Debye model

About: Debye model is a research topic. Over the lifetime, 7462 publications have been published within this topic receiving 133987 citations.

Dielectric relaxations and alternating current conductivity in manganese substituted cobalt ferrite

Abstract: Manganese (Mn) substituted cobalt ferrites (CoFe2-xMnxO4, referred to CFMO) have been synthesized by the solid state reaction method and their dielectric properties and ac conductivity have been evaluated as a function of applied frequency and temperature. X-ray diffraction measurements indicate that CFMO crystallize in the inverse cubic spinel phase with a lattice constant ∼8.38 A. Frequency dependent dielectric measurements at room temperature obey the modified Debye model with relaxation time of 10−4 s and spreading factor of 0.35(±0.05). The frequency (20 Hz–1 MHz) and temperature (T = 300–900 K) dependent dielectric constant analyses indicate that CFMO exhibit two dielectric relaxations at lower frequencies (1–10 kHz), while completely single dielectric relaxation for higher frequencies (100 kHz–1 MHz). The dielectric constant of CFMO is T-independent up to ∼400 K, at which point increasing trend prevails. The dielectric constant increase with T > 400 K is explained through impedance spectroscopy assuming a two-layer model, where low-resistive grains separated from each other by high-resistive grain boundaries. Following this model, the two electrical responses in impedance formalism are attributed to the grain and grain-boundary effects, respectively, which also satisfactorily accounts for the two dielectric relaxations. The capacitance of the bulk of the grain determined from impedance analyses is ∼10 pF, which remains constant with T, while the grain-boundary capacitance increases up to ∼3.5 nF with increasing T. The tan δ (loss tangent)-T also reveals the typical behavior of relaxation losses in CFMO.

Magnetic susceptibility, specific heat, and the spin-glass transition in Hg 1 − x Mn x Te

Abstract: A systematic study of the low-dc-field magnetic susceptibility and the specific heat has been carried out on mixed ${\mathrm{Hg}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}$ crystals, in the composition range $0l~xl~0.35$. The alloy with $x=0.35$ showed spin-glass behavior below $T=10.9$ K. At this Mn concentration the sample is a very poor conductor at low temperatures, so that the Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism cannot be responsible for the spin-glass transition. Also, since the Mn ions interact only antiferromagnetically, the observed spin-glass phase does not result from competition between ferro- and antiferromagnetic interaction. It must therefore be ascribed to the frustration of the antiferromagnetic interactions inherent in an fcc sublattice over which the Mn ions are distributed. For $xl~0.25$, the ${\mathrm{Hg}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}$ samples remain paramagnetic down to 1 K. Experimental results for the specific heat and the susceptibility for $xl0.1$ are discussed in terms of a cluster model which leads to an estimated value of the antiferromagnetic exchange constant $\frac{J}{k}\ensuremath{\approx}\ensuremath{-}0.7\ifmmode\pm\else\textpm\fi{}0.3$ K. When a random distribution of Mn ions over the fcc sublattice is assumed, calculated values for the specific heat and the susceptibility differ substantially from the experimental results for the low Mn concentration. This therefore indicates that in real crystals the distribution of Mn ions is very different from random. To obtain agreement between calculated and experimental results, the number of single ions has to be reduced to less than 30% of the number corresponding to a purely random distribution, leading to the conclusion that the magnetic ions prefer to cluster rather than to remain isolated in ${\mathrm{Hg}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}$. For HgTe, the Debye temperature ${\ensuremath{\Theta}}_{D}$ is 141 K at $T=0$ K, and goes through an anomalously low minimum value of ${\ensuremath{\Theta}}_{D}=77$ K at 7 K. A very small linear term in the low-temperature specific heat of HgTe gives an estimate of the electric charge carrier density in the ${10}^{18}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ range.

First-Principles Calculations to Investigate Structural, Electronic, Elastic, Magnetic, and Thermodynamic Properties of Full-Heusler Rh 2 MnZ (Z = Zr, Hf)

Abstract: The structural, electronic, elastic, magnetic, and thermodynamic properties of two new Heusler alloys Rh2MnZ (Z = Zr, Hf) are studied based on the first principal calculation using the scheme of the generalized gradient approximation (GGA) of density function theory. The investigation was carried out in ferromagnetic (FM), anti-ferromagnetic (AFM), and the non-magnetic (NM) phases of the Cu2MnAl-type structure (regular structure) and Hg2CuTi-type-structure (inverse structure). Both alloys were found to be more stable in the ferromagnetic phase of the Cu2MnAl-type structure. The equilibrium lattice parameter in this structure is equal to 6.39 Ǻ for Rh2MnZr and 6.35 Ǻ for Rh2MnHf. The electronic properties reveled the metallic nature of the Heusler Rh2MnZ (Z = Zr, Hf) alloys. The interpretation of the elastic properties confirmed the elastic stability of the two alloys in the studied structure with a good agreement between the resulting bulk modulus from the structural properties and that of resulting from the elastic properties. Other elastic parameters such as modulus B, shear modulus G, Young’s modulus E, Poisson’s ratio (ν) and Pugh’s ratio B/G, and the Zener anisotropy parameter A showed that the Rh2MnZ (Z = Zr, Hf) alloys are slightly deformed. They show high rigidity, anisotropic, and little deformation and behave in ductile way. The magnetic properties confirmed the ferromagnetic state of both compounds with computed total magnetic moment equal to 4.76 μB for Rh2MnZr and 4.60 μB for Rh2MnHf. The thermodynamic parameters were evaluated with various temperatures between 0 and 1200 K and a pressure from 0 to 50 GPa using the quasi-harmonic Debye model.

The low-temperature thermal expansion and Gruneisen parameters of some tetrahedrally bonded solids

Abstract: The results of thermal expansion measurements using a three-terminal capacitance dilatometer on Ge, GaAs, ZnS, ZnSe and CdTe below 30K and on ZnSe and CdTe between 57 and 90K are reported. Respective values of 0.49, 0.32, -0.14, 0.0 and -0.85 derived for gamma 0th, the zero temperature limit of the Gruneisen parameter gamma =3 alpha VBs/Cp are given. These are in good agreement with the values of gamma 0el calculated from the pressure dependence of the elastic constants. gamma (T) as a function of reduced temperature, T/ theta 0, where theta 0 is the Debye temperature determined at the zero temperature limit, is found to have an essentially universal form and passes through a negative minimum at T approximately 0.06 theta 0. The implications of this observation and the importance of the degree of ionicity in dictating gamma 0 are discussed.

Soft bulk metallic glasses based on cerium

Abstract: CeAlNiCu alloys can be readily cast into glassy rods with up to 5mm in diameter. The Ce-based bulk metallic glasses (BMGs) exhibit a wide supercooled region up to 78K, very low glass transition temperature (Tg=359K), melting temperature (Tm=637K), and Debye temperature (θD=144K). Ultrasonic measurements demonstrate that these Ce-based BMGs are very soft, having the lowest elastic moduli in known BMGs. These features suggest that the “soft” BMGs are an ideal model system for investigating physical problems in glass transition, supercooled liquid and melt states, and have potential applications as a functional material as well.