Defect ferromagnetism induced by lower valence cation doping: Li-doped SnO2 nanoparticles.

To explore the role of Li in establishing room-temperature ferromagnetism in SnO2, the structural, electronic and magnetic properties of Li-doped SnO2 compounds were studied for different size regimes, from nanoparticles to bulk crystals. Li-doped nanoparticles show ferromagnetic ordering plus a paramagnetic contribution for particle sizes in the range of 16-51 nm, while pure SnO2 and Li-doped compounds below and above this particular size range are diamagnetic. The magnetic moment is larger for compositions where the Li substitutes for Sn than for compositions where Li prevalently occupies interstitial sites. The observed ferromagnetic ordering in Li-doped SnO2 nanoparticles is mainly due to the holes created when Li substitutes at a Sn site. Conversely, Li acts as an electron donor and electrons from Li may combine with holes to decrease ferromagnetism when lithium mainly occupies interstitial sites in the SnO2 lattice.

Defect ferromagnetism in SnO2:Zn2+ hierarchical nanostructures: correlation between structural, electronic and magnetic properties.

We report on the ferromagnetism of Sn1-x Zn x O2 (x ≤ 0.1) hierarchical nanostructures with various morphologies synthesized by a solvothermal route. A room temperature ferromagnetic and paramagnetic response was observed for all compositions, with a maximum in ferromagnetism for x = 0.04. The ferromagnetic behaviour was found to correlate with the presence of zinc on substitutional Sn sites and with a low oxygen vacancy concentration in the samples. The morphology of the nanostructures varied with zinc concentration. The strongest ferromagnetic response was observed in nanostructures with well-formed shapes, having nanoneedles on their surfaces. These nanoneedles consist of (110) and (101) planes, which are understood to be important in stabilizing the ferromagnetic defects. At higher zinc concentration the nanostructures become eroded and agglomerated, a phenomenon accompanied with a strong decrease in their ferromagnetic response. The observed trends are explained in the light of recent computational studies that discuss the relative stability of ferromagnetic defects on various surfaces and the role of oxygen vacancies in degrading ferromagnetism via an increase in free electron concentration.

A neutron diffraction study of structural and magnetic transformations in AFeO(2) (A = K, Rb and Cs).

In continuation of our recent x-ray study of the structural phase transitions in the AFeO(2) (A = K, Rb, Cs) family, we have systematically investigated the respective structural and magnetic phase transitions by neutron powder diffraction. While the temperatures of the first-order structural phase transitions are strongly different for the three compounds (~1003, ~737 and ~350 K for A = K, Rb, Cs) and systematically decrease with increasing ionic radius of the A-cation, the magnetic transition temperatures in all three compounds have been found to be almost the same-slightly above 1000 K. The magnetic ordering type is similar in all three compounds-antiferromagnetic ordering of magnetic Fe(3 + ) ions within the system of the three-dimensional Fe-O-Fe linkages such that the Fe-Fe exchange between the nearest neighboring ions is always antiferromagnetic. The directions of magnetic Fe moments were found to be parallel to the crystallographic axis c in RbFeO(2) and CsFeO(2) and parallel to the axis b in KFeO(2) in notations of their low-temperature orthorhombic modifications.