Abstract

We have studied the effects of a magnetic field on the magnetism and transport properties of the layered manganites R_1/2Sr_3/2MnO₄ (R = La and Nd) in pulsed magnetic fields up to 40 T. The R = La crystal shows metamagnetic-like transitions above 30 T, concomitantly with a colossal magnetoresistance (CMR) effect as large as [ρ(0) - ρ(H)]/ρ(H) > 10³ with a field of µ&ogr;H = 38 T at low temperatures. These transitions can be ascribed to the field-induced melting of the real-space ordering of the e_g electrons (charge ordering). For the R = Nd crystal, a magnetic field along the c-axis enhances the two-dimensionality in the conductivity. Moreover, we observed metamagnetic-like transitions accompanied by the CMR effects at low temperatures, in spite of the absence of charge ordering.