Abstract
In this research, polycrystalline samples of (Nd2-xAx)MnO4 (A = Ca, Sr and Ba) with 1.40 ≦ x ≦ 1.70 for Ca, 1.30 ≦x ≦ 1.70 for Sr and 1.30 ≦ x ≦ 1.50 for Ba were synthesized by a polymeric polyprotic acid precursor method at 1450℃, 2 h. For A = Ca, single phase materials have orthorhombic symmetry with space group of Fmm2. For A = Sr, single phase materials have tetragonal symmetry with space group of I4/mmm. They all have K2NiF4 structure. Increasing x, valence of Mn4+ increases for all of the samples measured by the XANES. Increasing x, the grain size increases for the Ca and Ba series and decreases for the Sr series samples. In the Ca series, (Nd0.5Ca1.5)MnO4 has the largest magnetoresistance ratio {∣[ R (H, T)-R (0, T) ]∣/ R (0, T)} × 100%, which reaches to 69% at 95 K. In the Sr series, (Nd0.4Sr1.6)MnO4 has the largest magnetoresistance ratio, which reaches to 85% at 85 K. In addition, spin glass state probability exists at low temperature for the Sr and Ba series samples.
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