Resonant tunneling magnetoresistance in epitaxial metal-semiconductor heterostructures

We report on resonant tunneling magnetoresistance via localized states through a ZnSe semiconducting barrier which can reverse the sign of the effective spin polarization of tunneling electrons. Experiments performed on Fe/ZnSe/Fe planar junctions have shown that positive, negative or even its sign-reversible magnetoresistance can be obtained, depending on the bias voltage, the energy of localized states in the ZnSe barrier and spatial symmetry. The averaging of conduction over all localized states in a junction under resonant condition is strongly detrimental to the magnetoresistance

The satisfactory growth and development at 2 years of age of the INTERGROWTH-21st Fetal Growth Standards cohort support its appropriateness for constructing international standards.

BACKGROUND: The World Health Organization recommends that human growth should be monitored with the use of international standards. However, in obstetric practice, we continue to monitor fetal growth using numerous local charts or equations that are based on different populations for each body structure. Consistent with World Health Organization recommendations, the INTERGROWTH-21st Project has produced the first set of international standards to date pregnancies; to monitor fetal growth, estimated fetal weight, Doppler measures, and brain structures; to measure uterine growth, maternal nutrition, newborn infant size, and body composition; and to assess the postnatal growth of preterm babies. All these standards are based on the same healthy pregnancy cohort. Recognizing the importance of demonstrating that, postnatally, this cohort still adhered to the World Health Organization prescriptive approach, we followed their growth and development to the key milestone of 2 years of age. OBJECTIVE: The purpose of this study was to determine whether the babies in the INTERGROWTH-21st Project maintained optimal growth and development in childhood. STUDY DESIGN: In the Infant Follow-up Study of the INTERGROWTH-21st Project, we evaluated postnatal growth, nutrition, morbidity, and motor development up to 2 years of age in the children who contributed data to the construction of the international fetal growth, newborn infant size and body composition at birth, and preterm postnatal growth standards. Clinical care, feeding practices, anthropometric measures, and assessment of morbidity were standardized across study sites and documented at 1 and 2 years of age. Weight, length, and head circumference age- and sex-specific z-scores and percentiles and motor development milestones were estimated with the use of the World Health Organization Child Growth Standards and World Health Organization milestone distributions, respectively. For the preterm infants, corrected age was used. Variance components analysis was used to estimate the percentage variability among individuals within a study site compared with that among study sites. RESULTS: There were 3711 eligible singleton live births; 3042 children (82%) were evaluated at 2 years of age. There were no substantive differences between the included group and the lost-to-follow up group. Infant mortality rate was 3 per 1000; neonatal mortality rate was 1.6 per 1000. At the 2-year visit, the children included in the INTERGROWTH-21st Fetal Growth Standards were at the 49th percentile for length, 50th percentile for head circumference, and 58th percentile for weight of the World Health Organization Child Growth Standards. Similar results were seen for the preterm subgroup that was included in the INTERGROWTH-21st Preterm Postnatal Growth Standards. The cohort overlapped between the 3rd and 97th percentiles of the World Health Organization motor development milestones. We estimated that the variance among study sites explains only 5.5% of the total variability in the length of the children between birth and 2 years of age, although the variance among individuals within a study site explains 42.9% (ie, 8 times the amount explained by the variation among sites). An increase of 8.9 cm in adult height over mean parental height is estimated to occur in the cohort from low-middle income countries, provided that children continue to have adequate health, environmental, and nutritional conditions. CONCLUSION: The cohort enrolled in the INTERGROWTH-21st standards remained healthy with adequate growth and motor development up to 2 years of age, which supports its appropriateness for the construction of international fetal and preterm postnatal growth standards

Magnetic and structural properties of Mn5+xGe3+y thin films as a function of substrate orientation

International audienceMn 5 Ge 3 thin films were grown on GaAs(1 1 1) and GaAs(0 0 1) by Molecular Beam Epitaxy (MBE). The influence of stoichiometry and alloying of the samples was investigated using different Mn diffusion cell temperatures for each sample. X-ray diffraction (XRD) measurements reveal the following epitaxial relations: (0 0 1) Mn 5 Ge 3 // (1 1 1) GaAs and [1 1 0] Mn5Ge3//[2-1-1] GaAs for the sample grown on GaAs(1 1 1), and (1 1 1) Mn 5 Ge 3 //(0 0 1) GaAs and [ − 1 1 0] Mn5Ge3//[1 1 0] GaAs for the samples grown on GaAs(0 0 1). High-resolution transmission electron microscopy (HRTEM) results for a sample grown on GaAs(0 0 1) indicate that samples are around 70 nm thick and are composed of several grains with 2 different orientations corresponding to a 90 • rotation around the [1 1 1] axis. Analyzing the Mn map with energy-filtered transmission electron microscopy (EFTEM) we observed that all the grains look very similar, indicating that they contain roughly the same amount of manganese. The magnetocaloric effect found in both family of samples is spread in a wider temperature range when compared with bulk Mn5.1Ge2.9 and the magnetic entropy change peak is higher than Si-or Sb-modified Mn 5 Ge 3. This result makes the Mn 5 Ge 3 a possible candidate for magnetic refrigeration while being a rare earth-free material

Magnetic anisotropy of 3 nm diameter Co nanowires embedded in CeO 2

International audienceThe magnetic anisotropy of 3-nm wide cobalt nanowires embedded in epitaxial CeO2/SrTiO3(001) layers is investigated by ferromagnetic resonance measurements. The measured magnetic shape and the magnetocrystalline anisotropies confirm that the Co nanowires have their main axes perpendicular to the film surface, and they are composed of hcp Co grains with the c-axes oriented along one of the \textbackslashlangle 111\textbackslashrangle directions of the CeO2 matrix. The effects of such a peculiar structure on the magnetic anisotropy are addressed experimentally. The results show that the magnetic anisotropy of the wires is dominated by the magnetostatic term. The inhomogeneous structure of the wires leads to an effective magnetocrystalline anisotropy smaller than the bulk value of hcp Co

Magnetoresistance in granular magnetic tunnel junctions with Fe nanoparticles embedded in ZnSe semiconducting epilayer

International audienceWe have investigated transport properties of iron (Fe) nanoparticles embedded in zinc selenide (ZnSe) semiconducting epilayers prepared by molecular beam epitaxy. Both positive and negative tunneling magnetoresistances (TMRs) were measured depending on the applied voltage biases and on the temperature. A slow reduction of the TMR magnitude with temperature was detected and it could be explained in terms of a crossover between direct/resonant tunneling and variable range hopping. The temperature behavior of the magnetoresistance is a clear signature of tunneling and hopping mechanisms mediated by the ZnSe barrier localized states. (C) 2008 American Institute of Physics