Variation of structural and magnetic properties of mixed-valent manganites through A-site cationic ordering

Abstract

Effect of A-site cationic ordering on structural, transport and magnetic properties of mixed-valent perovskite manganite NdBaMn2O6 has been studied. Three samples of same chemical composition NdBaMn2O6 but with different degrees of A-site ordering (92 %, 60 %, 28 %) were prepared by tuning the synthesis conditions. At higher temperatures, all the compounds possess the same crystallographic phase (P4/mmm). However, temperature (T) dependent X-ray diffraction data reveals that different types of crystallographic phase transitions depends on the degree of ordering of the A-site cations Nd3+ and Ba2+. For example, the maximum ordered compound undergoes multiple structural phase transitions (from P4/mmm to Cmmm to P21am) with decreasing T, whereas the disordered compound retains the high-T crystal structure (P4/mmm) throughout the studied temperature range. Resistivity measurements shows that although both the compounds (ordered and disordered) are insulating, the mechanism of charge transport is different for the two compounds. Similarly the magnetic properties and colossal magnetoresistance also depend on the degree of ordering. Cationic disorder suppresses the long-range antiferromagnetic order and enhances ferromagnetic correlations at lower temperature; which (ferromagnetic correlation) is accompanied with larger value of saturation magnetic moment while ferromagnetic TC too shifts to higher temperature with increasing disorder. Disordered system also exhibits a larger value of magnetoresistance at low temperature compared to the ordered counterpart.