Relaxation process and ferromagnetic resonance investigation of ferrofluids with Mn–Zn and Mn–Fe mixed ferrite particles

The magnetic relaxation processes in two ferrofluids with Mn_0.4Zn_0.6Fe₂O₄ (sample F1) and Mn_0.6Fe_0.4Fe₂O₄ (sample F2) mixed ferrite particles, dispersed in n-decan and kerosene, respectively, are investigated through the determination of components χ′ and χ′′ of the complex magnetic susceptibility in the range of (2–30) MHz. The values of the saturation magnetization of the two ferrofluids are M_∞=5.28 kA/m for sample F1 and M_∞=10.99 kA/m for sample F2. A maximum of the imaginary component χ′′ was observed for both samples at frequencies of tens MHz. This maximum was assigned to relaxation processes of Néel type.The effective anisotropy constant K of the particles from the studied samples was evaluated, using both static and dynamic measurements and the values were found to be K₁=6.12×10³ J m⁻³ for the ferrofluid F1, and K₂=5.60×10³ J m⁻³ for the ferrofluid F2. From ferromagnetic resonance measurements, and based on the theoretical values computed for the Lande factor (g), the effective anisotropy constants for the mixed ferrite particles in the studied ferrofluids and the anisotropy field values were determined using a new method. The values obtained in this way for the anisotropy constants K₁ and K₂ are compared to the ones determined from magnetic relaxation measurements.     

Texture evolution in Mg–Mn and Mg–Mn–Ce alloys and the effect of as-cast grain size

Texture evolution in rolled (400°C) and annealed (450°C) Mg–1% Mn-based alloys containing different levels of Ce was examined. It was found that Ce refines the as-cast and rolled/annealed grain structure. The overall intensity of the basal texture decreases with Ce additions in both the rolled and subsequently annealed condition. A strong relationship was found between maximum intensity (M) of rolling and annealing textures and the as-cast effective grain size, d?′, i.e. M?∝?e^{0.4 d ?′} (R ^2?=?0.8), which was attributed to the role of plastic compatibility stresses which lead to grain boundary deformation altering the balance of deformation modes. Contributions from Ce solute effects and lattice parameter changes are also discussed.     

Laser ionization in β decay studies of Zn and Mn nuclei

Resonance ionization laser ion source (RILIS) technique has been used in the β-decay studies of ⁵⁹Mn and ⁵⁸Zn. The importance of the RILIS for production of these elements is discussed. The properties of the low-lying levels of the studied nuclei are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Magnetic properties and magnetocaloric effects of Mn5Ge3-xGax

We report on the magnetic properties and magnetocaloric effects of Mn5Ge3-xGax compounds with x=0.1,0.2,0.3,0.4,0.6 and 0.9. All samples crystallize in the hexagonal Mn5Si3-type structure with space group P63/mcm and order ferromagnetically.The Curie temperature of these compounds decreases with increasing x, from 306K (x=0.1) to 274K (x=0.9).The average Mn magnetic moments increases with increasing Ga content,reaching a maximum value at x=0.6.The magnetic entropy changes in these compounds are determined from the temperature and field dependence of the magnetization using the thermodynamic Maxwell relation.The Ga substitution has two kinds of influence on the magnetocaloric effect (MCE) of Mn5Ge3.One is that the magnitude of the magnetic entropy change decreases,the other is that the MCE peak becomes broadened.     

Study of technique and performance of laser beam welding for W18Cr4V and 65Mn

In this experiment, we used 7-kW continuous wave (CW) CO2 low order mode laser and YAG impulse laser to deeply weld high-speed steel (W18Cr4V) and spring steel (65Mn). To welded joint, we analyzed its microstructure, micro hardness and fracture appearance for different welding technology by metallography microscope, scanning electron microscopy and bending strength test, discussed its action of fracture and the relation between its structural transformation and strength. At last, we offered an optimum welding technology having no welding defect by the comparison and analysis of processing property.     

Synthesis of high quality and monodisperse CdS:Mn/ZnS and CdS:Mn/CdS core–shell nanoparticles

CdS:Mn²⁺/ZnS and CdS:Mn²⁺/CdS core–shell nanoparticles were synthesized in aqueous medium via chemical precipitation method in an ambient atmosphere. Polyvinylpyrrolidone (PVP) was used as a capping agent. The effect of the shell (ZnS and CdS) thickness on CdS:Mn²⁺ nanoparticles was investigated. Inorganically passivated core/shell nanocrystals having a core (CdS:Mn²⁺) diameter of 4 nm and a ZnS-shell thickness of ∼0.5 nm exhibited improved PL intensity. Optimum concentration of doping ions (Mn²⁺) was selected through optical study. For all the core–shell samples two emission peaks were observed, the first one is band edge emission in the lower wavelength side due to energy transfer to the Mn²⁺ ions in the crystal lattice; the second emission is characteristic peak of Mn²⁺ ions (⁴T₁ → ⁶A₁). The XRD, TEM and PL results showed that the synthesized core–shell particles were of high quality and monodisperse.     

Phase transformations and structure of Ni–Mn–In alloys with varying ratio Ni/Mn

The fine structure of Ni–Mn–In alloys has been studied when manganese atoms are substituted for nickel atoms in an annealing state. The concentration dependence of the critical temperatures and the structures of the alloys have been discussed. It has been found that, as manganese atoms replace nickel atoms, the structure after annealing is changed from a two-phase (L2₁ + martensite) to single-phase L2₁ structure. The martensitic transformation in Ni₄₇Mn₄₂In₁₁ alloy is accompanied by the formation of modulated 14M martensite.     

Theoretical and Experimental Study of the Conformation and Vibrational Frequencies of N‐Acetyl‐L‐alanine and N‐Acetyl‐L‐alaninate

The vibrational frequencies of N‐acetyl‐L‐alanine (NAAL), its potassium salt (NAALK) and its free anionic form (NAAL^?) are calculated using density functional theory (B3LYP) combined with the 6‐311 + + G(d,p) basis set. The experimental Raman spectrum of solid NAALK and the scaling factors for calculated values are discussed as well. The three species are characterized by intramolecular NH…O hydrogen bonds leading to the formation of a five‐membered ring. As indicated by the intramolecular (N)H…O distances and by the ν(NH) frequencies, the strength of the intramolecular hydrogen bond is ordered as follows: NAAL^? < NAALK < NAAL^?. Owing to their difference in the coupling with other vibrational modes, the in‐plane and out‐of‐plane vibrations do not reflect the strength of the hydrogen bond.     

Nonergodicity of blinking nanocrystals and other Lévy-walk processes

We investigate the nonergodic properties of blinking nanocrystals modeled by a Lévy-walk stochastic process. Using a nonergodic mean field approach we calculate the distribution functions of the time averaged intensity correlation function. We show that these distributions are not delta peaked on the ensemble average correlation function values; instead they are W or U shaped. Beyond blinking nanocrystals our results describe ergodicity breaking in systems modeled by Lévy walks , for example, certain types of chaotic maps and spin dynamics to name a few.     

Langevin Picture of Lévy Walks and Their Extensions

In this paper we derive Langevin picture of Lévy walks. Applying recent advances in the theory of coupled continuous time random walks we find a limiting process of the properly scaled Lévy walk. Next, we introduce extensions of Levy walks, in which jump sizes are some functions of waiting times. We prove that under proper scaling conditions, such generalized Lévy walks converge in distribution to the appropriate limiting processes. We also derive the corresponding fractional diffusion equations and investigate behavior of the mean square displacements of the limiting processes, showing that different coupling functions lead to various types of anomalous diffusion.     

Fuzzy modeling and impulsive control of hyperchaotic Lü system

This paper presents a novel approach to hyperchaos control of hyperchaotic systems based on impulsive control and the Takagi--Sugeno (T--S) fuzzy model. In this study, the hyperchaotic Lü system is exactly represented by the T--S fuzzy model and an impulsive control framework is proposed for stabilizing the hyperchaotic Lü system, which is also suitable for classes of T--S fuzzy hyperchaotic systems, such as the hyperchaotic R?ssler, Chen, Chua systems and so on. Sufficient conditions for achieving stability in impulsive T--S fuzzy hyperchaotic systems are derived by using Lyapunov stability theory in the form of the linear matrix inequality, and are less conservative in comparison with existing results. Numerical simulations are given to demonstrate the effectiveness of the proposed method.     

Study of electrical and thermoelecrical properties of sulfides Tm x Mn1–x S

Variable-valence Tm _x Mn_1–x S (0 ? x ? 0.15) compounds have been synthesized and their structural, electrical, and thermoelectrical properties have been studied in the temperature range of 80–1100 K. The regions of existence of solid solutions of sulfides Tm _x Mn_1–x S with the NaCl-type fcc lattice have been determined. It has been found that, as thulium ions are substituted for manganese cations, the electrical resistivity increases, and the lattice parameter increases more sharply than that corresponding to the Vegard’s law. The study of the temperature dependences of the thermopower coefficient has revealed that the current carrier sign is retained to 500 K for all the substitution concentrations, and the charge carrier type changes from the hole type to the electron type with variations in the temperature. The experimental data have been explained in terms of the exciton model.     

Magnetic and structural properties of Pd–Mn–Sn intermetallics compounds

Magnetic and structural properties of Heusler Pd_0.5Mn_{0.5- x} Sn _x with x = 0.05, 0.10, 0.17, 0.20 and 0.25, have been studied by magnetisation and X-ray diffraction measurements at room and low temperatures. The crystal structure at room temperature is L2₁ cubic phase for x = 0.17, 0.20, 0.25 and B2 cubic phase for x = 0.10. Martensite structure 10M, was observed at room temperature for x = 0.05. X-ray measurements at low temperatures revealed a structural transformation from B2 to 14M for the x = 0.10 case. The lattice parameter of the L2₁ phase decreases linearly with the concentration, x. A ferromagnetic behaviour has been detected for L2₁ compounds, but the ferromagnetic exchange characteristic of each composition is of different strength. This gives rise to different Curie temperatures.     

Crystal and Magnetic Structures of Granular Powder Spinel Mn–Zn and Ni–Zn Ferrites

Some specific features of the crystal and magnetic structures of granular powder spinel-like ferrites Mn_0.160Mg_0.404Zn_0.448Fe₂O₄, Mn_0.676Zn_0.227Fe_0.09Fe₂O₄, Mn_0.5792Zn_0.2597Fe_0.1612Fe₂O₄, and Ni_0.32Zn_0.68Fe₂O₄ have been studied by neutron diffraction. It has been established that the crystal structure of all the studied compounds has a cubic symmetry with space group Fd\(\bar 3\)m. Ferrimagnetic ordering is observed in all the studied structures. Based on the experimental data, the unit cell parameters and interatomic bond lengths of the studied compounds are determined alongside with the distribution of cations between octahedral and tetrahedral crystallographic positions in their cubic crystal structure. Corresponding average magnetic moments are calculated for different positions in their cubic structure. Some structural mechanisms of the formation of magnetic properties depending on the level of doping and the size of powder grains are discussed.     

Influence of manganese substitution and annealing temperature on the formation,microstructure and magnetic properties of Mn–Zn ferrites

The effects of annealing temperature and manganese substitution on the formation, microstructure and magnetic properties of Mn_xZn_1−xFe₂O₄ (with x varying from 0.3 to 0.9) through a solid-state method have been investigated. The correlation of the microstructure and the grain size with the magnetic properties of Mn–Zn ferrite powders was also reported. X-ray diffraction (XRD), a scanning electron microscope (SEM) and a vibrating sample magnetometer (VSM) were utilized in order to study the effect of variation of manganese substitution and its impact on crystal structure, crystalline size, microstructure and magnetic properties of the ferrite powders formed. The XRD analysis showed that pure single phases of Mn–Zn ferrites were obtained by increasing the annealing temperature to 1200–1300 °C. Increasing the annealing temperature to ?1300 °C led to abnormal grain growth with inter-granular pores and this led to a decrease in the saturation magnetization. Moreover, an increase in the Mn²⁺ ion substitution up to x=0.8 increased the lattice parameter of the formed powders due to the high ionic radii of the Mn²⁺ ion. Mn–Zn ferrites phases were formed and the positions of peaks were shifted by substituting manganese. The average crystalline size was increased by increasing the annealing temperature and decreased by increasing the substitution by manganese up to 0.8. The average crystalline size was in the range 95–137.3 nm. The saturation magnetization of the Mn–Zn-substituted ferrite powders increased continuously with an increase in the Mn concentration up to 0.8 at annealing temperatures of 1200–1300 °C. Further increase of Mn substitution up to 0.9 led to a decrease of saturation magnetization. The saturation magnetization increased from 17.3 emu/g for the Mn_0.3Zn_0.7Fe₂O₄ phase particles produced to 59.08 emu/g for Mn_0.8Mn_0.2Fe₂O₄ particles.     

Photoluminescence decay of silicon nanocrystals and Lévy stable distributions

We incorporate the tools of Lévy processes and distributions to describe the photoluminescence of silicon nanocrystals. The method relies on two novel features: first we use exact forms of one-sided Lévy distributions to get an excellent reproduction of experimental data. Then we show that the dynamics leading to photoluminescence decay can be modelled in terms of fractional Fokker–Planck equation.     

Hydrogen Atom and Equivalent Form of the Lévy-Leblond Equation

We discuss the equivalent form of the Lévy-Leblond equation such that the nilpotent matrices are two-dimensional.We show that this equation can be obtained in the non-relativistic limit of the(2+1)-dimensional Dirac equation.Furthermore, we analyze the case with four-dimensional matrices, propose a Hamiltonian for the equation in(3+1) dimensions, and solve it for a Coulomb potential. The quantized energy levels for the hydrogen atom are obtained, and the result is consistent with the non-relativistic quantum mechanics.