Orhan Yalçın

Size effects and origin of easy-axis in nickel nanowire arrays

High quality compact Ni nanowire (NW) arrays with aspect ratios (wire length/diameter) varying between 70–171 for a wire length of ∼6 μm, and between 3–400 for a constant wire diameter of 60 nm were successfully grown by direct current electrodeposition into free standing porous alumina templates having a lattice constant, i.e., interpore distance, of 105 nm. The NWs have been investigated using a combination of scanning- and transmission-electron microscopies, selected-area electron diffraction, x-ray diffraction analysis, ferromagnetic resonance (FMR), and vibration sample magnetometer techniques at room temperature. Microscopic and diffraction results show that the wires are uniform and mostly single-crystalline, being 220-oriented along the growth direction. Magnetic properties of samples are heavily dependent on the wire length as well as the diameter or packing factor, P (the volume fraction of wires/template). The FMR spectra and the field orientation dependence of the resonance field values were f...

Origin of the martensitic and austenitic phase transition in core-surface smart nanoparticles with size effects and hysteretic splitting

Blume-Emery-Griffiths model is used to study the effects of the bilinear and biquadratic exchange interactions as well as crystal field interaction on the magnetic properties of core-surface smart nanoparticles. From the minimization of the free energy in pair approximation in Kikuchi version, a set of equations for the bond variables and magnetization are derived. Based on the numerical solutions of these equations, magnetization and hysteresis curves are obtained. Besides the first- and second-order phase transitions, martensitic and austenitic phase regions are observed in the phase diagrams of homogeneous and composite nanoparticles and the origin of martensitic transitions (MT)-austenitic transitions (AT) is investigated. It is found that MT-AT occurred for a nonzero biquadratic exchange parameter. On the other hand, nonzero single-ion anisotropy caused the hysteretic splitting in core-surface type nanoparticles. Martensitic and austenitic results of nanoparticles agree with the experimental results ...

Perpendicular silver nanowire arrays on an ITO/glass substrate for plasmonic applications

Ordered metal nanoparticle arrays are currently considered for novel optoelectronic and magnetic applications due to their unique properties. Template-grown ordered nanorod or nanowire arrays in particular are very interesting since they can be produced inexpensively over large scales and have tailorable properties. In this study, synthesis and properties of silver (Ag) nanowire arrays grown on transparent conducting indium tin oxide/glass substrate using thin-film anodic alumina nano-templates are investigated. The optical (plasmonic) properties of free-standing Ag nanowire arrays characterized by absorption spectroscopy were explained by modelling via the finite element method. The plasmonic activity of such nanowire arrays was then demonstrated by the well-known surface-enhanced Raman scattering effect through Raman mapping versus optical microscopy. Simulations confirmed the dependence of plasmonic properties to sample geometry and suggested their tunability through parameters such as the wire diameter and packing density. Such plasmonic nanowire arrays can find use in novel applications, e.g. the third generation solar cells, nano-sensors, etc since they can be addressed both optically and electrically through the conducting, transparent substrate.

Spin-lattice relaxation within a dimerized Ising chain in a magnetic field

A qualitative study of the spin-lattice relaxation within a dimerized Ising chain in a magnetic field is presented. We have first determined the time dependence of the deviation of the lattice distortion parameter δΔ from the equilibrium state within framework of a technique combining the statistical equilibrium theory based on the transfer matrix method and the linear theory of irreversible thermodynamics. We have shown that the time dependence of the lattice distortion parameter is characterized by a single time constant ( τ) which diverges around the critical point in both dimerized ( Δ≠0) and uniform ( Δ=0) phase regions. When the temperature and magnetic field are fixed to certain values, the time τ depends only on exchange coupling between the spins. It is a characteristic time associated with the long wavelength fluctuations of distortion. We have also taken into account the effects of spatial fluctuations on the relaxation time using the full Landau-Ginzburg free energy functional. We have found a...

Magnetic and transport properties of chemical solution deposited (100)-textured La(0.7)Sr(0.3)MnO(3) and La(0.7)Ca(0.3)MnO(3) nanocrystalline thin films

A study of the magnetic and electrical properties of (100)-oriented La(0.7)(Sr,Ca)(0.3)MnO(3) thin films prepared by an optimized chemical solution deposition process on a (100) SrTiO(3) single-crystal substrate is reported in this paper. The films were studied by high-resolution scanning electron microscopy, x-ray diffraction, vibrating sample magnetometer, ferromagnetic resonance and four-point-probe electrical measurements. A characteristic nanocrystalline texture with similar to 15 nm crystallites is observed in both films. Remarkably, the resistivities of these films are three orders of magnitude smaller compared with unoptimized films grown even on the same substrate. The magnetotransport properties were determined as a function of temperature and applied field magnitude, and compared with bulk crystals and vacuum-deposited single crystalline (epitaxial) thin films as well as manganite nanostructures reported in the literature. Both films display large values of colossal magnetoresistance at around room temperature. Significantly, the magnetoresistance in (100) La(0.7)Sr(0.3)MnO(3) thin film is observed to be highly linear even at low fields near the metal-insulator transition temperature. However, the dependence of these promising properties on the nanocrystal size remains to be explored.

Origins of the Exchange-Bias Phenomenology, Coercivity Enhancement, and Asymmetric Hysteretic Shearing in Core-Surface Smart Nanoparticles

We have used a spin-1 Ising model Hamiltonian with dipolar (bilinear, ), quadrupolar (biquadratic, ), and dipolar-quadrupolar (odd, ) interactions in pair approximation to investigate the exchange-bias (EB), coercive field, and asymmetric hysteretic shearing properties peculiar to core/surface ( ) composite nanoparticles (NPs). Shifted hysteresis loops with an asymmetry and coercivity enhancement are observed only in the presence of the odd interaction term in the Hamiltonian expression and their magnitudes show strong dependence on the value of . The observed coercivity and EB in NPs originated from nonzero odd coupling energies and their dependence on temperature ( ) and particle size ( ) are also discussed in relation to experimental findings.

Thermal Hysteresis Due to the Structural Phase Transitions in Magnetization for Core-Surface Nanoparticles

One important issue raised in magnetism studies is the thermal response of various mag‐ netic properties. This topic is known as the magnetic thermal hysteresis (MTH) which is principally associated with magnetic phase transitions. The MTH is of particular interest for both quantum and applied physics researches on magnetization of nanomaterials. Hysteresis of the temperature-induced structural phase transitions in some materials and nanostructures with first-order phase transitions reduces useful magnetocaloric effect to transform cycling between martensite (M) and austenite (A) phases under application. In additional, the size, surface and boundary effects on thermal hysteresis loops have been under consideration for the development of research on nanostructured materials. Exper‐ imental data indicate that nanostructured materials offer many interesting prospects for the magnetization data and for understanding of temperature-induced M-A phase transi‐ tions. In this chapter, we have presented a review of the the latest theoretical develop‐ ments in the field of MTH related to the structural phase transitions for the core-surface nanoparticles based on the fundamental formulation of pair approximation in Kikuchi version.