Biaxial strain-induced suppression of spinodal decomposition in GaMnAs and GaCrAs

The thermodynamic properties of the magnetic semiconductors GaMnAs and GaCrAs are studied under biaxial strain. The calculations are based on the projector augmented wave method combined with the generalized quasichemical approach to treat the disorder and composition effects. Considering the influence of biaxial strain, we find a tendency to the suppression of binodal decomposition mainly for GaMnAs under compressive strain. For a substrate with a lattice constant 5% smaller than the one of GaAs, for GaMnAs, the solubility limit increases up to 40%. Thus, the strain can be a useful tool for tailoring magnetic semiconductors to the formation or not of embedded nanoclusters. (C) 2010 American Institute of Physics. [doi:10.1063/1.3448025]

Anisotropic Confinement, Electronic Coupling and Strain Induced Effects Detected by Valence-Band Anisotropy in Self-Assembled Quantum Dots

A method to determine the effects of the geometry and lateral ordering on the electronic properties of an array of one-dimensional self-assembled quantum dots is discussed. A model that takes into account the valence-band anisotropic effective masses and strain effects must be used to describe the behavior of the photoluminescence emission, proposed as a clean tool for the characterization of dot anisotropy and/or inter-dot coupling. Under special growth conditions, such as substrate temperature and Arsenic background, 1D chains of In(0.4)Ga(0.6) As quantum dots were grown by molecular beam epitaxy. Grazing-incidence X-ray diffraction measurements directly evidence the strong strain anisotropy due to the formation of quantum dot chains, probed by polarization-resolved low-temperature photoluminescence. The results are in fair good agreement with the proposed model.

Strain induced edge magnetism at zigzag edge of a graphene quantum dot

We study the temperature dependent magnetic susceptibility of a strained graphene quantum dot by using the determinant quantum Monte Carlo method. Within the Hubbard model on a honeycomb lattice, our unbiased numerical results show that a relative small interaction $U$ may lead to a edge ferromagnetic like behavior in the strained graphene quantum dot, and a possible room temperature transition is suggested. Around half filling, the ferromagnetic fluctuations at the zigzag edge is strengthened both markedly by the on-site Coulomb interaction and the strain, especially in low temperature region. The resultant strongly enhanced ferromagnetic like behavior may be important for the development of many applications.

Strain-induced stabilization of new magnetic spinel structures in epitaxial oxide heterostructures.

We report here on the growth of NiFe2O4 epitaxial thin films of different thickness (3 nm ¿ t ¿ 32 nm) on single crystalline substrates having spinel (MgAl2O4) or perovskite (SrTiO3) structure. Ultrathin films, grown on any of those substrates, display a huge enhancement of the saturation magnetization: we will show that partial cationic inversion may account for this enhancement, although we will argue that suppression of antiparallel collinear spin alignment due to size-effects cannot be excluded. Besides, for thicker films, the magnetization of films on MAO is found to be similar to that of bulk ferrite; in contrast, the magnetization of films on STO is substantially lower than bulk. We discuss on the possible mechanisms leading to this remarkable difference of magnetization.

Thermal and magnetic field-induced martensite-austenite transition in Ni50.3Mn35.3Sn14.4 ribbons

Thermal and field-induced martensite-austenite transition was studied in melt spun Ni50.3Mn35.3Sn14.4 ribbons. Its distinct highly ordered columnarlike microstructure normal to ribbon plane allows the direct observation of critical fields at which field-induced and highly hysteretic reverse transformation starts (H=17kOe at 240K), and easy magnetization direction for austenite and martensite phases with respect to the rolling direction. Single phase L21 bcc austenite with TC of 313K transforms into a 7M orthorhombic martensite with thermal hysteresis of 21K and transformation temperatures of MS=226K, Mf=218K, AS=237K, and Af=244K

Strain induced anomalous red shift in mesoscopic iron oxide prepared by a novel technique

Nano magnetic oxides are promising candidates for high density magnetic storage and other applications. Nonspherical mesoscopic iron oxide particles are also candidate materials for studying the shape, size and strain induced modifications of various physical properties viz. optical, magnetic and structural. Spherical and nonspherical iron oxides having an aspect ratio, ~2, are synthesized by employing starch and ethylene glycol and starch and water, respectively by a novel technique. Their optical, structural, thermal and magnetic properties are evaluated. A red shift of 0⋅24 eV is observed in the case of nonspherical particles when compared to spherical ones. The red shift is attributed to strain induced changes in internal pressure inside the elongated iron oxide particles. Pressure induced effects are due to the increased overlap of wave functions. Magnetic measurements reveal that particles are superparamagnetic. The marked increase in coercivity in the case of elongated particles is a clear evidence for shape induced anisotropy. The decreased specific saturation magnetization of the samples is explained on the basis of weight percentage of starch, a nonmagnetic component and is verified by TGA and FTIR studies. This technique can be modified for tailoring the aspect ratio and these particles are promising candidates for drug delivery and contrast enhancement agents in magnetic resonance imaging

Interaction between recrystallization and strain-induced precipitation in a high Nb- and N-bearing austenitic stainless steel: influence of the interpass time

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Effect of simvastatin on passive strain-induced skeletal muscle injury in rats

Introduction: HMG-CoA reductase inhibitors are the most frequently prescribed drugs for treatment of lipid imbalance, but they have side effects, such as myopathy. Our aim was to assess the effect of simvastatin on the inflammatory process induced by skeletal muscle injury. Methods: Rats were divided into experimental groups [control group, simvastatin (20 mg/kg) group, group treated with simvastatin (20 mg/kg) and subjected to injury, and group subjected to injury only]. Histological analysis and analyses of creatine kinase activity and C-reactive protein were performed. Results: Animals treated with simvastatin exhibited significantly greater morphological and structural skeletal muscle damage in comparison to the control group and injured animals without treatment. Conclusions: Although simvastatin has a small anti-inflammatory effect in the early stage after a muscle strain injury, the overall picture is negative, as simvastatin increases the extent of damage to muscle morphology. Further studies are needed. Muscle Nerve 46: 908-913, 2012

Infrared (810 nm) Low-Level Laser Therapy in Experimental Model of Strain-Induced Skeletal Muscle Injury in Rats: Effects on Functional Outcomes

Muscle strains are among the most prevalent causes for athletes absence from sport activities. Low-level laser therapy (LLLT) has recently emerged as a potential contender to nonsteroidal anti-inflammatory drugs in muscle strain treatment. In this work we investigated effects of LLLT and diclofenac on functional outcomes in the acute stage after muscle strain injury in rats. Muscle strain was induced by overloading the tibialis anterior muscle of rats during anesthesia. The injured groups received either no treatment, or a single treatment with diclofenac 30 min prior to injury, or LLLT (810 nm, 100 mW) with doses of 1, 3, 6 or 9 J, at 1 h after injury. Functional outcome measures included a walking index and assessment of electrically induced muscle performance. All treatments (except 9 J LLLT) significantly improved the walking index 12 h postinjury compared with the untreated group. The 3 J group also showed a significantly better walking index than the drug group. All treatments significantly improved muscle performance at 6 and 12 h. LLLT dose of 3 J was as effective as the pharmacological agent in improving functional outcomes in the early phase after a muscle strain injury in rats.

Strain-induced structural instability in FeRh

We perform density functional calculations to investigate the structure of the intermetallic alloy FeRh under epitaxial strain. Bulk FeRh exhibits a metamagnetic transition from a low-temperature antiferromagnetic (AFM) phase to a ferromagnetic phase at 350 K, and its strain dependence is of interest for tuning the transition temperature to the room-temperature operating conditions of typical memory devices. We find an unusually strong dependence of the structural energetics on the choice of exchange-correlation functional, with the usual local density approximation yielding the wrong ground-state structure, and generalized gradient (GGA) extensions being in better agreement with the bulk experimental structure. Using the GGA we show the existence of a metastable face-centered-cubic-like AFM structure that is reached from the ground-state body-centered-cubic-like AFM structure by following the epitaxial Bain path. We show that the behavior is well described using nonlinear elasticity theory, which captures the softening and eventual sign change of the orthorhombic shear modulus under compressive strain, consistent with this structural instability. Finally, we predict the existence of an additional unit-cell-doubling lattice instability, which should be observable at low temperature.

Experimental and computational investigation of the roll forming process

One of the first questions to consider when designing a new roll forming line is the number of forming steps required to produce a profile. The number depends on material properties, the cross-section geometry and tolerance requirements, but the tool designer also wants to minimize the number of forming steps in order to reduce the investment costs for the customer. There are several computer aided engineering systems on the market that can assist the tool designing process. These include more or less simple formulas to predict deformation during forming as well as the number of forming steps. In recent years it has also become possible to use finite element analysis for the design of roll forming processes. The objective of the work presented in this thesis was to answer the following question: How should the roll forming process be designed for complex geometries and/or high strength steels? The work approach included both literature studies as well as experimental and modelling work. The experimental part gave direct insight into the process and was also used to develop and validate models of the process. Starting with simple geometries and standard steels the work progressed to more complex profiles of variable depth and width, made of high strength steels. The results obtained are published in seven papers appended to this thesis. In the first study (see paper 1) a finite element model for investigating the roll forming of a U-profile was built. It was used to investigate the effect on longitudinal peak membrane strain and deformation length when yield strength increases, see paper 2 and 3. The simulations showed that the peak strain decreases whereas the deformation length increases when the yield strength increases. The studies described in paper 4 and 5 measured roll load, roll torque, springback and strain history during the U-profile forming process. The measurement results were used to validate the finite element model in paper 1. The results presented in paper 6 shows that the formability of stainless steel (e.g. AISI 301), that in the cold rolled condition has a large martensite fraction, can be substantially increased by heating the bending zone. The heated area will then become austenitic and ductile before the roll forming. Thanks to the phenomenon of strain induced martensite formation, the steel will regain the martensite content and its strength during the subsequent plastic straining. Finally, a new tooling concept for profiles with variable cross-sections is presented in paper 7. The overall conclusions of the present work are that today, it is possible to successfully develop profiles of complex geometries (3D roll forming) in high strength steels and that finite element simulation can be a useful tool in the design of the roll forming process.

Thermal analysis of the chip formation in austenitic stainless steel

The most important property of austenitic stainless steels is corrosion resistance. In these steels, the transition between paramagnetic and ferromagnetic conditions occurs at low temperatures. Therefore, the use of austenitic stainless steels in conditions in which ferromagnetism absence is important can be considered. On the other hand, the formation of strain-induced martensite is detected when austenitic stainless steels are deformed as well as machined. The strain-induced martensite formed especially in the machining process is not uniform through the chip and its formation can also be related to the Md temperature. Therefore, both the temperature distribution and the gradient during the cutting and chip formation are important to identify regions in which martensite formation is propitiated. The main objective here is evaluate the strain-induced martensite formation throughout machining by observing microstructural features and comparing these to thermal results obtained through finite element method analysis. Results show that thermal analysis can give support to the martensite identified in the microstructural analysis.