145 research outputs found
Proceedings of the Workshop on Adaptation of Plants to Soil Stresses
Sustainable production of food and forage with a focus on plant adaptation to stress environments will be a continued priority for developing countries in the future. Since many areas of the world which support substantial human populations are drought prone, such as the subsaharan African zone and others, the primary focus has been on drought. However, one of the greatest restraints to sustainability of agriculture worldwide is the lack of sufficient soil nutrients for crop growth, or other soil constraints such as acidity or salinity which hinder crop production substantially.
Optimizing soil fertility or amending acid and saline soils to achieve high production is difficult in areas of low economic stability since inputs are costly or quite often technically not feasible. The other obvious alternative to increase stability in stress areas is by genetic improvement of crops. Dr. Donald L. Plucknett, in a recent lecture on science and agricultural transformations, stated that while not all yield gains in the Green Revolutions can be attributed to plant breeding, it is doubtful such gains would have taken place without the new varieties or hybrids . Development and release of new and improved germplasm is probably the most economic method of technology transfer currently available. According to Dr. Plucknett, most studies indicate about half of yield gains can be attributed to genetic improvements. This statistic is undoubtedly argued in many circles, but regardless of the final figure, gains from genetic improvement are substantial
Delta Doping of Ferromagnetism in Antiferromagnetic Manganite Superlattices
We demonstrate that delta-doping can be used to create a dimensionally
confined region of metallic ferromagnetism in an antiferromagnetic (AF)
manganite host, without introducing any explicit disorder due to dopants or
frustration of spins. Delta-doped carriers are inserted into a manganite
superlattice (SL) by a digital-synthesis technique. Theoretical consideration
of these additional carriers show that they cause a local enhancement of
ferromagnetic (F) double-exchange with respect to AF superexchange, resulting
in local canting of the AF spins. This leads to a highly modulated
magnetization, as measured by polarized neutron reflectometry. The spatial
modulation of the canting is related to the spreading of charge from the doped
layer, and establishes a fundamental length scale for charge transfer,
transformation of orbital occupancy and magnetic order in these manganites.
Furthermore, we confirm the existence of the canted, AF state as was predicted
by de Gennes [P.-G. de Gennes, Phys. Rev. 118, 141 (1960)], but had remained
elusive
Fast strain wave induced magnetization changes in long cobalt bars: Domain motion versus coherent rotation
A high frequency (88 MHz) traveling strain wave on a piezoelectric substrate is shown to change the magnetization direction in 40 lm wide Co bars with an aspect ratio of 103. The rapidly alternating strain wave rotates the magnetization away from the long axis into the short axis direction, via magnetoelastic coupling. Strain-induced magnetization changes have previously been demonstrated in ferroelectric/ferromagnetic heterostructures, with excellent fidelity between the ferromagnet and the ferroelectric domains, but these experiments were limited to essentially dc frequencies. Both magneto-optical Kerr effect and polarized neutron reflectivity confirm that the traveling strain wave does rotate the magnetization away from the long axis direction and both yield quantitatively similar values for the rotated magnetization. An investigation of the behavior of short axis magnetization with increasing strain wave amplitude on a series of samples with variable edge roughness suggests that the magnetization reorientation that is seen proceeds solely via coherent rotation. Polarized neutron reflectivity data provide direct experimental evidence for this model. This is consistent with expectations that domain wall motion cannot track the rapidly varying strain
Precipitating Ordered Skyrmion Lattices from Helical Spaghetti
Magnetic skyrmions have been the focus of intense research due to their
potential applications in ultra-high density data and logic technologies, as
well as for the unique physics arising from their antisymmetric exchange term
and topological protections. In this work we prepare a chiral jammed state in
chemically disordered (Fe, Co)Si consisting of a combination of
randomly-oriented magnetic helices, labyrinth domains, rotationally disordered
skyrmion lattices and/or isolated skyrmions. Using small angle neutron
scattering, (SANS) we demonstrate a symmetry-breaking magnetic field sequence
which disentangles the jammed state, resulting in an ordered, oriented skyrmion
lattice. The same field sequence was performed on a sample of powdered Cu2OSeO3
and again yields an ordered, oriented skyrmion lattice, despite relatively
non-interacting nature of the grains. Micromagnetic simulations confirm the
promotion of a preferred skyrmion lattice orientation after field treatment,
independent of the initial configuration, suggesting this effect may be
universally applicable. Energetics extracted from the simulations suggest that
approaching a magnetic hard axis causes the moments to diverge away from the
magnetic field, increasing the Dzyaloshinskii-Moriya energy, followed
subsequently by a lattice re-orientation. The ability to facilitate an emergent
ordered magnetic lattice with long-range orientation in a variety of materials
despite overwhelming internal disorder enables the study of skyrmions even in
imperfect powdered or polycrystalline systems and greatly improves the ability
to rapidly screen candidate skyrmion materials
Measurement of instantaneous nitrogen use efficiency among pearl millet genotypes
Nitrogen is often a limiting factor in pearl millet [Penuisetum glaucum( L.) R. Br.] production. Genotypesa re knownth at differ their responset o N. In this study four pearl millet genotypes,w hich werep reviouslyi dentified as extremesi n N use efficiency (total above ground biomass/unit of N absorbed) in the field, were comparedi n
nutrient solution culture for their responset o N supply and the instantaneous measuremenot f photosynthetic N efficiency (~tmol CO2 g-~ N s-a). The latter componenpt ossibly contributes to N use efficiency
N-efficient genotypes, ’Souna B’ and 700112, and N-inefficient genotypes, ’BK560a’ nd ’B J104’, were growna t four N levels containing 60, 120, 180, and 240 mg N plant-~. Specific leaf area (cm2 g-a) was calculated from an accompanyingg rowth analysis. Leaf CO2 exchange rate was measured on several leaves as they became fully expanded. Photosynthetic N efficiency was derived using
values of leaf N concentration. Nitrogen-efficient genotypes
maintained thicker leaves (316.1 vs. 332.5 cm2 g-l) and were generally less responsive in terms of leaf N concentrations and leaf COs exchanger ate (CER)t o the N available in solution as comparetdo the N-inefficient genotypes. Souna B, the most efficient genotype,
maintained a stable CER across all N levels. Photosynthetic N efficiency was similar for all genotypese xcept BK560w, hich was ~]0 to 15% less efficient. The small difference in photosynthetic N efficiency among genotypes coupled with relatively high photosynthetic N efficiency values of BJI04 (N-inefficient genotype) suggests that differences in instantaneous measures of N use efficiency occurring
during photosynthesios ffer little explanationf or overall differences in N use efficiency among these genotypes in a previous field study. Nitrogen use efficiency was more related to the partitioning of N resources available into additional leaf are
Ionic Tuning of Cobaltites at the Nanoscale
Control of materials through custom design of ionic distributions represents
a powerful new approach to develop future technologies ranging from spintronic
logic and memory devices to energy storage. Perovskites have shown particular
promise for ionic devices due to their high ion mobility and sensitivity to
chemical stoichiometry. In this work, we demonstrate a solid-state approach to
control of ionic distributions in (La,Sr)CoO thin films. Depositing a Gd
capping layer on the perovskite film, oxygen is controllably extracted from the
structure, up-to 0.5 O/u.c. throughout the entire 36 nm thickness. Commensurate
with the oxygen extraction, the Co valence state and saturation magnetization
show a smooth continuous variation. In contrast, magnetoresistance measurements
show no-change in the magnetic anisotropy and a rapid increase in the
resistivity over the same range of oxygen stoichiometry. These results suggest
significant phase separation, with metallic ferromagnetic regions and
oxygen-deficient, insulating, non-ferromagnetic regions, forming percolated
networks. Indeed, X-ray diffraction identifies oxygen-vacancy ordering,
including transformation to a brownmillerite crystal structure. The unexpected
transformation to the brownmillerite phase at ambient temperature is further
confirmed by high-resolution scanning transmission electron microscopy which
shows significant structural - and correspondingly chemical - phase separation.
This work demonstrates room-temperature ionic control of magnetism, electrical
resistivity, and crystalline structure in a 36 nm thick film, presenting new
opportunities for ionic devices that leverage multiple material
functionalities
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