S. J. Gray

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.

MAGNETIC SWITCHING AND IN-PLANE UNIAXIAL ANISOTROPY IN ULTRATHIN AG/FE/AG(100) EPITAXIAL FILMS

We have studied the process by which the in‐plane magnetization of an ultrathin (4–11 ML) epitaxial iron film reverses under the action of an external magnetic field. Kerr effect measurements reveal a small in‐plane uniaxial anisotropy superimposed on the cubic magnetocrystalline anisotropy which greatly influences the reversal. In addition, we find that depending upon the field orientation, reversal can proceed either via a ‘‘1‐jump’’ mechanism, by the sweeping of 180° domain walls and which gives a classic square hysteresis loop, or by a ‘‘2‐jump’’ mechanism, by the sweeping of 90° domain walls at two distinct applied field strengths—this gives a more unusual hysteresis loop with two irreversible transitions. We have developed a simple phenomenological energy model which explains how so small a uniaxial anisotropy can play so significant a role in the reversal process. The model explains the two reversal mechanisms and predicts with good experimental agreement which should be observed for different appl...

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.

A new technique for measuring magnetic anisotropies in thin and ultrathin films by magneto-optics

A new technique for high precision measurement of magnetic anisotropy fields in thin and ultrathin films called modulated field magneto-optical anisometry (MFMA) is described. MFMA can be performed by a simple extension to a conventional magneto-optical Kerr effect magnetometer, and is therefore experimentally simple. It can resolve an arbitrary combination of anisotropies of different symmetries with a very high precision, even when there is little magneto-optical signal (e.g., ultrathin in-plane magnetized films) and high optical noise. It is spatially resolving and readily suited to the ultrahigh vacuum environment. MFMA thus offers many advantages over existing anisotropy measurement methods. A quantitative comparison is made between anisotropy field measurements made by MFMA and by Brillouin light scattering on an ultrathin Fe(001) epitaxial film. Agreement is found to within a high precision.

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.

Thickness-dependent in-plane magnetic anisotropy in epitaxial Fe films on GaAs substrates

Abstract We have studied the evolution of the magnetic in-plane anisotropy in epitaxial Fe GaAs films for both (001) and (110) orientations as a function of the Fe layer thickness using the in-situ longitudinal magneto-optic Kerr effect and ex-situ Brillouin light scattering. Magnetisation curves recorded during film growth reveal a continuous directional change of the anisotropy axes with increasing film thickness. This behaviour arises from the combination of a uniaxial and a cubic in-plane magnetic anisotropy which are both thickness dependent. We discuss the possible origins for this behaviour considering the substrate preparation and the structure of the Fe films.

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.