Fe-doped ZnO nanocrystals are successfully synthesized and structurally characterized by using x-ray diffraction and transmission electron microscopy. Magnetization measurements on the same system reveal a ferromagnetic to paramagnetic transition temperature above 450 K with a low-temperature transition from the ferromagnetic to the spin-glass state due to canting of the disordered surface spins in the nanoparticle system. Local magnetic probes like electron paramagnetic resonance and Mössbauer spectroscopy indicate the presence of Fe in both valence states Fe 2+ and Fe 3+ . We argue that the presence of Fe 3+ is due to possible hole doping in the system by cation ͑Zn͒ vacancies. In a subsequent ab initio electronic structure calculation, the effects of defects ͑e.g., O and Zn vacancies͒ on the nature and origin of ferromagnetism are investigated for the Fe-doped ZnO system. Electronic structure calculations suggest hole doping ͑Zn vacancy͒ to be more effective to stabilize ferromagnetism in Fe-doped ZnO and our results are consistent with the experimental signature of hole doping in ferromagnetic Fe-doped ZnO samples.

X-ray diffraction studies on bulk amount of chemically prepared nanocrystalline powder of Zn1−xTMxO (TM=Co, Mn, Fe, and Ni) show that the evolution of secondary phases (Co3O4, Mn3O4, Fe3O4, or NiO) along with the single phase Zn1−xTMxO strongly depend on growth temperature and doping concentration. The highest solubility limits of Co, Mn, Fe, and Ni in ZnO are 30%, 30%, 20%, and 3% (atomic weight), respectively. The magnetization measurement shows that the secondary phase formation reduces the magnetization of single phase Zn1−xTMxO, which may be the important clue that the secondary phase is not responsible for magnetism in Zn1−xTMxO.

A La(2)NiMnO(6) polycrystalline sample prepared by the sol-gel method showed monoclinic crystal structure with the P2(1)/n space group and a saturation magnetization of 4.63 μ(B)/f.u. at 5 K. Impedance spectroscopy results in the temperature range of 10 K < T < 300 K have revealed a distinct conduction process at grains and grain boundaries, where the grains followed the variable range hopping mechanism and the grain boundaries obeyed Arrhenius thermal activation. A negative magnetoresistance of 2.5% was observed at the paramagnetic to ferromagnetic transition, and this became temperature independent below the magnetic ordering. A marginal positive magnetodielectric (MD) effect that followed the dielectric relaxation was observed and its magnitude was found to decrease with increase of the frequency. A systematic study on the magnetic field induced dielectric properties, dc transport and dc bias effect on the dielectric permittivity has revealed the extrinsic origin of the MD effect in the bulk sample of La(2)NiMnO(6).

We probe nuclear and electron spins in a single molecule even beyond the electromagnetic dipole selection rules, at readily accessible magnetic fields (few mT) and temperatures (5 K) by resonant radio-frequency current from a scanning tunneling microscope. We achieve subnanometer spatial resolution combined with single-spin sensitivity, representing a 10 orders of magnitude improvement compared to existing magnetic resonance techniques. We demonstrate the successful resonant spectroscopy of the complete manifold of nuclear and electronic magnetic transitions of up to ΔI(z)=±3 and ΔJ(z)=±12 of single quantum spins in a single molecule. Our method of resonant radio-frequency scanning tunneling spectroscopy offers, atom-by-atom, unprecedented analytical power and spin control with an impact on diverse fields of nanoscience and nanotechnology.

We report magnetization, dielectric and dc transport properties of La(2)NiMnO(6) nanoparticles. Both dc and ac magnetization measurements indicated a metastable magnetic behaviour with random ferromagnetic and antiferromagnetic interactions below 110 K; critical slow-down, memory and rejuvenation properties signify the spin glass nature. The dc resistivity shows a semiconducting nature but the temperature dependent magnetoresistance (MR) shows a peak at the spin glass transition. The colossal dielectric property and its frequency dependence were interpreted using the Maxwell-Wagner (MW) interfacial polarization model. Impedance analysis along with magnetodielectric (MD) and magnetoresistance (MR) indicates that the observed MD originates from the combined effect of MR and MW interfacial polarization.