1,048 research outputs found
Dimensional reduction by pressure in the magnetic framework material CuF(DO)pyz: from spin-wave to spinon excitations
Metal organic magnets have enormous potential to host a variety of electronic
and magnetic phases that originate from a strong interplay between the spin,
orbital and lattice degrees of freedom. We control this interplay in the
quantum magnet CuF(DO)pyz by using high pressure to drive the
system through a structural and magnetic phase transition. Using neutron
scattering, we show that the low pressure state, which hosts a two-dimensional
square lattice with spin-wave excitations and a dominant exchange coupling of
0.89 meV, transforms at high pressure into a one-dimensional spin-chain
hallmarked by a spinon continuum and a reduced exchange interaction of 0.43
meV. This direct microscopic observation of a magnetic dimensional crossover as
a function of pressure opens up new possibilities for studying the evolution of
fractionalised excitations in low dimensional quantum magnets and eventually
pressure-controlled metal--insulator transitions
Structural properties in Sr0.61a0.39Nb2O6 in the temperature range 10 K to 500 K investigated by high-resolution neutron powder diffraction and specific heat measurements
We report high-resolution neutron powder diffraction on Sr0.61Ba0.39Nb2O6,
SBN61, in the temperature range 15-500 K. The results indicate that the
low-temperature anomalies (T<100K) observed in the dielectric dispersion are
due to small changes in the incommensurate modulation of the NbO6-octahedra, as
no structural phase transition of the average structure was observed. This
interpretation is supported by specific heat measurements, which show no latent
heat, but a glass-like behavior at low temperatures. Furthermore we find that
the structural changes connected with the ferroelectric phase transition at Tc
approx. 350K start already at 200K, explaining the anisotropic thermal
expansion in the temperature range 200-300K observed in a recent x-ray
diffraction study.Comment: Accepted by PRB (2006
Unconventional magnetic phase separation in -CoVO
We have explored the magnetism in the non-geometrically frustrated spin-chain
system -CoVO which possesses a complex magnetic exchange
network. Our neutron diffraction patterns at low temperatures (
= 6.6 K) are best described by a model in which two magnetic
phases coexist in a volume ratio 65(1) : 35(1), with each phase consisting of a
single spin modulation. This model fits previous studies and our observations
better than the model proposed by Lenertz in J. Phys. Chem. C 118,
13981 (2014), which consisted of one phase with two spin modulations. By
decreasing the temperature from , the minority phase of our
model undergoes an incommensurate-commensurate lock-in transition at =
5.6 K. Based on these results, we propose that phase separation is an
alternative approach for degeneracy-lifting in frustrated magnets
Observation of vortex-nucleated magnetization reversal in individual ferromagnetic nanotubes
The reversal of a uniform axial magnetization in a ferromagnetic nanotube
(FNT) has been predicted to nucleate and propagate through vortex domains
forming at the ends. In dynamic cantilever magnetometry measurements of
individual FNTs, we identify the entry of these vortices as a function of
applied magnetic field and show that they mark the nucleation of magnetization
reversal. We find that the entry field depends sensitively on the angle between
the end surface of the FNT and the applied field. Micromagnetic simulations
substantiate the experimental results and highlight the importance of the ends
in determining the reversal process. The control over end vortex formation
enabled by our findings is promising for the production of FNTs with tailored
reversal properties.Comment: 20 pages, 13 figure
Neutron scattering study of the field-dependent ground state and the spin dynamics in S=1/2 NH4CuCl3
Elastic and inelastic neutron scattering experiments have been performed on the dimer spin system NH4CuCl3, which shows plateaus in the magnetization curve at m=1/4 and m=3/4 of the saturation value. Two structural phase transitions at T1≈156 K and at T2=70 K lead to a doubling of the crystallographic unit cell along the b direction and as a consequence a segregation into different dimer subsystems. Long-range magnetic ordering is reported below TN=1.3 K. The magnetic field dependence of the excitation spectrum identifies successive quantum phase transitions of the dimer subsystems as the driving mechanism for the unconventional magnetization process in agreement with a recent theoretical model
Magnetoelastic coupling in triangular lattice antiferromagnet CuCrS2
CuCrS2 is a triangular lattice Heisenberg antiferromagnet with a rhombohedral
crystal structure. We report on neutron and synchrotron powder diffraction
results which reveal a monoclinic lattice distortion at the magnetic transition
and verify a magnetoelastic coupling. CuCrS2 is therefore an interesting
material to study the influence of magnetism on the relief of geometrical
frustration.Comment: 6 pages, 6 figures, 1 tabl
Magnetic inversion symmetry breaking and ferroelectricity in TbMnO3
TbMnO3 is an orthorhombic insulator where incommensurate spin order for
temperature T_N < 41K is accompanied by ferroelectric order for T < 28K. To
understand this, we establish the magnetic structure above and below the
ferroelectric transition using neutron diffraction. In the paraelectric phase,
the spin structure is incommensurate and longitudinally-modulated. In the
ferroelectric phase, however, there is a transverse incommensurate spiral. We
show that the spiral breaks spatial inversion symmetry and can account for
magnetoelectricity in TbMnO3.Comment: 4 pages revtex, accepted by Phys. Rev. Lett. on June 21, 200
Magnetic structure and spin dynamics of quasi-one-dimensional spin-chain antiferromagnet BaCo2V2O8
We report a neutron diffraction and muon spin relaxation muSR study of static
and dynamical magnetic properties of BaCo2V2O8, a quasi-one-dimensional
spin-chain system. A proposed model for the antiferromagnetic structure
includes: a propagation vector k_AF = (0, 0, 1), independent of external
magnetic fields for fields below a critical value H_c(T). The ordered moments,
of 2.18 \mu_B per Co ion, are aligned along the crystallographic c-axis. Within
the screw chains, along the c axis, the moments are arranged
antiferromagnetically. In the basal planes the spins are arranged
ferromagnetically (forming zig-zags paths) along one of the axis and
antiferromagnetically along the other. The temperature dependence of the
sub-lattice magnetization is consistent with the expectations of the 3D Ising
model. A similar behavior is observed for the internal static fields at
different muon stopping sites. Muon time spectra measured at weak longitudinal
fields and temperatures much higher than T_N can be well described using a
single muon site with an exponential muon spin relaxation that gradually
changes into an stretched exponential on approaching T_N. The
temperature-induced changes of the relaxation suggest that the Co fluctuations
dramatically slow down and the system becomes less homogeneous as it approaches
the antiferromagnetic state.Comment: 7 pages, 9 figure
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