Low-field colossal magnetoresistance in bandwidth-controlled manganites

Abstract

We have investigated magnetoresistance (MR) phenomena relevant to the charge ordering (CO), namely, the real space ordering of both Mn3+ and Mn4+, in single crystals of (Nd1−ySmy)1/2Sr1/2MnO3, in which the one-electron bandwidth (W) is systematically controlled by varying the average ionic radius of the A site. The low-field colossal MR is observed for the small-W region of y⩾0.5; e.g., ρ(0)/ρ(H)>103 in a field of 0.4 T at 115 K for the y=0.94 crystal. This is viewed as a first-order insulator-to-metal phase transition induced by a magnetic field, which accompanies a lattice-structural change. In the small-W region, the CO instability accompanying the antiferromagnetic spin correlations subsists even above the ferromagnetic transition temperature Tc. The competition between the ferromagnetic double-exchange and antiferromagnetic CO interactions gives rise to such a lattice-coupled first-order phase transition induced by a relatively low magnetic field.