ALBERT

Notes: Magnetic anisotropies of a 357-A(ring)-thick Co film in the bcc phase were examined using Brillouin light scattering. This film, one of the thickest known bcc Co structures, is found to have a fourfold magnetic anisotropy that is markedly different from those of thinner films. A large possibly strain-induced uniaxial anisotropy is also found. The film is thick enough so that the surface and n=1 bulk magnon are within a few GHz of each other and strongly hybridize. Unusual behavior of the scattering intensities suggest the possible presence of surface anisotropies and/or inhomogeneous internal fields. © 1996 American Institute of Physics.

Notes: Brillouin light scattering (BLS) in the backscattering configuration has been used to study the magnetic excitations in epitaxial Fe(38 A(ring))/Cr(13 A(ring))/Fe(38 A(ring)) thin-film sandwiches. Spin-wave frequency was measured versus in-plane wave vector k and static in-plane magnetic field H for both [100] and [110] direction fields and with k perpendicular to H. The range of applied fields was 50–1500 Oe. The wave-number range was 0.45×105–2.3×105 rad/cm. The BLS spectra correlate with magnetization versus field profiles and confirm the antiferromagnetic coupling between the Fe layers. Under low-field conditions and near-antiparallel alignment of the Fe layer magnetization vectors, the spin-wave frequencies from the Stokes and anti-Stokes sides of the BLS spectra are different. These frequencies merge at high field. The low-field splitting feature is very sensitive to small differences in thickness for the magnetic layers and indicates a thickness difference of 9%. Matchups between features of the BLS data and magnetization curves were obtained for the following parameters: magnetic layer saturation induction 4πMs=18.6 kG; cubic anisotropy field HA=550 Oe; in-plane [110] uniaxial anisotropy field HU=76 Oe; antiferromagnetic interlayer exchange coupling field HJ=120 Oe. Spin-wave frequency calculations based on these parameters show quantitative agreement with the measurements. The quantitative fits require inclusion in the theory of an iron layer surface anisotropy with an effective surface anisotropy field of 2 kOe.

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: We report on Brillouin scattering measurements on a CoFe/Mn/CoFe trilayer film characterized by unusually large biquadratic coupling. The magnetic field dependence of the exchange coupled in- and out-of-phase magnons as well as their in-plane directional dependence are determined. The saturation magnetization of the trilayer was measured independently through superconducting quantum interference device magnetometry. The spin wave data is well represented by a generalization of the model that takes into account the antiferromagnetic order in the Mn layer. © 1997 American Institute of Physics.

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

Notes: Co/Pd multilayers with modulation wavelengths between 4 and 220 A(ring) have been prepared by magnetically enhanced dc-triode sputtering on single-crystal sapphire substrates. Their saturation magnetization and volume and interface anisotropies have been investigated using Brillouin light scattering from collective spin waves and by SQUID magnetometry. The saturation magnetization of Co is found to be independent of the Co layer thickness and reduced by about 20% from the Co bulk value. From the comparison of the results of the two experimental methods, clear evidence for a Pd polarization is found and the polarization depth is estimated. Samples with Co thicknesses of 2 atomic layers and Pd thicknesses ≥5 atomic layers exhibit a perpendicular magnetization due to a large negative out-of-plane interface anisotropy. The properties of spin waves in Co/Pd multilayers with the direction of magnetization pointing out-of-plane are discussed with respect to an appropriate theoretical model.

Notes: The frequencies of long-wavelength spin wave modes are calculated for single and multilayer structures composed of ferromagnetic films with strong out-of-plane surface and interface anisotropies. The properties of these modes are examined from the point of view of possible Brillouin light-scattering studies and the evaluation of exchange constants and interface anisotropies. The frequencies of the collective modes on multilayer structures as well as their degree of localization to the surface of the stack are strongly affected by perpendicular anisotropies. This is demonstrated by studying the spectral density function obtained from an effective medium theory for semi-infinite superlattices.

Notes: Some models for exchange bias at the interface of a ferromagnet and antiferromagnet involve the formation of partial domain walls in the antiferromagnet layer. Numerical calculations of mean-field temperature dependence are used to examine thermally induced instabilities in the partial domain wall at ideal compensated and uncompensated antiferromagnet interfaces. At compensated interfaces, depinning of the partial wall results in a total loss of bias. At uncompensated interfaces, thermal effects at the interface cause the wall to move into the antiferromagnet. The critical fields for this partial depinning are different for the forward and reverse magnetization directions. This mechanism on uncompensated interfaces allows for simultaneous loop shift and coercivity, which is not found in the compensated case. © 2001 American Institute of Physics.

Notes: Long-wavelength spin waves are examined in an exchange biased ferromagnet/antiferromagnet bilayer. The frequencies of the excitations are calculated as a function of external field strength and orientation. It is suggested that the angular dependence of the frequencies can lead to linewidth enhancements, in agreement with recent experimental results. It is also shown that the study of the spin wave frequencies can yield the strengths of the antiferromagnet anisotropy and interlayer coupling. © 2001 American Institute of Physics.

Notes: Fast reversal processes in magnetic particles and arrays involve fundamental magnetic dynamic and relaxation processes. Exchange and dipolar interactions determine equilibrium ground states and strongly influence linear and nonlinear dynamics. Calculations are used to show how high frequency resonances in arrays of densely packed magnetic particles can affect reversal times, possibly leading to dramatic decreases in switching rates. High frequency excitations and dynamic processes in interface exchange coupled magnets are also discussed, with emphasis on exchange biased materials. The exchange bias effect is closely related to interface magnetic structure and magnetization processes in systems of ferromagnets exchange coupled to antiferromagnets. It is shown how magnetization processes in the antiferromagnet can be studied through observation of dynamic effects in the ferromagnetic component. © 2001 American Institute of Physics.