Investigation Of Magnetic Properties Of Perovskite Oxides Nd_1-xSr_xCoO₃ System

Recently, Ln1-xAxCoO3 (Ln=rare earths) system has received attention, due to the rich phenomena of the perovskite cobaltites. In this paper, the rare earth cobaltite Nd1-xSrxCoO3 (0.1≤x≤0.5) samples were prepared using conventional solid state reaction method. The crystal structures of Nd1-xSrxCoO3 have been investigated. Powder X-ray diffraction patterns show that the samples are single. dc magnetization, ac susceptibility, magnetic relaxation and exchange bias of polycrystalline Nd1-xSrxCoO3 are investigated in detail.After doping Sr, a proportional number of Co³⁺ is converted into Co⁴⁺ in the compound. The double-exchange interaction between Co³⁺ and Co⁴⁺ gives rise to ferromagnetic clusters. It has been well known that a spin-disordered interface/ surface layer is usually formed when a FM particle is embedded in a non-FM matrix or the magnetic particle size is small enough. Therefore, it is concluded that the phase-separated state in cobaltites consist of FM clusters and non-FM matrix, and spin glass-like regions. The presence of the typical features of spin/ cluster glass state in x≤0.2 samples was well revealed by ac susceptibility and magnetic relaxation. In contrary to the situation in manganites, spin glass phase and intercluster interactions contribute to the glassy behaviors. For the x=0.3⁰.5 samples, the number and size of FM cluster increase sharply with increasing Sr content, which show ferromagnetic ordering. The magnetization show large ferromagnetic-type magnetizations at high temperature, however, the magnetization exhibit a decrease at low temperature, which comes from the contribution of magnetic Nd³⁺ ions. This behavior has been observed in Nd1-x SrxCoO3 samples, which was revealed to be the result of a spontaneously antiferromagnetic order of the Nd³⁺ ions. Due to the weak Nd-Co magnetic coupling, the decrease is only observed for some ferrimagnetic temperature. We note that according to the short-range-order ferromagnetic model the hysteresis loops should not saturate due to the finite number of FM clusters in the non-FM matrix. It should be pointed out that the coercive field for these samples is large, which is driven by a higher magnetic anisotropy induced through the L-S coupling of the Nd³⁺ ion.In this context, Nd1-xSrxCoO3 has been found to exhibit a strong tendency toward magnetic phase separation. We present the rich magnetic phase diagram of Nd1-xSrxCoO3. At lower Sr doping (x≤0.2) the system exhibits SG/CG behavior, while at higher Sr doping the system presents ferromagnetic transition.Moreover, the exchange bias effect has been observed for the first time when we measured hysteresis loops of the lower doping samples with FC processes cooling. The horizontal and vertical shift occurs when the samples were cooled down in the external magnetic field through freezing temperature. Furthermore, unlike conventional exchange bias phenomenon, exchange bias in Nd1-xSrxCoO3 is strongly dependent on magnetic field. The influence of applied field on relative proportion of coexisting phases may be responsible for this phenomenon.