Tuning Exchange Coupling in NiO-Based Bimagnetic Heterostructured Nanocrystals

Abstract

A series of bimagnetic heterostructured nanocrystals having an antiferromagnetic NiO core and a ferrimagnetic Mn x Ni1–x O 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 Mn x Ni1–x O 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/Mn x Ni1–x O core–shell-like heterostructures at the pH of 2.4, mixed NiO/Mn x Ni1–x O 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 Mn x Ni1–x O phase has ferrimagnetic properties with a stable spin orientation along the ⟨111⟩ orientation. Furthermore, the calculations show that the magnetic anisotropy constant (K 1) of the Mn3O4 phase is considerably larger than that of the Mn x Ni1–x O 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 Mn x Ni1–x O 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.

Magnetic Coupling in Cobalt-Doped Iron Oxide Core–Shell Nanoparticles: Exchange Pinning through Epitaxial Alignment

“…Similarly, a drastic increase of the Néel temperature of FeO even beyond room temperature was observed for a nanoscale layer magnetically coupled to adjacent ferromagnetic Fe layers . Very recently, ferrimagnetic properties were indicated by DFT-based calculations for a mixed Mn x Ni 1– x O phase in the vicinity of NiO . A similar scenario might be attributed to our nanoscale rock-salt nanoparticle core surrounded by a ferrimagnetic spinel shell, supporting an ordering temperature above the ambient temperature of our magnetic SANS experiment.…”

“…79 Very recently, ferrimagnetic properties were indicated by DFT-based calculations for a mixed Mn x Ni 1−x O phase in the vicinity of NiO. 80 A similar scenario might be attributed to our nanoscale rock-salt nanoparticle core surrounded by a ferrimagnetic spinel shell, supporting an ordering temperature above the ambient temperature of our magnetic SANS experiment. In the antiferromagnetic phase of the binary compounds, rhombohedral (FeO) and tetragonal (CoO) crystallographic distortions have been observed that can serve as signatures for magnetic ordering.…”

Defects and Surface Chemistry of Novel PH-Tunable NiO-Mn3O4 ± MnxNi1-xO Heterostructured Nanocrystals as Determined Using X-ray Photoemission Spectroscopy

“…During hydrothermal synthesis, Mn 2+ and Mn 3+ ions are adsorbed on the NiO surface, competing with the highly reactive OH − ions present in the aqueous solutions. The adsorption of the metal ions on the NiO surface is controlled via the protonation and deprotonation processes, and thereby pH, as described in more detail in our previous work [26]. Accordingly, pH has a direct affect on the structural composition of the overgrowth phase and defect properties of the core/overgrowth interface [27][28][29].…”

“…We have been able to manipulate such effects in our previous work on AFM/FiM coupled Cr2O3/MxCr2-xO3 core-shell nanoparticles [20][21][22][23][24] and (Mn3O4 and/or MnxNi1-xO)/NiO bimagnetic HNCs [25]. Our previous study demonstrated that overgrowth of Mn3O4 and/or MnxNi1-xO phases and the resulting magnetic properties are tunable using pH in the hydrothermal synthesis of (Mn3O4 and/or MnxNi1-xO)/NiO HNCs [26]. In particular, it was shown that a substantially large coercivity and exchange bias is obtainable for the Mn-NiO-based HNCs upon tuning the pH value to about 5 in the second step hydrothermal synthesis process [26].…”

“…Our previous study demonstrated that overgrowth of Mn3O4 and/or MnxNi1-xO phases and the resulting magnetic properties are tunable using pH in the hydrothermal synthesis of (Mn3O4 and/or MnxNi1-xO)/NiO HNCs [26]. In particular, it was shown that a substantially large coercivity and exchange bias is obtainable for the Mn-NiO-based HNCs upon tuning the pH value to about 5 in the second step hydrothermal synthesis process [26]. During hydrothermal synthesis, Mn 2+ and Mn 3+ ions are adsorbed on the NiO surface, competing with the highly reactive OH − ions present in the aqueous solutions.…”