Magnetocaloric properties of the spin-S (S ≥ 1) Ising model on a honeycomb lattice

Using effective field theory, we have investigated magnetocaloric features of the spin-S Ising model for some spin values $S=1,3/2,2,5/2,3$ and 7/2 on a honeycomb lattice. Effects of the external magnetic field on the isothermal entropy change, its half-width and also cooling capacity have been determined and discussed in detail. The numerical results show that the isothermal entropy change decreases whereas its half-width and cooling capacity tends to increase when the spin magnitude of the system is increased starting from $S=1$ to 7/2 for a fixed value of the external field. Finally, we have also performed some calculations to check the applicability of magnetocaloric scaling procedure for the present system. It is found that the rescaled numerical data collapse onto the same universal curve for all considered values of spin component S.     

Tuning the magnetocaloric properties of La0.7Ca0.3MnO3 manganites through Ni-doping

The effect of Ni²⁺ doping on the magnetic and magnetocaloric properties of La_0.7Ca_0.3MnO₃ manganites synthesized via the auto-combustion method is reported. The aim of studying Ni²⁺-substituted La_0.7Ca_0.3Mn_1 ? xNi_xO₃ ($x=0,0.02,0.07$, and 0.1) manganites was to explore the possibility of increasing the operating temperature range for the magnetocaloric effect through tuning of the magnetic transition temperature. X-ray diffraction analysis confirmed the phase purity of the synthesized samples. The substitution of Mn³⁺ ions by Ni²⁺ ions in the La_0.7Ca_0.3MnO₃ lattice was also corroborated through this technique. The dependence of the magnetization on the temperature reveals that all the compositions exhibit a well-defined ferromagnetic to paramagnetic transition near the Curie temperature. A systematic decrease in the values of the Curie temperature is clearly observed upon Ni²⁺ doping. Probably the replacement of Mn³⁺ by Ni²⁺ ions in the La_0.7Ca_0.3MnO₃ lattice weakens the Mn³⁺–O–Mn⁴⁺ double exchange interaction, which leads to a decrease in the transition temperature and the magnetic moment in the samples. By using Arrott plots, it was found that the phase transition from ferromagnetic to paramagnetic is second order. The maximum magnetic entropy changes observed for the $x=0,0.02,0.07$, and 0.1 composites was 0.85, 0.77, 0.63, and 0.59 J/kg?K, respectively, under a magnetic field of 1.5 T. In general, it was verified that the magnetic entropy change achieved for La_0.7Ca_0.3Mn_1 ? xNi_xO₃ manganites synthesized via the auto-combustion method is higher than those reported for other manganites with comparable Ni²⁺-doping levels synthesized via standard solid state reaction. The addition of Ni²⁺ increases the value of the relative cooling power as compared to that of the parent compound. The highest value of this parameter (～60 J/kg) is found for a Ni-doping level of 2% around 230 K in a field of 1.5 T.     

Compression features of high density electron plasma in a long harmonic trap using a rotating wall technique

We studied radial compression features of electron plasmas in a Penning trap (an assembly of multi-ring electrodes (MRE) housed in a uniform magnetic field) using the rotating wall technique. The magnetic field was 5 T, and the MRE provide a long harmonic potential along the magnetic field axis. The compression features of the plasma were studied by varying the applied rotating electric field frequency, rotating electric field amplitude, compression time, as well as, the magnetic field strength in the Penning trap. The highest density achieved was $～5\times {10}^{10}$ cm^?3 and the compressed plasma shows slow expansion after the compression, with an expansion rate of $7\times {10}^{?4}$ s^?1.     

The β-expansion of periodic dressed chain models

We revisit the method of calculating the $\beta$-expansion of the Helmholtz free energy of any one-dimensional (1D) Hamiltonian with invariance under space translations, presented in [O. Rojas, S.M. de Souza, M.T. Thomaz, J. Math. Phys. 43 (2002) 1390], extending this method to 1-D Hamiltonians that are invariant under translations along super-sites (sequences of $l$ sites). The method is applicable, for instance, to spin models and bosonic/fermionic versions of Hubbard models, either quantum or classical. As an example, we focus on the staggered spin-$S$ Ising model in the presence of a longitudinal magnetic field, comparing some of its thermodynamic functions to those of the standard Ising model. We show that for arbitrary values of spin ($S\in \{1,3/2,2,\dots \}$) but distinct values of the coupling constant and the magnetic field, the specific heat and the $z$-component of the staggered and usual magnetizations can be well approximated by their respective thermodynamic function of the spin-$1/2$ models in a suitable interval of temperature. These approximations are valid for the standard Ising model as well as for the staggered model, the thermodynamics of which are known exactly.     

Application of medium energy plasma focus device in study of radioisotopes

Pulsed neutrons generated in a plasma focus device are used for the thermal neutron activation analysis (TNAA) of selected three elements having widely different half-lives varying from a few seconds to a few days [Dysprosium (Dy), Manganese (Mn) and Gold (Au)]. Neutron pulse having strength of $(1.2\pm 0.3)\times {10}^9$ neutrons/pulse with a pulse width of $46\pm 5\text{ns}$ is produced by “MEPF-12” device operated at a filling gas (deuterium + 0.5% krypton) pressure of 3 mbar. The fast 2.45 MeV D–D neutrons are thermalized before irradiating the sample. The decay gammas from the radioisotopes ^165mDy ($T_{1/2}=1.26\text{min.}$), ⁵⁶Mn ($T_{1/2}=2.58\text{hrs.}$), and ¹⁹⁸Au ($T_{1/2}=2.69\text{days}$) produced via reactions, ¹⁶⁴Dy($n,\gamma$)^165mDy, ⁵⁵Mn($n,\gamma$) ⁵⁶Mn, and ¹⁹⁷Au($n,\gamma$) ¹⁹⁸Au respectively are counted off-line in a lead shielded well type $76\times 76{\text{mm}}^2$ NaI(Tl) detector coupled to a calibrated 2048 channel analyzer. The values of half-lives evaluated from the measured decay gammas, $1.43\pm 0.3\text{min.}$, $2.56\pm 0.5\text{hrs.}$ and $2.84\pm 0.6\text{days}$ respectively for the radioisotopes of Dy, Mn and Au, are seen to be close to the values reported in literature.     

Zeeman photoluminescence spectroscopy of isoelectronic beryllium pairs in silicon

Isoelectronic beryllium (Be) pair centers in silicon have been studied by photoluminescence spectroscopy under a magnetic field. The photoluminescence of the bound-exciton recombination at this center shows that the number of Zeeman split peaks is the smallest for the magnetic field applied along a $〈100〉$ direction. This result provides direct evidence that the Be pairs orient themselves in $〈111〉$ directions. The $g$ values of the hole and electron in this bound exciton determined by fitting of the Zeeman diagrams support the shallow acceptor character of this isoelectronic center.     

ESR in a gapped Anderson model

We applied the numerical renormalization group method to study the electron spin resonance (ESR) of a single-impurity Anderson model with a gap $\Delta$ in the conduction electron density of states, centered at the Fermi level. We analyzed the relaxation rate $1/T_1$ of a magnetic probe located at a position $\overrightarrow{R}$ around the Anderson impurity. It presents different behaviors for the symmetric and the asymmetric case. For the symmetric case and any $\Delta >0$, $1/T_1$ goes to a constant for $T?{\Gamma }_{\text{k}}$ (Kondo resonance). $1/T_1$ decreases monotonically to zero only for $\Delta =0$. For the asymmetric case, there is a $\Delta$ under which $1/T_1$ decreases monotonically to zero as $T\to 0$, and above which $1/T_1$ saturates, as occurs in the symmetric case for $\Delta >0$. This behavior indicates a quantum phase transition from the quenched to the unquenched magnetic moment in the ground state of the Anderson ion.     

Structure and magnetic properties of In2RuMnO6 and In2RuFeO6 : Heavily transition-metal doped In2O3 -type bixbyites

The title compounds have been synthesized by a citrate technique. The crystal structure of these materials has been studied at room temperature from high-resolution neutron powder diffraction data. In, Ru, and M=Mn,Fe are distributed at random over the metal sites of a C-M₂O₃ bixbyite-type structure, space group $Ia\stackrel{?}{3}$, with $a=9.89000\left(3\right)\AA$ for M=Mn and $a=10.07536\left(4\right)\AA$ for M=Fe. It is well known that In₂O₃ is able to dissolve certain amounts of transition metals but, at ambient pressure, the solubility is rather small, lower than $s=0.5$ in In_2?sM_sO₃. Here, we describe a substantial incorporation of magnetic transition metals into In₂O₃ of half of the metal positions ($s=1$), by simple thermal treatment at temperatures in the 1300–1400 °C range. Although a bond-valence study shows unrealistically low valences for Ru ions, which statistically occupy the metal positions, replacing In, the magnetic properties suggest a valence equilibrium Ru³⁺/M³⁺?Ru⁴⁺/M²⁺. In₂RuFeO₆ shows antiferromagnetic interactions below $T_N\approx 95K$, with a very weak ferromagnetism effect at low temperatures.