  The effect of disorder on the critical properties of the ferromagnetic phase
transition in colossal magnetoresistive manganite La0.67Ca0.33MnO3 has been
studied by substituting Ga for Mn. It is found that, upon 10% Ga substitution,
the peak in the specific heat at the Curie point T_C changes drastically and
appears as a small anomaly. Static magnetization data analyzed in the
asymptotic critical region using modified Arrott plots and the Kouvel-Fisher
method give values for the critical exponents beta=0.387(6), gamma=1.362(2),
and delta=4.60(3). The results show that the first-order transition in
La0.67Ca0.33MnO3 becomes continuous by Ga substitution. The critical properties
of the rounded transition in La0.67Ca0.33Mn0.9Ga0.1O3 suggest that the magnetic
subsystem in this mixed-valent perovskite is close to that of a conventional
isotropic ferromagnet belonging to the Heisenberg universality class with
short-range interactions. It is concluded that the first-order magnetic
transition in pure La0.67Ca0.33MnO3 is induced by fluctuations from a competing
mode, which couples to the magnetic subsystem.
