We report the synthesis of nominal 2 and 5 at.% Mn-doped ZnO nanocrystalline particles by a co-precipitation method. Rietveld refinement of x-ray diffraction data revealed that Mn-doped ZnO crystallizes in the monophasic wurtzite structure and the unit cell volume increases with increasing Mn concentration. DC magnetization measurements showed ferromagnetic ordering above room temperature with Hc∼150 Oe for nominal 2 at.% Mn-doped ZnO nanoparticles annealed at 675 K. A distinct ferromagnetic resonance (FMR) signal was observed in the EPR spectra of the 2 at.% Mn-doped ZnO nanoparticles annealed at 675 K. EPR measurements were used to estimate the number of spins participating in ferromagnetic ordering. Of the total Mn present in the 2 at.% Mn ZnO lattice, 25% of the Mn2+ ions were responsible for ferromagnetic ordering, whereas nearly 5% of the Mn2+ ions remained uncoupled (isolated spins). A well resolved EPR spectrum of 5% Mn-doped ZnO samples annealed at 875–1275 K (g = 2.007, A = 80 G, D = 210 G and E = 15 G) confirmed that Mn was substitutionally incorporated into the ZnO lattice as Mn2+. On increasing the temperature of annealing beyond 1075 K an impurity phase emerges in both the 2 and 5 at.% Mn-doped ZnO samples, which has been identified as a variant of (Zn1−XMn(II)X)Mn(III)2O4 with Tc∼15 K. Our results indicate that the observed room temperature ferromagnetism in Mn-doped ZnO can be attributed to the substitutional incorporation of Mn at Zn-sites rather than due to the formation of any metastable secondary phases.

We investigate the chiral magnetic order in free-standing planar 3d-5d bi-atomic metallic chains (3d: Fe, Co; 5d: Ir, Pt, Au) using first-principles calculations based on density functional theory. We found that the antisymmetric exchange interaction, commonly known as Dzyaloshinskii-Moriya interaction (DMI), contributes significantly to the energetics of the magnetic structure. For the Fe-Pt and Co-Pt chains, the DMI can compete with the isotropic Heisenberg-type exchange interaction and the magneto-crystalline anisotropy energy (MAE), and for both cases a homogeneous left-rotating cycloidal chiral spin-spiral with a wave length of 51Å and 36Å, respectively, were found. The sign of the DMI, that determines the handedness of the magnetic structure changes in the sequence of the 5d atoms Ir(+), Pt(−), Au(+). We used the full-potential linearized augmented plane wave method and performed self-consistent calculations of homogeneous spin spirals, calculating the DMI by treating the effect of spin-orbit interaction (SOI) in the basis of the spin-spiral states in first-order perturbation theory. To gain insight into the DMI results of our ab initio calculations, we develop a minimal tight-binding model of three atoms and 4 orbitals that contains all essential features: the spin-canting between the magnetic 3d atoms, the spin-orbit interaction at the 5d atoms, and the structure inversion asymmetry facilitated by the triangular geometry. We found that spin-canting can lead to spin-orbit active eigenstates that split in energy due to the spin-orbit interaction at the 5d atom. We show that, the sign and strength of the hybridization, the bonding or antibonding character between d-orbitals of the magnetic and non-magnetic sites, the bandwidth and the energy difference between states occupied and unoccupied states of different spin projection determine the sign and strength of the DMI. The key features observed in the trimer model are also found in the first-principles results.

The interfacial engineering of magnetic nanoparticles (MNPs) with specific functional groups or targeting ligands is important for their in vivo applications. We report here the preparation and characterization of bifunctional magnetic nanoparticles (BMNPs) which contain a carboxylic moiety for drug binding and an amine moiety for folate mediated drug targeting. BMNPs were prepared by introducing bioactive cysteine molecules onto the surface of undecenoic acid coated FeO magnetic nanoparticles (UMNPs) via a thiol-ene click reaction and then, folic acid was conjugated with these BMNPs through an EDC-NHS coupling reaction. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis indicate the formation of highly crystalline single-phase FeO nanostructures. The changes in the interfacial characteristics of the nanoparticles and the presence of an organic coating are evident from Fourier transform infrared (FTIR) spectroscopy, dynamic light scattering (DLS), zeta-potential measurement, and thermogravimetric analysis (TGA). These nanocarriers have an average size of 10 nm, and have a pH dependent charge conversional feature and protein resistance characteristic in physiological medium. These nanoparticles also show high loading affinity for an anticancer drug, doxorubicin hydrochloride (DOX) and its pH dependent release. This is highly beneficial for cancer therapy as the relatively low pH in tumors will specifically stimulate the drug release at the site of interest. Furthermore, our fluorescence microscopy and flow cytometry studies confirmed the higher cellular internalization capability of these folic acid conjugated nanoparticles in cancer cells over-expressing folate receptors.