtering [5, 6]. The temperature dependence of the chemical ordering shows that a markedly lower temperature is required for complete ordering in thin film growth [7] as opposed to bulk material [8] (i. e. 500 'C vs. 1300 'C, respectively). This has been attributed to the enhanced surface mobility during thin film growth [7]. Here we present a detailed investigation of the magnetisation behaviour and the magnetic domain structure of fully chemically ordered FePt epitaxial thin films grown by magnetron sputtering. A more detailed investigation of the structural p
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