  A phenomenological mean-field theory is presented to describe the role of
external magnetic field, pressure and chemical substitution on the nature of
ferromagnetic (FM) to paramagnetic (PM) phase transition in manganites. The
application of external field (or pressure) shifts the transition, leading to a
field (or pressure) dependent phase boundary along which a tricritical point is
shown to exist where a first-order FM-PM transition becomes second-order. We
show that the effect of chemical substitution on the FM transition is analogous
to that of external perturbations (magnetic field and pressure); this includes
the existence of a tricritical point at which the order of transition changes.
Our theoretical predictions satisfactorily explain the nature of FM-PM
transition, observed in several systems. The modeling hypothesis has been
critically verified from our experimental data from a wide range of colossal
magnetoresistive manganite single crystals like Sm$_{0.52}$Sr$_{0.48}$MnO$_3$.
The theoretical model prediction of a tricritical point has been validated in
this experiment which provides a major ramification of the strength of the
model proposed.
