3,438 research outputs found
Origin of Electric Field Induced Magnetization in Multiferroic HoMnO3
We have performed polarized and unpolarized small angle neutron scattering
experiments on single crystals of HoMnO3 and have found that an increase in
magnetic scattering at low momentum transfers begins upon cooling through
temperatures close to the spin reorientation transition at TSR ~ 40 K. We
attribute the increase to an uncompensated magnetization arising within
antiferromagnetic domain walls. Polarized neutron scattering experiments
performed while applying an electric field show that the field suppresses
magnetic scattering below T ~ 50 K, indicating that the electric field affects
the magnetization via the antiferromagnetic domain walls rather than through a
change to the bulk magnetic order
Evidences of a consolute critical point in the Phase Separation regime of La(5/8-y)Pr(y)Ca(3/8)MnO(3) (y = 0.4) single crystals
We report on DC and pulsed electric field sensitivity of the resistance of
mixed valent Mn oxide based La(5/8-y)Pr(y)Ca(3/8)MnO(3) (y = 0.4) single
crystals as a function of temperature. The low temperature regime of the
resistivity is highly current and voltage dependent. An irreversible transition
from high (HR) to a low resistivity (LR) is obtained upon the increase of the
electric field up to a temperature dependent critical value (V_c). The
current-voltage characteristics in the LR regime as well as the lack of a
variation in the magnetization response when V_c is reached indicate the
formation of a non-single connected filamentary conducting path. The
temperature dependence of V_c indicates the existence of a consolute point
where the conducting and insulating phases produce a critical behavior as a
consequence of their separation.Comment: 5 pages, 6 figures, corresponding author: C. Acha ([email protected]
First-order nature of the ferromagnetic phase transition in (La-Ca)MnO_3 near optimal doping
Neutron scattering has been used to study the nature of the ferromagnetic
transition in single crystals of La_0.7Ca_0.3MnO_3 and La_0.8Ca_0.2MnO_3, and
polycrystalline samples of La_0.67Ca_0.33MnO_3 and La_5/8Ca_3/8MnO_3 where the
naturally occurring O-16 can be replaced with the O-18 isotope. Small angle
neutron scattering on the x=0.3 single crystal reveals a discontinuous change
in the scattering at the Curie temperature for wave vectors below ~0.065 A^-1.
Strong relaxation effects are observed for this domain scattering, for the
magnetic order parameter, and for the quasielastic scattering, demonstrating
that the transition is not continuous in nature. There is a large oxygen
isotope effect observed for the T_C in the polycrystalline samples. For the
optimally doped x=3/8 sample we observed T_C(O-16)=266.5 K and T_C(O-18)=261.5
K at 90% O-18 substitution. The temperature dependence of the spin-wave
stiffness is found to be identical for the two samples despite changes in T_C.
Hence, T_C is not solely determined by the magnetic subsystem, but instead the
ferromagnetic phase is truncated by the formation of polarons which cause an
abrupt transition to the paramagnetic, insulating state. Application of
uniaxial stress in the x=0.3 single crystal sharply enhances the polaron
scattering at room temperature. Measurements of the phonon density-of-states
show only modest differences above and below T_C and between the two different
isotopic samples.Comment: 13 pages, 16 figures, submitted to Phys. Rev.
Equivalent Circuit Modeling of the Dielectric Loaded Microwave Biosensor
This article describes the modeling of biological tissues at microwave frequency using equivalent lumped elements. A microwave biosensor based on microstrip ring resonator (MRR), that has been utilized previously for meat quality evaluation is used for this purpose. For the first time, the ring-resonator loaded with the lossy and high permittivity dielectric material, such as; biological tissue, in a partial overlay configuration is analyzed. The equivalent circuit modeling of the structure is then performed to identify the effect of overlay thickness on the resonance frequency. Finally, the relationship of an overlay thickness with the corresponding RC values of the meat equivalent circuit is established. Simulated, calculated and measured results are then compared for validation. Results are well agreed while the observed discrepancy is in acceptable limit
Relationship between macroscopic physical properties and local distortions of low doping La{1-x}Ca{x}MnO3: an EXAFS study
A temperature-dependent EXAFS investigation of La{1-x}Ca{x}MnO3 is presented
for the concentration range that spans the ferromagnetic-insulator (FMI) to
ferromagnetic-metal (FMM) transition region, x = 0.16-0.22. The samples are
insulating for x = 0.16-0.2 and show a metal/insulator transition for x = 0.22.
All samples are ferromagnetic although the saturation magnetization for the 16%
Ca sample is only ~ 70% of the expected value at 0.4T. We find that the FMI
samples have similar correlations between changes in the local Mn-O distortions
and the magnetization as observed previously for the colossal magnetoresistance
(CMR) samples (0.2 < x < 0.5) - except that the FMI samples never become fully
magnetized. The data show that there are at least two distinct types of
distortions. The initial distortions removed as the insulating sample becomes
magnetized are small and provides direct evidence that roughly 50% of the Mn
sites have a small distortion/site and are magnetized first. The large
remaining Mn-O distortions at low T are attributed to a small fraction of
Jahn-Teller-distorted Mn sites that are either antiferromagnetically ordered or
unmagnetized. Thus the insulating samples are very similar to the behavior of
the CMR samples up to the point at which the M/I transition occurs for the CMR
materials. The lack of metallic conductivity for x <= 0.2, when 50% or more of
the sample is magnetic, implies that there must be preferred magnetized Mn
sites and that such sites do not percolate at these concentrations.Comment: 27 pages, 8 figures, to be submitted to Phys. Rev.
Spin freezing and dynamics in Ca_{3}Co_{2-x}Mn_{x}O_{6} (x ~ 0.95) investigated with implanted muons: disorder in the anisotropic next-nearest neighbor Ising model
We present a muon-spin relaxation investigation of the Ising chain magnet
Ca_{3}Co_{2-x}Mn_{x}O_{6} (x~0.95). We find dynamic spin fluctuations
persisting down to the lowest measured temperature of 1.6 K. The previously
observed transition at around T ~18 K is interpreted as a subtle change in
dynamics for a minority of the spins coupling to the muon that we interpret as
spins locking into clusters. The dynamics of this fraction of spins freeze
below a temperature T_{SF}~8 K, while a majority of spins continue to
fluctuate. An explanation of the low temperature behavior is suggested in terms
of the predictions of the anisotropic next-nearest-neighbor Ising model.Comment: 4 pages, 2 figure
Raman scattering studies of temperature- and field-induced melting of charge order in (La,Pr,Ca)MnO
We present Raman scattering studies of the structural and magnetic phases
that accompany temperature- and field-dependent melting of charge- and
orbital-order (COO) in La0.5Ca0.5MnO3 and La0.25Pr0.375Ca0.375MnO3. Our results
show that thermal and field-induced COO melting in La0.5Ca0.5MnO3 exhibits
three stages in a heterogeneous melting process associated with a structural
change: a long-range, strongly JT distorted/COO regime; a coexistence regime;
and weakly JT distorted/PM or FM phase. We provide a complete structural phase
diagram of La0.5Ca0.5MnO3 for the temperature and field ranges 6<=T<=170 K and
0<=H<=9 T. We also investigate thermal and field-induced melting in
La0.25Pr0.375Ca0.375MnO3 to elucidate the role of disorder in melting of COO.
We find that while thermal melting of COO in La0.25Pr0.375Ca0.375MnO3 is quite
similar to that in La0.5Ca0.5MnO3, the field-induced transition from the COO
phase to the weakly JT-distorted/FM phase in La0.25Pr0.375Ca0.375MnO3 is very
abrupt, and occurs at significantly lower fields (H~2 T at T~0 K) than in
La0.5Ca0.5MnO3 (H~30 T at T=0 K). Moreover, the critical field H_c increases
with increasing temperature in La0.25Pr0.375Ca0.375MnO3 in contrast to
La0.5Ca0.5MnO3. To explain these differences, we propose that field-induced
melting of COO in La0.25Pr0.375Ca0.375MnO3 is best described as the
field-induced percolation of FM domains, and we suggest that Griffiths phase
physics may be an appropriate theoretical model for describing the unusual
temperature- and field- dependent transitions observed in
La0.25Pr0.375Ca0.375MnO3.Comment: 14 pages, 8 figures, to be published in PR
Structural and Magnetic Characterization of Large Area, Free-Standing Thin Films of Magnetic Ion Intercalated Dichalcogenides Mn0.25TaS2 and Fe0.25TaS2
Free-standing thin films of magnetic ion intercalated transition metal
dichalcogenides are produced using ultramicrotoming techniques. Films of
thicknesses ranging from 30nm to 250nm were achieved and characterized using
transmission electron diffraction and X-ray magnetic circular dichroism.
Diffraction measurements visualize the long range crystallographic ordering of
the intercalated ions, while the dichroism measurements directly assess the
orbital contributions to the total magnetic moment. We thus verify the
unquenched orbital moment in Fe0.25TaS2 and measure the fully quenched orbital
contribution in Mn0.25TaS2. Such films can be used in a wide variety of
ultrafast X-ray and electron techniques that benefit from transmission
geometries, and allow measurements of ultrafast structural, electronic, and
magnetization dynamics in space and time
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
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