T. Mydlarz

Magnetic properties of GdPdAl single crystals

Single crystals of the intermetallic ternary compound GdPdAl, crystallizing in the hexagonal ZrNiAl-type structure, were grown by the Czochralski method. Lattice parameters, electrical resistivity, magnetic susceptibility give evidence of an isostructural phase transition from a high temperature modification HTM I phase to HTM II phase at 180 K. The jump related to this transition causes a decrease of the lattice parameter a of 0.7% and an increase of the lattice parameter c of 1.3% during the cooling process. These changes are responsible for some microcracks in the single crystal. Besides the structural phase transition, magnetic ordering of the Gd sublattice is observed at 48 K. At about 20 K a spin reorientation process may occur. The XPS measurements of GdPdAl reveal a dominating contribution of d electrons at the Fermi level. For comparison the electronic structure of TbPdAl was also examined. Both compounds reveal formation of a narrow Pd 4d band. Further information on changes of the electronic structure at the isostructural phase transition were obtained from the temperature dependence of the ESR linewidth.

Antiferromagnetism of Zn1−xGa2x3Cr2Se4 (for 0.0⩽x⩽0.5)+

Abstract The results of extensive studies of magnetic properties for the spinels Zn1−xGa 2x 3 Cr2Se4 are summarized. They concern the isotherms of static magnetization in applied induction up to 14 T, the magnetization and its time derivative curves in pulsed fields up to 42 T, the Neel temperatures and the high-temperature susceptibility, all in the concentration range 0.0 ⩽x ⩽ 0.5. It turns out that the magnetic properties of these spinels can be explained only if we assume the existence of Cr ions in two valence states: Cr3+ and Cr2+, and the average ratio Ga:Cr2+ isabout 1:1. A hypothesis on a modification of the simple spin spiral produced by an increase of Ga composition and by a strong magnetic field is described.

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.

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.

MAGNETIC PROPERTIES OF UFEXCO10-XSI2 INTERMETALLICS

Abstract UFe 10 Si 2 and UCo 10 Si 2 form solid solution with the ThMn 12 type of tetragonal structure. The lattice parameter a follows the Vegard law and parameter c does not. The increase of cobalt content depresses a saturation magnetization at 4.2 K almost linearly, whereas the remanence for all samples is very low. The highest Curie point equal to 750 K is observed for UFe 6 Co 4 Si 2 . The annealing process does not affect the Curie point of these materials, however increases slightly the saturation magnetization. Two samples from the U(Fe, Co, Al) 10 Si 2 system have been investigated too.

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 properties of DyMn2 single crystals

Abstract Single crystals of the DyMn 2 compound were grown by the Czochralski method from a levitated melt. The electrical resistivity exhibits a magnetic transition at 40 K, a second transition at 25 K, a cubic temperature dependence at low temperatures and a saturation effect above 250 K. The magnetization in strong magnetic fields, and the DC and AC magnetic susceptibility measured along the principal crystallographic directions show an ordering of the dysprosium moments to a spin-canted magnetic structure at about 40 K and a spin reorientation below 25 K. The temperature dependence of the lattice parameter shows a lattice contraction below 40 K.

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 ErMn2 single crystals

Abstract Single crystals of the ErMn 2 compound were grown by the Czochralski method from a levitated melt. The thermal dependence of the lattice parameters exhibits a magnetovolume effect along the a axis, a contraction for both the c axis and unit cell volume below 20 K. The thermal dependence of the electrical resistivity exhibits a magnetic transition at 23 K. A second transition occurs at 13 K. Moreover, for ρ ( T ) a negative curvature above the ordering temperature and a saturation effect above 200 K occurs. The magnetization in strong magnetic fields, the d.c. and a.c. magnetic susceptibilities measured along the principal crystallographic directions show ordering of the erbium moments to a spin-canted magnetic structure at about 15 K and a spin reorientation about 10 K.