The The second maximum is obtained near TN

The physical quantities that measure the
magnetocaloric potential are the magnetic entropy

change DSm and the adiabatic
temperature change DTad. The former can be
evaluated by using

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the measurement of initial isothermal
magnetization versus magnetic field at several temperatures, while the latter
is measured by the adiabatic change of temperature under the application of a
magnetic field. We have used the first method. It has been shown that the
change in magnetic entropy (DSm) with temperature (T) and field (H) is sensitive to the magnetic state
of the system 53-54. According to the convention adapted in MCE
analysis, The
sign of (DSm) (H, T) depends on the type of magnetic interaction dominating within a
field and temperature range, the value of ?DSm is
positive for materials exhibiting a FM transition, because of the fully
magnetically ordered configuration with the application of an external magnetic
field 36. The value of ?DSm is negative for antiferromagnetic state 55.

?Sm
as a function of temperature,
at various magnetic field, has been numerically calculated by

using the equation where Mi
and Mi+1
are the magnetization values at Ti
and Ti+1
temperatures respectively under a magnetic applied field µ0H. The results of the calculation are presented in Fig. 8 which shows a remarkable difference between tS-S and GNP
samples .

Two maximums for La0.5Ca0.5Mn0.95V0.05O3
sample were observed. The first one, in the vicinity of TC = 208 K
and it has a negative value. The second maximum is obtained near TN =
150 K and it has positive value. The negative values of ?DSM are associated with
antifermagnetic ordering in systems due to orientational disorder of the
magnetic sublattice antiparallel to the applied magnetic field .

We notice that the peak value of DSM increases with the increase of magnetic field from 0 T

to 5 T. A positive maximum is observed
near the Curie temperature TC = 59 K, which is related to FM–PM phase transition. A second negative maximum
appears below T= 47 K, associated to the transition seen in the curve dM/dT
versus temperature, and becomes clearly visible by increasing the magnetic
field.

The reached value for an applied magnetic
field of 5 T is found to be about 0.82 Jkg-1K-1for GNP.
It is observed that it is 3 times less than that obtained for S-S sample.