127 research outputs found
I-V curves and intergranular flux creep activation energy in the magnetic superconductor RuSr2GdCu2O8
A systematic study of I-V characteristic curves for RuSr2GdCu2O8 [Ru-(1212)]
is presented, with magnetic fields up to 3 T and 5 K<T<30 K, in the region of
the superconducting transition. The activation energy E_{a}(H,T) for flux line
depinning was determined by fitting the nonlinear region of the curves using
the flux creep model. E_{a}(H,T) was found to vary linearly with temperature,
while a power-law dependence on the magnetic field was observed up to H=0.1 T,
where an abrupt reduction in its decreasing rate occurs. The extrapolated
value, E_{a}(0,0)=50 meV, is twice the reported value for YBa2Cu3O7, but the
critical current density J_{C}(0,0)=70 A/cm2 is about one order of magnitude
lower. These results are explained as a consequence of the contribution of the
magnetization in the grains to the effective field at the intergranular links
and to a spin-flop transition of the Ru-sub-lattice.Comment: 4 pages, 3 figure
Coupled magnetic and elastic properties in LaPr(CaSr)MnO manganites
We investigate a series of manganese oxides, the
La0.225Pr0.4(Ca1-xSrx)0.375MnO3 system. The x = 0 sample is a prototype
compound for the study of phase separation in manganites, where ferromagnetic
and charge ordered antiferromagnetic phases coexist. Replacing Ca2+ by Sr2+
gradually turns the system into a homogeneous ferromagnet. Our results show
that the material structure plays a major role in the observed magnetic
properties. On cooling, at temperatures below 100 K, a strong contraction of
the lattice is followed by an increase in the magnetization. This is observed
both through thermal expansion and magnetostriction measurements, providing
distinct evidence of magneto-elastic coupling in these phase separated
compounds
Magnetocaloric effect in manganites: metamagnetic transitions for magnetic refrigeration
We present a study of the magnetocaloric effect in La5/8-yPryCa3/8MnO3
(y=0.3) and Pr0.5Ca0.09Sr0.41MnO3 manganites. The low temperature state of both
ystems is the result of a competition between the antiferromagnetic and
ferromagnetic phases. The samples display magnetocaloric effect evidenced in an
adiabatic temperature change during a metamagnetic transition from an
antiferromagnetic to a ferromagnetic phase . As additional features,
La5/8-yPryCa3/8MnO3 exhibits phase separation characterized by the coexistence
of antiferromagnetic and ferromagnetic phases and Pr0.5Ca0.09Sr0.41MnO3
displays inverse magnetocaloric effect in which temperature decreases while
applying an external magnetic field. In both cases, a significant part of the
magnetocaloric effect appears from non-reversible processes. As the traditional
thermodynamic description of the effect usually deals with reversible
transitions, we developed an alternative way to calculate the adiabatic
temperature change in terms of the change of the relative ferromagnetic
fraction induced by magnetic field. To evaluate our model, we performed direct
measurement of the sample's adiabatic temperature change by means of a
differential thermal analysis. An excellent agreement has been obtained between
experimental and calculated data. These results show that metamagnetic
transition in manganites play an important role in the study of magnetic
refrigeration.Comment: Acepted to be published in Applied Physics Letter
Thermodynamic modeling of phase separation in manganites
We present a phenomenological model based on the thermodynamics of the phase
separated state of manganites, accounting for its static and dynamic
properties. Through calorimetric measurements on LaPrCaMnO the low temperature free energies of the coexisting
ferromagnetic and charge ordered phases are evaluated. The phase separated
state is modeled by free energy densities uniformly spread over the sample
volume. The calculations contemplate the out of equilibrium features of the
coexisting phase regime, to allow a comparison between magnetic measurements
and the predictions of the model. A phase diagram including the static and
dynamic properties of the system is constructed, showing the existence of
blocked and unblocked regimes which are characteristics of the phase separated
state in manganites.Comment: 7 pages, 5 figures, Submitted to Phys. Rev.
Correlation between magnetic and transport properties of phase separated LaCaMnO
The effect of low magnetic fields on the magnetic and electrical transport
properties of polycrystalline samples of the phase separated compound
LaCaMnO is studied. The results are interpreted in the
framework of the field induced ferromagnetic fraction enlargement mechanism. A
fraction expansion coefficient af, which relates the ferromagnetic fraction f
with the applied field H, was obtained. A phenomenological model to understand
the enlargement mechanism is worked out.Comment: 3 pages, 3 figures, presented at the Fifth LAW-MMM, to appear in
Physica B, Minor change
- …