123 research outputs found
Phase transitions and iron-ordered moment form factor in LaFeAsO
Elastic neutron scattering studies of an optimized LaFeAsO single crystal
reveal that upon cooling, an onset of the tetragonal (T)-to-orthorhombic (O)
structural transition occurs at K, and it exhibits a
sharp transition at K. We argue that in the
temperature range to , T and O structures may
dynamically coexist possibly due to nematic spin correlations recently proposed
for the iron pnictides, and we attribute to the formation of
long-range O domains from the finite local precursors. The antiferromagnetic
structure emerges at K, with the iron moment
direction along the O \emph{a} axis. We extract the iron magnetic form factor
and use the tabulated of Fe, Fe and Fe to
obtain a magnetic moment size of 0.8 at 9.5 K.Comment: 7 pages, 6 figures, 3 table
Temperature and magnetic field dependences of the elastic constants of Ni-Mn-Al magnetic Heusler alloys
We report on measurements of the adiabatic second order elastic constants of
the off-stoichiometric NiMnAl single crystalline Heusler
alloy. The variation in the temperature dependence of the elastic constants has
been investigated across the magnetic transition and over a broad temperature
range. Anomalies in the temperature behaviour of the elastic constants have
been found in the vicinity of the magnetic phase transition. Measurements under
applied magnetic field, both isothermal and variable temperature, show that the
value of the elastic constants depends on magnetic order, thus giving evidence
for magnetoelastic coupling in this alloy system.Comment: 7 pages, 5 figures. Accepted for publication in Physical the Review
Revisiting the ground state of CoAlO: comparison to the conventional antiferromagnet MnAlO
The A-site spinel material, CoAl2O4, is a physical realization of the
frustrated diamond-lattice antiferromagnet, a model in which is predicted to
contain unique incommensurate or `spin-spiral liquid' ground states. Our
previous single-crystal neutron scattering study instead classified it as a
`kinetically-inhibited' antiferromagnet, where the long ranged correlations of
a collinear Neel ground state are blocked by the freezing of domain wall motion
below a first-order phase transition at T* = 6.5 K. The current paper expands
on our original results in several important ways. New elastic and inelastic
neutron measurements are presented that show our initial conclusions are
affected by neither the sample measured nor the instrument resolution, while
measurements to temperatures as low as T = 250 mK limit the possible role being
played by low-lying thermal excitations. Polarized diffuse neutron measurements
confirm reports of short-range antiferromagnetic correlations and diffuse
streaks of scattering, but major diffuse features are explained as signatures
of overlapping critical correlations between neighboring Brillouin zones.
Finally, and critically, this paper presents detailed elastic and inelastic
measurements of magnetic correlations in a single-crystal of MnAl2O4, which
acts as an unfrustrated analogue to CoAl2O4. The unfrustrated material is shown
to have a classical continuous phase transition to Neel order at T_N = 39 K,
with collective spinwave excitations and Lorentzian-like critical correlations
which diverge at the transition. Direct comparison between the two compounds
indicates that CoAl2O4 is unique, not in the nature of high-temperature diffuse
correlations, but rather in the nature of the frozen state below T*. The higher
level of cation inversion in the MnAl2O4 sample indicates that this novel
behavior is primarily an effect of greater next-nearest-neighbor exchange.Comment: 13 pages, 8 figures, acccepted for publication in Physical Review
Suppression of antiferromagnetic order and orthorhombic distortion in superconducting Ba(Fe0.961Rh0.039)2As2
Neutron diffraction and high-resolution x-ray diffraction studies find that,
similar to the closely related underdoped Ba(Fe[1-x]Cox)2As2 superconducting
compounds, Ba(Fe0.961Rh0.039)2As2 shows strong evidence of competition and
coexistence between superconductivity and antiferromagnetic order below the
superconducting transition, Tc = 14 K. The transition temperatures for both the
magnetic order and orthorhombic distortion are in excellent agreement with
those inferred from resistivity measurements, and both order parameters
manifest a distinct decrease in magnitude below Tc. These data suggest that the
strong interaction between magnetism and superconductivity is a general feature
of electron-doped Ba(Fe[1-x]TMx)2As2 superconductors (TM = Transition Metal).Comment: 4 pages, 4 figure
Competition and coexistence of antiferromagnetism and superconductivity in underdoped Ba(Fe0.953Co0.047)2As2
Neutron and x-ray diffraction studies show that the simultaneous first-order
transition to an orthorhombic and antiferromagnetic (AFM) ordered state in
BaFe2As2 splits into two transitions with Co doping. For
Ba(Fe0.953Co0.047)2As2, a tetragonal-orthorhombic transition occurs at TS = 60
K, followed by a second-order transition to AFM order at TN = 47 K.
Superconductivity (SC) occurs in the orthorhombic state below TC = 15 K and
coexists with AFM. Below TC, the static Fe moment is reduced and a 4 meV spin
gap develops indicating competition between coexisting SC and AFM order.Comment: 15 pages, 4 figure
Delta Doping of Ferromagnetism in Antiferromagnetic Manganite Superlattices
We demonstrate that delta-doping can be used to create a dimensionally
confined region of metallic ferromagnetism in an antiferromagnetic (AF)
manganite host, without introducing any explicit disorder due to dopants or
frustration of spins. Delta-doped carriers are inserted into a manganite
superlattice (SL) by a digital-synthesis technique. Theoretical consideration
of these additional carriers show that they cause a local enhancement of
ferromagnetic (F) double-exchange with respect to AF superexchange, resulting
in local canting of the AF spins. This leads to a highly modulated
magnetization, as measured by polarized neutron reflectometry. The spatial
modulation of the canting is related to the spreading of charge from the doped
layer, and establishes a fundamental length scale for charge transfer,
transformation of orbital occupancy and magnetic order in these manganites.
Furthermore, we confirm the existence of the canted, AF state as was predicted
by de Gennes [P.-G. de Gennes, Phys. Rev. 118, 141 (1960)], but had remained
elusive
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