Photovoltaic amorphous feroxyhyte nanostructures synthesized by atmospheric AC microplasma.

Feroxyhite (δ-FeOOH) nanomaterials were successfully synthesized through the atmospheric AC microplasma method at room temperature from ferrous sulfate aqueous solutions. Various syntheses conditions, including electric voltage, electric field strength, ferrous concentration, hydrogen peroxide concentration, and reaction duration, were systematically investigated. The synthesized products were characterized through x-ray diffraction, UV-vis absorption spectroscopy, photoluminescence spectroscopy, infra-red spectroscopy, and electron microscopy. The bandgap of the produced materials were strongly dependent of the ferrous concentration while the product ratio was dependent on all experimental conditions. The synthesis mechanism was thoroughly discussed. The synthesized nanomaterials were amorphous nanospheres, showing superparamagnetic properties at room temperature. The synthesized oxyhydroxide is a potential photovoltaic material besides its reported applications in photocatalysts and supercapacitors. The application of this synthesis technique could be extended to synthesize other oxy-hydroxide nanomaterials for renewable energy applications facilely, scalablely, cost-effectively, and environmentally.

Printing Air-Stable High-Tc Molecular Magnet with Tunable Magnetic Interaction.

High-Tc molecular magnets have amassed much promise; however, the long-standing obstacle for its practical applications is the inaccessibility of high-temperature molecular magnets showing dynamic and nonvolatile magnetization control. In addition, its functional durability is prone to degradation in oxygen and heat. Here, we introduce a rapid prototyping and stabilizing strategy for high Tc (360 K) molecular magnets with precise spatial control in geometry. The printed molecular magnets are thermally stable up to 400 K and air-stable for over 300 days, a significant improvement in its lifetime and durability. X-ray magnetic circular dichroism and computational modeling reveal the water ligands controlling magnetic exchange interaction of molecular magnets. The molecular magnets also show dynamical and reversible tunability of magnetic exchange interactions, enabling a colossal working temperature window of 86 K (from 258 to 344 K). This study provides a pathway to flexible, lightweight, and durable molecular magnetic devices through additive manufacturing.

Tunable electroresistance and electro-optic effects of transparent molecular ferroelectrics.

Recent progress in molecular ferroelectrics (MOFEs) has been overshadowed by the lack of high-quality thin films for device integration. We report a water-based air-processable technique to prepare large-area MOFE thin films, controlled by supersaturation growth at the liquid-air interface under a temperature gradient and external water partial pressure. We used this technique to fabricate ImClO4 thin films and found a large, tunable room temperature electroresistance: a 20-fold resistance variation upon polarization switching. The as-grown films are transparent and consist of a bamboo-like structure of (2,[Formula: see text],0) and (1,0,[Formula: see text]) structural variants of R3m symmetry with a reversible polarization of 6.7 µC/cm2. The resulting ferroelectric domain structure leads to a reversible electromechanical response of d33 = 38.8 pm/V. Polarization switching results in a change of the refractive index, n, of single domains, [Formula: see text]. The remarkable combination of these characteristics renders MOFEs a prime candidate material for new nanoelectronic devices. The information that we present in this work will open a new area of MOFE thin-film technologies.

Unique magnetostriction of Fe68.8Pd31.2 attributable to twinning.

Fe68.8Pd31.2 exhibits an anomalously large magnetostriction of ~400 ppm at room temperature as well as linear, isotropic, and hysteresis free magnetization behavior. This near perfectly reversible magnetic response is attributable to the presence of a large number of premartensitic magnetoelastic twin clusters present in the system made possible through the elastic softening that occurs near a martensitic transformation temperature of 252 K. It is proposed that the twin clusters in the material reduce both internal elastic and magnetic energy, causing the elastic and magnetic behavior of the material to be intimately linked. In such a framework, the anisotropy energy becomes extremely low causing the material to bear no crystalline dependence on magnetization, and application of a magnetic field causes simultaneous magnetic and twin domain movement which relaxes the system.

Room Temperature Multiferroicity of Charge Transfer Crystals.

Room temperature multiferroics has been a frontier research field by manipulating spin-driven ferroelectricity or charge-order-driven magnetism. Charge-transfer crystals based on electron donor and acceptor assembly, exhibiting simultaneous spin ordering, are drawing significant interests for the development of all-organic magnetoelectric multiferroics. Here, we report that a remarkable anisotropic magnetization and room temperature multiferroicity can be achieved through assembly of thiophene donor and fullerene acceptor. The crystal motif directs the dimensional and compositional control of charge-transfer networks that could switch magnetization under external stimuli, thereby opening up an attractive class of all-organic nanoferronics.

Shape memory alloys. Ultralow-fatigue shape memory alloy films.

Functional shape memory alloys need to operate reversibly and repeatedly. Quantitative measures of reversibility include the relative volume change of the participating phases and compatibility matrices for twinning. But no similar argument is known for repeatability. This is especially crucial for many future applications, such as artificial heart valves or elastocaloric cooling, in which more than 10 million transformation cycles will be required. We report on the discovery of an ultralow-fatigue shape memory alloy film system based on TiNiCu that allows at least 10 million transformation cycles. We found that these films contain Ti2Cu precipitates embedded in the base alloy that serve as sentinels to ensure complete and reproducible transformation in the course of each memory cycle.

Non-Joulian magnetostriction.

All magnets elongate and contract anisotropically when placed in a magnetic field, an effect referred to as Joule magnetostriction. The hallmark of Joulian magnetostriction is volume conservation, which is a broader definition applicable to self-accommodation of ferromagnetic, ferroelectric or ferroelastic domains in all functional materials. Here we report the discovery of 'giant' non-volume-conserving or non-Joulian magnetostriction (NJM). Whereas Joulian strain is caused by magnetization rotation, NJM is caused by facile (low-field) reorientation of magnetoelastically and magnetostatically autarkic (self-sufficient) rigid micro-'cells', which define the adaptive structure, the origin of which is proposed to be elastic gradients ultimately caused by charge/spin density waves. The equilibrium adaptive cellular structure is responsible for long-sought non-dissipative (hysteresis-free), linearly reversible and isotropic magnetization curves along all directions within a single crystal. Recently discovered Fe-based high magnetostriction alloys with special thermal history are identified as the first members of this newly discovered magnetic class. The NJM paradigm provides consistent interpretations of seemingly confounding properties of Fe-based alloys, offers recipes to develop new highly magnetostrictive materials, and permits simultaneously large actuation in longitudinal and transverse directions without the need for stacked composites.

Multiferroicity of carbon-based charge-transfer magnets.

A new type of carbon charge-transfer magnet, consisting of a fullerene acceptor and single-walled carbon nanotube donor, is demonstrated, which exhibits room temperature ferromagnetism and magnetoelectric (ME) coupling. In addition, external stimuli (electric/magnetic/elastic field) and the concentration of a nanocarbon complex enable the tunabilities of the magnetization and ME coupling due to the control of the charge transfer.

Phase transformation-induced tetragonal FeCo nanostructures.

Tetragonal FeCo nanostructures are becoming particularly attractive because of their high magnetocrystalline anisotropy and magnetization achievable without rare-earth elements, . Yet, controlling their metastable structure, size and stoichiometry is a challenging task. In this study, we demonstrate AuCu templated FeCo shell growth followed by thermally induced phase transformation of AuCu core from face-centered cubic to L10 structure, which triggers the FeCo shell to transform from the body-centered cubic structure to a body-centered tetragonal phase. High coercivity, 846 Oe, and saturation magnetization, 221 emu/g, are achieved in this tetragonal FeCo structure. Beyond a critical FeCo shell thickness, confirmed experimentally and by lattice mismatch calculations, the FeCo shell relaxes. The shell thickness and stoichiometry dictate the magnetic characteristics of the tetragonal FeCo shell. This study provides a general route to utilize phase transformation to fabricate high performance metastable nanomagnets, which could open up their green energy applications.

Charge-transfer magnetoelectrics of polymeric multiferroics.

The renaissance of multiferroics has yielded a deeper understanding of magneto-electric coupling of inorganic single-phase multiferroics and composites. Here, we report charge-transfer polymeric multiferroics, which exhibit external field-controlled magnetic, ferroelectric, and microwave response, as well as magneto-dielectric coupling. The charge-transfer-controlled ferroic properties result from the magnetic field-tunable triplet exciton which has been validated by the dynamic polaron-bipolaron transition model. In addition, the temperature-dependent dielectric discontinuity and electric-field-dependent polarization confirms room temperature ferroelectricity of crystalline charge-transfer polymeric multiferroics due to the triplet exciton, which allows the tunability of polarization by the photoexcitation.

Morphotropic phase boundaries in ferromagnets: Tb(1-x)Dy(x)Fe2 alloys.

The structure and properties of the ferromagnet Tb(1-x)Dy(x)Fe(2) are explored through the morphotropic phase boundary (MPB) separating ferroic phases of differing symmetry. Our synchrotron data support a first order structural transition, with a broadening MPB width at higher temperatures. The optimal point for magnetomechanical applications is not centered on the MPB but lies on the rhombohedral side, where the high striction of the rhombohedral majority phase combines with the softened anisotropy of the MPB. We compare our findings with single ion crystal field theory and with ferroelectric MPBs, where the controlling energies are different.

All conjugated copolymer excitonic multiferroics.

A substantial magnetoelectric coupling effect of an excitonic all-conjugated block copolymer multiferroics consisting of electronically distinct polythiophene derivatives is reported. The observations open new avenues for the multifunctional all-conjugated block copolymer synthesis and electric field tunable multiferroic devices.

Giant magnetostriction in annealed Co(1-x)Fe(x) thin-films.

Chemical and structural heterogeneity and the resulting interaction of coexisting phases can lead to extraordinary behaviours in oxides, as observed in piezoelectric materials at morphotropic phase boundaries and relaxor ferroelectrics. However, such phenomena are rare in metallic alloys. Here we show that, by tuning the presence of structural heterogeneity in textured Co(1-x)Fe(x) thin films, effective magnetostriction λ(eff) as large as 260 p.p.m. can be achieved at low-saturation field of ~10 mT. Assuming λ(100) is the dominant component, this number translates to an upper limit of magnetostriction of λ(100)≈5λ(eff) >1,000 p.p.m. Microstructural analyses of Co(1-x)Fe(x) films indicate that maximal magnetostriction occurs at compositions near the (fcc+bcc)/bcc phase boundary and originates from precipitation of an equilibrium Co-rich fcc phase embedded in a Fe-rich bcc matrix. The results indicate that the recently proposed heterogeneous magnetostriction mechanism can be used to guide exploration of compounds with unusual magnetoelastic properties.

Giant nonhysteretic responses of two-phase nanostructured alloys.

A new class of functional materials with giant nonhysteretic strain responses to applied fields is considered. They are decomposed two-phase systems consisting of single-domain nanoprecipitates of a low-symmetry phase. Their strain response is caused by the field-induced change of structural orientation of the domain states of these precipitates. The superresponse follows from the novel concept of structural anisotropy that is analogous to the magnetic anisotropy. Its vanishing produces a new glasslike structural state. The developed phase field theory and modeling allow us to formulate criteria for searching superresponsive two-phase nanostructured alloys.

Demonstration of magnetoelectric scanning probe microscopy.

A near-field room temperature scanning magnetic probe microscope has been developed using a laminated magnetoelectric sensor. The simple trilayer longitudinal-transverse mode sensor, fabricated using Metglas as the magnetostrictive layer and polyvinylidene fluoride as the piezoelectric layer, shows an ac field sensitivity of 467+/-3 microV/Oe in the measured frequency range of 200 Hz-8 kHz. The microscope was used to image a 2 mm diameter ring carrying an ac current as low as 10(-5) A. ac fields as small as 3 x 10(-10) T have been detected.

Combinatorial search of thermoelastic shape-memory alloys with extremely small hysteresis width.

Reversibility of structural phase transformations has profound technological implications in a wide range of applications from fatigue life in shape-memory alloys (SMAs) to magnetism in multiferroic oxides. The geometric nonlinear theory of martensite universally applicable to all structural transitions has been developed. It predicts the reversibility of the transitions as manifested in the hysteresis behaviour based solely on crystal symmetry and geometric compatibilities between phases. In this article, we report on the verification of the theory using the high-throughput approach. The thin-film composition-spread technique was devised to rapidly map the lattice parameters and the thermal hysteresis of ternary alloy systems. A clear relationship between the hysteresis and the middle eigenvalue of the transformation stretch tensor as predicted by the theory was observed for the first time. We have also identified a new composition region of titanium-rich SMAs with potential for improved control of SMA properties.