Iain Will

Magnetostriction and magnetocrystalline anisotropy constants of ultrathin epitaxial Fe films on GaAs, with Au overlayers

In this paper, we present the first measurements on the variation of the saturation magnetostriction constant with film thickness of ultrathin epitaxial Fe films on GaAs(100) substrates. Furthermore, we explore whether there is a link between magnetostriction and the uniaxial anisotropy in these Fe films. The Fe film thickness ranged from seven monolayers (ML) (having only uniaxial anisotropy) to 50 ML (almost pure cubic anisotropy). The anisotropy constants were determined from the normalized magnetization loops, using a magneto-optic Kerr effect (MOKE) fitting technique that convolutes a magnetic energy density model with the dependence of the MOKE signal on the angle between the pass plane of the analyser and the plane of incidence of the laser light on the sample. Each film was uniformly strained along the [011] direction, while the magnetization was measured along the direction using a MOKE magnetometer. From the change in anisotropy field as a function of strain (Villari effect), the magnetostriction constant in the [011] direction was calculated. It is demonstrated that the saturation magnetostriction constant in the [011] direction is significantly different to the equivalent value in bulk Fe, and increases in magnitude as the thickness of the Fe film decreases. It will also be shown that the uniaxial anisotropy constant has a linear dependence on the magnetostriction constant for each film.

Hysteretic properties of epitaxial Fe/GaAs(100) ultrathin films under external uniaxial strain

We report an experimental and theoretical study of the influence of uniaxial strain on the magnetic properties of Fe/GaAs(100) ultrathin film systems. In-plane tensile strains have been applied along different directions relative to the intrinsic uniaxial magnetic anisotropy axes of the Fe/GaAs(100) films, and the M-H loops were measured. In an attempt to interpret the observed behavior, a phenomenological energy model is considered, which incorporates both intrinsic and strain-induced uniaxial anisotropy terms. Theoretical magnetization reversal loops were obtained from the model, assuming that the magnetoelastic coefficients of the thin films are same as the bulk bcc Fe. The model qualitatively predicted the magnetization loops for all the experimental data, assuming a positive uniaxial anisotropy constant.

Magnetocrystalline anisotropies and magnetostriction of ultrathin Fe films on GaAs with Cr overlayers

In this article we present data which show that Cr overlayers on epitaxial Fe films on GaAs substrates change the magnetic properties of the films in comparison to Au overlayers. For films thinner than 50 ML, the presence of the Cr overlayer reduced the magnitude of the uniaxial anisotropy in the Fe film, while for a 150 ML film, there was still a strong uniaxial anisotropy present. The anisotropy constants were determined using a modified energy density model, and were found to be smaller than those for Au overlayer films. The absolute magnetizations were measured on a vibrating sample magnetometer. It was determined that for the same thickness of Fe the magnetization with a Cr overlayer was 75% of that for a Au overlayer. This was either due to the Cr alloying with the Fe at the interface, or the Cr forming CrO2 on the surface of the film. The magnetostriction constant was determined using the Villari effect. For all the films the saturation magnetostriction constant was more negative than the bulk Fe v...

Reduction of In-Plane Uniaxial Magnetic Anisotropy in Patterned Single-Crystal Fe Dot Arrays

Single-crystal Fe dot arrays with the lateral size varying from 400 nm to 50 nm are fabricated by focused ion beam (FIB) direct writing from a single-crystalline 10 monolayer (ML) Fe (100) continuous thin film grown on GaAs substrate. The Kerr hysteresis loops of both dot arrays and continuous thin film are measured by focused magnetooptical Kerr effect (MOKE) measurements along four major crystal directions: [0-11] [010] [011] and [001]. It is found that the in-plane uniaxial anisotropy has been greatly reduced down to zero in the dot arrays when the size is less than 150 nm. The micromagnetic simulations confirm the reduction of this intrinsic in-plane uniaxial anisotropy in the patterned dots by separating the effect of the shape anisotropy. The experimental and simulation results further indicate an additional magnetic uniaxial anisotropy along the [010] direction.

Comparison between the in-plane anisotropies and magnetostriction constants of thin epitaxial Fe films grown on GaAs and Ga0.8In0.2As substrates, with Cr overlayers

Thin epitaxial Fe films grown on GaAs(100) and Ga0.8In0.2As(100) substrates were investigated to determine how tuning the lattice constant mismatch between the Fe and the substrate may change the in-plane anisotropies and the magnetostriction. Two sets of Fe films were grown using molecular-beam epitaxy, each capped with a Cr overlayer. For each film, the in-plane anisotropy constants were determined from the normalized magnetization loops measured using a magneto-optic Kerr effect magnetometer. The lattice mismatch was found to give no contribution to the in-plane anisotropies. For all the films the magnetostriction constants, determined by the Villari method, were negative and became more negative as the Fe thickness decreased.

Spin-Dependent Transport in Fe/GaAs(100)/Fe Vertical Spin-Valves

The integration of magnetic materials with semiconductors will lead to the development of the next spintronics devices such as spin field effect transistor (SFET), which is capable of both data storage and processing. While the fabrication and transport studies of lateral SFET have attracted greatly attentions, there are only few studies of vertical devices, which may offer the opportunity for the future three-dimensional integration. Here, we provide evidence of two-terminal electrical spin injection and detection in Fe/GaAs/Fe vertical spin-valves (SVs) with the GaAs layer of 50 nanometers thick and top and bottom Fe electrodes deposited by molecular beam epitaxy. The spin-valve effect, which corresponds to the individual switching of the top and bottom Fe layers, is bias dependent and observed up to 20 K. We propose that the strongly bias- and temperature-dependent MR is associated with spin transport at the interfacial Fe/GaAs Schottky contacts and in the GaAs membranes, where balance between the barrier profiles as well as the dwell time to spin lifetime ratio are crucial factors for determining the device operations. The demonstration of the fabrication and spin injection in the vertical SV with a semiconductor interlayer is expected to open a new avenue in exploring the SFET.

Element specific spin and orbital moments of nanoscale CoFeB amorphous thin films on GaAs(100)

CoFeB amorphous films have been synthesized on GaAs(100) and studied with X-ray magnetic circular dichroism (XMCD) and transmission electron microscopy (TEM). We have found that the ratios of the orbital to spin magnetic moments of both the Co and Fe in the ultrathin amorphous film have been enhanced by more than 300% compared with those of the bulk crystalline Co and Fe, and specifically a large orbital moment of 0.56 μB from the Co atoms has been observed and at the same time the spin moment of the Co atoms remains comparable to that of the bulk hcp Co. The results indicate that the large uniaxial magnetic anisotropy (UMA) observed in the ultrathin CoFeB film on GaAs(100) is related to the enhanced spin-orbital coupling of the Co atoms in the CoFeB. This work offers experimental evidences of the correlation between the UMA and the element specific spin and orbital moments in the CoFeB amorphous film on the GaAs(100) substrate, which is of significance for spintronics applications.

DC-contact RF-MEMS cantilever switch on a PCB substrate for monolithic integration with microstrip phased array antennas

This paper presents electrostatic DC-contact RF-MEMS cantilever switches based on an alternative and cheap fabrication process on a PCB, suitable for monolithic integration with other elements on the same substrate. The switches are implemented on a Rogers 4003c substrate, based on a 1μm-thick copper cantilevers, with an average voltage actuation of 90V. Stable cantilevers have been demonstrated in the off-state and during the actuation process. Also this work presents the design, simulation, and measurements of the S-parameters of the RF-MEMS at 12.5 GHz. The design has a promising application on switched-line phase shifters for receive-only phased array antennas in the Ku-band frequencies.

Discontinuous properties of current-induced magnetic domain wall depinning

The current-induced motion of magnetic domain walls (DWs) confined to nanostructures is of great interest for fundamental studies as well as for technological applications in spintronic devices. Here, we present magnetic images showing the depinning properties of pulse-current-driven domain walls in well-shaped Permalloy nanowires obtained using photoemission electron microscopy combined with x-ray magnetic circular dichroism. In the vicinity of the threshold current density (Jth = 4.2 × 1011 A.m−2) for the DW motion, discontinuous DW depinning and motion have been observed as a sequence of “Barkhausen jumps”. A one-dimensional analytical model with a piecewise parabolic pinning potential has been introduced to reproduce the DW hopping between two nearest neighbour sites, which reveals the dynamical nature of the current-driven DW motion in the depinning regime.

Influence of Rare Earth Ho3+ Doping on Structural, Microstructure and Magnetic Properties of ZnO Bulk and Thin Film Systems

We have investigated the doping behavior of rare earth element holmium (Ho3+) in ZnO semiconductor. The structural, microstructure, and magnetic properties of Zn1−xHoxO (x=0.0, 0.04, and 0.05) thin films deposited on Si(100) substrate by thermal evaporation technique were studied. The ceramic targets were prepared by conventional solid state ceramic technique. The pallets used as target were final sintered at 900 °C in the presence of N2 atmosphere. The experimental results of X-ray diffraction (XRD) spectra, surface morphology, and magnetic properties show that the Ho3+ doped ZnO thin films has a strong influence on the materials properties. The higher angle shift in peak position and most preferred (101) orientation were observed in XRD pattern. These spectra confirmed the substitution of Ho3+ in ZnO lattice. The surface morphology and stoichiometry for both bulk and thin films were analyzed by scanning electron microscopy and energy dispersive spectroscopy. It was observed that grain size decreases with the increase of Ho3+. Room temperature ferromagnetism was observed for Zn0.95Ho0.05O films. The ferromagnetism might be attributed to the substitution of Ho ions for Zn2+ in ZnO lattices.