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Notes: We have used the magneto-optical Kerr effect to study the thickness dependence of the perpendicular magnetic anisotropy, coercivity, nucleation field and remanence in an ultrathin Co wedge (0–9 monolayers) grown epitaxially on Pt(111) and capped by a thin Pt overlayer. In addition, the spin-reversal mechanism in the Co layers has been investigated using Kerr domain imaging. The sample showed perpendicular magnetization up to an extrapolated Co thickness of 15 monolayers (ML), with derived volume and interface anisotropy constants of −0.77 MJ/m3 and 1.15 mJ/m2, respectively. The Kerr rotation and Kerr ellipticity demonstrated linear dependences on the Co thickness (tCo), with sizeable extrapolated offsets at tCo=0, attributed to polarization of the Pt by the Co. The coercivity (Hc) and nucleation field both rose to peak values at tCo=1.5 ML, with a subsequent monotonic fall-off for higher Co thicknesses. In the case of Hc, this fall-off did not demonstrate the tCo−5/2 dependence shown by a similar Pd(111)/Co/Pd sample investigated earlier.

Notes: A unique sample was prepared on a Cu(100) single crystal, consisting of three Co layers separated by two Cu layers in the form of wedges oriented perpendicular to each other: Cu(100)/80 A(ring) Co/Cu wedge A/30 A(ring) Co/Cu wedge B/30 A(ring) Co/7 A(ring) Cu/30 A(ring) Au. Position-sensitive magneto-optical Kerr effect measurements along Cu wedge B, at a fixed position on Cu wedge A corresponding to maximum antiferromagnetic (AF) coupling, enabled us to investigate not only the AF but also the ferromagnetic (F) coupling between the two 30 A(ring) Co layers as a function of the Cu thickness. The measurements confirmed both long and short period coupling oscillations in the AF regime, and revealed the predicted extension of the short period through the F regime.

Notes: A Pd/Co double layer has been grown epitaxially in ultrahigh vacuum on a Pd(111) single crystal, with the Co in the shape of a wedge of thickness varying from 0 to 10 monolayers (ML). The polar magneto-optical Kerr effect has been used to make a nearly continuous set of measurements of the magnetic properties as a function of Co thickness tCo. The coercive fields Hc and nucleation fields Hn show large, well-defined maxima at tCo∼2.3 ML. Between 4 and 9 ML, we find that Hc falls with increasing thickness according to a tCo−5/2 dependence. The Kerr ellipticity and Kerr rotation depend linearly on Co thickness above ∼2 ML Co, but show appreciable offsets when extrapolated to tCo=0. These offsets are clear evidence of the polarization of Pd near the interfaces. Extensive in situ growth studies of the structure of the films are also reported.

Notes: Magnetic anisotropies of ultrathin transition-metal films are inherently related to their structural properties. In ultrathin films the large fraction of atoms located at the film interface generates strong interface anisotropies, whereas elastic strain fields caused by the forced registry of atoms at the substrate/film interface induce magnetoelastic anisotropy contributions. So far the experimental confirmation of the transition from these thin-film properties to bulk anisotropy properties, characterized by a dominating magnetocrystalline anisotropy, has not yet been presented. Magnetic anisotropies reflect, depending on their origin, both the crystallographic symmetry and the symmetry of the film geometry. For a clear separation between magnetoelastic, magnetocrystalline and Néel-type interface anisotropy contributions, the film symmetry and thickness must be chosen such that the respective different anisotropy contributions appear with different symmetries and film thickness dependencies. This is the case for (110)-oriented fcc Co films. In the present study we use the Brillouin light-scattering technique for the determination of the anisotropy contributions. An analysis of the spin-wave frequency measured as a function of the in-plane direction of the external field and the film thickness yields information about all relevant anisotropies. The samples used were molecular-beam-epitaxy grown in ultrahigh vacuum. Onto a Cu (110) single-crystal substrate a wedge-type sample and two staircase-shaped samples with distinct thicknesses in the range of 8–110 A(ring) were grown.To obtain symmetric Co/Cu interfaces the Co layers were covered with a 12 A(ring) Cu layer. Finally, a 25-A(ring)-thick Au protective layer was deposited. Low-energy electron-diffraction studies were used to obtain the structural data of the films. All relevant anisotropy contributions—the magnetocrystalline anisotropy, and the uniaxial in-plane and out-of-plane anisotropy contributions—were determined. Three different anisotropy regimes are observed as a function of the Co layer thickness dCo. This thickness regime up to 13 A(ring) is dominated by the magnetoelastic anisotropy contributions as a result of the pseudomorphic film growth of the Co layer. For Co layer thicknesses larger than 13 A(ring) we find a reduction of the magnetoelastic anisotropy contributions. This is structurally correlated to an anisotropic relaxation of the in-plane Co lattice constant. In the regime of dCo(approximately-greater-than)50 A(ring) we observe a thickness-independent value for the magnetocrystalline anisotropy contribution K1=−8.5×105 erg/cm3. This anisotropy contribution is largely suppressed for dCo〈50 A(ring). This finding might either indicate a breakdown of the usually postulated linear superposition principle of magnetic anisotropy contributions to the free anisotropy energy, or it might point to a subtle modification of the electronic band structure. At the onset of the magnetocrystalline anisotropy we find a change in the easy magnetization direction from 〈001〉 for thin Co films to 〈111〉 for thicker ones. For a more detailed discussion see Ref. .

Notes: A dependence of the strength of the antiferromagnetic coupling across Cu on the Co layer thickness has been observed. The Co thickness dependence displays two clear peaks consistent with the recently predicted oscillation period of 6.2 A(ring) Co. Apart from the two peaks also several small peaks are visible on a scale of about 1 monolayer Co. Free-electron calculations indicate that these rapid variations in strength may result from slight differences between the slopes and starting points of the two Co wedges that were involved in the experiment.

Notes: The exchange biasing field (Heb) and coercive field (Hc) of molecular-beam-epitaxy-grown Cu/Ni80Fe20/Fe50Mn50 samples in [111], [001], and [110] orientations have been investigated by longitudinal Kerr effect measurements. Ni80Fe20 and Fe50Mn50 were deposited as orthogonal wedge-shaped layers on single-crystal Cu substrates in a magnetic field, enabling the study of the thickness dependence of Heb and Hc on a single sample for each orientation. A strong dependence of Heb and Hc on the growth orientation is observed. The results are interpreted in terms of the observed noncollinear spin structure of the antiferromagnet and a comparison is given with the predictions from recent theoretical models.

Notes: We discuss the fabrication and microstructuring techniques of pillar structures made of high vacuum sputtered Fe/Cr multilayers and of molecular beam epitaxy evaporated Co/Cu multilayers, for which we measured the giant magnetoresistance effect with the current perpendicular to the multilayer plane from 4 K to 300 K. Using optical lithography and reactive ion etching techniques we obtained structures with a typical height of 0.5 μm and a width ranging between 3 and 10 μm. For both Fe/Cr and Co/Cu multilayers we find an enhanced magnetoresistance with respect to the in-plane case. The perpendicular magnetoresistance of the Fe/Cr pillars strongly decreases with temperature, while for the Co/Cu samples the temperature dependence is weaker, indicating electron-magnon scattering processes of different strength.

Notes: Magnetic anisotropies and misfit strain relaxations have been investigated in Cu/Ni-wedge/Cu (100) and (111) sandwiches deposited by molecular beam epitaxy on single-crystal Cu substrates. Our results reveal a clear distinction in the nature of the measured anisotropy at Ni thicknesses below and above the critical value tc, where the growth becomes incoherent. Below tc, coherent lattice strain modifies only the volume anisotropy, while interface anisotropy is Néel type; above tc, magnetoelastic effects are found to contribute to the interface anisotropy.

Notes: An oscillatory interlayer coupling in Co/Au(111)/Co has been obtained from polar Kerr hysteresis loop measurements on a molecular-beam-epitaxy-grown 50 A(ring) Au/2 A(ring) Co/0–40 A(ring) Au wedge/7 A(ring) Co/20 A(ring) Au sample deposited on a Cu(111) single crystal. The period of the coupling is seen to increase from 11 to 14 A(ring) with increasing Au thickness, indicating a preasymptotic behavior. The asymptotic period of 14±1 A(ring) is larger than the theoretically expected period of 11.4 A(ring). The observation of coupling up to 40 A(ring) Au will be discussed in relation to the general absence of coupling in evaporated Co/Cu(111)/Co samples.

Notes: Abstract The potential of natural enemies to influence the rate of herbivore adaptation to resistance factors in plants is examined using conceptual and mathematical models. Results indicate that natural enemies could increase or decrease the rate of herbivore adaptation. The specific behavioral and physiological effects of a resistance factor on the herbivore, as well as the behavior of the natural enemy, and the population dynamics of the natural enemy/herbivore system are important in assessing the extent to which the natural enemies will affect the rate of herbivore adaptation to a resistance factor. Herbivore adaptation to partial resistance in a host-plant is generally expected to be slower than adaptation to high levels of resistance, even in the presence of natural enemies, if genetic variance is not limiting.