M. Gester

Continuous evolution of the in‐plane magnetic anisotropies with thickness in epitaxial Fe films

We have studied the evolution of the magnetic in‐plane anisotropy in epitaxial Fe/GaAs films of both (001) and (110) orientation as a function of the Fe layer thickness using the longitudinal magneto‐optic Kerr effect and Brillouin light scattering. Magnetization curves which are recorded in situ during film growth reveal a continuous change of the net anisotropy axes with increasing film thickness. This behavior can be understood to arise from the combination of a uniaxial and a cubic in‐plane magnetic anisotropy which are both thickness dependent. Structural analysis of the substrate and Fe film surfaces provides insight into the contribution of atomic steps at the interfaces to the magnetic anisotropy. Changing the degree of crystalline order at the Fe–GaAs interface allows us to conclude that the magnetic anisotropies are determined by atomic scale order.

Magnetoresistance and magnetization in submicron ferromagnetic gratings

A technique for engineering micron and submicron scale structures from magnetic films of transition metals has been developed using a combination of electron‐ and ion‐beam lithography enabling high‐quality arrays of submicron magnetic Fe wires to be fabricated. This process can be used to fabricate novel devices from a variety of metal combinations which would not be possible by the usual liftoff metallization method. The structure and magnetic properties are reported of an epitaxial 25 nm Fe(001)/GaAs(001) film and the wire gratings which are fabricated from it. The width of the wires in the grating is 0.5 μm for all structures studied, but the separation of each wire is varied in the range 0.5 to 16 μm. An artificially induced shape anisotropy field of around 1 kG, consistent with a magnetostatic calculation, was observed for all separations studied. The field dependence of the magneto‐optic Kerr effect and magnetoresistance (MR) data is consistent with a twisted magnetization configuration across the w...

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.

The magnetoresistance of sub-micron Fe wires

Abstract A novel combination of electron- and ion-beam lithography has been used to prepare Fe gratings with wire widths of 0.5 μm and wire separations in the range 0.5–4 μm from an Fe/GaAs (001) film of thickness 25 nm. With an in-plane magnetic field applied perpendicular to the length of the wires, a harder magnetisation loop is observed using the magneto-optic Kerr effect (MOKE), compared with that observed in the unprocessed film. We observe a strong effect in the magnetoresistance (MR) when the magnetic field is applied transverse to the wires. It is believed that this effect originates from the highly non-uniform demagnetising field in each wire of the grating. These results demonstrate that the combination of MOKE and MR measurements can provide important information about the magnetisation reversal processes in magnetic gratings and can be used to understand the effect of shape anisotropy on magnetic properties.

Electron‐energy‐loss spectroscopy of Fe thin films on GaAs(001)

An electron‐microscopy‐based technique of electron‐energy‐loss spectroscopy (EELS) has been used to characterize electronic and magnetic properties of ultrathin Fe films grown on GaAs(100) surface, as a function of the film thickness. Large‐area electron transparent membranes for microscopic analysis are prepared by ion‐beam thinning or chemical etching from the substrate side, and the top surface of the ultrathin Fe film is protected by a thin Cr layer. Analysis of the Fe 2p, Cr 2p, and O 1s absorption spectra confirms that only the Cr layer is oxidized. The local magnetic moments of the ultrathin Fe films are deduced from the ‘‘white line’’ branching ratio in the Fe 2p absorption spectra. For Fe films as thin as 150 A, the magnetic moment is not different from that found in bulk α‐Fe. For a 70‐A Fe film, the local magnetic moment is enhanced although the average magnetization is reduced. As doping is suspected to be the cause for the departure from bulk α‐Fe properties. In the case where the 50‐A film i...

Formation of pyramid-like structures in the growth of epitaxial Fe/GaAs(001) films

Abstract We present a low energy electron diffraction (LEED) study of the surface structure of epitaxial Fe films on GaAs(001) substrates during deposition. Layers and islands grow simultaneously and the substrate is totally covered after the equivalent of 3 monolayers of Fe are deposited. Analysis of LEED spot shapes reveals that the Fe surface contains steps which are aligned predominantly parallel to the 〈110〉 direction. At Fe thicknesses exceeding 50 A, the steps form regular arrays which give rise to pyramid-like structures.

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.

Magnetic domains in epitaxial Fe/GaAs micro-patterned wires

The magnetic domain evolution during the magnetization reversal in in-plane magnetized epitaxial Fe/GaAs(001) wire elements with dimensions of 15 μm width × 500 μm length × 300 A thickness has been studied using a scanning Kerr microscope. The two jump switching processes, characteristic of the magnetization reversal in continuous epitaxial Fe(001) films with four-fold in-plane anisotropy, are observed. However, the domain nucleation and growth processes which mediate the discontinuous and irreversible magnetization switching at the two critical reverse fields Hc1 and Hc2 are different from those in the continuous film. They are found to be determined by the orientation of the applied field with respect to the short and the long wire axis and hence are “shape”-anisotropic. This shape-anisotropy is a combined local dipolar field and anisotropic finite size effect, allowing the growth of domains only from the long/short wire axis and the propagation of domains only in direction of the short/long wire axis r...