Large Elasto-Optic Effect in Epitaxial PbTiO(3) Films.

First-principles calculations are performed to investigate the elasto-optic properties of four different structural phases in (001) epitaxial PbTiO(3) films under tensile strain: a tetragonal (T) phase and an orthorhombic (O) phase, which are the ground states for small and large strain, respectively, and two low-symmetry, monoclinic phases of Cm and Pm symmetries that have low total energy in the intermediate strain range. It is found that the refractive indices of the T and O phases respond differently to epitaxial strain, evidenced by a change of sign of their effective elasto-optic coefficients, and as a result of presently discovered correlations between refractive index, axial ratio, and magnitude of the ferroelectric polarization. The difference in refractive indices between T and O and the existence of such correlations naturally lead to large elasto-optic coefficients in the Cm and Pm states in the intermediate strain range, because Cm structurally bridges the T and O phases (via polarization rotation and a rapid change of its axial ratio) and Pm adopts a similar axial ratio and polarization magnitude to Cm. The present results therefore broaden the palette of functionalities of ferroelectric materials, and suggest new routes to generate systems with unprecedentedly large elasto-optic conversion.

Crafting the magnonic and spintronic response of BiFeO3 films by epitaxial strain.

Multiferroics are compounds that show ferroelectricity and magnetism. BiFeO3, by far the most studied, has outstanding ferroelectric properties, a cycloidal magnetic order in the bulk, and many unexpected virtues such as conductive domain walls or a low bandgap of interest for photovoltaics. Although this flurry of properties makes BiFeO3 a paradigmatic multifunctional material, most are related to its ferroelectric character, and its other ferroic property--antiferromagnetism--has not been investigated extensively, especially in thin films. Here we bring insight into the rich spin physics of BiFeO3 in a detailed study of the static and dynamic magnetic response of strain-engineered films. Using Mössbauer and Raman spectroscopies combined with Landau-Ginzburg theory and effective Hamiltonian calculations, we show that the bulk-like cycloidal spin modulation that exists at low compressive strain is driven towards pseudo-collinear antiferromagnetism at high strain, both tensile and compressive. For moderate tensile strain we also predict and observe indications of a new cycloid. Accordingly, we find that the magnonic response is entirely modified, with low-energy magnon modes being suppressed as strain increases. Finally, we reveal that strain progressively drives the average spin angle from in-plane to out-of-plane, a property we use to tune the exchange bias and giant-magnetoresistive response of spin valves.

Thickness-dependent polarization of strained BiFeO3 films with constant tetragonality.

We measure the ferroelectric polarization of BiFeO3 films down to 3.6 nm using low energy electron and photoelectron emission microscopy. The measured polarization decays strongly below a critical thickness of 5-7 nm predicted by continuous medium theory whereas the tetragonal distortion does not change. We resolve this apparent contradiction using first-principles-based effective Hamiltonian calculations. In ultrathin films, the energetics of near open circuit electrical boundary conditions, i.e., an unscreened depolarizing field, drive the system through a phase transition from single out-of-plane polarization to nanoscale stripe domains. It gives rise to an average polarization close to zero as measured by the electron microscopy while maintaining the relatively large tetragonal distortion imposed by the nonzero polarization state of each individual domain.

Antiferromagnetic textures in BiFeO3 controlled by strain and electric field.

Antiferromagnetic thin films are currently generating considerable excitement for low dissipation magnonics and spintronics. However, while tuneable antiferromagnetic textures form the backbone of functional devices, they are virtually unknown at the submicron scale. Here we image a wide variety of antiferromagnetic spin textures in multiferroic BiFeO3 thin films that can be tuned by strain and manipulated by electric fields through room-temperature magnetoelectric coupling. Using piezoresponse force microscopy and scanning NV magnetometry in self-organized ferroelectric patterns of BiFeO3, we reveal how strain stabilizes different types of non-collinear antiferromagnetic states (bulk-like and exotic spin cycloids) as well as collinear antiferromagnetic textures. Beyond these local-scale observations, resonant elastic X-ray scattering confirms the existence of both types of spin cycloids. Finally, we show that electric-field control of the ferroelectric landscape induces transitions either between collinear and non-collinear states or between different cycloids, offering perspectives for the design of reconfigurable antiferromagnetic spin textures on demand.

Strain and Magnetic Field Induced Spin-Structure Transitions in Multiferroic BiFeO3.

The magnetic-field-dependent spin ordering of strained BiFeO3 films is determined using nuclear resonant scattering and Raman spectroscopy. The critical field required to destroy the cycloidal modulation of the Fe spins is found to be significantly lower than in the bulk, with appealing implications for field-controlled spintronic and magnonic devices.

Large elasto-optic effect and reversible electrochromism in multiferroic BiFeO3.

The control of optical fields is usually achieved through the electro-optic or acousto-optic effect in single-crystal ferroelectric or polar compounds such as LiNbO3 or quartz. In recent years, tremendous progress has been made in ferroelectric oxide thin film technology-a field which is now a strong driving force in areas such as electronics, spintronics and photovoltaics. Here, we apply epitaxial strain engineering to tune the optical response of BiFeO3 thin films, and find a very large variation of the optical index with strain, corresponding to an effective elasto-optic coefficient larger than that of quartz. We observe a concomitant strain-driven variation in light absorption--reminiscent of piezochromism--which we show can be manipulated by an electric field. This constitutes an electrochromic effect that is reversible, remanent and not driven by defects. These findings broaden the potential of multiferroics towards photonics and thin film acousto-optic devices, and suggest exciting device opportunities arising from the coupling of ferroic, piezoelectric and optical responses.

Suppression of creep-regime dynamics in epitaxial ferroelectric BiFeO3 films.

Switching dynamics of ferroelectric materials are governed by the response of domain walls to applied electric field. In epitaxial ferroelectric films, thermally-activated 'creep' motion plays a significant role in domain wall dynamics, and accordingly, detailed understanding of the system's switching properties requires that this creep motion be taken into account. Despite this importance, few studies have investigated creep motion in ferroelectric films under ac-driven force. Here, we explore ac hysteretic dynamics in epitaxial BiFeO3 thin films, through ferroelectric hysteresis measurements, and stroboscopic piezoresponse force microscopy. We reveal that identically-fabricated BiFeO3 films on SrRuO3 or La0.67Sr0.33MnO3 bottom electrodes exhibit markedly different switching behaviour, with BiFeO3/SrRuO3 presenting essentially creep-free dynamics. This unprecedented result arises from the distinctive spatial inhomogeneities of the internal fields, these being influenced by the bottom electrode's surface morphology. Our findings further highlight the importance of controlling interface and defect characteristics, to engineer ferroelectric devices with optimised performance.

[Acute pancreatitis: clinical-tomographic correlations]. / Pancreatite acuta: correlazioni clinico-tomografiche.

The authors are interested in verifying the correlation between clinical findings and CT scan imaging in acute pancreatitis. Forty-one patients with acute episodes of pancreatitis are studied according to classic clinical Ranson prognostic factors and radiological Hill classification. All patients are investigated with G.E. 9800 Quick TC scanner. Our data show a significant correlation especially for patients of extreme classes: 78.3% of patients in first Ranson class are included in 0-I-II Hill grades, 74% of the patients in the cumulative third and fourth Ranson classes are correlated with IV and V Hill grades. Finally the authors underline the usefulness of clinical and tomographic evaluation for prognostic and therapeutical trends in patients with acute pancreatitis.

[Value of cardiac scintigraphy using thallium dipyridamole in the preoperative diagnosis of vascular diseases]. / Validità della scintigrafia cardiaca al tallio dipiridamolo nello studio preoperatorio dei pazienti vascolari.

The authors assess the value of cardiac scintigraphy using thallium dipyridamidole in the preoperative examination of vascular patients. Data from the literature indicate the real possibility of cardiac complications in major vascular surgery. These complications, which range from simple arrhythmias to lethal myocardial infarctions, have an incidence of 0.5-2% in aortic surgery, 0.6-3% in peripheral revascularisation increasing to 2.6-8.3% in distal revascularisation, although they are less important in carotid surgery. From January 1993 to March 1994 cardiac risk was evaluated in 44 patients due to undergo major vascular surgery using cardiac scintigraphy with thallium dipyridamidole. The test was negative in 14 cases. The presence of one or more redistribution areas was observed in 23 cases, and a fixed zone in 7 cases. In line with the proposals made by Eagle, the authors conclude that it is necessary to restrict the indications for this test to those cases presenting at least two risk factors.

[The assessment of the operative risk in vascular surgery]. / Valutazione del rischio operatorio in chirurgia vascolare.

Atherosclerosis is a polydistrectual disease. The patients with peripheral, carotid or aortoiliac pathology suffer frequently from coronary disease; so, a careful evaluation of operative risk is indispensable related to the anatomic district that we intend submit to surgery. In this work we separately explore the risk related to peripheral revasculations, aortic surgery and carotid surgery following the current literature and our experience.

BiFeO3 epitaxial thin films and devices: past, present and future.

The celebrated renaissance of the multiferroics family over the past ten years has also been that of its most paradigmatic member, bismuth ferrite (BiFeO3). Known since the 1960s to be a high temperature antiferromagnet and since the 1970s to be ferroelectric, BiFeO3 only had its bulk ferroic properties clarified in the mid-2000s. It is however the fabrication of BiFeO3 thin films and their integration into epitaxial oxide heterostructures that have fully revealed its extraordinarily broad palette of functionalities. Here we review the first decade of research on BiFeO3 films, restricting ourselves to epitaxial structures. We discuss how thickness and epitaxial strain influence not only the unit cell parameters, but also the crystal structure, illustrated for instance by the discovery of the so-called T-like phase of BiFeO3. We then present its ferroelectric and piezoelectric properties and their evolution near morphotropic phase boundaries. Magnetic properties and their modification by thickness and strain effects, as well as optical parameters, are covered. Finally, we highlight various types of devices based on BiFeO3 in electronics, spintronics, and optics, and provide perspectives for the development of further multifunctional devices for information technology and energy harvesting.

Control of ferroelectricity and magnetism in multi-ferroic BiFeO3 by epitaxial strain.

Recently, strain engineering has been shown to be a powerful and flexible means of tailoring the properties of ABO3 perovskite thin films. The effect of epitaxial strain on the structure of the perovskite unit cell can induce a host of interesting effects, these arising from either polar cation shifts or rotation of the oxygen octahedra, or both. In the multi-ferroic perovskite bismuth ferrite (BiFeO3-BFO), both degrees of freedom exist, and thus a complex behaviour may be expected as one plays with epitaxial strain. In this paper, we review our results on the role of strain on the ferroic transition temperatures and ferroic order parameters. We find that, while the Néel temperature is almost unchanged by strain, the ferroelectric Curie temperature strongly decreases as strain increases in both the tensile and compressive ranges. Also unexpected is the very weak influence of strain on the ferroelectric polarization value. Using effective Hamiltonian calculations, we show that these peculiar behaviours arise from the competition between antiferrodistortive and polar instabilities. Finally, we present results on the magnetic order: while the cycloidal spin modulation present in the bulk survives in weakly strained films, it is destroyed at large strain and replaced by pseudo-collinear antiferromagnetic ordering. We discuss the origin of this effect and give perspectives for devices based on strain-engineered BiFeO3.

Three years of HIV/AIDS care and treatment services in Tanzania: achievements and challenges.

The Tanzania HIV Care and Treatment Plan was launched in October 2004 aiming at providing 440,000 AIDS patients with antiretroviral drugs (ARVs) and track disease progression in 1.2 million HIV+ persons by the end of the 2008. This paper is intended to provide information to stake holders of the achievements and challenges of the HIV Care and Treatment Plan since its inception in 2004. Facility patient reports are aggregated at district and then regional level before being sent to the national level where they are aggregated to form a national report. By December 2007, 210 health facilities were offering HIV care and treatment services in Tanzania. About 123,147 (5%) of the 2,636,785 estimated people living with HIV and AIDS were enrolled, and 71,439 (13.6%) of the estimated 527,357 AIDS cases commenced ART. More females than males started ART, F:M ratio being 3:2. Most (49%) patients were started ART due to low CD4 counts (< 200). About 6,618 patients had their initial ARV regimen changed due to starting anti-TB treatment 679 (10%), peripheral neuropathy 812 (12%), skin rash 378 (6%), and stock out 247 (4%) or other reasons (18%), while 2,653 (42%) had no reason recorded. The proportion of patients still alive and on ART at 6, 12 and 24 months after initiation of treatment was 60%, 60%, and 50%, respectively, while those collecting ARVs on schedule was 34%, 25% and 10% respectively. About 3,084 patients developed TB after starting ART, of whom 1,557 (approximately 50%) patients during the first three months of treatment. During the three years (2004-2007) of HIV care and treatment services in Tanzania, there has been an increase in the number of CTC facilities, geographical coverage of services, the number of enrolled patients and those on ART. However, the set target for ART services has not been achieved and there are significant geographical variations in these achievements, which do not correspond with either population density or disease burden. Efforts should be made to i) ensure equitable accessibility when scaling up ART services in Tanzania, ii) improve the recording and reporting system and iii) armonize the activities of various stakeholders.