36 research outputs found
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
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
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
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
Quantized spin waves in the metallic state of magnetoresistive manganites
High resolution spin waves measurements have been carried out in
ferromagnetic (F) La(1-x)(Sr,Ca)xMnO3 with x(Sr)=0.15, 0.175, 0.2, 0.3 and
x(Ca)=0.3. In all q-directions, close to the zone boundary, the spin wave
spectra consist of several energy levels, with the same values in the metallic
and the x\approx 1/8 ranges. Mainly the intensity varies, jumping from the
lower energy levels determined in the x\approx 1/8 range to the higher energy
ones observed in the metallic state. On the basis of a quantitative agreement
found for x(Sr)=0.15 in a model of ordered 2D clusters, the spin wave anomalies
of the metallic state can be interpreted in terms of quantized spin waves
within the same 2D clusters, embedded in a 3D matrix.Comment: 4 pages, 5 figure
Phase diagram and magnetic properties of LaCaMnO compound for
In this article a detailed study of LaCaMnO () phase diagram using powder x-ray diffraction and magnetization
measurements is presented. Unfortunately, in the related literature no properly
characterized samples have been used, with consequence the smearing of the real
physics in this complicated system. As the present results reveal, there are
two families of samples. The first family concerns samples prepared in
atmosphere ( Atm) which are all ferromagnetic with Curie
temperature rising with . The second family concerns samples, where a post
annealing in nearly zero oxygen partial pressure is applied. These samples show
a canted antiferromagnetic structure for below , while
for an unconventional ferromagnetic insulated phase is
present below . The most important difference between nonstoichiometric
and stoichiometric samples concerning the magnetic behavior, is the anisotropy
in the exchange interactions, in the stoichiometric samples putting forward the
idea that a new orbital ordered phase is responsible for the ferromagnetic
insulating regime in the LaCaMnO compound
Temperature Dependence of Low-Lying Electronic Excitations of LaMnO_3
We report on the optical properties of undoped single crystal LaMnO_3, the
parent compound of the colossal magneto-resistive manganites. Near-Normal
incidence reflectance measurements are reported in the frequency range of
20-50,000 cm-1 and in the temperature range 10-300 K. The optical conductivity,
s_1(w), is derived by performing a Kramers-Kronig analysis of the reflectance
data. The far-infrared spectrum of s_1(w) displays the infrared active optical
phonons. We observe a shift of several of the phonon to high frequencies as the
temperature is lowered through the Neel temperature of the sample (T_N = 137
K). The high-frequency s_1(w) is characterized by the onset of absorption near
1.5 eV. This energy has been identified as the threshold for optical
transitions across the Jahn-Teller split e_g levels. The spectral weight of
this feature increases in the low-temperature state. This implies a transfer of
spectral weight from the UV to the visible associated with the paramagnetic to
antiferromagnetic state. We discuss the results in terms of the double exchange
processes that affect the optical processes in this magnetic material.Comment: 7 pages, 5 figure