Thermodynamic properties of the ferromagnetic spinel ZnpCd1−pCr2Se4

The magnetic properties of the B-spinel ferromagnetic Zn_pCd_1−pCr₂Se₄ compound are studied via a cluster series expansion approximation with nearest and next-nearest exchange integrals J₁ and J₂, respectively. Using the minimization of the free-energy expressions, the magnetization, the magnetic susceptibility, the two-spin correlation functions and the specific heat are obtained and computed numerically as a function of temperature and for each composition of the system. The magnetization curves are used to determine the critical temperatures T_c. Using the power laws in the vicinity of the critical regions the critical exponents β, γ and α associated, respectively, with the magnetization, the magnetic susceptibility and the specific heat are numerically calculated. The critical temperatures T_c obtained are in very good agreement with those predicted by the magnetic measurements and the values of the critical exponents may be compared with other theoretical results based on the 3D Heisenberg model.     

Magnetic properties of (GdxY1−x)Co2B2 compounds

Magnetic measurements were performed on the (Gd_xY_1−x)Co₂B₂ compounds, in the temperature range 2–800 K and fields up to 70 kOe. YCo₂B₂ is a paramagnet. The (Gd_xY_1−x)Co₂B₂ compounds with x≥0.2 shows a ferromagnetic type ordering. The saturation magnetization at 2 K coincides only with the contribution of gadolinium. The Curie temperatures are nearly linearly dependent on the composition. Above the Curie points, the thermal variations of the magnetic susceptibility can be described as a superposition of a temperature independent term ϰ₀ on a Curie-Weiss behavior. The Curie constants are determined by the contribution of Gd³⁺ ions only. The ϰ₀ values increase when the gadolinium content is greater. The observed properties are discussed in the wider framework of the magnetic behavior of cobalt in GdCo_xB_y compounds.     

Vibrational properties of one- and two-mode behaviour in spinel type mixed systems Zn1−xCdxCr2S4

The infrared reflectivity spectra of spinel type mixed crystals Zn_1−xCd_xCr₂S₄ have been measured from 40 to 900 cm⁻¹. Four or five bands were observed at both end members in the region of intermediate composition. The three bands in higher energy show one-mode behaviour and the other two bands show two-mode behaviour. The concentration dependences of these mode frequencies are interpreted by a model proposed already, in which the 2 basic units and the 6 force constants are contained. By comparing the force constants associated with the Zn-site in several spinels, it is suggested that the two-mode behaviour arises from the large difference in the force constants between the two end members. The E_g (260 cm⁻¹) and T_2g (160 cm⁻¹) Raman modes for ZnCr₂S₄ were also observed and used for the determination of the force constants.     

Magnetic properties of solid solutions of the system ErxY1−xFe2

The magnetic properties of the system Er_xY_1–xFe₂ (MgCu₂ structure) in the temperature range 80–800°K are investigated. The magnetization and the coercive force H_c are measured as a function of T. For alloys with x= 0.5–1 there is a magnetic compensation point _c. The composition dependence of _c, the magnetic moment of the alloy and of the iron atoms Fe are determined. An H_c anomaly is observed in _c. In order to explain the results obtained, the presence of two types of magnetically active atoms and three types of exchange interaction are included.Translated from Izvestiya Vysshykh Uchebnykh Zavedenii, Fizika, No. 10, pp. 27–30, October, 1982.     

Optical properties of Li1−xNb1−xEu2xO3 solid solutions

Abstract

The effects of europium substitution on crystalline solubility, structural changes and optical properties of LiNbO₃ is reported. Li_1?xNb_1?xEu_2xO₃ solid solutions exist over a very limited range of europium concentrations (x ≤ 0.01). The solid solutions were characterized by X-ray powder diffraction and density measurements. Within the range of compositions of the solid solutions, the optical properties (emission and excitation spectra) of the Eu³⁺ ions have been characterized. The obtained optical data indicate that two equally abundant europium luminescent species exist in the stoichiometric speciments; these correspond to the Eu³⁺ ions occupying the Li⁺ and Nb⁵⁺ sites, in agreement with the replacement mechanism which is inferred from density measurements. Other Eu³⁺ luminescent species which appear to be closely related with lithium deficiency were found to exist in nonstoichiometric samples, in addition to those which are present in the stoichiometric materials.     

Crystallographic and optical properties of Li1−xTa1−xEu2xO3 solid solutions

Abstract

The effects of europium substitution on crystalline solubility, structural changes and optical properties of Li_1?xTa_1?xEu_2xO₃ solid solutions are reported. The solid solutions exist over a very limited range of europium concentrations (x ≤ 0.02). They were characterized by X-ray powder diffraction and density measurements. The liquid nitrogen fluorescence and excitation spectra of Eu³⁺ in the solid solutions were measured in the frequency range 12,200–45,450 cm^?1. The emission mainly arises from the ⁵ D ₀ level to Stark components of the five lowest ⁷ F _j (j = 0, 1, 2, 3, 4) states. It was inferred from the analyses of the optical and density data that the Eu³⁺ ions may lie on both Li and Ta sites, but not on the intrinsic vacant lattice sites.     

Magnetic properties and specific heat of Tm1−xLuxCu2Si2

The tetragonal TmCu₂Si₂ compound is the only magnetically ordered material of the RECu₂Si₂ group for which crystal field parameters were determined. Quadrupole splitting measured by means of Mössbauer spectroscopy showed that two lowest lying states are nonmagnetic singlets. Therefore, this material is likely to have an induced magnetic moment, mainly due to mixing of the two lowest states. We performed specific heat and magnetic susceptibility measurements of Tm_1−xLu_xCu₂Si₂ (x = 0, 0.025, 0.050, 0.10, 0.25, 0.50 and 1) alloys to determine the crystal field level scheme and compare it with the Mössbauer data. The saturation magnetization of the antiferromagnetic phase was calculated to be 3.2 μ_B and the moment is directed along the tetragonal c axis. No direct experimental evidence is known to support this prediction.     

Magnetic properties of Nd3+ in Nd−Ba−Cu−O-compounds

Neutron diffraction, neutron spectroscopy and magnetization measurements have been employed to study the structural, electronic, and magnetic behavior of eleven compounds with the general formula Nd_1+y Ca _v Ba_2–y–v Cu₃O _x (0y0.5; 0v0.25; 6x7). The structure turned out to react to oxygen reduction similar as other 123-compounds, yielding discontinuities close to the metal-insulator-transition and the well-known relations of bond lengths as a function ofT _c. The crystalline electric field (CEF) interaction, splitting the 10-fold degenerate ground-state J-multiplet of the Nd³⁺-ions into five doublet states, was investigated by neutron spectroscopy. The derived CEF parameters have been used to determine changes in the electronic surroundings of the Nd³⁺ ions. In addition, with the help of the CEF parameters the thermodynamic magnetic properties of these compounds were calculated which turn out to be in good agreement with the experimental data.     

Magnetic and transport properties of CuCr1−x VxS2 compounds

A comparison of experimental and calculated effective magnetic moments shows that the sulfur in CuCr_1−x V_xS₂ compounds has two different oxidation states, S¹⁻ and S²⁻, while the vanadium has a higher degree of oxidation (V³⁺) than the chromium (Cr²⁺). A model is proposed to explain the reduction in the activation energy for defect formation in the Cu-sublattice and the rise in the Cu⁺-cation conductivity when chromium is replaced by vanadium. Fiz. Tverd. Tela (St. Petersburg) 41, 1450–1451 (August 1999)     

Magnetic properties of FexMn1−x S sulfides exhibiting the magnetoresistive effect

The magnetic, electrical, and thermal (derived from DTA data) properties of Fe_xMn_1?x S polycrystalline sulfides (0≤x≤0.38) synthesized based on α-MnS (NaCl cubic lattice) and exhibiting colossal magnetoresistance were studied. The studies were conducted at temperatures from 77 to 1000 K and magnetic fields of up to 30 kOe. As the degree of cation substitution in the Fe_xMn_1?x S system was increased, the magnetic order was found to change from antiferromagnetic to ferromagnetic. In the high-temperature domain (550–850 K), the samples undergo two phase transitions with critical temperatures $T_{c_1 }$ and $T_{c_2 }$ , which are accompanied by reversible anomalies in the magnetization and thermal (DTA) properties and by a semiconductor-metal transition.     

Magnetic ordering in Fe2−xZnxMoO4(X=0.1–1) spinel

We have studied the diluted Fe_2−xZn_xMoO₄ spinel ferrite which shows a frozen disordered magnetic state at low temperature. Magnetic properties are examined by DC magnetisation measurements as a function of temperature, field and time and AC susceptibility experiment. Our measurements show that this disordered magnetic system at low fields, shares many common features with spin glass or cluster glass like phases. Results suggest that the interaction gradually changes as the magnetic ion concentration decreases by the substitution of non-magnetic Zn on A site and causing a perturbation to the magnetically ordered spins and magnetic order decreases.     

Magnetic properties and structure of Fe2 − x Mg x CrO4 chromites

The structural and magnetic properties of Fe_{2 ? x} Mg _x CrO₄ chromite synthesized by the ceramic method to receive analogs of natural minerals and to solve the rock magnetism problems are investigated. The dependences of cubic-lattice parameters and magnetic characteristics on composition, with slight deviation from linearity, are obtained. It is established in the course of experiments related to the partial thermal remanent magnetization in weak fields that some compositions are characterized by self-reversal under sample heating to 600°C in air. It is suggested that the self-reversal is caused by a nonuniform distribution of cations in chromite structure and phase transformation under oxidation.     

Dielectric properties of NaCl1−xBr and KCl1−xBrx mixed ionic solution

In the present paper a three-body potential model (TBPM) has been employed for the analysis of dielectric behaviour of NaCl-NaBr and and KCl-KBr mixed crystals with varying compositions. The physical properties like dielectric constants ( ₀ and ), optic mode frequencies ( _OLO and _TO), effective charge parameter (e _s ^* ), optic mode Grüneisen parameters and strain derivatives of ₀ and dielectric constants have been calculated. The results achieved in the present study are found in fairly good agreement with the available experimental data. The results obtained by previous investigators are also shown for the sake of comparison.     

Synchrotron X-ray diffraction studies on the phase transitions in the spinel LixMn3−xO4 intercalation compounds

Detailed investigations have been undertaken of the lithium for manganese substitution effect on the LiMn₂O₄, in the system Li_xMn_3−xO₄, for 0.95≤x≤1.05, that is for the nearly stoichiometric lithium content. Synchrotron X-ray measurements have been performed in the temperature range 10–300 K. The diffraction experiments were carried out at the DESY-HASYLAB high-resolution powder diffractometer (beamline B2), equipped with a closed-cycle He-cryostat. Very small changes in the lithium content influence clearly the low-temperature crystal structure of Li_xMn_3−xO₄, spinels and the nature of phase transitions. It was found that for x=0.95 the sample remains tetragonal in the whole 10–300 K temperature range. The stoichiometric LiMn₂O₄ transforms from cubic to orthorhombic at about 280 K. For x=1.0125 the temperature of phase transition from cubic to orthorhombic decreases down to about 260 K, whereas for x=1.025 the transformation goes from cubic to tetragonal phase, at the temperature 220 K. No phase transition has been observed for the cubic sample with x=1.0375. These results partly explain the divergences in recent reports on the low-temperature structure and phase transformations of lithium manganese oxides.