104 research outputs found
Incommensurate magnetic ordering in (X=Cl,Br) studied by neutron diffraction
We present the results of the first neutron powder and single crystal
diffraction studies of the coupled spin tetrahedra systems {\CuTeX} (X=Cl,
Br). Incommensurate antiferromagnetic order with the propagation vectors
{\bf{k}_{Cl}}\approx[0.150,0.422,\half],
{\bf{k}_{Br}}\approx[0.158,0.354,\half] sets in below =18 K for X=Cl
and 11 K for X=Br. No simple collinear antiferromagnetic or ferromagnetic
arrangements of moments within Cu tetrahedra fit these observations.
Fitting the diffraction data to more complex but physically reasonable models
with multiple helices leads to a moment of 0.67(1)/Cu at 1.5 K
for the Cl-compound. The reason for such a complex ground state may be
geometrical frustration of the spins due to the intra- and inter-tetrahedral
couplings having similar strengths. The magnetic moment in the Br- compound,
calculated assuming it has the same magnetic structure as the Cl compound, is
only 0.51(5)/Cu at 1.5 K. In neither compound has any evidence
for a structural transition accompanying the magnetic ordering been found
Origin and stability of the dipolar response in a family of tetragonal tungsten bronze relaxors
A new family of relaxor dielectrics with the tetragonal tungsten bronze
structure (nominal composition Ba6M3+Nb9O30, M3+ = Ga, Sc or In) were studied
using dielectric spectroscopy to probe the dynamic dipole response and
correlate this with the crystal structure as determined from powder neutron
diffraction. Independent analyses of real and imaginary parts of the complex
dielectric function were used to determine characteristic temperature
parameters, TVF, and TUDR, respectively. In each composition both these
temperatures correlated with the temperature of maximum crystallographic
strain, Tc/a determined from diffraction data. The overall behaviour is
consistent with dipole freezing and the data indicate that the dipole stability
increases with increasing M3+ cation size as a result of increased
tetragonality of the unit cell. Crystallographic data suggests that these
materials are uniaxial relaxors with the dipole moment predominantly restricted
to the B1 cation site in the structure. Possible origins of the relaxor
behaviour are discussed.Comment: Main article 32 pages, 8 figures; Supplementary data 24 pages, 4
figure
Commensurate structural modulation in the charge- and orbitally-ordered phase of the quadruple perovskite (NaMn)MnO
By means of synchrotron x-ray and electron diffraction, we studied the
structural changes at the charge order transition =176 K in the
mixed-valence quadruple perovskite (NaMn)MnO. Below we
find satellite peaks indicating a commensurate structural modulation with the
same propagation vector q =(1/2,0,-1/2) of the CE magnetic order that appears
at low temperature, similarly to the case of simple perovskites like
LaCaMnO. In the present case, the modulated structure
together with the observation of a large entropy change at gives
evidence of a rare case of full Mn/Mn charge and orbital order
consistent with the Goodenough-Kanamori model.Comment: Accepted for publication in Phys. Rev. B Rapid Communication
Magnetic order and the electronic ground state in the pyrochlore iridate Nd2Ir2O7
We report a combined muon spin relaxation/rotation, bulk magnetization,
neutron scattering, and transport study of the electronic properties of the
pyrochlore iridate Nd2Ir2O7. We observe the onset of strongly hysteretic
behavior in the temperature dependent magnetization below 120 K, and an abrupt
increase in the temperature dependent resistivity below 8 K. Zero field muon
spin relaxation measurements show that the hysteretic magnetization is driven
by a transition to a magnetically disordered state, and that below 8 K a
complex magnetically ordered ground state sets in, as evidenced by the onset of
heavily damped spontaneous muon precession. Our measurements point toward the
absence of a true metal-to-insulator phase transition in this material and
suggest that Nd2Ir2O7 lies either within or on the metallic side of the
boundary of the Dirac semimetal regime within its topological phase diagram.Comment: 21 pages, 7 figure
Incommensurate magnetic ordering in Cu2Te2O5X2 (X=Cl, Br) studied by single crystal neutron diffraction
Polarized and unpolarized neutron diffraction studies have been carried out
on single crystals of the coupled spin tetrahedra systems Cu2Te2O5X2 (X=Cl,
Br). A model of the magnetic structure associated with the propagation vectors
k'Cl ~ -0.150,0.422,1/2 and k'Br ~ -0.172,0.356,1/2 and stable below TN=18 K
for X=Cl and TN=11 K for X=Br is proposed. A feature of the model, common to
both the bromide and chloride, is a canted coplanar motif for the 4 Cu2+ spins
on each tetrahedron which rotates on a helix from cell to cell following the
propagation vector. The Cu2+magnetic moment determined for X=Br, 0.395(5)muB,
is significantly less than for X=Cl, 0.88(1)muB at 2K. The magnetic structure
of the chloride associated with the wave-vector k' differs from that determined
previously for the wave vector k~0.150,0.422,1/2 [O. Zaharko et.al. Phys. Rev.
Lett. 93, 217206 (2004)]
Muon spin rotation and neutron scattering study of the non-centrosymmetric tetragonal compound CeAuAl3
We have investigated the non-centrosymmetric tetragonal heavy-fermion
compound CeAuAl3 using muon spin rotation (muSR), neutron diffraction (ND) and
inelastic neutron scattering (INS) measurements. We have also revisited the
magnetic, transport and thermal properties. The magnetic susceptibility reveals
an antiferromagnetic transition at 1.1 K with a possibility of another magnetic
transition near 0.18 K. The heat capacity shows a sharp lambda-type anomaly at
1.1 K in zero-filed, which broadens and moves to higher temperature in applied
magnetic field. Our zero-field muSR and ND measurements confirm the existence
of a long-range magnetic ground state below 1.2 K. Further the ND study reveals
an incommensurate magnetic ordering with a magnetic propagation vector k = (0,
0, 0.52) and a spiral structure of Ce moments coupled ferromagnetically within
the ab-plane. Our INS study reveals the presence of two well-defined crystal
electric field (CEF) excitations at 5.1 meV and 24.6 meV in the paramagnetic
phase of CeAuAl3 which can be explained on the basis of the CEF theory.
Furthermore, low energy quasi-elastic excitations show a Gaussian line shape
below 30 K compared to a Lorentzian line shape above 30 K, indicating a
slowdown of spin fluctuation below 30 K. We have estimated a Kondo temperature
of TK=3.5 K from the quasi-elastic linewidth, which is in good agreement with
that estimated from the heat capacity. This study also indicates the absence of
any CEF-phonon coupling unlike that observed in isostructural CeCuAl3. The CEF
parameters, energy level scheme and their wave functions obtained from the
analysis of INS data explain satisfactorily the single crystal susceptibility
in the presence of two-ion anisotropic exchange interaction in CeAuAl3.Comment: 28 pages and 17 figure
Spontaneous magnetization above TC in polycrystalline La0.7 Ca0.3 MnO3 and La0.7 Ba0.3 MnO3
In the present work, spontaneous magnetization is observed in the inverse magnetic susceptibility of La0.7Ca0.3MnO3 and La0.7Ba0.3MnO3 compounds above TC up to a temperature T*. From information gathered from neutron diffraction, dilatometry, and high-field magnetization data, we suggest that T* is related to the transition temperature of the low-temperature (high magnetic field) magnetic phase. In the temperature region between T* and TC, the application of a magnetic field drives the system from the high-temperature to low-temperature magnetic phases, the latter possessing a higher magnetization. © 2014 American Physical Society
Incommensurate magnetism in the coupled spin tetrahedra system Cu₂Te₂O₅Cl₂
Neutron scattering studies on powder and single crystals have provided new evidences for unconventional
magnetism in Cu₂Te₂O₅Cl₂. The compound is built from tetrahedral clusters of S = 1/2
Cu²⁺ spins located on a tetragonal lattice. Magnetic ordering, emerging at TN = 18.2 K, leads to a
very complex multi-domain, most likely degenerate, ground state, which is characterized by an incommensurate
(ICM) wave vector k ~ [0.15, 0.42, 1/2]. The Cu²⁺ ions carry a magnetic moment
of 0.67(1) μB/Cu²⁺ at 1.5 K and form a four helices spin arrangement with two canted pairs
within the tetrahedra. A domain redistribution is observed when a magnetic field is applied in the
tetragonal plane (Hc ≈ 0.5 T), but not for H||c up to 4 T. The excitation spectrum is characterized
by two well-defined modes, one completely dispersionless at 6 meV, the other strongly dispersing
to a gap of 2 meV. The reason for such complex ground state and spin excitations may be geometrical
frustration of the Cu²⁺ spins within the tetrahedra, intra- and inter-tetrahedral couplings having
similar strengths and strong Dzyaloshinski–Moriya anisotropy. Candidates for the dominant
intra- and inter-tetrahedral interactions are proposed
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