Magnetic ordering and magnetoresistance of layered perovskite Nd1+xSr2-xMn2O7

Magnetic and transport properties of layered perovskite Nd1+xSr2-xMn2O7 crystals have been studied. The material with x=0 is insulating and antiferromagnetic with spin-glass-like behavior at low temperature. On increasing x, the antiferromagnetic behavior is suppressed and ferromagnetic behavior appears, and the resistivity decreases. The material with x=0.2 shows two characteristic temperatures, Tc1=230 K and Tc2=80 K. Two-dimensional ferromagnetic correlation within c-plane appears below Tc1 and three-dimensional ordering with easy-axis anisotropy is established below Tc2. The resistivity increases steeply below Tc2 and a magnetic field of 7 T causes a colossal magnetoresistance of 97.5%. On further lowering the temperature, the anisotropic spin-glass state appears. These results are ascribed to the frustration of random competing double-exchange ferromagnetic and super-exchange antiferromagnetic interactions with the anisotropy originating from layered structure