199 research outputs found
Magnetic phase transitions in SmCoAsO
Magnetization, x-ray diffraction and specific-heat measurements reveal that
SmCoAsO undergoes three magnetic phase transitions. A ferromagnetic transition
attributed to the Co ions, emerges at TC=57 K with a small saturation moment of
0.15muB/Co. Reorientation of the Co moment to an antiferromagnetic state is
obtained at TN2=45 K. The relative high paramagnetic effective moment Peff=1.57
MuB/Co indicates an itinerant ferromagnetic state of the Co sublattice. The
third magnetic transition at TN1=5 K is observed clearly in the specific-heat
study only. Both magnetic and 57Fe Mossbauer studies show that substitution of
small quantities of Fe for Co was unsuccessful.Comment: 10pages text+Figures: comments welcome ([email protected]
The possible origin of the higher magnetic phase transition in RuSr2Eu1.5Ce0.5Cu2O10 (Ru-1222)
Two magnetic transitions are observed in the magneto-superconducting
RuEu2-xCexSr2Cu2O10 (Ru-1222), at TM 160 K and TM2~80 K. Below TM2 the Ru
moments are weak-ferromagnetically ordered and wide ferromagnetic hysteresis
loops are observed, they become narrow and disappear at ~ 60-70 K. Above TM2,
(i) small antiferromagnetic-like hysteresis loops reappear with a peak in the
coercive fields around 120 K. (ii) A small peak at ~120 K is also observed in
the dc and ac susceptibility curves. The two phenomena are absent in the non-SC
x=1 samples. For x<1, the decrease of the Ce4+ content, is compensated by
non-homogeneous oxygen depletion, which may induce a reduction of Ru5+ ions to
Ru4+. The higher ordering temperature, TM, which does not change with x, may
result from Ru4+ rich clusters, in which the Ru4+-Ru4+ exchange interactions
are stronger than the Ru5+-Ru5+ interactions. In the superconducting
Ru1-xMoxSr2Eu1.5Ce0.5Cu2O10 (x=0-0.4) system, TM2 shifts to low temperature
with x (14 K for x=0.4), whereas TM is not affected by the Mo content,
indicating again that TM may not correspond to the main phase. Two scenarios
are suggested to explain the magnetic phenomena at TM2<T< TM. (i) They are due
to a small fraction of nano-size islands inside the crystal grains, in which
the Ru4+ concentration is high and are magnetically ordered below TM. (ii) The
presence of nanoparticles of a foreign minor extra Ru4+ magnetic phase of
Sr-Cu-Ru-O3, which orders at TM, in which Cu is distributed inhomogeneously in
both the Ru and Sr sitesComment: 11 pages, 10 figure
Self doping effect and successive magnetic transitions in superconducting SrVFeAsO
We have studied a quinary Fe-based superconductor SrVFeAsO by the
measurements of x-ray diffraction, x-ray absorption, M\"{o}ssbauer spectrum,
resistivity, magnetization and specific heat. This apparently undoped
oxyarsenide is shown to be self doped via electron transfer from the V
ions. We observed successive magnetic transitions within the VO layers: an
antiferromagnetic transition at 150 K followed by a weak ferromagnetic
transition at 55 K. The spin orderings within the VO planes are discussed
based on mixed valence of V and V.Comment: One Table and more references are adde
Correlation between intercalated magnetic layers and superconductivity in pressurized EuFe2(As0.81P0.19)2
We report comprehensive high pressure studies on correlation between
intercalated magnetic layers and superconductivity in EuFe2(As0.81P0.19)2
single crystal through in-situ high pressure resistance, specific heat, X-ray
diffraction and X-ray absorption measurements. We find that an unconfirmed
magnetic order of the intercalated layers coexists with superconductivity in a
narrow pressure range 0-0.5GPa, and then it converts to a ferromagnetic (FM)
order at pressure above 0.5 GPa, where its superconductivity is absent. The
obtained temperature-pressure phase diagram clearly demonstrates that the
unconfirmed magnetic order can emerge from the superconducting state. In stark
contrast, the superconductivity cannot develop from the FM state that is
evolved from the unconfirmed magnetic state. High pressure X-ray absorption
(XAS) measurements reveal that the pressure-induced enhancement of Eu's mean
valence plays an important role in suppressing the superconductivity and tuning
the transition from the unconfirmed magnetic state to a FM state. The unusual
interplay among valence state of Eu ions, magnetism and superconductivity under
pressure may shed new light on understanding the role of the intercalated
magnetic layers in Fe-based superconductors
57Fe Mossbauer spectroscopy and magnetic measurements of oxygen deficient LaFeAsO
We report on the magnetic behavior of oxygen deficient LaFeAsO1-x (x-0.10)
compound, prepared by one-step synthesis, which crystallizes in the tetragonal
(S.G. P4/nmm) structure at room temperature. Resistivity measurements show a
strong anomaly near 150 K, which is ascribed to the spin density wave (SDW)
instability. On the other hand, dc magnetization data shows paramagnetic-like
features down to 5 K, with an effective moment of 0.83 mB/Fe. 57Fe Mossbauer
studies (MS) have been performed at 95 and 200 K. The spectra at both
temperatures are composed of two sub-spectra. At 200 K the major one (88%), is
almost a singlet, and corresponds to those Fe nuclei, which have two oxygen
ions in their close vicinity. The minor one, with a large quadrupole splitting,
corresponds to Fe nuclei, which have vacancies in their immediate neighborhood.
The spectrum at 95 K, exhibits a broadened magnetic split major (84%)
sub-spectrum and a very small magnetic splitting in the minor subspectrum. The
relative intensities of the subspectra facilitate in estimating the actual
amount of oxygen vacancies in the compound to be 7.0(5)%, instead of the
nominal LaFeAsO0.90. These results, when compared with reported 57Fe MS of
non-superconducting LaFeAsO and superconducting LaFeAsO0.9F0.1, confirm that
the studied LaFeAsO0.93 is a superconductivity-magnetism crossover compound of
the newly discovered Fe based superconducting family.Comment: 7 pages text + Figs : Comments/suggestions welcome
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