Compensation temperatures and exchange bias in La1.5Ca0.5CoIrO6

We report on the study of magnetic properties of the La1.5Ca0.5CoIrO6 double perovskite. Via ac magnetic susceptibility we have observed evidence of weak ferromagnetism and reentrant spin glass behavior on an antiferromagnetic matrix. Regarding the magnetic behavior as a function of temperature, we have found that the material displays up to three inversions of its magnetization, depending on the appropriate choice of the applied magnetic field. At low temperature the material exhibit exchange bias effect when it is cooled in the presence of a magnetic field. Also, our results indicate that this effect may be observed even when the system is cooled at zero field. Supported by other measurements and also by electronic structure calculations, we discuss the magnetic reversals and spontaneous exchange bias effect in terms of magnetic phase separation and magnetic frustration of Ir4+ ions located between the antiferromagnetically coupled Co ions.Comment: 10 pages, 8 figures and supplemental materia

Pressure and chemical substitution effects in the local atomic structure of BaFe2As2

The effects of K and Co substitutions and quasi-hydrostatic applied pressure (P<9 GPa) in the local atomic structure of BaFe2As2, Ba(Fe{0.937}Co{0.063})2As2 and Ba{0.85}K{0.15}Fe2As2 superconductors were investigated by extended x-ray absorption fine structure (EXAFS) measurements in the As K absorption edge. The As-Fe bond length is found to be slightly reduced (<~ 0.01 Angstroms) by both Co and K substitutions, without any observable increment in the corresponding Debye Waller factor. Also, this bond is shown to be compressible (k = 3.3(3)x10^{-3} GPa^{-1}). The observed contractions of As-Fe bond under pressure and chemical substitutions are likely related with a reduction of the local Fe magnetic moments, and should be an important tuning parameter in the phase diagrams of the Fe-based superconductors.Comment: 7 pages, 6 figure

Absence of zero-field-cooled exchange bias effect in single crystalline La2−xAx CoMnO6 (A=Ca,Sr) compounds

Magnetic properties of A2BB'O6 (A = rare or alkaline-earth ions; B, B' = transition-metal ions) double perovskites are of great interest due to their potential spintronic applications. Particularly fascinating is the zero-field-cooled exchange bias effect observed for the hole-doped La2−xAxCoMnO6 polycrystalline samples. In this paper we synthesize La2CoMnO6, La1.5Ca0.5CoMnO6, and La1.5Sr0.5CoMnO6 single crystals by the floating zone method and study their magnetic behavior. The three materials are ferromagnetic. Surprisingly, we observe no zero or even conventional exchange bias effect for the Ca- and Sr-doped single crystals, in sharp contrast to polycrystalline samples. This absence indicates that the lack of grain boundaries and spin-glass-like behavior, not observed in our samples, might be key ingredients for the spontaneous exchange bias phenomena seen in polycrystalline samples

Pressure and chemical substitution effects in the local atomic structure of BaFe2As2

The effects of K and Co substitutions and quasihydrostatic applied pressure (P ~0.01 Å) by both Co and K substitutions, without any observable increment in the corresponding Debye-Waller factor. Also, this bond is shown to be compressible [κ = 3.3(3) × 10−3 GPa−1]. The observed contractions of As-Fe bond under pressure and chemical substitutions are likely related with a reduction of the local Fe magnetic moments, and should be an important tuning parameter in the phase diagrams of the Fe-based superconductors

Pressure and chemical substitution effects in the local atomic structure of BaFe2As2

The effects of K and Co substitutions and quasihydrostatic applied pressure (P ~0.01 Å) by both Co and K substitutions, without any observable increment in the corresponding Debye-Waller factor. Also, this bond is shown to be compressible [κ = 3.3(3) × 10−3 GPa−1]. The observed contractions of As-Fe bond under pressure and chemical substitutions are likely related with a reduction of the local Fe magnetic moments, and should be an important tuning parameter in the phase diagrams of the Fe-based superconductors