Phase Separation Phenomena in Lightly Cu-Doped A-Site-Ordered Quadruple Perovskite NdMn7O12

A-site-ordered quadruple perovskite manganites, AMn₇O₁₂, show many interesting physical phenomena, including orbital and spin modulations, spin-induced multiferroic properties, and competitions between different magnetic ground states. Doping with Cu²⁺ 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 ACu₃Mn₄O₁₂ compositions. Small-concentration doping has been investigated in a limited number of systems, e.g., in BiCu<i>_x</i>Mn_7-<i>x</i>O₁₂. In this work, we investigated solid solutions of NdCu<i>_x</i>Mn_7-<i>x</i>O₁₂ with <i>x</i> = 0.1, 0.2, and 0.3, prepared at 6 GPa and 1500 K. Specific heat measurements detected three magnetic transitions at <i>x</i> = 0 (at <i>T</i>_N3 = 9 K, <i>T</i>_N2 = 12 K, and <i>T</i>_N1 = 84 K) and two transitions at <i>x</i> = 0.1 (at <i>T</i>_N2 = 10 K and <i>T</i>_N1 = 78 K), while only one transition was found at <i>x</i> = 0.2 (<i>T</i>_N1 = 72 K) and <i>x</i> = 0.3 (<i>T</i>_N1 = 65 K). Differential scanning calorimetry (DSC) measurements showed sharp and strong peaks near <i>T</i>_OO = 664 K at <i>x</i> = 0, corresponding to an orbital-order (OO) structural transition from <i>I</i>2/<i>m</i> to <i>Im</i>-3 symmetry. DSC anomalies were significantly broadened and their intensities were significantly reduced at <i>x</i> = 0.1-0.3, and structural transitions were observed near <i>T</i>_OO = 630 K at <i>x</i> = 0.1, <i>T</i>_OO = 600 K at <i>x</i> = 0.2, and <i>T</i>_OO = 570 K at <i>x</i> = 0.3. The <i>x</i> = 0.1 sample clearly showed double-peak features on the DSC curves near <i>T</i>_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 <i>x</i> = 0.1-0.3 samples, where two <i>I</i>2/<i>m</i> phases with an approximate ratio of 1:1 were present (e.g., <i>a</i> = 7.47143 Å, <i>b</i> = 7.36828 Å, <i>c</i> = 7.46210 Å, and <i>β</i> = 90.9929° for one phase and <i>a</i> = 7.46596 Å, <i>b</i> = 7.37257 Å, <i>c</i> = 7.45756 Å, and <i>β</i> = 90.9328° for the second phase at <i>x</i> = 0.3). The Curie-Weiss temperature changed from negative (for <i>x</i> = 0, 0.1, and 0.2) to positive (for <i>x</i> = 0.3). <i>T</i>_OO, <i>T</i>_N1, the Curie-Weiss temperature, and magnetization (at 5 K and 70 kOe) changed almost linearly with <i>x</i>.