T. Groń

Darstellung, elektrische und magnetische eigenschaften von Zn1−xGa0,667xCr2Se4-spinell-einkristallen

Abstract Single crystals of the helimagnetic semiconductors Zn 1−x Ga 0.667x Cr 2 Se 4 (x = 0, 0.05, 0.4) were grown by a vapor-liquid transport method, and the crystallographic, electrical, and magnetic properties studied. The resultant magnetic moments of gallium containing crystals are lower than theoretical ones, indicating that divalent chromium, which is coupled antiferromagnetically to the trivalent chromium ions, is present. The electrical properties are interpreted with the aid of the presented band structure model.

Dielectric and magnetic permittivities of three new ceramic tungstates MPr2W2O10 (M = Cd, Co, Mn)

Broadband dielectric spectroscopy measurements revealed an anomalously large relative permittivity value (ε r = 884) for MnPr2W2O10, a smaller value (ε r = 156) for CoPr2W2O10 and the smallest value (ε r = 22) for CdPr2W2O10 at low frequency (ν = 0.1 Hz) and above room temperature in the insulating and paramagnetic state. Below 273 K, the relative permittivity (ε r ∼ 24) did not depend significantly on frequency for all the tungstates under study. Electrical resistivity, thermoelectric power, electron paramagnetic resonance, magnetic susceptibility and magnetization provided experimental evidence that the studies tungstates were paramagnetic insulators with low n-type conduction. Only in the case of MnPr2W2O10 was a ferrimagnetic order below 45 K observed. These effects are discussed within the framework of Maxwell–Wagner polarization, chemical covalent bonds and porosity mechanism.

Influence of Temperature Independent Contribution of Magnetic Susceptibility on the Curie-Weiss Law

A fitting procedure of the Curie-Weiss law, eliminating the temperature independent contribution (0) from the experimental susceptibility data, was used for determination of the magnetic parameters, i.e. a Curie constant, a Curie-Weiss temperature and an effective magnetic moment, because the theoretical considerations showed that the Curie-Weiss law is invalid for 0  0, even small. This method revealed for the HgCo(NCS)4 paramagnet, commonly accepted as a magnetic susceptibility standard, 0 = 2.43710-6 cm3/g, while for the ZnCr2Se4 antiferromagnet, known as a matrix of various diluted systems, – 0 = -2.56610-6 cm3/g, for comparison.

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.

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 and electrical properties of Cu substituted La1−xCaxCoO3 crystals

Abstract La 1− x Ca x Co 1− y Cu y O 3-δ polycrystals have been prepared in the compositional range 0 y x y = 0, a transition to metallic (impurity band) conduction is observed at x c = 0.45. All other compounds are semiconducting ( ∂ρ / ∂T ρ ∼ T − 1 4 ) and a spin-glass-like behaviour except for T = 4.2 K. A field-induced antiferromagnetic low spin/spin-glass-like mixed spin transition is observed, yielding 0.12 meV as an estimate for the ls/hs energy gap.

Correlation between the negative magnetoresistance effect and magnon excitations in single-crystalline CuCr1.6V0.4Se4

Structural, electrical and magnetic measurements, as well as electron spin resonance (ESR) spectra, were used to characterise the single-crystalline CuCr1.6V0.4Se4 spinel and study the correlation between the negative magnetoresistance effect and magnon excitations. We established the ferromagnetic order below the Curie temperature T C ≈ 193 K, a p-type semiconducting behaviour, the ESR change from paramagnetic to ferromagnetic resonance at T C, a large ESR linewidth value and its temperature dependence in the paramagnetic region. Electrical studies revealed negative magnetoresistance, which can be enhanced with increasing magnetic field and decreasing temperature, while a detailed thermopower analysis showed magnon excitations at low temperatures. Spin–phonon coupling is explained within the framework of a complex model of paramagnetic relaxation processes as a several-stage relaxation process in which the V3+ ions, the exchange subsystem and conduction electron subsystem act as the intermediate reservoirs.

Paramagnetic Behaviour in RE2W2O9 Tungstates (RE = Pr, Nd, Sm – Gd)

Magnetic susceptibility measurements revealed a disordered state of magnetic moments above 4.2 K for all compounds under study, and a weak response to the magnetic field and the temperature for Sm2W2O9 and Eu2W2O9 tungstates. The temperature independent component of magnetic susceptibility has a positive value for RE2W2O9 (RE = Pr, Nd and Gd) indicating a domination of van Vleck contribution. Only for Gd2W2O9 the magnetization is a universal function of µ0H/T, characteristic for the superparamagnetism.

Electrical and Magnetic Properties of CuEu2W2O10 and Cu3Eu2W4O18

Magnetic susceptibility measurements showed both a weak response to magnetic field and a lack of the Curie-Weiss region for CuEu2W2O10 and Cu3Eu2W4O18 tungstates characteristic for the multiplet widths comparable to thermal energy. Magnetization measurements displayed the linear temperature dependence with the lower magnetic moment for Cu3Eu2W4O18 in comparison with CuEu2W2O10, indicating that the effect of the electric charges associated with the surrounding ligands can change the multiplet width of individual states. It is affecting the electrical properties of examined tungstates which reveal the insulating state and low relative permittivity εr ~ 29 in case of CuEu2W2O10 and the thermally activated p-type electrical conduction for Cu3Eu2W4O18 with the activation energy of 1.11 eV and the large value of εr ~ 217 above the room temperature.

Electrical resistivity dip in Sb x V y Mo z O t phases

Electrical resistivity dips have been discovered in the temperature range 100–500 K both in the SbVO4.96 matrix and the Sb x V y Mo z O t phases for 10 mol% solubility of MoO3 in SbVO5. As the Sb content increases and simultaneously the V content decreases, the value of the resistivity at the dip, ρ d, decreases and shifts the dip to higher temperatures. The magnetic measurements showed a spontaneous magnetization and parasitic magnetism of the solid solutions under study. Characteristic for parasitic magnetism is a small value of the magnetic moment, here 0.014 μ B/f.u. at 4.2 K and at a magnetic field of 14 T as well as a small value of the mass susceptibility, here 10−5 cm3/g. The value of the Néel temperature, T N ≤ 8 K, and the Curie–Weiss temperature, θ CW ≤ −208 K, indicate a collinear antiferromagnetic (AFM) order. We suggest that neither the magnetism nor the Mo-content can be correlated with the resistivity anomalies. Therefore, these effects may rather be interpreted in terms of a small-polaron gas in the resistivity dip area. Alternatively, they could mark a lattice/electronic entropy-driven incomplete metal–insulator transition.