96 research outputs found
Measurement of the neutron electric dipole moment via spin rotation in a non-centrosymmetric crystal
We have measured the neutron electric dipole moment using spin rotation in a
non-centrosymmetric crystal. Our result is d_n = (2.5 +- 6.5(stat) +-
5.5(syst)) 10^{-24} e cm. The dominating contribution to the systematic
uncertainty is statistical in nature and will reduce with improved statistics.
The statistical sensitivity can be increased to 2 10^{-26} e cm in 100 days
data taking with an improved setup. We state technical requirements for a
systematic uncertainty at the same level.Comment: submitted to Phys. Lett.
Extended skyrmion lattice scattering and long-time memory in the chiral magnet FeCoSi
Small angle neutron scattering measurements on a bulk single crystal of the
doped chiral magnet FeCoSi with =0.3 reveal a pronounced effect
of the magnetic history and cooling rates on the magnetic phase diagram. The
extracted phase diagrams are qualitatively different for zero and field cooling
and reveal a metastable skyrmion lattice phase outside the A-phase for the
latter case. These thermodynamically metastable skyrmion lattice correlations
coexist with the conical phase and can be enhanced by increasing the cooling
rate. They appear in a wide region of the phase diagram at temperatures below
the -phase but also at fields considerably smaller or higher than the fields
required to stabilize the A-phase
Magnetic Fluctuations, Precursor Phenomena and Phase Transition in MnSi under Magnetic Field
The reference chiral helimagnet MnSi is the first system where skyrmion
lattice correlations have been reported. At zero magnetic field the transition
at to the helimagnetic state is of first order. Above , in a region
dominated by precursor phenomena, neutron scattering shows the build up of
strong chiral fluctuating correlations over the surface of a sphere with radius
, where is the pitch of the helix. It has been suggested that
these fluctuating correlations drive the helical transition to first order
following a scenario proposed by Brazovskii for liquid crystals. We present a
comprehensive neutron scattering study under magnetic fields, which provides
evidence that this is not the case. The sharp first order transition persists
for magnetic fields up to 0.4 T whereas the fluctuating correlations weaken and
start to concentrate along the field direction already above 0.2 T. Our results
thus disconnect the first order nature of the transition from the precursor
fluctuating correlations. They also show no indication for a tricritical point,
where the first order transition crosses over to second order with increasing
magnetic field. In this light, the nature of the first order helical transition
and the precursor phenomena above , both of general relevance to chiral
magnetism, remain an open question
Universality of the helimagnetic transition in cubic chiral magnets: Small angle neutron scattering and neutron spin echo spectroscopy studies of FeCoSi
We present a comprehensive Small Angle Neutron Scattering (SANS) and Neutron
Spin Echo Spectroscopy (NSE) study of the structural and dynamical aspects of
the helimagnetic transition in FeCoSi with = 0.30. In contrast
to the sharp transition observed in the archetype chiral magnet MnSi, the
transition in FeCoSi is gradual and long-range helimagnetic
ordering coexists with short-range correlations over a wide temperature range.
The dynamics are more complex than in MnSi and involve long relaxation times
with a stretched exponential relaxation which persists even under magnetic
field. These results in conjunction with an analysis of the hierarchy of the
relevant length scales show that the helimagnetic transition in
FeCoSi differs substantially from the transition in MnSi and
question the validity of a universal approach to the helimagnetic transition in
chiral magnets
Magnetic Fluctuations and Correlations in MnSi - Evidence for a Skyrmion Spin Liquid Phase
We present a comprehensive analysis of high resolution neutron scattering
data involving Neutron Spin Echo spectroscopy and Spherical Polarimetry which
confirm the first order nature of the helical transition and reveal the
existence of a new spin liquid skyrmion phase. Similar to the blue phases of
liquid crystals this phase appears in a very narrow temperature range between
the low temperature helical and the high temperature paramagnetic phases.Comment: 11 pages, 16 figure
On the sign of the linear magnetoelectric coefficient in CrO
We establish the sign of the linear magnetoelectric (ME) coefficient,
, in chromia, CrO. CrO is the prototypical linear ME
material, in which an electric (magnetic) field induces a linearly proportional
magnetization (polarization), and a single magnetic domain can be selected by
annealing in combined magnetic (H) and electric (E) fields. Opposite
antiferromagnetic domains have opposite ME responses, and which
antiferromagnetic domain corresponds to which sign of response has previously
been unclear. We use density functional theory (DFT) to calculate the magnetic
response of a single antiferromagnetic domain of CrO to an applied
in-plane electric field at 0 K. We find that the domain with nearest neighbor
magnetic moments oriented away from (towards) each other has a negative
(positive) in-plane ME coefficient, , at 0 K. We show that this
sign is consistent with all other DFT calculations in the literature that
specified the domain orientation, independent of the choice of DFT code or
functional, the method used to apply the field, and whether the direct
(magnetic field) or inverse (electric field) ME response was calculated. Next,
we reanalyze our previously published spherical neutron polarimetry data to
determine the antiferromagnetic domain produced by annealing in combined E and
H fields oriented along the crystallographic symmetry axis at room temperature.
We find that the antiferromagnetic domain with nearest-neighbor magnetic
moments oriented away from (towards) each other is produced by annealing in
(anti-)parallel E and H fields, corresponding to a positive (negative) axial ME
coefficient, , at room temperature. Since
at 0 K and at room temperature are known to be of opposite
sign, our computational and experimental results are consistent.Comment: 11 pages, 5 figure
Multiple low-temperature skyrmionic states in a bulk chiral magnet
Magnetic skyrmions are topologically protected nanoscale spin textures with particle-like properties. In bulk cubic helimagnets, they appear under applied magnetic fields and condense spontaneously into a lattice in a narrow region of the phase diagram just below the magnetic ordering temperature, the so-called A-phase. Theory, however, predicts skyrmions to be locally stable in a wide range of magnetic fields and temperatures. Our neutron diffraction measurements reveal the formation of skyrmion states in large areas of the magnetic phase diagram, from the lowest temperatures up to the A-phase. We show that nascent and disappearing spiral states near critical lines catalyze topological charge changing processes, leading to the formation and destruction of skyrmionic states at low temperatures, which are thermodynamically stable or metastable depending on the orientation and strength of the magnetic field. Skyrmions are surprisingly resilient to high magnetic fields: the memory of skyrmion lattice states persists in the field polarized state, even when the skyrmion lattice signal has disappeared. These findings highlight the paramount role of magnetic anisotropies in stabilizing skyrmionic states and open up new routes for manipulating these quasi-particles towards energy-efficient spintronics applications
Magnetization jump in the XXZ chain with next-nearest-neighbor exchange
We study the dependence of the magnetization M with magnetic field B at zero
temperature in the spin-1/2 XXZ chain with nearest-neighbor (NN) J1 and next-NN
J2 exchange interactions, with anisotropies Delta1 and Delta2 respectively. The
region of parameters for which a jump in M(B) exists is studied using numerical
diagonalization, and analytical results for two magnons on a ferromagnetic
background in the thermodynamic limit. We find a line in the parameter space
(J2/J1, Delta1/J1, Delta2/J2) (determined by two simple equations) at which the
ground state is highly degenerate. M(B) has a jump near this line, and at or
near the isotropic case with ferromagnetic J1 and antiferromagnetic J2, with
|J2/J1| near 1/4. These results are relevant for some systems containing CuO
chains with edge-sharing CuO4 units.Comment: 9 pages, 8 figures, submitted to Phys. Rev.
Elucidating Individual Magnetic Contributions in Bi-Magnetic Fe3O4/Mn3O4 Core/Shell Nanoparticles by Polarized Powder Neutron Diffraction
Heterogeneous bi-magnetic nanostructured systems have had a sustained interest during the last decades owing to their unique magnetic properties and the wide range of derived potential applications. However, elucidating the details of their magnetic properties can be rather complex. Here, a comprehensive study of Fe3O4/Mn3O4 core/shell nanoparticles using polarized neutron powder diffraction, which allows disentangling the magnetic contributions of each of the components, is presented. The results show that while at low fields the Fe3O4 and Mn3O4 magnetic moments averaged over the unit cell are antiferromagnetically coupled, at high fields, they orient parallel to each other. This magnetic reorientation of the Mn3O4 shell moments is associated with a gradual evolution with the applied field of the local magnetic susceptibility from anisotropic to isotropic. Additionally, the magnetic coherence length of the Fe3O4 cores shows some unusual field dependence due to the competition between the antiferromagnetic interface interaction and the Zeeman energies. The results demonstrate the great potential of the quantitative analysis of polarized neutron powder diffraction for the study of complex multiphase magnetic materials
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