Electrical and magnetic properties of Y-doped La_0.5Sr_0.5MnO₃ manganite system: Observation of step-like magnetization

The magnetic and transport properties of bulk doped manganite, La_0.5-yY_ySr_0.5MnO₃ have been studied as a function of (Y) yttrium doping, y, for 0 ≤ y ≤ 0.25. In the absence of the yttrium this is a well studied half-doped manganite which is a metallic ferromagnet. With successive doping of Y the system undergoes a transformation from rhombohedral (space group R3¯-C) to orthorhombically (space group Pnma) distorted perovskite structure, due to the large distortions caused by the yttrium; this is seen directly from the softening of the Mn-O stretching mode (around 600 cm^-1) as a function of Y³⁺ doping which has relatively lower ionic radius compared to La³⁺ and Sr²⁺. These distortions make this series of compounds particularly interesting. The magnetization obtained in zero field cooled (ZFC) and field cooled (FC) studies show dramatic changes as the value of y is raised above 0.05. This is accompanied by a transition from the metallic and into a semiconducting state as is seen from the studies of the electrical resistivity. The magnetic behavior is further investigated by a study of the hysteresis loops, frequency dependent susceptibility and studies of the magnetic relaxation. The results are understood in terms of the average size of the cation on the A site, (r_A), and the variance, σ² (=∑y_i²r_i² - (r_A)²). The value of the variance, σ², increases rapidly with doping, y, induces weakening of the long range ferromagnetic ordering, exhibiting a stepwise variation of magnetization (metamagnetic like). Based on the present results a magnetic phase diagram has been drawn demonstrating different magnetic phase sequences with the variation of σ².