Tuning Exchange Coupling in NiO-Based Bimagnetic Heterostructured Nanocrystals

A series of bimagnetic heterostructured nanocrystals having an antiferromagnetic NiO core and a ferrimagnetic MnxNi1-xO and/or FiM Mn3O4 island nanophase overgrowth has been synthesized under varying aqueous solution pH conditions. The two-step self-assembly process employs a thermal decomposition method to synthesize NiO nanoparticles, followed by growth of the MnxNi1-xO and/or Mn3O4 nanophase over the NiO core using hydrothermal synthesis at pH values ranging from 2.4-7.0. The environmentally benign hydrothermal process involves pH control of the protonation vs hydroxylation reactions occurring at the nanoparticle surface. TEM analysis and Rietveld refinement of XRD data show that three distinct types of heterostructured nanocrystals occur: NiO/MnxNi1-xO core-shell-like heterostructures at the pH of 2.4, mixed NiO/MnxNi1-xO and/or/Mn3O4 core-overgrowth structures for 2.4 < pH < 4.5, and predominantly NiO/Mn3O4 core-island structures for pH > 4.5. The magnetic coercivity and exchange bias of the heterostructured nanocrystals vary systematically with the pH of the aqueous solution used to synthesize the samples. The temperature-dependent magnetization and hysteresis loop data are consistent with the nature of overlayer coverage of the NiO core. Our DFT based calculations show that the MnxNi1-xO phase has ferrimagnetic properties with a stable spin orientation along the ⟨111⟩ orientation. Furthermore, the calculations show that the magnetic anisotropy constant (K1) of the Mn3O4 phase is considerably larger than that of the MnxNi1-xO phase, which is confirmed by our experimental results. The coercivity and exchange bias field are the largest for the NiO/Mn3O4 core-island nanocrystals, synthesized at a pH value of 5.0, with robust values of nearly 6 kOe and 3 kOe, respectively. This work demonstrates the tunability of hydrothermal deposition, and concomitant magnetic coercivity and exchange bias properties, of MnxNi1-xO and/or Mn3O4 nanophase overgrowth over a NiO core with pH, that makes these heterostructured nanocrystals potentially useful for magnetic device, biomedical, and other applications.