The Structural and Magnetic Properties of NiO/Mn(Ni)-Oxide Magnetic Heterostructured Nanocrystals Synthesized Under Varying pH Values

The role of pH on the structural, morphological, and magnetic properties of hydrothermally synthesized NiO-based magnetic heterostructured nanocrystals (MHNCs) is investigated. The NiO nanocrystals were synthesized using a two-step thermal decomposition process whereas the deposition of the surrounding Mn-based phases was accomplished by hydrothermal means at pH values ranging from 2.4 to 7.0. The resulting heterostructured nanocrystals consist of inverted AFM-FiM NiO/±MnxNi1-xO/±Mn3O4 bimagnetic/trimagnetic systems. A complete characterization of the MHNCs was carried using XRD, TEM, EDS, MPMS magnetometry and XPS analysis. Structural investigations revealed predominantly faceted MHNCs ranging in size from 24-30 nm with expitaxially grown MnxNi1-xO overlayers and/or Mn3O4 decorated nanoislands. Magnetic data reveal a significant enhancement in the coercivity, exchange bias and remanent magnetization values with increasing pH values and peaking at a pH of ~5, followed by a substantial decline with further increasing pH. The XRD, TEM and magnetometry data are consistent with predominantly MnxNi1-xO shell formation at lower pH, mixed deposition of MnxNi1-xO shell and Mn3O4 islands at intermediate pH, and desposition of predominantly Mn3O4 islands at higher pH values. Finally, the XPS spectra analysis confirm Mn2+ incorporation at low pH values and an admixture of Mn2+ and Mn2+ at higher pH values in the MHNCs