L. R. Fleet

Uniaxial anisotropy of two-magnon scattering in an ultrathin epitaxial Fe layer on GaAs

We report an on-chip, electrically detected ferromagnetic resonance study on microbars made from GaAs/Fe(1 nm)/GaAs layers. Our experiments, performed at several different microwave frequencies and static magnetic field directions, enable us to observe a strong in-plane uniaxial anisotropy of the linewidth. We attribute the linewidth anisotropy to the two magnon scattering process, supporting this by calculations of possible linewidth broadening mechanisms. Our findings are useful for designing future high-performance spintronic devices based on nanoscale magnetic structures.

Growth and characterization of thin Cu-phthalocyanine films on MgO(001) layer for organic light-emitting diodes

Surface morphology and thermal stability of Cu-phthalocyanine (CuPc) films grown on an epitaxially grown MgO(001) layer were investigated by using atomic force microscope and X-ray diffractometer. The (002) textured β phase of CuPc films were prepared at room temperature beyond the epitaxial MgO/Fe/MgO(001) buffer layer by the vacuum deposition technique. The CuPc structure remained stable even after post-annealing at 350°C for 1 h under vacuum, which is an important advantage of device fabrication. In order to improve the device performance, we investigated also current-voltage-luminescence characteristics for the new top-emitting organic light-emitting diodes with different thicknesses of CuPc layer.

Effect of grain size on exchange-biased Heusler alloys

We report on an investigation into the effects of grain size of both antiferromagnetic IrMn and ferromagnetic Heusler alloy layers on the magnetic properties of exchange-biased films. IrMn/Co2FeSi was grown by a HiTUS sputtering system which allows control of the grain size. We found that small IrMn grains (?7?nm) could not generate an exchange bias Hex, while those above 8?nm in size showed Hex between 100 and 200?Oe. Hex showed a minor decrease with increasing Co2FeSi grain sizes (up to 15?20?nm) but Ms gradually increased. Our results show that sharp interfacial matching is required between 8 and 10?nm IrMn grains and 15?nm Co2FeSi grains to exhibit both large Hex and Ms for device applications.

Schottky Barrier Height in Fe/GaAs Films

We discuss the effect of annealing on the interfacial structure of Fe/GaAs films, with 2 × 4 surface reconstructions, and the subsequent effect on the Schottky barrier height. Images of the interfaces indicate that the annealing process can greatly reduce the level of atomic mixing. A study of the I-V characteristics of Fe/GaAs Schottky barrier diodes, in a wide temperature range of 10-300 K, reveals a strong temperature dependence for the unannealed case, arising from the presence of mixed surface states. The annealing process reduces the existence of the interfacial states, leading to more ideal behavior, with a reduced temperature dependence of the Schottky behavior.

Effect of Interface Structure on Exchange Biased Heusler Alloy Films

We report on the effects of grain size in antiferromagnetic IrMn layers exchange bias to Co2FeSi. We also report on an enhanced effect where Mn layers are inserted in the interface. The exchange-biased IrMn/Co2FeSi samples were grown by a HiTUS system, which allows us to control the grain size. The smaller IrMn grains were too small to give a large Hex while an Mn layer 0.5 nm thick dramatically increased Hex. This significant increase is attributed to optimization of the Mn concentration at the interface. This grain-size and interface tuning offers a way to control the exchange bias in such systems.

Interfacial structure and transport properties of Fe/GaAs(001)

The interfacial atomic structure of post-annealed Fe/GaAs(001) films, grown by molecular beam epitaxy, has been investigated using high resolution transmission electron microscopy. The images show a single plane of Fe atoms partially mixing between the Fe film and GaAs, along with the presence of vacancies above the As. Transport properties of three terminal devices based on these films are also presented, employing an all electrical method to confirm the injection and detection of a spin-polarized current using the Hanle effect. The effect of differing interfacial atomic ordering on the barrier heights is discussed.

Over 50% reduction in the formation energy of Co-based Heusler alloy films by two-dimensional crystallisation

Crystalline formation of high magnetic-moment thin films through low-temperature annealing processes compatible with current semiconductor technologies is crucial for the development of next generation devices, which can utilise the spin degree of freedom. Utilising in-situ aberration corrected electron microscopy, we report a 235 °C crystallisation process for a Co-based ternary Heusler-alloy film whose initial nucleation is initiated by as few as 27 unit cells. The crystallisation occurs preferentially in the ⟨111⟩ crystalline directions via a two-dimensional (2D) layer-by-layer growth mode; resulting in grains with [110] surface normal and [111] plane facets. This growth process was found to reduce the crystallisation energy by more than 50% when compared to bulk samples whilst still leading to the growth of highly ordered grains expected to give a high degree of spin-polarisation. Our findings suggest that the 2D layer-by-layer growth minimises the crystallisation energy allowing for the possible implementation of highly spin-polarised alloy films into current chip and memory technologies.

Activation Volumes in Co

Magnetic measurements and TEM analysis have been carried out in order to investigate the activation volume and its correlation with physical grain size within plasma sputtered Co2FeSi thin films. This has led to a new technique for estimating the volumes of ordered and disordered interfacial regions within granular Heusler alloy films. It has been shown that the activation volume has very little grain-size dependence, while the physical grain volume is seen to increase with bias voltage. This suggests that reversal within the films is a domain wall process, and the multistage reversal seen in those films with larger grain sizes is due to pinning of domain walls within the grains.

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MR effects in epitaxial Fe:MgO anti-granular systems, which consist of MgO nanoparticles in an Fe matrix, have been studied at room temperature. Fe and MgO were co-evaporated on MgO(001) substrate in the ratio of Fe:MgO = 4:1, 3:1, and 2:1. The samples were annealed for up to 5 h inducing a uniaxial anisotropy. The saturation magnetization of the samples was found to be up to 20% greater than the value for bulk Fe. This is induced by a moment enhancement at the Fe/MgO interfaces. The corresponding magnetoresistance shows strong isotropic behavior with anisotropic components up to ±0.09% for a magnetic field applied along the hard axis. Very strong uniaxial anisotropy is observed with a hysteresis loop squareness of up to 0.97 for the annealed samples.

FeSi Thin Films

We discuss the effect of the atomic interfacial structure on the Schottky barrier height in Fe/GaAs films. HRTEM image simulations, produced using the microscopy software JEMS, were used to predict the interfacial structure of Fe/GaAs thin films. Comparisons between experimental images, obtained using a JEOL FS2200 microscope, and the image simulations show the interfaces to contain various structures. This leads to regions with different barrier properties giving a distribution of barrier heights. This would create preferential regions for tunnelling across the film which would dominate device characteristics.