Colossal magnetoresistance in manganites as a multicritical phenomenon

The colossal magnetoresistance in manganites AMnO_3 is studied from the viewpoint of multicritical phenomena. To understand the complicated interplay of various phases, we study the Ginzburg-Landau theory in terms of both the mean-field approximation and the renormalization-group analysis to compare with the observed phase diagram. Several novel features, such as the first-order ferromagnetic transition, and the dip in the transition temperature near the multicritical point, can be understood as driven by enhanced fluctuations near the multicritical point. Furthermore, we obtain a universal scaling relation for the H/M-M^2 plot (Arrott plot), which fits rather well with the experimental data, providing the further evidence for the enhanced fluctuation.