Effect of surface pinning on magnetic nanostuctures

Magnetic nanostructures are often considered as highly functional materials because they exhibit unusual magnetic properties under different external conditions. We study the effect of surface pinning on the core-shell magnetic nanostuctures of different shapes and sizes considering the spin-interaction to be Ising-like. We explore the hysteresis properties and find that the exchange bias, even under zero field cooled conditions, increases with increase of, the pinning density and the fraction of up-spins among the pinned ones. We explain these behavior analytically by introducing a simple model of the surface. The asymmetry in hysteresis is found to be more prominent in a inverse core-shell structure, where spin interaction in the core is antiferromagnetic and that in the shell is ferromagnetic. These studied of inverse core-shell structure are extended to different shapes, sizes, and different spin interactions, namely Ising, XY- and Heisenberg models in three dimension. We also briefly discuss the pinning effects on magnetic heterostructures.Comment: 12 pages, 12 figure

Large structure-dependent room temperature exchange bias in self-assembled BiFeO3 nanoparticles

We studied the magnetic properties of self-assembled aggregates of BiFeO3 nanoparticles (~ 20-40 nm). The aggregates formed two different structures - one with limited and another with massive cross-linking - via `drying-mediated self-assembly' process following dispersion of the nanoparticles within different organic solvents. They exhibit large coercivity H_C (>1000 Oe) and exchange bias field H_E (~ 350-900 Oe) in comparison to what is observed in isolated nanoparticles (H_C ~ 250 Oe; H_E ~ 0). The H_E turns out to be switching from negative to positive depending on the structure of the aggregates with |H_E| being larger. The magnetic force microscopy reveals the magnetic domains (extending across 7-10 nanoparticles) as well as the domain switching characteristics and corroborate the results of magnetic measurements. Numerical simulation of the `drying-mediated-self-assembly' process shows that the nanoparticle-solvent interaction plays an important role in forming the `nanoparticle aggregate structures' observed experimentally. Numerical simulation of the magnetic hysteresis loops, on the other hand, points out the importance of spin pinning at the surface of nanoparticles as a result of surface functionalization of the particles in different suspension media. Depending on the concentration of pinned spins at the surface pointing preferably along the easy-axis direction - from greater than 50\% to less than 50% - H_E switches from negative to positive. Quite aside from bulk sample and isolated nanoparticle, nanoparticle aggregates - resulting from surface functionalization - therefore, offer remarkable tunability of properties depending on structures.Comment: 14 pages, 33 pdf figures. Contact authors for the supplementary data and movie

Secondary Structure-Dependent Physicochemical Interaction of Oligonucleotides with Gold Nanorod and Photothermal Effect for Future Applications: A New Insight

We investigate the physicochemical interactions of gold nanorod (GNR) with single-stranded, double-stranded, and hairpin DNA structures to improve the biological compatibility as well as the therapeutic potential, including the photothermal effect of the conjugates. Studies have demonstrated that different DNA secondary structures, containing thiol group, have different patterns of physicochemical interaction. Conjugation efficiency of paired oligonucleotides are significantly higher than that of oligonucleotides with naked bases. Furthermore, hairpin-shaped DNA structures are most efficient in terms of conjugation and increased dispersion, with least interference on GNR near-infrared absorbance and photothermal effect. Our conjugation method can successfully exchange the overall coating of the GNR, attaching the maximum number of DNA molecules, thus far reported. Chemical mapping depicted uniform attachment of thiolated DNA molecules without any topological preference on the GNR surface. Hairpin DNA-coated GNR are suitable for intracellular uptake and remain dispersed in the cellular environment. Finally, we conjugated GNR with 5-fluoro-2'-deoxyuridine-containing DNA hairpin and the conjugate demonstrated significant cytotoxic activity against human cervical cancer cell line (KB). Thus, hairpin DNA structures could be utilized for optimal dispersion and photothermal effect of GNR, along with the delivery of cytotoxic nucleotides, developing the concept of multimodality approach

Shape-dependent magnetoelectric coupling in nanoscale BiFeO3

The nanorods and nanocuboids of multiferroic BiFeO3 exhibit remarkable shape dependence of crystallographic, ferroelectric, and magnetoelectric properties. The nanorods, for example, assume R3m space group (as against R3c observed both in bulk and nearly spherical nanosized particles) and are found to be oriented preferentially with long axis parallel to 101]. The magnetic and electric coercive fields as well as magnetoelectric coupling - measured from change in remanent polarization under a magnetic field - are found to be larger in nanocuboids. The shape dependence of multiferroic properties originates from that of microstrain and anisotropy. (C) 2018 Elsevier B.V. All rights reserved

Effect of surface pinning on magnetic nanostuctures

We show that pinning of surface spins affects the hysteresis properties of the core-shell magnetic nanostuctures of different shapes, sizes, and different spin interactions, namely, Ising, XY, and Heisenberg models. The asymmetry in hysteresis loops occuring due to pinning turns out to be more prominent in an inverse core-shell structure where spin interaction in the core is antiferromagnetic and that in the shell is ferromagnetic. Monte Carlo simulations of the inverse core-shell nanostructures show that the exchange bias, even under zero-field-cooled conditions, increases with increase of both the pinning density and the fraction of up spins among the pinned ones. The exchange bias also exhibits a switch-from negative to positive-depending on the fraction of up spins pinned. These results are remarkably well reproduced by a simple model of the outermost surface layer. The surface spin pinning appears to affect the magnetic properties of heterostructures as well, besides nanostructures

Shape dependent multiferroic behavior in Bi2Fe4O9 nanoparticles

Ferroelectric and magnetic properties are investigated for Bi2Fe4O9 nanoparticles with different shapes (cuboid and sphere-like) synthesized by hydrothermal and sol-gel method. The magnetic study reveals that coercivity, Neel temperature and remanent magnetization strongly depend on shape of the particle. The nanoparticle with sphere-like shape exhibits magnetization curve of antiferromagnetic (AFM) ordering with ferromagnetic (FM) component. As the particle shape changes from sphere-like to cuboid, the AFM component is dominating over the ferromagnetic component. A small exchange bias is also observed at low temperature in both the sphere-like and cuboid nanoparticle. The coercivity, remanent magnetization and Neel temperature of sphere-like nanoparticle is greater than cuboid nanoparticle. Ferroelectric measurement shows the remanent polarization of cuboid is greater than sphere-like nanoparticle but the coercivity is almost same. This Bi2Fe4O9 nanoparticle shows a small change in polarization under magnetic field. The polarization value decreases with magnetic field increases. The magnetoelectric coupling-measured by change of remanent polarization under magnetic field are found to be greater in Bi2Fe4O9 sphere-like nanoparticles. These shape dependent magnetic and ferroelectric properties are coming because of shape anisotropy

Crystal Structure of 27R-SiAlON Synthesized Under Carbothermal Nitridation

Laboratory synthesized (carbothermal-reduction-nitridation) 27R-SiAlON is studied in terms of crystallographic and microscopic features by detailed X-ray diffraction and electron microscopy experiments. 27R- SiAlON is analyzed to possess rhombohedral R3m structure and is best represented as Si1.1Al7.9O1.9N8.1. Lattice parameters, ion positions, bond lengths, angles as well as the crystallite size and strain have been determined for the constituent phase. The R3m phase in hexagonal setting exhibits enormous anisotropy with pseudo-tetragonality ratio c/a root 6 similar to 10. The electron microscopy too offers evidence of preferential growth of platelike crystals of SiAlON due to its large c-axis

Temperature-dependent dielectric properties of CsPb2Br5: a 2D inorganic halide perovskite

Two dimensional (2D) CsPb2Br5 have been successfully synthesized via the chemical precipitation method. Detailed structural, morphological, optical, and dielectric studies of these materials have been performed. These 2D CsPb2Br5 plates (of thickness around 200-300 nm) are ascribed to a tetragonal lattice system with I4/mcm space group. The dielectric attributes such as dielectric constant, electrical modulus, loss factor, and the DC, and AC conductivities, are observed to be varying appreciably with temperature over an extensive frequency window of 10 Hz-50 MHz. The Nyquist plots are investigated using the Maxwell-Wagner equivalent circuit model, which shows the impact of grains and grain boundaries on the overall impedance. Both the free charge conductivity and space charge increase with an increment in temperature, as revealed from the modified Cole-Cole plot. The relaxation time and relaxation mechanism of 2D CsPb2Br5 are estimated using the Kohlrausch-Williams-Watts equation. Variation in DC conductivity and relaxation time, as a function of temperature, closely resembles Arrhenius' behavior. Value of activation energy calculated from the DC conductivity corroborates with the same derived from relaxation time. The observation of high dielectric constant and nominal dielectric loss for CsPb2Br5 perovskite offers enormous potential in energy harvesting and storage devices

Hydrothermal synthesis of Bi2Fe0O9 nanochains and study of their multiferroic coupling

We report synthesis of self-assembled Bi2Fe4O9 nanochains by hydrothermal method using Oleic acid as a surfactant which plays an important role in the self-assembled nanochain formation. Temperature dependent dc magnetization study suggests downward shift of the Neel temperature TN -63 K for the particles of size -22 nm while it is reported to be -260 K in the bulk sample. The magnetic hysteresis (M-H) loops offer evidence of weak ferromagnetism across the entire temperature range 10-300 K. Exchange coupling across the interface between coexisting ferromagnetic and antiferromagnetic orders gives rise to a small amount of exchange bias at low temperature. The sample also exhibits substantial magnetoelectric multiferroic coupling at room temperature with-77% suppression of ferroelectric polar-ization under 10 kOe magnetic field. (c) 2021 Elsevier B.V. All rights reserved. Superscript/Subscript Available</commen

Large structure-dependent room temperature exchange bias in self-assembled BiFeO3 nanoparticles

We studied the magnetic properties of self-assembled aggregates of BiFeO3 nanoparticles (similar to 20 nm-40 nm). The aggregates formed two different structures-one with limited and another with massive crosslinking-via the ``drying-mediated self-assembly'' process following dispersion of the nanoparticles within different organic solvents. They exhibit large coercivity H-C (>1000 Oe) and exchange bias field H-E (similar to 350-900 Oe) in comparison to what is observed in isolated nanoparticles (H-C similar to 250 Oe; H-E similar to 0). H-E turns out to be switching from negative to positive depending on the structure of the aggregates, with |+H-E| being larger. Magnetic force microscopy reveals the magnetic domains (extending across 7-10 nanoparticles) as well as the domain switching characteristics and corroborates the results of magnetic measurements. Numerical simulation of the ``drying-mediated self-assembly'' process shows that the nanoparticle-solvent interaction plays an important role in forming the ``nanoparticle aggregate structures'' observed experimentally. Numerical simulation of the magnetic hysteresis loops, on the other hand, points out the importance of spin pinning at the surface of nanoparticles as a result of surface functionalization of the particles in different suspension media. Depending on the concentration of pinned spins at the surface pointing preferably along the easy-axis direction-from greater than 50% to less than 50%-H-E switches from negative to positive. Quite aside from the bulk sample and isolated nanoparticle, nanoparticle aggregates-resulting from surface functionalization-therefore offer remarkable tunability of properties depending on structures