Temperature-induced magnetic anisotropies in Co/Cu(1 1 17)

Abstract

The temperature dependence of magnetic anisotropy in cobalt films grown on Cu(1 1 17) has been studied by means of the magneto-optic Kerr effect. An in-plane uniaxial anisotropy is found in the as-grown films. At elevated temperatures the films exhibit changes in anisotropy. At temperatures around 100'C a transition to nearly biaxial behavior is found which transforms again into uniaxial behavior at higher temperatures. Both transitions generate pronounced secondary maxima in the temperature-dependent susceptibility.One of today's most interesting topics in magnetism is ferromagnetism in thin 6lms and multilayers. Due to recent developments in the 6eld of epitaxy it becomes feasible to create materials in diferent forms of condensation, e. g., regarding their crystal structure and/or lattice constant. Such tailoring of material parameters allows one to investigate the dependence of magnetism on distinct, well-characterized properties of ferromagnets. This tendency in experimental magnetism is accompanied by strong activities in theory. 2 Some of the theoretically modeled systems may be realized as thin films now. Thus 6rst-principals calculations of magnetic properties may be compared with experimental findings giving a strong impetus to the understanding of magnetism in general.The dependence of magnetization on particular directions is known as magnetic anisotropy, which includes crystalline (i.e., symmetry), strain as well as shape effects. Hence, structure and morphology determine the anisotropy. Because the selection of a substrate and/or growth conditions ofFers the possibility of manipulating the magnetic behavior, a huge number of papers are dealing with anisotropy behavior of thin 6lms. The major point with the studies on magnetic anisotropies is to figure out which structural properties are predominantly reflected in the magnetic behavior.A strong impetus on activities in thin-film magnetism is related to the concept of surface anisotropies proposed by Neel.Neel pointed out the importance of the reduced symmetry at the surface of a ferromagnet. A new facet of Neel's approach of anisotropy has been proposed by Albrecht et al. recently. The reduced symmetry at surface steps caused additional twofold contributions to surface anisotropy. The authors could extrapolate such step anisotropy from their results.An elegant way to demonstrate the infIuence of steps on anisotropy is to study ferromagnetic 6lms grown on vicinal surfaces.Because of the step alignment and high step density on such surfaces, the influence of steps can be observed immediately as a strong twofold anisotropy. Recently it was demonstrated that first-principals calculations of electronic structure in fcc lattices are in good agreement with a Neel ansatz considering only nearest-neighbor interactions. Symmetry-based contributions to the anisotropy energy have been given. Based on this, a complete Neel ansatz including all energy contributions for a stepped ultrathin ferromagnet has been worked out. i The rigorous description shows twofold vo...

Magnetic ordering in Co films on stepped Cu(100) surfaces

“…4 The anisotropy switches to biaxial in-plane at increased temperatures. 13 This may be due to Cu atoms decorating the step edges, 14 or to restructuring of the step edges with rectangular protrusions perpendicular to the original step. 15 In the latter case, micromagnetics simulations reported here predict a small ͑ϳ3%͒ out-of-plane component to the magnetization.…”

Unidirectional anisotropy in ultrathin transition-metal films

“…[8][9][10] In the context of ultrathin-film magnetism, unidirectional Kerr hysteresis loops have been observed for stepped Co surfaces: Co/Cu͑1117͒ films vicinal to fcc ͑001͒ exhibit an unambiguous but unexplained unidirectional shift of order 1 mT. 11,12 Although nonferromagnetic spin structures cannot be ruled out in low-symmetry 3d films, 13 it is not possible to ascribe the observed loops to the well-known exchange between ferromagnetic and antiferromagnetic phases as in Co/CoO.…”

“…Furthermore, the unidirectional shift of the hysteresis loops is most pronounced if the field is parallel rather than perpendicular to the steps. 11,12 …”

“…͑4͒ with the behavior of fcc (11n) surfaces vicinal to fcc ͑001͒. 11,12,[22][23][24] Figure 4 shows the atomic structure of fcc (11n) surfaces, which involve four types of atoms: bulk atoms, surface atoms, step-edge atoms, and step-corner atoms. 22 Since there are no second-order bulk contributions, we have to deal with N s surface atoms, N c step-corner atoms, and N e step-edge atoms.…”

Anisotropic susceptibility of ferromagnetic ultrathin Co films on vicinal Cu

“…2 The singularity of χ(T ) corresponds to the onset of a ferromagnetic state, i.e. to the occurrence of a nonvanishing magnetization m(T ) = |m(T )| for temperatures below the (ferromagnetic) Curie temperature T C .…”