Abstract
A-site-ordered quadruple perovskite manganites, AMn(7)O(12), show many interesting physical phenomena, including orbital and spin modulations, spin-induced multiferroic properties, and competitions between different magnetic ground states. Doping with Cu(2+) can result in colossal magnetoresistance properties, ferrimagnetism, and additional structural modulations producing electric-dipole helicoidal textures. Many previous works have focused on large-concentration doping, reaching ACu(3)Mn(4)O(12) compositions. Small-concentration doping has been investigated in a limited number of systems, e.g., in BiCu(x)Mn(7-x)O(12). In this work, we investigated solid solutions of NdCu(x)Mn(7-x)O(12) with x = 0.1, 0.2, and 0.3, prepared at 6 GPa and 1500 K. Specific heat measurements detected three magnetic transitions at x = 0 (at T(N3) = 9 K, T(N2) = 12 K, and T(N1) = 84 K) and two transitions at x = 0.1 (at T(N2) = 10 K and T(N1) = 78 K), while only one transition was found at x = 0.2 (T(N1) = 72 K) and x = 0.3 (T(N1) = 65 K). Differential scanning calorimetry (DSC) measurements showed sharp and strong peaks near T(OO) = 664 K at x = 0, corresponding to an orbital-order (OO) structural transition from I2/m to Im-3 symmetry. DSC anomalies were significantly broadened and their intensities were significantly reduced at x = 0.1-0.3, and structural transitions were observed near T(OO) = 630 K at x = 0.1, T(OO) = 600 K at x = 0.2, and T(OO) = 570 K at x = 0.3. The x = 0.1 sample clearly showed double-peak features on the DSC curves near T(OO) because of the presence of two close phases. High-resolution synchrotron powder X-ray diffraction studies gave strong evidence that phase separation phenomena took place in the x = 0.1-0.3 samples, where two I2/m phases with an approximate ratio of 1:1 were present (e.g., a = 7.47143 Å, b = 7.36828 Å, c = 7.46210 Å, and β = 90.9929° for one phase and a = 7.46596 Å, b = 7.37257 Å, c = 7.45756 Å, and β = 90.9328° for the second phase at x = 0.3). The Curie-Weiss temperature changed from negative (for x = 0, 0.1, and 0.2) to positive (for x = 0.3). T(OO), T(N1), the Curie-Weiss temperature, and magnetization (at 5 K and 70 kOe) changed almost linearly with x.