Magnetic Properties and Charge Ordering in Polycrystalline La1-xCaxMnO3 (x=0.2, 0.5) Manganite

In this work, we report the attractive magnetic properties of La1-xCaxMnO3 manganites with x = 0.2 (LCMO-2) and x = 0.5 (LCMO-5). X-ray diffraction (XRD) measurements show that the prepared samples are single phase and they crystallize in the orthorhombic structure with Pnma space group. Scanning electron microscope (SEM) measurements reveal that the particles of samples are homogeneous and possess the diameter of several microns. Magnetization versus temperature measurements show paramagnetic (PM) to ferromagnetic (FM) transition at T-C approximate to 200 K for LCMO-2 and at T-C approximate to 250 K for LCMO-5. Below T-C, FM nature is observed in LCMO-2. However, LCMO-5 displays the complex magnetic phase transition. Below T-C, LCMO-5 shows a FM to charge-ordered antiferromagnetic (COAFM) transition at CO temperature T-CO and a novel, unexpected FM phase transition at characteristic temperature T* (near 35 K). The FM and COAFM phases coexist in LCMO-5, and the competition of FM and AFM phases lead to the unexpected FM transition at T*. Furthermore, the inverse of susceptibility versus temperature measurements confirm the similar results. Magnetic hysteresis loop measurements suggest the typical FM feature of LCMO-2. However, LCMO-5 exhibits the hysteresis loops with butterfly-type structure at 100 and 150 K, suggesting the presence of COAFM phase. These attractive magnetic properties are explained by the existence of COAFM phase and the competition between FM and COAFM phases.