2,447 research outputs found
Neutron diffraction, magnetization and ESR studies of pseudocubic Nd(0.75)Ba(0.25)MnO3 and its unusual critical behavior above Tc
Results of structural neutron diffraction study, magnetization and ESR
measure-ments are presented for insulating Nd0.75Ba0.25MnO3, Tc = 129 K. The
crystal structure is refined in the range 4.2-300 K. The compound is found to
exhibit the Jahn-Teller (JT) transition at 250 K. The field cooled (FC)
magnetization data are in a reasonable agreement with the predictions for a 3D
isotropic ferromagnet above Tc. However, these measurements reveal a difference
between the FC and zero FC data in the paramagnetic region. ESR results are
also in a correspondence with behavior of a cubic ferromagnet above T* = 143 K.
It is shown that an anisotropic exchange coupling of the Mn and Nd magnetic
moments can give a substantial contribution in ESR linewidth masking its
critical enhan-cement. The different temperature treatments of the sample
reveal a temperature hysteresis of the ESR spectra below T* indicating an
anomalous response in the paramagnetic region. The study of phase transition in
this manganite suggests change in its character from the second to first order
at T*. The conventional free energy including the magnetization and magnetic
field is not found to describe the first order transition. This suggests that
the charge, orbital and JT phonon degrees of freedom, in addition to
magnetization, may be the critical variables, the unusual character of the
transition being determined by their coupling. Unconventional critical behavior
is attributed to orbital liquid metallic phase that coexists with the initial
orbital ordered phase below T*.Comment: 18 pages, 5 figures, submitted to Phys. Rev.
Generalized van der Waals theory of liquid-liquid phase transitions
In the framework of the thermodynamic perturbation theory for fluids we study
how the phase diagram of an isotropic repulsive soft-core attractive potential,
where a liquid-liquid phase transition exists in addition to the standard
gas-liquid phase transition, changes by varying the parameters of the
potential. We show that existence of the liquid-liquid transition is determined
by the interplay of the parameters of the potential and the structure of a
reference liquid.Comment: 5 pages, 6 figure
Inversion of Sequence of Diffusion and Density Anomalies in Core-Softened Systems
In this paper we present a simulation study of water-like anomalies in
core-softened system introduced in our previous publications. We investigate
the anomalous regions for a system with the same functional form of the
potential but with different parameters and show that the order of the region
of anomalous diffusion and the region of density anomaly is inverted with
increasing the width of the repulsive shoulder.Comment: 8 pages, 10 figure
Core-Softened System With Attraction: Trajectory Dependence of Anomalous Behavior
In the present article we carry out a molecular dynamics study of the
core-softened system and show that the existence of the water-like anomalies in
this system depends on the trajectory in space along which the
behavior of the system is studied. For example, diffusion and structural
anomalies are visible along isotherms as a function of density, but disappears
along the isochores and isobars as a function of temperature. On the other
hand, the diffusion anomaly may be seen along adiabats as a function of
temperature, density and pressure. It should be noted that it may be no
signature of a particular anomaly along a particular trajectory, but the
anomalous region for that particular anomaly can be defined when all possible
trajectories in the same space are examined (for example, signature of
diffusion anomaly is evident through the crossing of different isochors.
However, there is no signature of diffusion anomaly along a particular
isochor). We also analyze the applicability of the Rosenfeld entropy scaling
relations to this system in the regions with the water-like anomalies. It is
shown that the validity of the Rosenfeld scaling relation for the diffusion
coefficient also depends on the trajectory in the space along which
the kinetic coefficients and the excess entropy are calculated.Comment: 16 pages, 21 figures. arXiv admin note: this contains much of the
content of arXiv:1010.416
Spin-glass instability of short-range spherical ferromagnet
In structurally disordered ferromagnets the weak random dipole-dipole
exchange may transform the polydomain state into a spin-glass one. To some
extent the properties of such phase in disordered isotropic ferromagnet can be
qualitatively described by the spherical model with the short-range
ferromagnetic interaction and weak frustrated infinite-range random-bond
exchange. This model is shown to predict that spin-glass phase substitute the
ferromagnetic one at the arbitrary small disorder strength and that its
thermodynamics has some similarity to that of polydomain state along with some
significant distinctions. In particular, the longitudinal susceptibility at
small fields becomes frozen below transition point at a constant value
depending on the disorder strength, while the third order nonlinear magnetic
susceptibilitiy exhibits the temperature oscillations in small field near the
transition point. The relation of these predictions to the experimental data
for some disordered isotropic ferromagnets is discussed.Comment: 7 pages, 5 figure
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