507 research outputs found
Long-range Ni/Mn structural order in epitaxial double perovskite La2NiMnO6 thin films
We report and compare the structural, magnetic, and optical properties of
ordered La2NiMnO6 thin films and its disordered LaNi0.5Mn0.5O3 counterpart. An
x-ray diffraction study reveals that the B-site Ni/Mn ordering induces
additional XRD reflections as the crystal symmetry is transformed from a
pseudocubic perovskite unit cell in the disordered phase to a monoclinic form
with larger lattice parameters for the ordered phase. Polarized Raman
spectroscopy studies reveal that the ordered samples are characterized by
additional phonon excitations that are absent in the disordered phase. The
appearance of these additional phonon excitations is interpreted as the
clearest signature of Brillouin zone folding as a result of the long-range
Ni/Mn ordering in La2NiMnO6. Both ordered and disordered materials display a
single ferromagnetic-to-paramagnetic transition. The ordered films display also
a saturation magnetization close to 4.8 mB/f.u. and a transition temperature
(FM-TC) around 270 K, while the disordered ones have only a 3.7 mB/f.u.
saturation magnetization and a FM-TC around 138 K. The differences in their
magnetic behaviours are understood based on the distinct local electronic
configurations of their Ni/Mn cations.Comment: 15 pages, 5 fig
Phase formation, phonon behavior, and magnetic properties of novel ferromagnetic La3BAlMnO9 (B = Co or Ni) triple perovskites
In the quest for novel magnetoelectric materials, we have grown, stabilized
and explored the properties of La3BAlMnO9 (B = Co or Mn) thin films. In this
paper, we report the influence of the growth parameters that promote B/Al/Mn
ordering in the pseudo-cubic unit cell and their likely influence on the
magnetic and multiferroic properties. The temperature dependence of the
magnetization shows that La3CoAlMnO9 is ferromagnetic up to 190 K while
La3NiAlMnO9 shows a TC of 130 K. The behavior of these films are compared and
contrasted with related La2BMnO6 double perovskites. It is observed that the
insertion of AlO6 octahedra between CoO6 and MnO6 suppresses significantly the
strength of the superexchange interaction, spin-phonon and spin-polar coupling.Comment: 13 pages, 3 fig
Ferromagnetism and magneto-dielectric effect in insulating LaBiMn4/3Co2/3O6 thin films
High quality epitaxial thin films of LaBiMn4/3Co2/3O6 perovskite were
fabricated on (001)-oriented SrTiO3 and LaAlO3 substrates by the pulsed laser
deposition technique. Magnetization measurements reveal a strong magnetic
anisotropy and a ferromagnetic behavior that is in agreement with a
super-exchange interaction between Mn4+ and Co2+ ions, which are randomly
distributed in the B-site. A distinct anomaly is observed in the dielectric
measurements at 130K corresponding to the onset of the magnetic ordering,
suggesting a coupling. Above this temperature, the extrinsic Maxwell-Wagner
effect is dominating. Theses results are explained using the Raman
spectroscopic studies indicating a weak spin-lattice interaction around this
magnetic transition.Comment: Submitted to Appl. Phys. Lett. (2008
Investigation of phonon behavior in Pr2NiMnO6 by micro-Raman spectroscopy
The temperature dependence of phonon excitations and the presence of spin
phonon coupling in polycrystalline Pr2NiMnO6 samples were studied using
micro-Raman spectroscopy and magnetometry. Magnetic properties show a single
ferromagnetic-to-paramagnetic transition at 228 K and a saturation
magnetization close to 4.95 \muB/f.u.. Three distinct Raman modes at 657, 642,
and 511 cm-1 are observed. The phonon excitations show a clear hardening due to
anharmonicity from 300 K down to 10 K. Further, temperature dependence of the
657 cm-1 mode shows only a small softening. This reflects the presence of a
relatively weak spin-phonon coupling in Pr2NiMnO6 contrary to other double
perovskites previously studied.Comment: 10 pages, 4 fig
Infrared optical properties of Pr2CuO4
The ab-plane reflectance of a Pr2CuO4 single crystal has been measured over a
wide frequency range at a variety of temperatures, and the optical properties
determined from a Kramers-Kronig analysis. Above ~ 250 K, the low frequency
conductivity increases quickly with temperature; the resistivity follows the
form e^(E_a/k_BT), where E_a ~ 0.17 eV is much less than the inferred optical
gap of ~ 1.2 eV. Transport measurements show that at low temperature the
resistivity deviates from activated behavior and follows the form
e^[(T_0/T)^1/4], indicating that the dc transport in this material is due to
variable-range hopping between localized states in the gap. The four
infrared-active Eu modes dominate the infrared optical properties. Below ~ 200
K, a striking new feature appears near the low-frequency Eu mode, and there is
additional new fine structure at high frequency. A normal coordinate analysis
has been performed and the detailed nature of the zone-center vibrations
determined. Only the low-frequency Eu mode has a significant Pr-Cu interaction.
Several possible mechanisms related to the antiferromagnetism in this material
are proposed to explain the sudden appearance of this and other new spectral
features at low temperature.Comment: 11 pages, 7 embedded EPS figures, REVTeX
Ru–Ag and Ru–Au dicarbene complexes from an abnormal carbene ruthenium system
The first complexes containing an anionic dicarbene connected to two different d-block elements are reported. The synthetic pathway entails metalation and transmetalation reactions, starting from a cationic abnormal Ru-NHC complex
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