J. A. C. Bland

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...

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...

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.

Direct measurement of magnetic anisotropies in epitaxial FeNi/Cu/Co spin-valve structures by Brillouin light scattering

Abstract We have determined all the relevant magnetic anisotropy fields of epitaxial FeNi/Cu/Co spin-valve structures by Brillouin light scattering (BLS). A dominant and reproducible fourfold anisotropy was found in the cobalt layer, while a weak in-plane anisotropy was observed for the permalloy layer. BLS and magnetometry measurements have been performed over the temperature range of 10–300 K.

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...

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...

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...

Temperature dependence of spin waves in Co/CoO bilayers

Brillouin light scattering measurements of spin-wave frequencies in an exchange coupled ferromagnet/antiferromagnet epitaxial Co/CoO bilayer are reported. A striking temperature dependence of the measured spin-wave frequencies in the cobalt layer in the range 77–300 K was observed which has been demonstrated to be due to exchange coupling to the ultrathin (7 A) CoO layer as antiferromagnetic order develops. The temperature dependence of the spin-wave frequency demonstrates that interface exchange coupling occurs in the absence of the unidirectional anisotropy. A study of the mode line width shows a broadening with reducing temperature which indicates that locally ordered antiferromagnet regions persist above the Neel temperature and play a central role in determining the magnetic behavior of the bilayer system.