32 research outputs found
Transition from Mixed-Valence to Trivalent Cerium State in Ce(Ni,Cu)Al Series
In this work we present the study of the part of Ce(Ni,Cu)Al series from the pure CeNiAl to 30% of copper concentration, which illustrates the transition from mixed-valence state of CeNiAl to the trivalent state in CeCuAl. The work is based on X-ray diffraction, magnetization and specific heat measurement. The results indicate smooth transition between the mixed-valence and trivalent cerium state. The specific-heat data reveal increase of the Sommerfeld γ coefficient with copper concentration
Metal-insulator transition and the Pr/Pr valence shift in (PrY)CaCoO
The magnetic, electric and thermal properties of the
(Y)CaCoO perovskites (~=~Pr, Nd) were
investigated down to very low temperatures. The main attention was given to a
peculiar metal-insulator transition, which is observed in the praseodymium
based samples with and 0.15 at and 132~K, respectively.
The study suggests that the transition, reported originally in
PrCaCoO, is not due to a mere change of cobalt ions from
the intermediate- to the low-spin states, but is associated also with a
significant electron transfer between Pr and Co/Co sites,
so that the praseodymium ions occur below in a mixed
Pr/Pr valence. The presence of Pr ions in the insulating
phase of the yttrium doped samples (PrY)CaCoO
is evidenced by Schottky peak originating in Zeeman splitting of the ground
state Kramers doublet. The peak is absent in pure PrCaCoO
in which metallic phase, based solely on non-Kramers Pr ions, is
retained down to the lowest temperature.Comment: 10 figure
Magnetodielectric effect and optic soft mode behaviour in quantum paraelectric EuTiO3 ceramics
Infrared reflectivity and time-domain terahertz transmission spectra of
EuTiO3 ceramics revealed a polar optic phonon at 6 - 300K, whose softening is
fully responsible for the recently observed quantum paraelectric behaviour.
Even if our EuTiO3 ceramics show lower permittivity than the single crystal due
to a reduced density and/or small amount of secondary pyrochlore Eu2Ti2O7
phase, we confirmed the magnetic field dependence of the permittivity, also
slightly smaller than in single crystal. Attempt to reveal the soft phonon
dependence at 1.8K on the magnetic field up to 13T remained below the accuracy
of our infrared reflectivity experiment
Infrared and magnetic characterization of the multiferroic Bi2FeCrO6 thin films in a broad temperature range
Infrared reflectance spectra of an epitaxial Bi2FeCrO6 thin film prepared by
pulsed laser deposition on LaAlO3 substrate were recorded between 10 and 900 K.
No evidence for a phase transition to the paraelectric phase was observed, but
some phonon anomalies were revealed near 600 K. Most of the polar modes exhibit
only a gradual softening, which results in a continuous increase of the static
permittivity on heating. It indicates that the ferroelectric phase transition
should occur somewhere above 900 K. Magnetic measurements performed up to 1000
K, revealed a possible magnetic phase transition between 600 and 800 K, but the
exact critical temperature cannot be determined due to a strong diamagnetic
signal from the substrate. Nevertheless, our experimental data show that the
B-site ordered Bi2FeCrO6 is one of the rare high-temperature multiferroics.Comment: subm. to PR
Heat capacity studies of magnetic phase transition in sodium-rich NaxCoO₂ (0.73 ≤ x ≤ 0.87)
Specific heat measurements in the temperature region from 2 to 50 K in magnetic field up to 10 T, oriented parallel and perpendicularly to the CoO₂ layers were carried out on a series of high-quality single- crystals of NaxCoO₂ (x = 0.73, 0.76, 0.77, 0.78 and 0.87). Surprisingly, sharp lambda type anomaly was observed only for the concentration x = 0.76 at temperature (21.80 ± 0.02) K, for all the remaining doping levels round anomaly in experimental data was visible at temperature ~ 20 K, indicating a smeared magnetic phase transition. While the magnetic field oriented perpendicularly to the CoO₂ layers shifts the temperature of this anomaly to lower values, parallel magnetic field has no influence on it, what indirectly supports the idea of A-type antiferromagnetic ordering in studied systems
Magnetic Phase Transitions in TbNi(Al,In) Compounds
The magnetic phase transitions in compounds were investigated by ac-susceptibility measurements. Our data reveal magnetic ordering with transition temperatures between 40 and 70 K, depending on the In concentration. All the studied compounds exhibit two or more phase transitions accompanied by distinct anomalies in the real and imaginary part of the ac-susceptibility. The paramagnetic Curie temperatures remain positive in the whole series
Transition from Mixed-Valence to Trivalent Cerium State in Ce(Ni,Cu)Al Series
In this work we present the study of the part of Ce(Ni,Cu)Al series from the pure CeNiAl to 30% of copper concentration, which illustrates the transition from mixed-valence state of CeNiAl to the trivalent state in CeCuAl. The work is based on X-ray diffraction, magnetization and specific heat measurement. The results indicate smooth transition between the mixed-valence and trivalent cerium state. The specific-heat data reveal increase of the Sommerfeld γ coefficient with copper concentration
Structural and Magnetic Properties of the Series
Polycrystalline samples of series were studied by X-ray diffraction, magnetization and specific heat. The hexagonal ZrNiAl-type structure is preserved in the whole series. Compounds with x up to 0.4 order antiferromagnetically with the Néel temperatures between 3 and 5 K. The rest of the compounds (x = 0.5-0.9) exhibits a transition into a spin glass state below the freezing temperatures around 4 K. The analysis of the specific heat data reveals a quasi-doublet ground state well separated from higher crystal field levels in the whole series
Magnetic Anisotropy of Single Crystals
Magnetic anisotropy of single crystals grown by the Czochralski method was investigated. The homogeneity range of Si substitution for Co extends up to x = 3.4 in solid solutions. The unit cell volume, Curie temperature, and spontaneous magnetic moment decrease monotonously with increasing Si content. has the easy-plane type of magnetic anisotropy in the ground state, which changes into the easy-axis type by two spin-reorientation transitions of the second-order, the easy-plane-easy-cone at ~680 K and the easy-cone - easy-axis at ~730 K. Upon Si substitution, the observed spin-reorientations shift towards the lower temperatures for ~75 K and ~130 K in ) and vanish for compounds with 1<x≤3.4, which have the uniaxial type of magnetic anisotropy in the whole temperature range of magnetic order