Journal of Contemporary Physics (Armenian Academy of Sciences)最新文献

The dimensional magnetic effects of single-domain Ni nanoparticles encapsulated by a carbon shell (Ni@C nanocomposites) have been studied. The studied samples were obtained by solid-phase pyrolysis of solid solutions of nickel phthalocyanine (NiPc) and metal-free phthalocyanine (H₂Pc): (NiPc)_x(H₂Pc)_1 – x, where 0 ⩽ x ⩽ 1. The Ni concentration in the carbon matrix varied in the range of 0–12 wt %, the sizes of the average diameter of nanoparticles in different samples were from 4 to 40 nm. The paramagnetism of the surface and near-surface atoms of Ni nanoparticles, which is due to the charge transfer from the carbon matrix with the formation of Ni ions has been studied in detail. A method for determining the blocking temperature of superparamagnets in temperature measurements of the paramagnetic susceptibility in the case of high magnetic fields is considered. The magnetic characteristics, which reflect size effects in SQUID magnetometry and FMR, exhibit “anomalous” deviations in the range of ultrasmall particles. These are high values of the blocking temperature, high values of the coercive force, and the ferromagnetic resonance linewidth, as well as a significant shift of the effective g-factor in the FMR spectra. Generalizing equations are presented that include the contributions of the surface magnetic anisotropy along with the bulk magnetocrystalline anisotropy, which is consistent with the experimental results over the entire investigated range from 4  to 40 nm. It is shown that the parameters of the total magnetic resonance spectra (FMR + ESR) are caused by the ferromagnetism of the core of nickel nanoparticles and the paramagnetism of surface and near-surface Ni ions, as well as π-electrons of the organic matrix. A general diagram of the dependence of the coercive force on particle sizes for temperatures lower than the blocking temperature (T < T_b) is also presented.

The intermetallic compound Ni₃Fe was obtained using the solution combustion synthesis (SCS) method, based on heating an aqueous solution containing nitrates of the corresponding metals and hexamethylenetetramine (HMTA) in a nitrogen atmosphere. The limits for the implementation of a self-propagating reaction in the metal nitrates-HMTA system were determined depending on the amount of HMTA in the reacting mixture (n). It has been established that by changing the value of n it is possible to control phase composition and microstructure of the obtained SCS products. The X-ray examinations have shown that at n = 6, a single-phase target product Ni₃Fe is obtained, and electron microscopic studies indicated that the product is characterized by a pronounced porous structure because of abundant gas evolution during the SCS process. Based on thermogravimetric analyses of the individual compounds and the mixture of reagents (iron and nickel nitrates with HMTA), a possible mechanism for forming the target Ni₃Fe intermetallic compound is discussed. The magnetic characteristics of the synthesized substance have been studied.

Monochromatic electromagnetic fields are considered inside an arbitrarily chosen volume both in homogeneous and inhomogeneous media. Formulas for the fields in the selected volume are obtained using the vector Green’s function of the medium. Two forms of the vector Green’s function are found for homogeneous media. The Kirchhoff and Kirchhoff–Kottler vector formulas for the problem of diffraction by an aperture in an opaque screen are obtained. Because of the use of the vector Green’s function, the Kirchhoff–Kottler formula is obtained directly, without using additional assumptions.

The processes of heat propagation in a three-layered detection pixel of a thermoelectric single-photon detector, consisting of an absorber (W), a thermoelectric sensor (La_0.99Ce_0.01B₆), and a heat sink (W) were studied using computer simulation. The absorption of UV photons with an energy of 7.1–124 eV has been studied. The simulation was based on the heat propagation equation from a limited volume. The temporal dependencies of the signal generated on the sensor were examined, and its power was determined. The equivalent power of Johnson and thermal noise was calculated. The signal-to-noise ratio was determined using different values of heat capacity, thermal conductivity, and the Seebeck coefficient of the sensor material. It was shown that the signal-to-noise ratio can be significantly greater than one. The obtained results reveal the prospects of using thermoelectric single-photon detectors as components of photonic integrated circuit.

Experimentally, under laboratory conditions in the nitrogen acoustic plasma in the region of the forbidden lines at 654.81 and 658.36 nm, a spectral emission line of high intensity was obtained (up to 17 stronger than the neighboring lines of the first positive system of nitrogen). The results were obtained both in pure low-pressure nitrogen acoustic plasma (several hundreds of Pa) and various mixtures containing nitrogen, including the CO₂ : N₂ : He = 1 : 1 : 8 mixture. The results obtained are explained by the acoustic plasma state of the discharge and an analog of Rahman scattering, which remove some of the quantum mechanical prohibitions. The possible influence of the Coriolis force is also considered.