Electronic band structure and exchange coupling constants in ACr2X4 spinels

We present the results of band structure calculations for ACr2X4 (A=Zn, Cd, Hg and X=O, S, Se) spinels. Effective exchange coupling constants between Cr spins are determined by fitting the energy of spin spirals to a classical Heisenberg model. The calculations reproduce the change of the sign of the dominant nearest-neighbor exchange interaction J1 from antiferromagnetic in oxides to ferromagnetic in sulfides and selenides. It is verified that the ferromagnetic contribution to J1 is due to indirect hopping between Cr t2g and eg states via X p states. Antiferromagnetic coupling between 3-rd Cr neighbors is found to be important in all the ACr2X4 spinels studied, whereas other interactions are much weaker. The results are compared to predictions based on the Goodenough-Kanamori rules of superexchange.Comment: 15 pages, 10 figures, 3 table

EXCHANGE INTEGRALS FOR THE FIRST AND SECOND COORDINATION SPHERES IN THE NEW SPINEL SERIES Cu1-xZnxCr2Te4 (where x = 0.00, 0.01, 0.02)

The exchange integrals for the first and second coordination spheres of the spinels under study were determined. The former turned out to be positive and the latter-negative. Thus, the ferromagnetic structure of these spinels is a result of the competitition of the ferro- and antiferro-interactions

DOUBLE-EXCHANGE INTERACTION AS THE MAIN MECHANISM DRIVING A VERY STRONG FERROMAGNETIC COUPLING IN THE SPINEL SYSTEM Cd1-xCuxCr2Se4

Each Cu+ ion introduced in place of a Cd++ ion in the tetrahedral sublattice of the spinels under study causes a transformation of a Cr3+ ion into a Cr4+ ion in the octahedral sublattice. It follows, that apart from the superexchange interaction there appears in these compounds a double exchange interaction which consists in a jump of an electron between Cr3+ and Cr4+ and brings a strong ferromagnetic coupling

Hopkinson-Like Effect in Single-Crystalline CdCr2Se4CdCr_2Se_4 and Cd[Cr1.89Ti0.08]Se4Cd[Cr_{1.89}Ti_{0.08}]Se_4

The static (dc) and dynamic (ac) magnetic measurements of $CdCr_2Se_4$ and $Cd[Cr_{1.89}Ti_{0.08}]Se_4$ showed their ferromagnetic properties with a Curie temperature $T_{C}$ ≈ 130 K and revealed on the real component of ac susceptibility curve, the peaks near $T_{C}$ at 200 Oe, 450 Oe and 1 kOe, characteristic for the Hopkinson ones. The meaningful reduction of saturation moment to 4.73 $μ_{B}$/f.u. for $Cd[Cr_{1.89}Ti_{0.08}]Se_4$ suggests the diamagnetic configuration 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 $T_{C}$ are pointing out to the lack of the spin fluctuations in the paramagnetic state

High Spin-Low Spin Transitions in Cu0.2Co0.76Cr1.83Se4Cu_{0.2}Co_{0.76}Cr_{1.83}Se_4 Semiconductor

Magnetization, ac and dc magnetic susceptibility measured in the zero-field-cooled mode were used to study the high spin-low spin transitions in polycrystalline $Cu_{0.2}Co_{0.76}Cr_{1.83}Se_4$ semiconductor. The real part component of fundamental susceptibility $χ_1^\prime$ (T) and its second $(χ_2)$ and third $(χ_3)$ harmonics revealed two spectacular peaks at 128 K and at 147 K, confirming the appearance of the spin-crossover phenomenon

Influence of Temperature on Critical Fields in ZnxSbyCrzSe4Zn_{x}Sb_{y}Cr_{z}Se_{4}

The electrical and complex ac dynamic magnetic susceptibility measurements were used to study an influence of temperature on critical fields in single-crystalline $Zn_{x}Sb_{y}Cr_{z}Se_{4}$ spinel system with y = 0.11, 0.16 and 0.20. The p-type conduction and a shift both of the Néel temperature $T_{N}$ to lower temperatures and a susceptibility peak at $T_{m}$ in the paramagnetic region - to higher temperatures were established. Below $T_{N}$ the magnetic field dependence of susceptibility, $χ_{ac}(H)$, shows two peaks. First peak at the critical field $H_{c1}$ slightly decreases with temperature and remains almost constant as Sb content increases. Second peak at the critical field $H_{c2}$ drops rapidly with temperature and remains almost unchanged as Sb content increases. At $T_{N}$ both critical fields disappear

Electrical and Magnetic Studies of the CdxCryVzSe4Cd_{x}Cr_{y}V_{z}Se_{4} Spinels

The electrical and magnetic investigations carried out on the polycrystalline spinels with the general chemical formula $Cd_{x}Cr_{y}V_{z}Se_{4}$ (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, σ(T), was observed for the polycrystal containing 6% V ions in the octahedral sites while the remaining samples showed a typical Arrhenius behaviour of σ(T). These effects are interpreted in terms of ferromagnetic spin clusters which finally dissolve on going towards the paramagnetic limit including non-stoichiometry