Structural, electrical and magnetic properties of two-dimensional La_1.2(Sr_1.8-xCa_x)Mn₂O₇ manganites

The effects of structural, electrical and magnetic properties with the isovalent chemical substitution of Ca²⁺ into the Sr²⁺ sites in La_1.2(Sr_1.8-xCa_x)Mn₂O₇ (x = 0-0.8) are investigated. The highest magnetoresistance ratio [ρ(0)/ρ(H)] of 208% (H = 1.5 T) at a temperature of 102 K was observed for the x = 0.4 sample. The Curie temperatures decreased from 135 to 102 K for x = 0-0.4, respectively. Moreover, the Weiss constants θ were varied from the positive to negative value with increasing Ca concentration. The antiferromagnetic behavior with Néel temperature around 30 K was found in the x = 0.8 sample. The magnetization measurements show that the hysteresis phenomenon appeared at the temperatures below the Curie or Néel temperatures.