A. Van Esch

Nanometer‐scale magnetic MnAs particles in GaAs grown by molecular beam epitaxy

Spherical MnAs ferromagnetic particles with controllable diameters (5–30 nm) are embedded in a high quality GaAs matrix. The particles are formed in a two step process consisting of the epitaxy of a homogeneous Ga1−xMnxAs layer at low temperatures using molecular beam epitaxy followed by phase separation upon annealing. During the annealing step, the excess arsenic in the as‐grown film forms magnetic MnAs precipitates with the Mn from the Ga1−xMnxAs lattice. Structural and room‐temperature magnetic properties of the heterogeneous GaAs:MnAs films are described. The magnetic MnAs rich layers can be incorporated into semiconductor heterostructures as demonstrated by growing (GaAs/AlAs) multiple quantum well structures in combination with GaAs:MnAs layers.

Automatic segmentation of thoracic and pelvic CT images for radiotherapy planning using implicit anatomic knowledge and organ-specific segmentation strategies

Automatic segmentation of anatomical structures in medical images is a valuable tool for efficient computer-aided radiotherapy and surgery planning and an enabling technology for dynamic adaptive radiotherapy. This paper presents the design, algorithms and validation of new software for the automatic segmentation of CT images used for radiotherapy treatment planning. A coarse to fine approach is followed that consists of presegmentation, anatomic orientation and structure segmentation. No user input or a priori information about the image content is required. In presegmentation, the body outline, the bones and lung equivalent tissue are detected. Anatomic orientation recognizes the patients position, orientation and gender and creates an elastic mapping of the slice positions to a reference scale. Structure segmentation is divided into localization, outlining and refinement, performed by procedures with implicit anatomic knowledge using standard image processing operations. The presented version of algorithms automatically segments the body outline and bones in any gender and patient position, the prostate, bladder and femoral heads for male pelvis in supine position, and the spinal canal, lungs, heart and trachea in supine position. The software was developed and tested on a collection of over 600 clinical radiotherapy planning CT stacks. In a qualitative validation on this test collection, anatomic orientation correctly detected gender, patient position and body region in 98% of the cases, a correct mapping was produced for 89% of thorax and 94% of pelvis cases. The average processing time for the entire segmentation of a CT stack was less than 1 min on a standard personal computer. Two independent retrospective studies were carried out for clinical validation. Study I was performed on 66 cases (30 pelvis, 36 thorax) with dosimetrists, study II on 52 cases (39 pelvis, 13 thorax) with radio-oncologists as experts. The experts rated the automatically produced structures on the scale 1-excellent (no corrections necessary, maximum time saving), 2-good (corrections necessary for up to 1/3 of slices), 3-acceptable (major corrections necessary, but still time saving), 4-not acceptable (manual redrawing more efficient, no time saving). A rating<or=3 indicates a time saving in the treatment planning process and was given for pelvis segmentation in 70% (I) and 68% (II) of the cases, with average ratings 2.9 (I) and 2.6 (II). For the thorax, a rating<or=3 was given in 94% and 91% of the cases, with average ratings 2.1 and 1.9, respectively. For quantitative validation, automatically generated structures were compared geometrically in 2D and 3D to manually drawn structures created by experts on seven randomly selected cases. The quantitative validation was limited to pelvis structures. The results indicate that the accuracy of the algorithms is within the bandwidth of manual segmentation by experts, except for specific erroneous situations. Even though manual review and corrections of automatically segmented structures are still mandatory, it can be expected that due to the speed of the presented software and the quality of its results, its introduction in the radiotherapy treatment planning process will lead to a considerable amount of time being saved.

Study of the demagnetization and optimization of the magnetic field of perpendicular ferromagnetic thin films usingesub-μ m lithography

Abstract The succesful fabrication using holographic lithography of a submicron grating reducing the demagnetizing field of a perpendicular ferromagnetic thin film is reported. An increase in the extraordinary Hall effect by a factor 3 is measured. The presence of holes in the film lowers the coercive field and causes a stepwise switching.

Structural and magnetic investigations of epitaxial ferromagnetic τ MnAl films grown on GaAs/AlAs by molecular‐beam epitaxy

The crystalline quality and magnetic properties of epitaxial ferromagnetic τ MnxAl100−x (x=50–70) films with perpendicular anisotropy grown on AlAs/GaAs by molecular‐beam epitaxy are improved by ex situ rapid thermal annealing compared to as‐grown thin films. An increase in magnetization of up to 230% is observed for moderate annealing temperatures (≊450 °C). This is strongly related to an improved ordering in the occupation of the two sublattices, which are antiferromagnetically coupled. At the same time a strong reduction in coercive field (up to a factor of 4) upon annealing is attributed to a decreased density of antiphase boundaries in the metal film. Annealing at higher temperatures (≊550 °C and above) results in the partial relaxation of the τ phase, and eventually in the transformation of the entire film to the nonmagnetic e phase. The composition has a strong influence since the presence of excess Mn (x≳50) reduces the magnetization. The remanent magnetization Mr=465 kA/m for τ  Mn50Al50 is close...

Controlled formation of nanoscale MnAs magnetic clusters in GaAs

Abstract Nanocrystalline MnAs magnetic clusters are formed controllably in GaAs by annealing Molecular Beam Epitaxial grown Ga 1- x Mn x As (0.03 x

Comparison of build-up dose between Elekta and Varian linear accelerators for high-energy photon beams using radiochromic film and clinical implications for IMRT head and neck treatments

Skin toxicity has been reported for IMRT of head and neck cancer. The purpose of this study was to investigate the dose in the build-up region delivered by a 6 MV treatment plan for which important skin toxicity was observed. We also investigated if the different designs of the treatment head of an Elekta and a Varian linear accelerator, especially the lower position of the Varian multi-leaf collimator, give rise to different build-up doses. For regular square open beams, the build-up dose along the central beam axis is higher for the Varian machine than for the Elekta machine, both for 6 MV and 18 MV. At the Elekta machine at 18 MV, the superficial dose of a diamond shaped 10 x 10 cm2 field is 3.6% lower than the superficial dose of a regular 10 x 10 cm2 field. This effect is not seen at 6 MV. At the Varian machine, the superficial dose of the diamond shaped field is respectively 3.5 and 14.2% higher than the superficial dose of the regular 10 x 10 cm2 field for 6 MV and 18 MV. Despite the differences measured in build-up dose for single beams between the Elekta and the Varian linear accelerator, there were no measurable differences in superficial dose when a typical IMRT dose plan of 6 MV for a head and neck tumour is executed at the two machines.

Epitaxy of Mn-based magnetic thin films on semiconductors

Abstract We review the epitaxy of Mn-based magnetic thin films and III–V semiconductors. It is demonstrated that by exploiting the epitaxial relationship to GaAs, novel artificial magnetic films can be fabricated. Metastable magnetic τ MnAl, forced bcc Co crystal structures and thin films with controlled magnetic anisotropy are feasible due to coherent growth on the semiconductor substrate. Novel magnetic superlattices and nano-scale magnetic particles embedded in a semiconductor matrix are described. The future synergism between semiconductor and magnetic thin films in research and applications is briefly discussed.

Magnetotransport and magnetization properties of p-type , a new III - V diluted magnetic semiconductor

A new III - V diluted magnetic semiconductor (DMS), , can be grown by molecular beam epitaxy. When a low temperature growth procedure is followed, homogeneous material of excellent crystal quality with Mn concentrations up to can be grown. Hole transport in these compounds is strongly affected by the antiferromagnetic exchange interaction between holes and Mn 3d spins. At a critical temperature , with a value of around 50 K for Mn concentrations of 3 - 5%, magnetic long-range order of Mn - hole complexes induces a paramagnetic - ferromagnetic transition. Above , it is shown that transport behaviour on both sides of the metal - insulator transition can be observed. Below , due to the rising spontaneous magnetization, carrier mobility increases and the relative position of the Fermi level towards the mobility edge changes. Also, a negative magnetoresistance is measured.

Co/CoAl magnetic superlattices on GaAs

Co/CoAl multilayers are grown by molecular beam epitaxy on AlAs/GaAs (001). CoAl is used as a template for the epitaxy of Co. From RHEED and lattice matching considerations bcc Co is expected, but thicker Co layers are probably fcc with stacking faults. The crystallographic structure of the Co layers is unclear at present. Room‐temperature magnetization and magnetoresistance data are presented. Co/CoAl multilayers with various CoAl thickness all show in‐plane magnetic anisotropies along 〈110〉. Uniaxial anisotropy along 〈110〉 is found to increase with increasing thickness of the CoAl layers in the multilayers. The magnetoresistive effect as a function of the orientation of the current path and the applied field is ascribed to domain‐wall effects and internal Lorentz magnetoresistance.

Ferromagnetic Mn-based thin films with perpendicular magnetization on semiconductors

Intermetallic compounds of Mn and Group III elements constitute an interesting class of magnetic material, suitable for epitaxy on semiconductor substrates. By exploiting the epitaxial relationship to GaAs, e.g. metastable magnetic τ MnAl can be fabricated with the c-axis of the tetragonal crystal parallel to the substrate normal. Consequently, perpendicular magnetization due to the large magnetocrystalline anisotropy in τ MnAl is found. The resulting magneto-optic Kerr and extraordinary Hall effect has generated interest in applications such as optical storage and non-volatile magnetic solid state memories. Tayloring the magnetic properties of the epitaxial heterostructures can be done by post-growth annealing or micro-patterning. More flexibility is achieved with multilayers (e.g. τ MnAl/NiAl) and superlattices (e.g. τ MnAl/Co). In the latter all-magnetic superlattice on GaAs, a forced bcc Co structure and strong coupling effects are under study. Further, we recently introduced heterogeneous magnet-semiconductor materials based on GaAs with a high density of nanometer-scale spherical magnetic MnAs clusters grown by MBE. Some of the interesting physics to be explored in magnetic material and semiconductor combinations will be discussed.