N. A. Morley

Engineering spin propagation across a hybrid organic/inorganic interface using a polar layer

Spintronics has shown a remarkable and rapid development, for example from the initial discovery of giant magnetoresistance in spin valves to their ubiquity in hard-disk read heads in a relatively short time. However, the ability to fully harness electron spin as another degree of freedom in semiconductor devices has been slower to take off. One future avenue that may expand the spintronic technology base is to take advantage of the flexibility intrinsic to organic semiconductors (OSCs), where it is possible to engineer and control their electronic properties and tailor them to obtain new device concepts. Here we show that we can control the spin polarization of extracted charge carriers from an OSC by the inclusion of a thin interfacial layer of polar material. The electric dipole moment brought about by this layer shifts the OSC highest occupied molecular orbital with respect to the Fermi energy of the ferromagnetic contact. This approach allows us full control of the spin band appropriate for charge-carrier extraction, opening up new spintronic device concepts for future exploitation.

Room temperature organic spintronics

The magnetoresistance (MR) of organic spin valves containing the spacer layer regioregular poly(3-hexylthiophene) (RR-P3HT), which is a conjugated polymer, has been studied at room temperature. Investigations have been made of the influence of the thickness of organic spacer layer on the magnetoresistance of Fe50Co50∕RR-P3HT∕Ni81Fe19 spin valves. It was determined that the MR decreased as the layer thickness increased, consistent with the spin coherence length in this material. It has been observed at room temperature that there is the co-occurrence of two magnetoresistance effects, the spin-valve magnetoresistance, and anisotropic magnetoresistance in the magnetic electrodes.

Effect of a forming field on the magnetic and structural properties of thin Fe–Ga films

The structural, magnetic, and magnetostrictive properties of 75nm Fe100−xGax films (0⩽x⩽30) have been studied to determine how a forming field during growth affects these properties. Two film sets were grown using a cosputtering-evaporation technique with and without a forming field of 65kAm−1. Using x-ray diffraction, the texture, lattice parameter, and the grain size were determined. An out-of-plane ⟨110⟩ texture was found for all films. The magnetization loops were measured on a magneto-optic Kerr effect magnetometer, from which the anisotropy symmetry was inferred from the remanent magnetization. It was found that the forming field produced a pronounced uniaxial anisotropy. The effective magnetostriction constant was measured using the Villari effect. The effective magnetostriction constant was compared with that calculated for a polycrystalline film with ⟨110⟩ out-of-plane texture using bulk constants, and the results are discussed.

Development of a cosputter-evaporation chamber for Fe–Ga films

A new deposition technique involving the dc magnetron sputtering of Fe simultaneously with Ga evaporation has been developed to fabricate thin Fe(1−x)Gax (0<x<0.4) films. Simultaneous sputtering and evaporation has been developed as an alternative to the sputtering of alloy targets, where the need for compositional variation can be hard to address. The new technique allows the freedom of composition that cosputtering would offer, but mitigates the issue of being unable to sputter Ga. The need for the preparation of alloy targets is also avoided. To control the concentration of the Ga in the films, the evaporation rate, the dc magnetron power, and the chamber pressure were varied. The films fabricated were studied to determine their magnetic and microstructural properties. The Fe–Ga films fabricated had magnetostriction constants which had increased by a factor of 5 compared to the same thickness of Fe film.

Comparison of Room Temperature Polymeric Spin-Valves With Different Organic Components

We have compared the magnetoresistance of organic spin-valve (SV) structures with regioregular and regiorandom poly (3-hexylthiophene) (RR-P3HT and RRa-P3HT), hole transporting polymers, as the spacer layers sandwiched between two ferromagnetic electrodes. A SV structure of Fe50Co50 /RRa-P3HT/Ni81 Fe19 was the test structure used. At room temperature, we observed the co-occurrence of two magnetoresistance effects, the spin magnetoresistance and magnetoresistance in the electrodes.

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.

Thickness dependence of magnetic and structural properties in Fe80Ga20 thin films

This work focuses on the effect of film thickness on the microstructural and magnetic properties of polycrystalline Fe80Ga20 films of different thickness (20 nm≤t≤200 nm) fabricated on Si⟨100⟩ substrates. From x-ray diffraction, a ⟨110⟩ texture was normal to the film plane in all films. For all film thicknesses, the lattice constant perpendicular to the film plane was less than the bulk, and first decreased and then increased toward the bulk value with increasing t. From atomic force microscopy data, both surface roughness and grain size were found to increase with film thickness. Using the Villari effect, the effective magnetostriction constant λeff was measured. Using the Szymczak model, it was found that λbulk=91±5 ppm, close to the bulk isotropic polycrystalline value (λiso=98 ppm). The surface magnetostriction λs contribution to λeff was found to be −1065±271 ppm nm. It was found that the 20 nm thick film had weak uniaxial anisotropy, while all the other films were magnetically isotropic. An extensio...

Effect of polymer processing on spin magnetoresistance in organic structures

Results from studies of spin dependent transport in the hole transporting organic semiconductor regioregular poly 3-hexylthiophene (RR-P3HT) are reported. The organic spin valves were grown on glass, with the structure Fe50Co50∕RR‐P3HT∕Ni81Fe19. RR-P3HT was cast from five different solvents with different boiling points; 1,2,4 trichlorobenzene, toluene, xylene, chloroform, and chlorobenzene. Magnetoresistance measurements were carried out at room temperature. Spin dependent transport was only observed when toluene, xylene, and annealed 1,2,4 trichlorobenzene were used as solvents. Results are interpreted on the basis of differences in the RR-P3HT morphology induced by different solvents under fixed processing conditions.

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...