Rıza Erdem

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 ...

Magnetic Properties and Size Effects of Spin-1/2 and Spin-1 Models of Core-Surface Nanoparticles in Different Type Lattices

Dimension in the range of 1 to 100 nm, is called the nano regime. In recent years, nanoparticles/quantum dots are in a class of magnetic nanostructures (Aktas et al., 2003, 2006; Kartopu & Yalcin, 2010). Nanoparticles (NPs) have been steadily interesting in Physics, Chemistry, Biology, Biomedicine, Spintronics, etc. As the dimensions of magnetic NPs decrease down to the nanometer scale, these core-surface NPs start to exhibit new and interesting physical properties mainly due to quantum size effects. Even the intrinsic physical characteristics of NPs are observed to change drastically compared to their macroscopic counterparts. The potential applications of NPs are very attractive for magnetosensor, bio-sensor, magneto-electronics, data storage media, computer hard disks, microwave electronic devices, nano-transistors, etc. Especially, the studies of core-surface NPs are extremely important for technology because of transmission of data at high density to optical computer, nanorobot to assemble, compose rigid disk. The nanoparticles have relevance to thin film devices in the new breed of magnetoelectronics, spin-valve, spintransistors, spin-dependence tunneling devices and etc. (Babin, et al., 2003). The hysteresis in fine magnetic particles applied to new technologies such as Magnetic Random Access Memory (MRAM).

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...

Size effects on the open probability of two-state ion channel system in cell membranes using microcanonical formalism based on gamma function

Ion channel systems are a class of proteins that reside in the membranes of all biological cells and forms conduction pores that regulate the transport of ions into and out of cells. They can be investigated theoretically in the microcanonical formalism since the number of accessible states can be easily evaluated by using the Stirling approximation to deal with factorials. In this work, we have used gamma function (Γ (n)) to solve the two-state or open-close channel model without any approximation. New values are calculated for the open probability (p0) and the relative error between our numerical results and the approximate one using Stirling formula is presented. This error (p0 app — p0)/p0 is significant for small channel systems.

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.