J. Krok-Kowalski

Correlations of the magnetic and electrical properties with the ionic radii of cations and anions for the series of ternary and quaternary spinel-type chromium compounds

Abstract For the compounds ACr 2 X 4 (where: A=Cd, Zn, Hg, Ga, Cu; X=O, S, Se, Te), ACr 2 x B 2−2 x X 4 (where: A=Cd, Zn; B=Ga, In; X=S, O), ZnCr 2− x In x Se 4 and Cr 2 V 4− x Mo x O 13+0.5 x correlations were observed of the magnetic and electrical properties with the ionic radii of their cations and anions, whereas for the solid solutions of the boundary compounds ACr 2 X 4 (where: A=Cd, Zn; X=S, Se, Te) and CuCr 2 X 4 (were X=S, Se, Te) not only such correlations but also the metallic conductivity and a semiconductor–metal phase transition were observed. In particular for Zn 1− x Cu x Cr 2 Se 4 this phase transition is accompanied by the magnetic AF-F phase transition, while for Cd 1− x Cu x Cr 2 Se 4 one deals with the accompanying Fi-F phase transition. In the latter spinel series there exists the ferrimagnetic ordering at low copper concentrations, whereas at high copper concentrations one deals with the ferromagnetic ordering.

Influence of the substitution of Cu ions for Cd ions on the crystallographic and magnetic properties of Cd1−xCuxCr2Se4 spinels

Abstract All the compounds under study show the spinel normal structure. The lattice parameters decrease with an increase in the Cu concentration. The samples with low Cu concentrations ( x ≤ 0.2) exhibit ferrimagnetic ordering and Curie temperatures of 150 K. The samples with high Cu concentrations ( x ≥ 0.9) are ordered ferromagnetically, and their Curie temperatures are much higher and close to 480 K. Moreover, for the four samples under investigation one can observe a significant lowering of the saturation magnetic moment in comparison with the theoretical one. This lowering seems to be connected with the high degree of defects in the crystal structure. An attempt is made to interpret this fact with the use of the vacancy parameter.

Electrical conductivity in the antiferromagnetic compounds FeVO4, FeVMoO7 and Fe4V2Mo3O20

Abstract The compounds under study are n-type antiferromagnetic semiconductors. Their electrical properties depend strongly on the metallic component concentration in the compound. The big changes of the electrical conductivity vs. temperature are connected with the processes strongly thermally activated.

Magnetic, electrical and structural properties of some ternary and quaternary spinels with chromium

Abstract Spinel compounds with the general formulas ACr 2 X 4 and ACr 2− x B x X 4 (where A=Cd, Zn, Hg, Ga, Cu; X=S, Se, Te, O; B=Ga, In, Sb), including also substitution of two different cations in the tetrahedral subarray, exhibit different correlations of the magnetic and electrical properties. These properties are correlated with the ionic radii of cations and anions. The solid solutions of the boundary compounds reveal, e.g. correlations of metallic conductivity and a semiconductor–metal phase transition. In some spinel solid solutions the latter phase transition is accompanied by the magnetic phase transitions, i.e. either AF-F or Fi-F. Magnetic and electrical properties and their correlations in different spinel series with chromium are discussed in the frames of the phase diagram of the spinel-type compounds with chromium. Different mechanisms leading to the spin-glass state are discussed and illustrated.

Double exchange magnetic interaction and giant negative magnetoresistivity in the spin glass state of the new compound CuCr1.6Sb0.4S4

Abstract CuCr 1.6 Sb 0.4 S 4 crystallizes in the spinel structure with the lattice parameter a =9.9680(1) A and the anion parameter u =0.3820(1). The very hard magnetization of the sample, the shape of the magnetization isotherms as well as the lack of magnetic saturation even in a field of B =14 T indicate that the antiferromagnetic superexchange interaction dominates the long-range magnetic interactions between the magnetic moments localized on the chromium ions. A high positive value of the paramagnetic Curie–Weiss temperature Θ C–W (equal to 318 K) testifies that the short-range interaction is ferromagnetic and strong which means that in the corresponding microregions the double exchange interaction is present and predominates. A suggestion is made that these microregions behave like superparamagnetic clusters. As the result of the competition of the interactions mentioned above the sample under study reveals a spin glass state at low temperatures, which is confirmed by the temperature dependence of the magnetic susceptibility curves. The spin-freezing temperature is lower than 50 K. CuCr 1.6 Sb 0.4 S 4 is a semiconductor with a giant negative magnetoresistivity equal to 74% at 38 T and 3 K. The driving force of this effect is the double exchange magnetic interaction. Three values for the activation energy were observed: E 1 =0.118 eV, E 2 =0.004 eV and E 3 =0.033 eV. These are most probably connected with the energy levels generated by the crystal field.

Ferrimagnetism and metamagnetism in Cd1−xCuxCr2S4 spinels

Abstract Although the compounds CdCr2S4 and CuCr2S4 are ferromagnetic, solid solutions of the system CdCr2S4-CuCr2S4 with the spinel normal structure show a ferrimagnetic ordering of their magnetic moments with Curie temperatures of about 90 K. For all the samples under investigation one can observe a significant lowering of the saturation magnetic moment in comparison with the theoretical one. Moreover, metamagnetic thresholds were observed at different temperatures. In particular at 250 K the values of the induction of the magnetic field for these thresholds are equal to 0.1 T for x = 0.05, 0.4 T for x = 0.1, 1.1 T for x = 0.15 and 0.4 T for x = 0.2, respectively. The lowering of the saturation magnetic moment and the existence of the metamagnetic thresholds for all the samples under study seem to be connected both with the high defectiveness of the crystal structure and with the mixed valence of the chromium ions. An attempt is made to interpret this fact in terms of the RKKY, double-exchange and superexchange magnetic interactions as well as with the use of the vacancy parameter.

Magnetic superexchange interactions and metamagnetic phase transitions in the single crystals of the spinels ZnCr2−xInxSe4 (0 ≤ x ≤ 0.14)

Abstract The experimental data for the magnetization and the magnetic susceptibility, as well as their dependence on temperature, on the induction of the external magnetic field and on the concentration x have been analyzed. The metamagnetic phase transitions induced by the external magnetic field have been observed as plateau areas and jump-like increments on the magnetization curves at 4.2 K. High values of the effective magnetic moment μ eff and of the Curie constant C M point to the existence of Cr 2+ ions as well as Cr 3+ ions. The exchange constants, transfer integrals and the spiral angles for the solid solutions under study have been evaluated.

Magnetic properties of the single crystals of ZnCr2−xInxSe4 (0.0<x<0.15)☆

From the magnetic measurements it follows that there is a noncollinear ordering of the magnetic moments localized on the chromium ions. The Neel temperature is of the order of 20 K and the Curie-Weiss paramagnetic temperature is positive of the order of 50 K. The latter value points to the existence of the short-range strong ferromagnetic interactions, the long-range interaction being antiferromagnetic.

Electrical and optical properties of AVO4 (A = Fe, Cr, Al) compounds

Abstract Measurements of the electrical resistivity, the Seebeck effect and the UV/VIS absorption were carried out for samples of AVO4 (A = Fe, Cr, Al) compounds. It was observed that: 1) chromium and aluminum vanadates are insulators at room temperature and they have p and n-type electrical conductivity at higher temperatures, respectively, 2) the shift of the absorption edge to lower wavelengths in iron and aluminum vanadates and to higher ones in chromium vanadate in comparison with their constituents is connected with the type of the crystal structure, 3) the charge transport occurs via anion-deficient centers in iron and aluminum vanadates and via cation-deficient centers in chromium vanadate.

Quantitative evaluation of the contribution of the double-exchange magnetic interaction in the total exchange integral in spinels Cd1−xCuxCr2Se4 (x = 0.05, 0.15, 0.20, 0.90, 0.95)

Abstract Using the high-temperature expansion of the magnetic susceptibility the hopping integrals for the first and second coordination sphere were evaluated. The two integrals are positive and several times greater than the superexchange integrals.