101 research outputs found
Large magnetocapacitance in electronic ferroelectric manganite systems
We have observed a sizable positive magnetocapacitance () in
perovskite PrCaMnO and bilayer
Pr(SrCa)MnO system under 5T magnetic field across
20-100 K below the magnetic transition point T. The magnetodielectric
effect, on the other hand, exhibits a crossover: (a) from positive to negative
for the perovskite system and (b) from negative to positive for the bilayer
system over the same temperature range. The bilayer
Pr(SrCa)MnO system exhibits a sizable anisotropy as
well. We have also noticed the influence of magnetic field on the dielectric
relaxation characteristics of these systems. These systems belong to a class of
improper ferroelectrics and are expected to exhibit charge/orbital order driven
ferroelectric polarization below the transition point T. Large
magnetocapacitance in these systems shows typical multiferroic behavior even
though the ferroelectric polarization is small in comparison to that of other
ferroelectrics.Comment: 6 pages with 5 embedded figures; accepted for publication in J. Appl.
Phy
Confined spin waves reveal an assembly of nanosize domains in ferromagnetic La(1-x)CaxMnO3 (x=0.17,0.2)
We report a study of spin-waves in ferromagnetic LaCaMnO,
at concentrations x=0.17 and x=0.2 very close to the metallic transition
(x=0.225). Below T, in the quasi-metallic state (T=150K), nearly
q-independent energy levels are observed. They are characteristic of standing
spin waves confined into finite-size ferromagnetic domains, defined in {\bf a,
b) plane for x=0.17 and in all q-directions for x=0.2. They allow an estimation
of the domain size, a few lattice spacings, and of the magnetic coupling
constants inside the domains. These constants, anisotropic, are typical of an
orbital-ordered state, allowing to characterize the domains as "hole-poor". The
precursor state of the CMR metallic phase appears, therefore, as an assembly of
small orbital-ordered domains.Comment: 4 pages, 5 figure
Spin dynamics of strongly-doped La_{1-x}Sr_xMnO_3
Cold neutron triple-axis measurements have been used to investigate the
nature of the long-wavelength spin dynamics in strongly-doped
LaSrMnO single crystals with =0.2 and 0.3. Both systems
behave like isotropic ferromagnets at low T, with a gapless ( meV)
quadratic dispersion relation . The values of the spin-wave
stiffness constant are large ( = 166.77 meV for =0.2
and D = 175.87 meV for =0.3), which directly shows that the
electron transfer energy for the band is large. exhibits a power law
behavior as a function of temperature, and appears to collapse as T -> T_C.
Nevertheless, an anomalously strong quasielastic central component develops and
dominates the fluctuation spectrum as T -> T_C. Bragg scattering indicates that
the magnetization near exhibits power law behavior, with for both systems, as expected for a three-dimensional ferromagnet.Comment: 4 pages (RevTex), 3 figures (encapsulated postscript
Electric field driven destabilization of the insulating state in nominally pure LaMnO3
We report an electric field driven destabilization of the insulating state in
nominally pure LaMnO3 single crystal with a moderate field which leads to a
resistive state transition below 300 K. The transition is between the
insulating state in LaMnO3 and a high resistance bad metallic state that has a
temperature independent resistivity. The transition occurs at a threshold field
(Eth) which shows a steep enhancement on cooling. While at lower temperatures
the transition is sharp and involves large change in resistance but it softens
on heating and eventually absent above 280K. When the Mn4+ content is increased
by Sr substitution up to x=0.1, the observed transition though observable in
certain temperature range, softens considerably. The observation has been
explained as bias driven percolation type transition between two coexisting
phases, where the majority phase is a charge and orbitally ordered polaronic
insulating phase and the minority phase is a bad metallic phase. The mobile
fraction f of the bad metallic phase deduced from the experimental data follows
an activated kinetics with the activation energy nearly equal to 200 meV and
the prefactor fo is a strong function of the field that leads to a rapid
enhancement of f on application of field leading to the resistive state
transition. We suggest likely scenarios for such co-existing phases in
nominally pure LaMnO3 that can lead to the bias driven percolation type
transition.Comment: Accepted in JPC
Nonequilibrium 1/f Noise in Low-doped Manganite Single Crystals
1/f noise in current biased La0.82Ca0.18MnO3 crystals has been investigated.
The temperature dependence of the noise follows the resistivity changes with
temperature suggesting that resistivity fluctuations constitute a fixed
fraction of the total resistivity, independently of the dissipation mechanism
and magnetic state of the system. The noise scales as a square of the current
as expected for equilibrium resistivity fluctuations. However, at 77 K at bias
exceeding some threshold, the noise intensity starts to decrease with
increasing bias. The appearance of nonequilibrium noise is interpreted in terms
of bias dependent multi-step indirect tunneling.Comment: 4pages, 3figures,APL accepte
Magnetic and Optical properties of strained films of multiferroic GdMnO3
The effects of strain on a film of mulitferroic GdMnO3 are investigated using
both magnetometry and magneto-optic spectroscopy. Optical spectra, in the
energy range 1.5eV - 3.5eV, were taken in Faraday geometry in an applied
magnetic field and also at remanence. This yielded rich information on the
effects of strain on the spin ordering in these films. Epitaxial films of
GdMnO3 were grown on SrTiO3 and LaAlO3 substrates. The LaAlO3 was twinned and
so produced a highly strained film whereas the strain was less for the film
grown on SrTiO3. The Ne\'el temperatures and coercive fields were measured
using zero field data and hysteresis loops obtained using a SQUID magnetometer.
Optical absorption data agreed with earlier work on bulk materials. The two
well known features in the optical spectrum, the charge transfer transition
between Mn d states at ~2eV and the band edge transition from the oxygen p band
to the d states at ~3eV are observed in the magnetic circular dichroism;
however they behaved very differently both as a function of magnetic field and
temperature. This is interpreted in terms of the magnetic ordering of the Mn
spins.Comment: 9 pages of text including figure
Ferromagnetic domain structure of La0.78Ca0.22MnO3 single crystals
The magneto-optical technique has been employed to observe spontaneous ferromagnetic domain structures in La0.78Ca0.22MnO3 single crystals. The magnetic domain topology was found to be correlated with the intrinsic twin structure of the investigated crystals. With decreasing temperature the regular network of ferromagnetic domains undergoes significant changes resulting in apparent rotation of the domain walls in the temperature range of 70–150 K. The apparent rotation of the domain walls can be understood in terms of the Jahn-Teller deformation of the orthorhombic unit cell, accompanied by additional twinning
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