28 research outputs found
Temperature-dependent properties of the magnetic order in single-crystal BiFeO3
We report neutron diffraction and magnetization studies of the magnetic order
in multiferroic BiFeO3. In ferroelectric monodomain single crystals, there are
three magnetic cycloidal domains with propagation vectors equivalent by
crystallographic symmetry. The cycloid period slowly grows with increasing
temperature. The magnetic domain populations do not change with temperature
except in the close vicinity of the N{\P}eel temperature, at which, in
addition, a small jump in magneti- zation is observed. No evidence for the
spin-reorientation transitions proposed in previous Raman and dielectric
studies is found. The magnetic cycloid is slightly anharmonic for T=5 K. The
an- harmonicity is much smaller than previously reported in NMR studies. At
room temperature, a circular cycloid is observed, within errors. We argue that
the observed anharmonicity provides important clues for understanding
electromagnons in BiFeO3.Comment: In Press at PR
Spin and Lattice Structure of Single Crystal SrFe2As2
We use neutron scattering to study the spin and lattice structure on single
crystals of SrFe2As2, the parent compound of the FeAs based superconductor
(Sr,K)Fe2As2. We find that SrFe2As2 exhibits an abrupt structural phase
transitions at 220K, where the structure changes from tetragonal with lattice
parameters c > a = b to orthorhombic with c > a > b. At almost the same
temperature, Fe spins in SrFe2As2 develop a collinear antiferromagnetic
structure along the orthorhombic a-axis with spin direction parallel to this
a-axis. These results are consistent with earlier work on the RFeAsO (R = rare
earth elements) families of materials and on BaFe2As2, and therefore suggest
that static antiferromagnetic order is ubiquitous for the parent compound of
these FeAs-based high-transition temperature superconductors.Comment: 14 pages with 4 figure
Evolution of the bulk properties, structure, magnetic order, and superconductivity with Ni doping in CaFe2-xNixAs2
Magnetization, susceptibility, specific heat, resistivity, neutron and x-ray
diffraction have been used to characterize the properties of single crystalline
CaFe2-xNixAs2 as a function of Ni doping for x varying from 0 to 0.1. The
combined first-order structural and magnetic phase transitions occur together
in the undoped system at 172 K, with a small decrease in the area of the a-b
plane along with an abrupt increase in the length of the c-axis in the
orthorhombic phase. With increasing x the ordered moment and transition
temperature decrease, but the transition remains sharp at modest doping while
the area of the a-b plane quickly decreases and then saturates. Warming and
cooling data in the resistivity and neutron diffraction indicate hysteresis of
~2 K. At larger doping the transition is more rounded, and decreases to zero
for x=0.06. The susceptibility is anisotropic for all values of x. Electrical
resistivity for x = 0.053 and 0.06 shows a superconducting transition with an
onset of nearly 15 K which is further corroborated by substantial diamagnetic
susceptibility. For the fully superconducting sample there is no long range
magnetic order and the structure remains tetragonal at all temperature, but
there is an anomalous increase in the area of the a-b plane in going to low T.
Heat capacity data show that the density of states at the Fermi level increases
for x > 0.053 as inferred from the value of Sommerfeld coefficient. The regime
of superconductivity is quite restrictive, with a maximum TC of 15 K and an
upper critical field Hc2=14 T. Superconductivity disappears in the overdoped
region.Comment: 14 pages, 12 figures. Submitted to Phys. Rev.
3:1 magnetization plateau and suppression of ferroelectric polarization in an Ising chain multiferroic
Ferroelectric Ising chain magnet CaCoMnO (0.96)
was studied in magnetic fields up to 33 T. Magnetization and neutron scattering
measurements reveal successive metamagnetic transitions from the zero-field
spin configuration to the state with a broad magnetization plateau, and
then to the state. The absence of
hysteresis in these plateaus reveals an intriguing coupling between the
intra-chain state and the three-dimensional geometrically frustrated magnetic
system. Inversion symmetry, broken in the state, is restored in the
state, leading to the complete suppression of the electric polarization driven
by symmetric superexchange.Comment: accepted for publication as a Brief Report in Physical Review
Magnetic Order versus superconductivity in the Iron-based layered La(O1-xFx)FeAs systems
In high-transition temperature (high-Tc) copper oxides, it is generally
believed that antiferromagnetism plays a fundamental role in the
superconducting mechanism because superconductivity occurs when mobile
electrons or holes are doped into the antiferromagnetic parent compounds. The
recent discovery of superconductivity in the rare-earth (R) iron-based oxide
systems [RO1-xFxFeAs] has generated enormous interest because these materials
are the first noncopper oxide superconductors with Tc exceeding 50 K. The
parent (nonsuperconducting) LaOFeAs material is metallic but shows anomalies
near 150 K in both resistivity and dc magnetic susceptibility. While optical
conductivity and theoretical calculations suggest that LaOFeAs exhibits a
spin-density-wave (SDW) instability that is suppressed with doping electrons to
form superconductivity, there has been no direct evidence of the SDW order.
Here we use neutron scattering to demonstrate that LaOFeAs undergoes an abrupt
structural distortion below ~150 K, changing the symmetry from tetragonal
(space group P4/nmm) to monoclinic (space group P112/n) at low temperatures,
and then followed with the development of long range SDW-type antiferromagnetic
order at ~134 K with a small moment but simple magnetic structure. Doping the
system with flourine suppresses both the magnetic order and structural
distortion in favor of superconductivity. Therefore, much like high-Tc copper
oxides, the superconducting regime in these Fe-based materials occurs in close
proximity to a long-range ordered antiferromagnetic ground state. Since the
discovery of longComment: 15 pages, 4 figures, and 3 table
Studies Of The Three-dimensional Frustrated Antiferromagnetic Zncr2o4
Results of studies of the susceptibility, magnetic specific heat, and electron paramagnetic resonance spectrum of the geometrically frustrated antiferromagnetic ZnCr2O4 are presented. The temperature dependence of the susceptibility and the specific heat are in good agreement with the predictions of the quantum tetrahedral mean field model for exchange-coupled spin-3/2 ions on a pyrochlore lattice. The origin of the anomalous behavior of the resonance intensity below 90 K is discussed. © 2001 American Institute of Physics.8911 II70507052Proceedings of the Conference Highly Frustrated Magnetism 2000 Can. J. Phys., , in pressRamirez, A.P., (1994) Annu. Rev. Mater. Sci., 24, p. 453Schiffer, P., Ramirez, A.P., (1996) Comments Condens. Matter Phys., 18, p. 21Kino, Y., Luthi, B., (1971) Solid State Commun., 9, p. 805Plumier, R., Lecomte, M., Sougi, M., (1977) J. Phys., 38, pp. L-149. , ParisLee, S.-H., Broholm, C., Kim, T.H., Ratcliff W. II, Cheong, S.-W., (2000) Phys. Rev. Lett., 84, p. 3718Martinho, H., cond-mat/0011171Garcia-Adeva, A.J., Huber, D.L., (2000) Phys. Rev. Lett., 85, p. 4598Baltzer, P.K., Wojtowicz, P.J., Robbins, M., Lopatin, E., (1966) Phys. Rev., 151, p. 367noteOhta, H., Okubo, S., Kikuchi, H., Ono, S., Proceedings of the Conference Highly Frustrated Magnetism 2000 Can. J. Phys., , in pressHuber, D.L., Alejandro, G., Caneiro, A., Causa, M.T., Prado, F., Tovar, M., Oseroff, S.B., (1999) Phys. Rev. B, 60, p. 1215
Nernst effect of the new iron-based superconductor LaOFFeAs
We report the first Nernst effect measurement on the new iron-based
superconductor LaOFFeAs . In the normal state, the
Nernst signal is negative and very small. Below a large positive peak
caused by vortex motion is observed. The flux flowing regime is quite large
compared to conventional type-II superconductors. However, a clear deviation of
the Nernst signal from normal state background and an anomalous depression of
off-diagonal thermoelectric current in the normal state between and 50
K are observed. We propose that this anomaly in the normal state Nernst effect
could correlate with the SDW fluctuations.Comment: 8 pages, 4 figures; Latex file changed, references adde
Superconductivity and Phase Diagram in the Iron-based Arsenic-oxides ReFeAsO1-delta (Re = rare earth metal) without F-Doping
Here we report a new class of superconductors prepared by high pressure
synthesis in the quaternary family ReFeAsO1-delta (Re = Sm, Nd, Pr, Ce, La)
without fluorine doping. The onset superconducting critical temperature (Tc) in
these compounds increases with the reduction of Re atom size, and the highest
Tc obtained so far is 55 K in SmFeAsO1-delta. For the NdFeAsO1-delta system
with different oxygen concentration a dome-shaped phase diagram was found