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An antiferromagnetic order with a Néel temperature TN = 17:5 K, a strong ferromagnetic exchange evidenced by a positive Curie Weiss temperature CW = 77:3 K, the fuzzy peaks in the real component of susceptibility x′ (T) and the disappearance of the second critical eld were established. The curvature of speci c heat C(T) and C(T)=T in surrounding of TN indicated a broad peak, characteristic for the system with inhomogeneous magnetic state (spin-glass-like phase). The calculated magnetic entropy showed the value of S(T) 1 J/(mol K) which is extremely small; i.e., much lower than the magnetic contribution Rln(2S + 1) = 11:52 J/(mol K) calculated for the spin 3/2.

The static (dc) and dynamic (ac) magnetic measurements of CdCr2Se4 and Cd[Cr1:89Ti0:08]Se4 showed their ferromagnetic properties with a Curie temperature TC 130 K and revealed on the real component of ac susceptibility curve, the peaks near TC at 200 Oe, 450 Oe and 1 kOe, characteristic for the Hopkinson ones. The meaningful reduction of saturation moment to 4.73 B/f.u. for Cd[Cr1:89Ti0:08]Se4 suggests the diamagnetic con guration of Ti ions, which dilutes the ferromagnetic sublattice of Cr ones and causes reducing of the energy losses visible on the imaginary components of ac susceptibility curve. Close for zero values of higher susceptibility harmonics above TC are pointing out to the lack of the spin uctuations in the paramagnetic state.

Magnetization, M, and susceptibility, , measurements showed both strong lowering of magnetic moment in comparison with CuCr2Se4 matrix and zero- eld-cooling eld-cooling susceptibility splitting characteristic for the spin-glass behaviour. Isothermal magnetization curves, M(H), easy saturate and large values both of the Curie TC = 253 K and Curie Weiss = 283:5 K temperatures indicate the ferromagnetic order which coexists with the spin-glass state. The critical behaviour investigated around the paramagnetic ferromagnetic phase transition revealed that the values of critical exponents are close to those predicted by the mean eld model for long-range magnetic interactions.

Measurements of the magnetic properties, the electrical conductivity and the Seebeck effect were performed on single crystals (CdxAly)[Cr2]Sez between 77 and 300 K. All samples have a ferromagnetic order with the Curie temperature of 130 K and the paramagnetic Curie–Weiss temperature of 155 K. Both these temperatures do not depend significantly on the Al substitution. The electrical conductivity of single crystals CdCr2Se4 doped with Al. was p-type and showed the change of log ¾ versus 1=T slope above 150 K. This fact is interpreted as being due to the onset of impurity conduction and structural defects.

The complex ac dynamic magnetic susceptibility was used to study the mictomagnetic-like behavior in polycrystalline Cd0:87Cr1:93V0:06Se4 spinel. The temperature dependences of the zero field in-phase (real part) and out-of-phase (imaginary part) components of fundamental susceptibility measured at the oscillating field Hac = 0:5 Oe and at the constant frequency of 125 Hz are characteristic for the mictomagnetic order. These results well correlate with the anomalies in the second and third harmonics of the ac susceptibility indicating the cluster glass.

Taking into account both the dc magnetic susceptibility and the magnetization measurements as well as the high-temperature expansion of the magnetic susceptibility procedure the hopping integrals and superexchange integrals for the first and second coordination sphere were evaluated. The two hopping integrals are positive and many times greater than the superexchange ones. The obtained results testify to that in the stoichiometric compounds under study the double-exchange magnetic interaction is the main mechanism responsible for very strong, ferromagnetic coupling. Using the total hopping integral B the bandwidth of mixed valence band of chromium ions was determined.

The electrical and complex ac dynamic magnetic susceptibility measurements were used to study an influence of temperature on critical fields in single-crystalline ZnxSbyCrzSe4 spinel system with y = 0:11, 0.16 and 0.20. The p-type conduction and a shift both of the Néel temperature TN to lower temperatures and a susceptibility peak at Tm in the paramagnetic region — to higher temperatures were established. Below TN the magnetic field dependence of susceptibility, Âac(H), shows two peaks. First peak at the critical field Hc1 slightly decreases with temperature and remains almost constant as Sb content increases. Second peak at the critical field Hc2 drops rapidly with temperature and remains almost unchanged as Sb content increases. At TN both critical fields disappear.

Single crystals of ZnCr2Se4 spinel doped with vanadium were prepared by chemical vapour transport. The chemical compositions of three crystals have been determined by X-ray diffraction. The structure refinement using the SHELXL-93 program system determine the cation distribution in the system as ZnCr2¡xVxSe4. For x values equal to 0.03, 0.1 and 0.13 the observed symmetry was cubic, space group Fd3m. Based on the structural data, influence of the V ions on the magnetic and electrical properties has been analyzed.

Both the dc and ac magnetic susceptibilities as well as magnetization measurements were used to study the influence of the dilution of the magnetic chromium subarray by nonmagnetic antimony and aluminium ions on the magnetization processes for four spinel families under investigation. Substitution of the chromium ions by the nonmagnetic Sb and Al ions in the compounds under study leads to the very hard magnetization in the case of the compounds with Sb and very easy magnetization in the compounds with Al. This effect is connected with the electronic configurations of the Sb and Al ions as well as with ionic radii of these cations.

The electrical and magnetic investigations carried out on the polycrystalline spinels with the general chemical formula CdxCryVzSe4 (where z = 0:06, 0.12, 0.24, and 0.31) revealed semiconducting and ferromagnetic properties with a Curie temperature of 127.5 K. A step-like structure of the electrical conductivity, ¾(T), was observed for the polycrystal containing 6% V ions in the octahedral sites while the remaining samples showed a typical Arrhenius behaviour of ¾(T). These effects are interpreted in terms of ferromagnetic spin clusters which finally dissolve on going towards the paramagnetic limit including non-stoichiometry.

The main research objective of the presented doctoral dissertation was to develop a technology for the preparation of new nanostructured sulfide (ZnCr₂S₄, CuCr₂S₄) and selenide (ZnCr₂Se₄, CuCr₂Se₄) spinels and to determine the influence of crystallite size on the physicochemical properties of this group of compounds. The implementation of this goal included: 1) obtaining nanospinels using mechanical alloying and high-energy ball mining methods; 2) determination of lattice parameters, crystallite sizes, and lattice strains using X-ray structural analysis; 3) imaging the morphology of the obtained nanospinels using electron spectroscopy; 4) determination of magnetic parameters using temperature tests of changes in direct current (dc) and alternating current (ac) susceptibility as well as magnetization as a function of an external magnetic field; 5) calculation of the exchange integrals related to the magnetic interactions of superexchange and double exchange; 6) carrying out tests of electrical conductivity and thermopower. The conducted studies of magnetic and electrical properties has shown that reducing the size of crystallites to the nano scale causes: 1) disturbance of the long-range and short-range superexchange interactions, which leads to the appearance of different orderings of magnetic moments, 2) weakening of the mechanisms of magnetic interactions of superexchange and double exchange, as indicated by the calculated values of the exchange integrals associated with these interactions; 3) disappearance of the magnetic saturation state; 4) changing the nature of the electrical conductivity of CuCr₂S₄ and CuCr₂Se₄ nanospinels from metallic to semiconductor, which is related to the narrowing of the mixed-valence band of the chromium ions Wd (3d t₂g); 5) changing the type of electric conductivity from p type to n type caused by an increase in the advantage of anionic vacancies over cationic ones. As part of the research work, nanostructured spinel materials with unique physicochemical properties were obtained and the possibility of their modulation by appropriate selection of the size of crystallites was shown, which may result in their potential application in science and technology in the future.