Combined effects of hole doping and off-stoichiometry on the structures, transport, and magnetic properties of Sr(2−y)NayFe(1−x)Mo(1+x)O6 (x = 0/5x = y; y = 0, 0.05, 0.1, 0.15, 0.2, and 0.3)

Abstract

A series of double perovskite, Sr_(2−y)Na_yFeMoO₆ (y = 0, 0.05, 0.1, 0.15, 0.2, and 0.3) and Sr_(2−y)Na_yFe_(1−x)Mo_(1+x)O₆ (5x = y; y = 0.05, 0.1, 0.15, 0.2, and 0.3), ceramics were synthesized via a solid-state reaction. The influences of the doped Na (y) and off-stoichiometry (excess Mo: x) content on the structure, magnetization, chemical states, resistivity, magnetoresistance (MR), and the Curie temperature (T_C) were systematically and comparatively investigated. The X-ray diffraction results indicated that the Fe/Mo ordering degree in Sr_(2−y)Na_yFeMoO₆ and Sr_(2−y)Na_yFe_(1−x)Mo_(1+x)O₆ exhibited a slight suppression for y = 0–0.15, whereas a significant decrease was observed for y = 0.2 and 0.3. X-ray photoelectron spectroscopy results confirmed that the chemical states of Fe and Mo cations in all the samples had +3 and +5 valences. With the decreasing y values, it was observed that the saturated magnetization M_s demonstrated a gradual decrease, in good agreement with the suppression of the Fe/Mo ordering degree. This close correlation between the magnetic properties and the Fe/Mo ordering degree indicates that the magnetic behaviors are predominantly controlled by the ordering degree of Fe/Mo in all the ceramics. The MR behavior in the samples is mainly determined by the grain boundary conditions. Upon comparing Sr_(2−y)Na_yFeMoO₆ with Sr_(2−y)Na_yFe_(1−x)Mo_(1+x)O₆ (5x = y) for the same y value, Sr_(2−y)Na_yFe_(1−x)Mo_(1+x)O₆ (5x = y) showed a decreased grain boundary strength (the resistivity); thus, a diminished MR performance was observed, indicating that introducing excess Mo into the hole-doped Sr₂FeMoO₆ system was disadvantageous for practical applications. Interestingly, the T_C of both Sr_(2−y)Na_yFeMoO₆ and Sr_(2−y)Na_yFe_(1−x)Mo_(1+x)O₆ (5x = y) gradually decreased until y = 0.15, but improved on further increasing the y value. This composition-dependent tendency of T_C cannot be explained only on the basis of the itinerant electron density, which is responsible for the optimized T_C in the electron-doped Sr₂FeMoO₆ systems.