7 research outputs found
Magnetic anisotropy in hole-doped superconducting Ba 0.67K 0.33Fe 2As2 probed by polarized inelastic neutron scattering
We use polarized inelastic neutron scattering (INS) to study spin excitations
of optimally hole-doped superconductor BaKFeAs
( K).
In the normal state, the imaginary part of the dynamic susceptibility,
, shows magnetic anisotropy for energies below
7 meV with c-axis polarized spin excitations larger than that of the
in-plane component. Upon entering into the superconducting state, previous
unpolarized INS experiments have shown that spin gaps at 5 and 0.75 meV
open at wave vectors and , respectively, with a
broad neutron spin resonance at meV. Our neutron polarization analysis
reveals that the large difference in spin gaps is purely due to different spin
gaps in the c-axis and in-plane polarized spin excitations, resulting resonance
with different energy widths for the c-axis and in-plane spin excitations. The
observation of spin anisotropy in both opitmally electron and hole-doped
BaFeAs is due to their proximity to the AF ordered BaFeAs where
spin anisotropy exists below .Comment: 5 pages, 4 figure
Strong short-range magnetic order in a frustrated FCC lattice and its possible role in the iron structural transformation
We investigate magnetic properties of a frustrated Heisenberg antiferromagnet
with a face-centered cubic (FCC) lattice and exchange interactions between the
nearest- and next-nearest neighbours, J1 and J2. In a collinear phase with the
wave vector Q = (pi,pi,pi) the equations of the self-consistent spin-wave
theory for the sublattice magnetization and the average short range order
parameter are obtained and numerically solved. The dependence of the Neel
temperature T_N on the ratio J2/J1 is obtained. It is shown, that at strong
enough frustration there is a wide temperature region above T_N with strong
short range magnetic order. Application of this result to description of
structural phase transition between alpha and gamma-phase of Fe is considered
Distinguishing and electron pairing symmetries by neutron spin resonance in superconducting NaFeCoAs
A determination of the superconducting (SC) electron pairing symmetry forms
the basis for establishing a microscopic mechansim for superconductivity. For
iron pnictide superconductors, the -pairing symmetry theory predicts the
presence of a sharp neutron spin resonance at an energy below the sum of hole
and electron SC gap energies () below . On the other hand,
the -pairing symmetry expects a broad spin excitation enhancement at an
energy above below . Although the resonance has been observed in
iron pnictide superconductors at an energy below consistent with the
-pairing symmetry, the mode has also be interpreted as arising from the
-pairing symmetry with due to its broad energy width and
the large uncertainty in determining the SC gaps. Here we use inelastic neutron
scattering to reveal a sharp resonance at E=7 meV in SC
NaFeCoAs ( K). On warming towards , the mode
energy hardly softens while its energy width increases rapidly. By comparing
with calculated spin-excitations spectra within the and
-pairing symmetries, we conclude that the ground-state resonance in
NaFeCoAs is only consistent with the -pairing, and
is inconsistent with the -pairing symmetry.Comment: 9 pages, 8 figures. submitted to PR
Spin excitations in cubic maghemite nanoparticles studied by time-of-flight neutron spectroscopy
We have determined the field dependence of collective magnetic excitations in iron oxide nanoparticles of cubic shape with 8.42(2) nm edge length and a narrow log normal size distribution of 8.2(2)% using time-of-flight neutron spectroscopy. The energy dependence of the uniform precession modes was investigated up to 5 T applied field and yields a Landé factor g=2.05(2) as expected for maghemite (γ-Fe2O3) nanoparticles. A large effective anisotropy field of BA,eff=0.45(16) T was determined, in excellent agreement with macroscopic measurements. This anisotropy is attributed to enhanced shape anisotropy in these monodisperse cubic nanoparticles. The combination of our results with macroscopic magnetization information provides a consistent view of the energy scales of superparamagnetic relaxation and collective magnetic excitations in magnetic nanoparticles. © 2014 American Physical Society.1441sciescopu