P. J. van der Zaag

A domain size effect in the magnetic hysteresis of NiZn‐ferrites

The domain size and the ac‐hysteresis of NiZn‐ferrites have been investigated as a function of grain size. It is found that the ac‐hysteresis or energy dissipation at MHz frequencies is substantially reduced in fine‐grained, monodomain NiZn‐ferrites, which is attributed to the absence of intragranular domain walls.

A consistent interpretation of the magneto-optical spectra of spinel type ferrites (invited)

Through a systematic investigation of the complete dielectric tensor, between 0.5 and 5.0 eV, of Fe 3 O 4 and of related spinel ferrites, i.e., MgFe 2 O 4 , Li 0.5 Fe 2.5 O 4 , NiFe 2 O 4 , and CoFe 2 O 4 , we have established that intervalence charge transfer and intersublattice charge transfer transitions dominate the optical and magneto-optical spectrum (between 0.5 and 5.0 eV) of all spinel ferrites of the general composition Me x Fe 3−x O 4 . In all cases examined the same set of intersublattice charge transfer transitions was observed. These are the only transitions observed in the cases where Me is a nonmagnetic ion ( Mg 2+ , Li + ). In the cases where Me is a magnetic ion ( Fe 2+ , Ni 2+ , Co 2+ ) additional intervalence charge transfer transitions are observed. CoFe 2 O 4 is the only spinel ferrite with a major contribution of crystal field transitions to the magneto-optical spectrum. The observed presence of only two intense crystal field transitions in specifically CoFe 2 O 4 is explained. The observed relative strengths of these two transitions in CoFe 2 O 4 , in which remarkably the upper transition at 1.82 eV is more intense than the lower transition at 0.83 eV is also explained in a crystal field analysis.

New views on the dissipation in soft magnetic ferrites

Neutron depolarization experiments show that below a certain grain size, polycrystalline ferrites are composed of mono-domain grains. These mono-domain ferrites exhibit low dissipation at MHz frequencies. This is attributed to the absence of intragranular domain wall movement, i.e. a new microscopic origin for dissipation in ferrites has been identified. The implication of this finding for the domain wall contribution to hysteresis is discussed.

The initial permeability of polycrystalline MnZn ferrites : the influence of domain and microstructure

The validity of nonmagnetic grain‐boundary (NMGB) models for the initial permeability of polycrystalline ferrites is examined. The domain size in a series of wet‐chemically prepared polycrystalline MnZn ferrites, in the demagnetized state, has been determined by neutron depolarization. A transition in the intragranular domain structure from mono‐ to two domain is observed at grain size D≊4 μm. An expression for this transition has been derived for a dense magnetic material. The grain size dependence of the initial permeability of the ferrites studied, particularly for monodomain grains, is consistent with the NMGB model. The grain‐boundary width and composition have been determined with a nanoprobe (1.5 nm resolution). Composition variations extend ≊10 nm into the grain. The findings suggest that, although the NMGB model considers ‘‘nonmagnetic’’ grain boundaries, in practice the grain boundary may be hard magnetic.

Thermally assisted reversal of exchange biasing in NiO and FeMn based systems

The stability of the exchange bias field Heb has been studied for magnetron sputtered NiO/Ni66Co18Fe16 and Ni66Co18Fe16/FeMn bilayers. A forced antiparallel alignment of the ferromagnetic magnetization to Heb results in a gradual decrease of Heb as a function of time for NiO as well as FeMn based samples. The observed decrease of Heb increases with temperature and is interpreted as a thermally assisted reversal of magnetic domains in the antiferromagnetic layer.

FE L2,3 LINEAR AND CIRCULAR MAGNETIC DICHROISM OF FE3O4

Abstract Measurements of circular and linear magnetic dichroism at the Fe 2p edges of Fe 3 O 4 are presented. The dichroism results from the ferrimagnetic ordering of Fe d 5 and d 6 ions in octahedral and tetrahedral sites. Atomic calculations taking into account the different crystal fields have been fitted to all L -edge spectra of this compound. A chemical shift of 1.5 eV between the dominant L 3 peak of the octahedral Fe 2+ and Fe 3+ ions gives the best fit of linear and circular dichroism. We investigate whether these parameters can be transferred to predict the Fe 2p dichroism in other ferrites.

A study of the magnitude of exchange biasing in [111] Fe3O4/CoO bilayers

Abstract Exchange biasing has been studied for a series of [111]-oriented Fe 3 O 4 /CoO bilayers with constant Fe 3 O 4 and varying CoO thicknesses. The magnitude of exchange biasing in this oxidic system is compared with the value calculated under the assumption of nearest-neighbour exchange at a flat and magnetically uncompensated interface.

Exchange biasing in MBE-grown Ni80Fe20/Fe50Mn50 bilayers

The exchange biasing field (Heb) and coercive field (Hc) of molecular-beam-epitaxy (MBE) grown Cu/Ni80Fe20/Fe50Mn50 samples in [111], [001] and [110] orientations were investigated by Kerr effect measurements. A strong dependence of Heb and Hc on the growth orientation is observed. A strong uniaxial in-plane anisotropy introduced by the Fe50Mn50 layer was found for the [110]-oriented sample. Conversion electron Mossbauer studies revealed a roughness of 2–3 A and no significant moment reduction at the Ni80Fe20/Fe50Mn50 interface.

Explanation for the leakage current in polycrystalline-silicon thin-film transistors made by Ni-silicide mediated crystallization

The source of the leakage current in polycrystalline-silicon (poly-Si) thin-film transistors (TFTs) made by Ni-mediated crystallization has been investigated. Studies of TFTs and of the crystallization process by in situ transmission electron microscopy show that the crystallization process is a two-stage process and that the cause of the leakage problem is associated with incomplete crystallization of amorphous-Si. By removing the last pockets of amorphous-Si, for instance, by long anneals, poly-Si TFTs can be made with adequately low leakage current <1 pA/μm (at a source–drain voltage of 5 V) for display applications, despite the presence of Ni up to 2.5×1019 atoms/cm3.

ON THE ORIGIN OF THE MAGNETO-OPTICAL EFFECTS IN LI, MG, NI, AND CO FERRITE

A reexamination of the magneto-optical (MO) spectra of Li0.5Fe2.5O4, MgFe2O4, NiFe2O4, and CoFe2O4 is presented. Thus far the MO spectra of these compounds have been explained by either orbital promotion processes, oxygen to iron charge transfer transitions or crystal field transitions and, in the case of CoFe2O4, a single intervalence charge transfer (IVCT) transition. For Li0.5Fe2.5O4 and MgFe2O4, the major transitions in the MO–Kerr spectra between 0.5 and 5.0 eV are assigned to intersublattice charge transfer (ISCT) transitions. The MO–Kerr spectrum of NiFe2O4 is more complex as in this case both Fe3+ and Ni2+ contribute to the spectrum. In NiFe2O4 an additional IVCT transition is observed at 2.9 eV. The complex MO spectrum of CoFe2O4 could be resolved using the dielectric tensors of Co2+Alx3+Fe2−x3+O4 (x=0, 0.1, 0.6, and 1) and rigorously fitting both the diagonal and the off-diagonal elements of the dielectric tensor with one set of transitions which was consistent for all degrees of Al3+ substituti...