20 research outputs found
Distinct high-T transitions in underdoped BaKFeAs
In contrast to the simultaneous structural and magnetic first order phase
transition previously reported, our detailed investigation on an
underdoped BaKFeAs single crystal unambiguously
revealed that the transitions are not concomitant. The tetragonal (:
I4/mmm) - orthorhombic (: Fmmm) structural transition occurs at
110 K, followed by an adjacent antiferromagnetic (AFM) transition
at 102 K. Hysteresis and coexistence of the and
phases over a finite temperature range observed in our NMR
experiments confirm the first order character of the structural transition and
provide evidence that both and are strongly correlated. Our
data also show that superconductivity (SC) develops in the phase
below = 20 K and coexists with long range AFM. This new observation,
, firmly establishes another similarity between the hole-doped
BaFeAs via K substitution and the electron-doped iron-arsenide
superconductors.Comment: 4 pages, 3 figure
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
data for "A phenomenological model for the spontaneous exchange bias effect"
these data will be published in the "A phenomenological model for the spontaneous exchange bias effect" articleTHIS DATASET IS ARCHIVED AT DANS/EASY, BUT NOT ACCESSIBLE HERE. TO VIEW A LIST OF FILES AND ACCESS THE FILES IN THIS DATASET CLICK ON THE DOI-LINK ABOV
The spontaneous exchange bias effect in La2-xCaxCoMnO6 series
Structural, electronic and magnetic properties of polycrystalline
La2-xCaxCoMnO6 (0 x 0.75) compounds are investigated by x-ray
diffraction and magnetometry. All the samples have an orthorhombic structure
and show a slight decrease in the unit cell with Ca-doping.
Temperature-dependent magnetization measurements reveal a complex magnetic
behavior with two ferromagnetic transitions. These transitions are ascribed to
Co2+--Mn4+ and Co3+--Mn3+ couplings and suggest the presence of additional
antiferromagnetic couplings in these disordered compounds. Field-dependent
magnetization curves, measured after cooling the samples in a zero external
magnetic field, reveal the spontaneous exchange bias (SEB) effect for the
Ca-doped samples. The strengthening of the uncompensated magnetic coupling at
the interfaces, caused by the increased antiferromagnetic phase, explains the
increase of SEB with increasing the Ca-content.Comment: 16 pages, 4 figure