ALBERT

Notes: We have measured the elastic properties of saturated ODF, and of unpolymerized and polymerized regular ODF, ODF/ODM, and ω-ODF Langmuir–Blodgett films by means of Brillouin spectroscopy. Whereas the ODF/ODM films show a slight increase in the elastic constants, the regular ODF and ω-ODF films exhibit elastic constants which are significantly softer in the polymerized films due to the formation of microcracks on polymerization.

Notes: We have studied the magnetic properties of Co/Cr/Fe multilayers by means of magneto-optic methods. These multilayers are of special interest because they combine different anisotropies and exchange coupling through the nonmagnetic Cr-spacer layer which was found to oscillate from ferromagnetic to antiferromagnetic coupling. The samples were grown by MBE methods on MgO(100) substrates at temperatures of about 300 °C. The fourfold crystalline symmetry is also obtained in the magnetic easy axis structure of Fe and Co. From the results of magneto-optic Kerr hysteresis loops we were able to distinguish between crystalline anisotropy and exchange coupling contributions. Additional monitoring of the domain structure by applying longitudinal Kerr microscopy was necessary to identify the regime were magnetization reversal occurs via domain wall motion or coherent rotation of the magnetization of the entire layer, respectively. From these results we were able to perform numerical simulations of the micromagnetic properties of trilayer samples. We discuss the results in terms of bilinear and biquadratic coupling. © 1996 American Institute of Physics.

Notes: [Auszug] Bose–Einstein condensation is one of the most fascinating phenomena predicted by quantum mechanics. It involves the formation of a collective quantum state composed of identical particles with integer angular momentum (bosons), if the particle density exceeds a critical value. To achieve ...

Notes: We report on studies of the profiles of the strain field components in epitaxially grown (110)-oriented Co/Pt superlattices with fixed Pt layer thickness (nominally 18 A(ring)) and varying Co layer thickness. The strain profiles in the growth direction as well as fluctuations in lattice parameters at the interfaces are obtained using a parameter refinement method for the quantitative analysis of high angle aitch-theta–2aitch-theta x-ray diffraction scans. We find that all strain components originate only from the Co-Pt interfaces. Off-axial x-ray scattering measurements provide for the in-plane strain components, which are found to increase with increasing Co layer thickness. From the obtained strain fields the in-plane uniaxial magnetic anisotropy contributions caused by magnetoelastic interactions are calculated, which agree well with experimentally observed values.

Notes: The stabilization of long-range ferromagnetic order in two-dimensional systems at finite temperatures is presently discussed very controversially. Among the mechanisms dipolar interactions or magnetic anisotropies are currently most considered. For experimental clarification all relevant magnetic anisotropy constants of ultrathin Co layers on Cu(001) as well as on Cu(1 1 13) substrates using Brillouin light scattering.1,2 Due to the fourfold symmetry of Co(001) films the relevant anisotropy contributions are a fourfold in-plane anisotropy Kin-plane(4)=Kp(4)+2kp(4)/d and a perpendicular anisotropy Kperp=Ks+2ks/d with Kp(4) and Ks (kp(4) and ks) the in-plane and out-of-plane volume (surface) anisotropy constants, respectively, and d the film thickness. For Co films prepared on Cu(1 1 13) substrates an additional uniaxial in-plane anisotropy (Kin-plane(2)) is generated by the rotation of the (001) surface about the [11¯0]-in-plane axis by 6.2°. The symmetry axis of this anisotropy lies along the [11¯0] axis. Co films with film thicknesses between 1.5 and 14 monolayers (ML) were prepared in an ultrahigh vacuum (UHV) system and characterized with low-energy electron diffraction (LEED) and Auger spectroscopy as well as by in situ magneto-optic Kerr effect, as described elsewhere.1,2 The Brillouin light scattering experiments were performed in situ in the UHV system for Co/Cu (001) with the external field aligned parallel to the [100] hard axisThe light scattering measurements on Co/Cu (1 1 13) with a Cu overlayer were performed ex situ with the external field direction aligned along different in-plane directions. From a fit to the measured spin wave frequencies as a function of the applied field and the Co film thickness the anisotropy constants are obtained. The Co/Cu (001) system is discussed first. Ferromagnetic order is observed for film thickness d larger than dc=(1.6±0.3) ML for uncovered Co films and dc=(1.9±0.3) ML for Co films covered by a 2-ML thick Cu overlayer. From the fits to data for different film thicknesses it was found that Kp(4)=(−2.32±0.15)×106 erg/cm3 and kp(4)=(0.034±0.004)erg/cm2 for the uncovered films, and Kp(4)=(−2.17±0.15)×106 erg/cm3 and kp(4)=(0.031±0.003)erg/cm2 for the Co layers covered with 2-ML Cu.1 Due to their opposite sign, the contributions of Kp(4) and kp(4) to Kin-plane(4) cancel each other at dc*=(1.55±0.3)ML for the uncovered films and at dc*=(1.7±0.3)ML for the films covered with 2-ML Cu. The out-of-plane anisotropy constant ks was found as ks=(−1.06±0.17)erg/cm2 for the Co/vacuum interface and ks=(0.15±0.04)erg/cm2 for the Co/Cu interface. The negative sign indicates that the surface normal is a magnetic hard axis for this anisotropy. No volume out-of-plane contribution, Ks, was found.From the observed agreement between the critical thickness for ferromagnetic order dc, with the thickness dc* at which the contributions to the in-plane anisotropy cancel, it can be concluded that the symmetry breaking interaction for stabilizing ferromagnetic order in Co(001) films at room temperature is indeed given by the magnetic in-plane anisotropy contribution. For Co/Cu(1 1 13) an additional large uniaxial in-plane anisotropy contribution of Kin-plane(2)=(−4.4±0.4)×105 erg/cm3 was found.2 The negative sign indicates that the easy axis is parallel to the step edges. The fourfold in-plane anisotropy constant, Kin-plane(4), is reduced by a factor of 2 to 4 compared to the (001)-oriented films. For the Co/Cu(1 1 13) system the uniaxial in-plane anisotropy can be described as magnetoelastic in origin: The determined value of Kin-plane(2) agrees well with the calculated in-plane magnetoelastic anisotropy constant of Kin-plane(2)=−3.5×105 erg/cm3.

Notes: The exchange stiffness constant A of evaporated (Gd,Tb)-(Fe,Co) amorphous alloys with constant Curie temperature of 465±25 K was determined with two different methods for a large composition and temperature range. One is the excitation of standing spin waves using Brillouin light scattering. The other method is the deviation of A from measurements of the wall energy density σw and the uniaxial anisotropy Ku using the Bloch wall energy equation. σw could be determined from the applied magnetic field difference for collapsing and expanding of thermomagnetically written domains. The compositional and experimental limits of this method are described. The uniaxial anisotropy Ku was measured with a torque magnetometer. In GdTb-FeCo the resulting exchange stiffness constant of A=(2.6±0.6)×10−12 J/m was found to be independent of the Tb content. The results obtained from Brillouin light-scattering measurements on Gd21.2 Co78.8 and Gd13.5Tb6.2Fe80.3 are in good agreement.

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: Recently the interlayer exchange coupling strength in Co/Ru multilayered structures was found to oscillate between ferromagnetic and antiferromagnetic coupling as a function of the Ru layer thickness.1,2 However, in the ferromagnetic coupling regimes, the determination of the interlayer coupling constant, A12, cannot be performed using standard magnetometry methods. Here we demonstrate that Brillouin light scattering from thermally activated spin waves in multilayered structures is applicable for the determination of the interlayer exchange coupling strength both in the ferromagnetic and antiferromagnetic regimes.2 In multilayered structures consisting of alternating magnetic and nonmagnetic layers, dipolar spin-wave modes exist within each magnetic layer (so-called Damon–Eshbach modes), which couple across the intervening nonmagnetic layer. Due to the coupling between the magnetic layers, which is dipolar as well as of exchange type, the spin-wave modes form a band of collective spin-wave excitations.3–5 Two different types of collective modes exist: (i) The so-called stack surface mode, for which the spins of all magnetic layers precess in phase. The frequency of this mode is independent of any exchange coupling, but is sensitive to the net magnetization of the multilayer stack. (ii) The collective bulk modes. Their frequencies depend both on the interlayer exchange constant as well as on the layer-to-layer distribution of the directions of the magnetization.6,7 In addition, in the regime of large antiferromagnetic coupling, a new collective spin-wave mode is found in theoretical investigations, which is reminiscent of the "optic'' high-frequency spin-wave mode of antiferromagnetic bulk material.6,7 This mode goes soft with decreasing canting angle between neighboring magnetic layers.The spin-wave frequencies, and therefore A12, are found to oscillate as a function of the Ru layer thickness in the Co/Ru multilayers with a period of 11.5 A(ring) and in the permalloy/Ru multilayered system with a period of 12 A(ring). In comparison to the Co/Ru multilayers we find for the permalloy/Ru multilayers characteristic differences: First, the amplitude of the oscillation is smaller by a factor of two compared to the Co/Ru system. This effect may be attributed to the reduced saturation magnetization of permalloy. Second, we find evidence for an additional, short-period oscillation with the first minimum in the spin-wave frequencies, i.e., correspondingly in A12, at dRu=8 A(ring). Its periodicity is estimated as between 5–8 A(ring). To our knowledge this is first evidence for the presence of a short-period oscillation in a sputtered multilayered system. From the data, however, the decay in oscillation amplitude cannot be extracted due to the rather small number of observed oscillations and the comparably large error in the determination of A12. A more detailed presentation of these data is reported elsewhere.8