M. Venkatesan

Anisotropic ferromagnetism in substituted zinc oxide.

Room-temperature ferromagnetism is observed in (110) oriented ZnO films made from targets containing 5 at. % of Sc, Ti, V, Fe, Co, or Ni, but not Cr, Mn, or Cu ions. There are large moments, 2.6 micro(B) and 0.5 micro(B)/dopant atom for Co- and Ti-containing oxides, respectively. There is also a moment of 0.3 micro(B)/Sc. Magnetization is very anisotropic, with variations of up to a factor of 3 depending on the orientation of the applied field relative to the substrate. Results are interpreted in terms of a spin-split donor impurity-band model, which can account for ferromagnetism in insulating or conducting high-k oxides with concentrations of magnetic ions that lie far below the percolation threshold. Magnetic moments are associated with two-electron defects in the films as well as unpaired electrons of the 3d ions.

Ferromagnetism in Fe-doped SnO2 thin films

Thin films grown by pulsed-laser deposition from targets of Sn0.95Fe0.05O2 are transparent ferromagnets with Curie temperature and spontaneous magnetization of 610 K and 2.2 A m2 kg−1, respectively. The 57Fe Mossbauer spectra show the iron is all high-spin Fe3+ but the films are magnetically inhomogeneous on an atomic scale, with only 23% of the iron ordering magnetically. The net ferromagnetic moment per ordered iron ion, 1.8 μB, is greater than for any simple iron oxide with superexchange interactions. Ferromagnetic coupling of ferric ions via an electron trapped in a bridging oxygen vacancy (F center) is proposed to explain the high Curie temperature.

Half-metallic ferromagnetism: Example of CrO2 (invited)

A broad classification scheme is proposed for half-metallic ferromagnets which embraces the possibilities of itinerant and localized electrons, as well as semimetallic and semiconducting electronic structure. Examples of each type are given. The problems of defining and measuring spin polarization are discussed and some characteristics of half-metals are reviewed with reference to chromium dioxide.

Ferromagnetism in defect-ridden oxides and related materials

The existence of high-temperature ferromagnetism in thin films and nanoparticles of oxides containing small quantities of magnetic dopants remains controversial. Some regard these materials as dilute magnetic semiconductors, while others think they are ferromagnetic only because the magnetic dopants form secondary ferromagnetic impurity phases such as cobalt metal or magnetite. There are also reports in d0 systems and other defective oxides that contain no magnetic ions. Here, we investigate TiO2 (rutile) containing 1–5% of iron cations and find that the room temperature ferromagnetism of films prepared by pulsed-laser deposition is not due to magnetic ordering of the iron. The films are neither dilute magnetic semiconductors nor hosts to an iron-based ferromagnetic impurity phase. A new model is developed for defect-related ferromagnetism, which involves a spin-split defect band populated by charge transfer from a proximate charge reservoir—in the present case a mixture of Fe2+ and Fe3+ ions in the oxide lattice. The phase diagram for the model shows how inhomogeneous Stoner ferromagnetism depends on the total number of electrons Ntot, the Stoner exchange integral I and the defect bandwidth W; the band occupancy is governed by the d–d Coulomb interaction U. There are regions of ferromagnetic metal, half-metal and insulator as well as non-magnetic metal and insulator. A characteristic feature of the high-temperature Stoner magnetism is an anhysteretic magnetization curve, which is practically temperature independent below room temperature. This is related to a wandering ferromagnetic axis, which is determined by local dipole fields. The magnetization is limited by the defect concentration, not by the 3d doping. Only 1–2% of the volume of the films is magnetically ordered.

Charge-transfer ferromagnetism in oxide nanoparticles

A new model is proposed for ferromagnetism associated with defects in the bulk or at the surface of nanoparticles. The basic idea is that a narrow, structured local density of states Ns(E) is associated with the defects, but the Fermi level (which may lie above or below a mobility edge) will not normally coincide with a peak in Ns(E). However, if there is a local charge reservoir, such as a dopant cation coexisting in two different charge states or a charge-transfer complex at the surface, then it may be possible for electron transfer to raise the Fermi level to a peak in the local density of states, leading to Stoner splitting of Ns(E). Spontaneous Stoner ferromagnetism can arise in percolating defect-rich regions, such as the nanoparticle surface. The charge-transfer ferromagnetism model may be applicable to a wide range of nanoparticles and thin films of dilute magnetic oxides have previously been regarded as dilute magnetic semiconductors.

SnO2 doped with Mn, Fe or Co: Room temperature dilute magnetic semiconductors

Room temperature ferromagnetism is found in (Sn1−xMx)O2 (M=Mn, Fe, Co, x=0.05) ceramics where x-ray diffraction confirms the formation of a rutile-structure phase. Room temperature saturation magnetization of 0.2 and 1.8 Am2 kg−1 for (Sn0.95Mn0.05)O2 and (Sn0.95Fe0.05)O2, respectively, corresponds to a moment of 0.11 or 0.95 μB per Mn or Fe atom. The Curie temperatures are 340 and 360 K, respectively. The magnetization cannot be attributed to any identified impurity phase. 57Fe Mossbauer spectra of the Fe-doped SnO2 samples, recorded at room temperature and 16 K, show that about 85% of the iron is in a magnetically ordered high spin Fe3+ state, the remainder being paramagnetic.

High spin polarization in epitaxial films of ferrimagnetic Mn3Ga

Ferrimagnetic Mn

Ferromagnetism of a graphite nodule from the Canyon Diablo meteorite.

{}_{3}

Magnetic, magnetotransport, and optical properties of Al-doped Zn0.95Co0.05O thin films

Ga exhibits a unique combination of low saturation magnetization (

Soft-x-ray spectroscopic investigation of ferromagnetic Co-doped ZnO

{M}_{s}=0.11