A. J. R. Ives

Structure induced magnetic anisotropy behavior in Co/GaAs(001) films

Epitaxial Co has been grown on GaAs(001) and studied by both low‐energy electron diffraction (LEED) and reflection high‐energy electron diffraction (RHEED), and by the magneto‐optic Kerr effect (MOKE) and polarized neutron reflection (PNR). Three samples were fabricated using different growth procedures: (1) ‘‘interrupted’’ growth (including an anneal); (2) and (3) continuous growth of similar thicknesses. For sample 1, RHEED patterns indicate an initial growth in the bcc phase followed by a relaxation into a distorted single phase at completion of growth, whereas samples 2 and 3 showed a multicrystalline structure after growth. LEED patterns were used to check the existence of the 2×4 reconstruction patterns before growth, but no LEED patterns could be obtained after more than 2 A Co was deposited, in contrast to the RHEED patterns which remained visible throughout the growth. Structural analysis of the completed films indicates the formation of a ∼10 A CoO layer on the Co/air interface, and gives thickn...

Magnetization reversal processes in epitaxial Fe/GaAs(001) films

In this article we present the results of a detailed study of the switching behavior observed in epitaxial single Fe films of thickness between 30 and 450 A, and a wedge shaped Fe film with a thickness range of 10–60 A grown on GaAs (001). These films have cubic and uniaxial anisotropies which change with film thickness. For the fixed thickness films the values of the anisotropy constants were accurately determined by Brillouin light scattering (BLS) measurements together with polar magneto‐optic Kerr effect (MOKE) measurements that gave the value of the magnetization. The switching behavior of these samples was observed with in‐plane MOKE magnetometry as a function of the angle between the applied field and the in‐plane crystallographic axes. Measurements of the component of magnetization perpendicular to the applied field allow a precise determination of the relative orientation of the hard and easy in‐plane anisotropy axes. This can be used to accurately determine the ratio of uniaxial to cubic anisotr...

Brillouin light scattering studies of magnetic anisotropy in epitaxial Fe/GaAs films

Abstract Brillouin Light Scattering (BLS) measurements have been performed upon epitaxial Fe/GaAs(100) and Fe/GaAs( 1 10) films with particular emphasis on Fe thicknesses less than 100 A. A continuum theory that is required to describe the experimental data is presented. The values of the magnetic anisotropy fields measured by BLS agree well with those obtained from detailed Magneto Optical Kerr Effect (MOKE) studies. An in-plane uniaxial anisotropy is observed in addition to the cubic anisotropy for both the (100) and ( 1 10) orientations. For Fe/GaAs(100), both the cubic anisotropy field and the effective demagnetizing field increase with film thickness while the uniaxial anisotropy field decreases with film thickness. For Fe/GaAs( 1 10) the uniaxial anisotropy field changes sign as the thickness is increased. In light of these findings we discuss how the Fe/GaAs interface may influence the values of the magnetization and anisotropy constants in the Fe films.

Magnetic switching, relaxation, and domain structure of a Co/Si(111) film

We have used scanning magneto‐optic Kerr effect (MOKE) microscopy to investigate the magnetic relaxation of a polycrystalline hcp 125 A Co/Si(111) film with planar uniaxial anisotropy, on time scales between 10 and 2400 s and with a spatial resolution of 15 μm. In a static magnetic field slightly less than the coercive field and applied along the easy axis direction, domains develop and the magnetization reversal proceeds via displacements of 180° domain walls. Microscopic images of this metastable state allow the 180° domains to be identified by calibration of the MOKE signal with respect to that for the saturated magnetization states. The 180° reversed domains are observed to grow in the direction of the field in the form of narrow fingers, extending via short Barkhausen jumps, randomly spaced in time over the entire time‐scale range investigated, with typical distances between pinning sites of the order of microns. This reversal behavior is qualitatively similar to that reported for Au/Co perpendicular...

High‐field polar MOKE magnetometry as a probe of interlayer exchange coupling in MBE‐grown Co/Cu/Co(111) and Fe/Cr/Fe(001) wedged trilayers

We discuss the use of room temperature polar magneto‐optic Kerr effect (MOKE) measurements at high field (≤7 T) in investigating antiferromagnetic (AFM) and ferromagnetic (FM) exchange coupling in MBE‐grown wedged trilayers. In the case of Co/Cu/Co(111), the polar MOKE revealed the first AFM coupling peak at 9 A Cu thickness and the second weaker AFM coupling peak at 20 A. This is an important result because it helps in resolving the present controversy over whether oscillatory coupling exists in (111) oriented MBE‐grown Co/Cu/Co structures. For Fe/Cr/Fe(001), polar MOKE is found to be less sensitive than in‐plane MOKE for extracting the detailed form of the coupling. However, polar MOKE reveals additional variations in the perpendicular saturation fields as a function of interlayer thickness, which are not found in the in‐plane MOKE saturation fields.

Magnetization of ultrathin ferromagnetic films at finite temperatures

The in-plane magnetization of a two-dimensional film may be stabilized by either dipolar interactions or by an in-plane anisotropy in the absence of an external field. We have calculated the conditions for either of these effects to dominate and show that there is only a restricted range of q values in which dipolar effects are important before the exchange dominates. We report measurements of all relevant anisotropy fields for an epitaxial Ag/2 ML Co/Ag(001) film structurally characterized by angle-resolved Auger spectroscopy and low-energy electron diffraction, together with a measurement of the ground state moment per Co atom (enhanced from the bulk value) using polarized neutron reflection. The results allow us to extend our previous study of the temperature dependence of the magnetization by including the perpendicular fields and the field dependence of the magnetization at 300 K. We show that the field dependence of the magnetization is not consistent with a spin wave gap due to dipolar interactions but is consistent with an anisotropy-induced spin wave gap, confirming the results of a preliminary study.

Interlayer exchange coupling in epitaxial Fe/Cr/Fe/Ag/GaAs(100) structures

The interlayer exchange coupling has been investigated in epitaxial Fe(20 A)/Cr/Fe(20 A)/Ag/GaAs(100) structures that contain a wedge‐shaped (0–40 A) Cr layer. Longitudinal and polar magneto‐optical Kerr‐effect (MOKE) and Brillouin light‐scattering measurements have been combined to determine values for the relevant anisotropy constants and both the bilinear and biquadratic coupling strengths. The phase and period of the oscillations in the interlayer coupling are found to agree well with those reported by other researchers while the total coupling strength is found to be reduced. This reduction is presumably due to the presence of structural imperfections in our samples, and our results may therefore be of use in testing some of the recently proposed extrinsic biquadratic coupling mechanisms. Specifically, we find that for the Cr thicknesses studied the biquadratic coupling strength in our samples varies as d−1.4Cr where dCr is the thickness of the Cr layer. We also present results that show how the ultr...

Bilinear and biquadratic exchange coupling in epitaxial Fe/Cr/Fe/Ag/GaAs(100) structures

Abstract The interlayer exchange coupling has been investigated in epitaxial Fe(20 A)/Cr/Fe(20 A)/Ag/GaAs(100) structures that contain a wedge-shaped (0–40 A) Cr layer. Longitudinal and polar magneto optical Kerr effect (MOKE) and Brillouin light scattering (BLS) measurements have been combined to determine values for the bilinear and biquadratic coupling strengths. While the phase and period of the oscillations in the interlayer coupling agree well with those reported by other researchers, the magnitude of the coupling strength is found to be reduced. The dependence of the biquadratic coupling strength upon the Cr thickness is well described by a simple power law and may provide a useful test of extrinsic biquadratic coupling models. We show that the combination of MOKE and BLS measurements are very effective in investigating the ultrathin Cr region and that ferromagnetic (FM) coupling produces large changes in the polar MOKE saturation field.

A magneto‐optic technique for studying magnetization reversal processes and anisotropies applied to Co/Cu/Co trilayer structures

We report the magnetization reversal and magnetic anisotropy behavior of ultrathin Co/Cu(111)/Co (dCu=20 and 27 A) trilayer structures prepared by MBE on a 500‐A Ge/GaAs(110) epilayer. We describe an arrangement in which the magnetization components parallel and perpendicular to the applied field are both determined from longitudinal MOKE measurements. For the samples examined, coherent rotation of the magnetization vector is observed when the magnetic field is applied along the hard in‐plane anisotropy axis, with the magnitude of the magnetization vector constant and close to its bulk value. Results of micromagnetic calculations closely reproduce the observed parallel and perpendicular magnetization loops, and yield strong uniaxial magnetic anisotropies in both layers while the interlayer coupling appears to be absent or negligible in comparison with the anisotropy strengths. An absence of antiferromagnetic (AF) coupling has been observed previously [W. F. Egelhoff, Jr. and M. T. Kief, Phys. Rev. B 45, 7...

Exchange-coupled MBE-grown Co/Cu/Co(111) trilayers

Abstract It is shown that the high-field curvature in the in-plane and polar magnetization curves of exchange-coupled MBE-grown Co/Cu/Co(111) structures can be accurately fitted by assuming a sizeable biquadratic coupling in addition to the bilinear coupling, and the existence of a large areal fraction of ferromagnetic inclusions. Measurements on a sample with a single Co layer confirm that the curvature does not originate from the individual Co layers. Analysis of two Co/Cu/Co(111) trilayers with different CoCu interface qualities, as measured by NMR, shows significantly higher bilinear and biquadratic coupling strengths for the trilayer with the better defined CoCu interfaces. While NMR indicates reasonably good structural quality on a short lateral length scale, polar Kerr measurements as a function of interlayer thickness, and polarized neutron reflection measurements, indicate the existence of structural imperfections on a long lateral length scale.