K. A. Shaw

Interdiffusion study of magnesium in magnetite thin films grown on magnesium oxide (001) substrates

Magnetite (Fe3O4) films and multilayers were grown using plasma-assisted molecular beam epitaxy and result in single-phase films grown in registry with a MgO substrate. No evidence of interdiffusion is detected on as-grown films. Both structural and magnetic probes indicate behaviors expected for a magnetite thin film. A thermal stability study of these films was performed by annealing these films under ultrahigh vacuum conditions at temperatures below 900 K. Bulk techniques such as x-ray diffraction, superconducting quantum interference device magnetometry, and energy dispersive spectroscopy confirm that the magnesium interdiffuses throughout the entire film, and surface techniques such as x-ray photoelectron spectroscopy and scanning tunneling microscopy/ion scattering spectroscopy show changes in the surface structure and stoichiometry of the film caused by the magnesium intermixing.

Magnetic structure determination for Fe3O4/NiO superlattices

Neutron diffraction measurements reveal the nature of the magnetic structure in Fe3O4/NiO superlattices grown by molecular beam epitaxy. Taking advantage of differences between the Fe3O4 and NiO crystalline symmetries, we have determined independently the magnetic order parameters of the bilayer components. The NiO antiferromagnetic order propagates coherently through several superlattice bilayers, while the magnetic coherence of the ferrimagnetic Fe3O4 is restricted to a single interlayer due to the random stacking of the spinel unit cells at the interfaces. A model for the diffraction data, based upon a Hendricks–Teller description of the interfacial disorder, demonstrates that the observed broadening of selected reflections originates directly from these stacking faults.

Investigations of the interplay between crystalline and magnetic ordering in Fe3O4/NiO superlattices

Using SQUID magnetometry and both x‐ray‐ and neutron‐diffraction techniques, we have studied the structural and magnetic ordering of a series of Fe3O4/NiO superlattices grown by MBE. X‐ray diffraction reveals that the superlattices are coherent, single phase crystals with narrow interfaces. Symmetry differences between the Fe3O4 spinel and NiO rocksalt structures lead to interfacial stacking faults, manifested in some diffraction intensities. Analysis of the neutron‐diffraction spectra show that the NiO antiferromagnetic ordering is coherent through several superlattice bilayers, while the Fe3O4 magnetic ordering is confined to individual interlayers by stacking faults in all superlattices but those with thinnest (≤10 A) NiO interlayers. Neutron diffraction and SQUID magnetometry have been used to study the Fe3O4 Verwey phase transition in thin‐layered superlattices. The charge ordering in superlattices such as [Fe3O4 (75 A)‖NiO (9 A)]500, below the Verwey transition, directly observable in (4, 0, 1/2) ne...

Structural characterization of Fe3O4 -NiO superlattices using high-resolution transmission electron microscopy

Superlattices of Fe3O4 –NiO layers have been studied by high-resolution transmission electron microscopy (HRTEM). These superlattices are grown by oxygen-plasma-assisted molecular-beam epitaxy (MBE) on (001) oriented MgO substrates, and exhibit a high degree of ordering at the interfaces between the interlayers. The lack of misfit dislocations at the Fe3O4 –NiO interfaces suggests that lattice strain is largely accommodated by changes in the lattice spacing. By quantitative HRTEM analysis of Fe3O4 –NiO interfaces, possible atomic models are discussed, having implications in magnetic ordering and spin exchange mechanisms for such interlayer systems.

Magnetic ordering in layered oxide structures: Fe3O4 thin films and Fe3O4/NiO superlattices

Abstract We report magnetic ordering studies of iron oxide and nickel oxide layered structures using SQUID magnetometry and neutron and X-ray diffraction techniques. This work focuses on the influence of interlayer coupling on the Neel ordering in the NiO layers and the Verwey ordering in the Fe 3 O 4 layers, and well as field dependence of moments in each layer.

Magnetic and crystallographic properties of molecular beam epitaxially grown Fe3O4/NiO superlattices and Fe3O4 films

Ferromagnetic resonance, SQUID magnetometry, and x‐ray diffraction have been used to characterize a set of [Fe3O4(68 A)/NiO(17 A)]N superlattices (SL) with N=3, 10, 30, and 100, as well as a 1.5‐μm‐thick Fe3O4 film. For this NiO thickness, Fe3O4 layers are strongly coupled and the in‐plane anisotropy is much less than the 330‐Oe ferromagnetic resonance (FMR) linewidth at 35 GHz. Both in‐plane and perpendicular FMR at 9.5 and 35 GHz have been used, with the 9.5‐GHz data showing significant hysteresis associated with the sample magnetization. X‐ray diffraction indicates that both the film and SL’s are nearly cubic single‐crystalline structures with long‐range coherence. The 300 K magnetization data indicate the presence of small cubic anisotropy in the SL’s, although bulklike Fe3O4 magnetic ordering in the thick single film. When the Fe3O4 film is cooled below the Verwey transition in a 10 kOe field (aligned along 〈100〉), the FMR shows that the sample develops a large uniaxial (Ku=1.8 kOe) in‐plane anisotro...

Magnesium outdiffusion through magnetite films grown on magnesium oxide (001) (abstract)

Scanning tunneling microscopy (STM) studies of 1 μm thick films of single crystalline Fe3O4 grown on MgO(001) indicate that repeated annealing of the sample in UHV causes Mg diffusion through the Fe3O4 film. The onset of this effect was clearly seen by STM at room temperature for samples raised above 400–430 °C. It appears that the annealing process causes the migration of Mg from the substrate entirely through the Fe3O4 lattice, and that the migration tends to fill the surface layer first, with lower layers filling as anneal time is increased. Upon detection of this effect, several complementary sample analysis techniques were employed to determine the extent of the changes observed. X-ray diffraction studies indicate shifts in the lattice constant from the cubic constant of magnetite, Fe3O4, (8.396 A), which is already strained in thin-film growth on a substrate, further toward the cubic lattice constant of magnesioferrite, MgFe2O4, (8.375 A) in order to accommodate the Mg that has migrated to the surfa...

Spin‐resolved electron spectroscopies of epitaxial magnetite (001) (abstract)

We will present the first spin‐resolving electron spectroscopic studies of a magnetite (Fe3O4)(001) surface. Magnetite is a semimetal with a high density of states in the minority band, but a large band gap in the majority states at the Fermi energy. The polarization of the secondary emission cascade is measured using spin‐resolved secondary electronemission spectroscopy (SRSEES), and reflects the semimetallic spin structure of Fe3O4. The polarization plateau of spin‐resolved secondary emission (29.8%) matches the average 3D band polarization of stoichiometric Fe3O4 as determined from spin‐resolved band structure calculations (34.2%). An enhancement of the polarization of the secondary electrons at lowest energies will also be discussed. Spin‐resolved Auger emission spectroscopy (SRAES) of the Fe3O4 films have been measured and show correlation effects in the valence‐valence Auger transitions. Suppressed intensity and polarization of M 23 M 45 M 45 Auger emission relative to M 1 M 45 M 45 Auger emission is observed, as well as strong resonant emission with shake‐up. Conversely, no spin polarization is detected in the spin‐resolved oxygen LMM Auger features, although oxygen Auger emission (in which we can distinguish between adsorbed and bonded oxygen) is used to verify surface cleanliness of the samples. The synthesis of Fe3O4 films grown on magnesium oxide (001) substrates using oxygen plasma‐assisted molecular beam epitaxy will be discussed, as will thin‐film characterization using SQUID magnetometry and x‐ray and electron diffraction. A unique angle‐, energy‐, and spin‐resolved electron spectrometer has been designed and built for the study of magnetic surfaces, and these studies represent its’ first use. That spectrometer is based on a tandem configuration of an energy‐dispersive energy analyzer and Mott spin polarimeter.

Domain structure in NiO biasing layers (abstract)

Fe3O4/NiO multilayers exhibit long‐range antiferromagnetic order with the magnetite ferrimagnetic correlations confined to a single layer due to stacking faults of the spinel structure at the interfaces. We are studying the field dependence of the interlayer coupling and magnetic structure in a series of Fe3O4/NiO multilayers using neutron diffraction. Both NiO and Fe3O4 thin films were included in the measurements for comparison. In a single thick Fe3O4 film, intensity changes with magnetic field are consistent with the alignment of the net ferrimagnetic moment parallel to the applied field. In a single thick NiO film, we observe no intensity changes with magnetic field. For multilayers where the ratio of NiO to Fe3O4 is far from unity, the field dependence approximates that of the thick film of the majority constituent. However, for a Fe3O4 (68 A)/NiO (34 A) multilayer the NiO antiferromagnetic intensity decreases with increasing field, applied parallel to a [110] axis in the film plane. This indicates ...

Studies of stoichiometric variations of epitaxially grown Fe3−δO4 (abstract)

Using plasma‐assisted molecular beam epitaxy, films of iron oxide cubic spinel phases have been prepared on single crystal MgO (100). Preparation was by deposition of Fe from an elemental e− gun in a reactive oxygen plasma (primarily O+) from an ECR source. Sample stoichiometry during synthesis was controlled by variation of growth parameters, including substrate temperature, oxygen flux, plasma power, and deposition rate. We present the results of structural and magnetic studies of these materials using electron and x‐ray diffraction, as well as SQUID magnetometry. Lattice spacing, saturation magnetization, and magnetic anisotropy all provide strong evidence that the variation of growth parameters does indeed produce a range of ‘‘cubic’’ defect spinel structures ranging from Fe3O4 to one approximating γ‐Fe2O3. Both structural and magnetic probes indicate that strong ordering changes occur in these systems as they are cooled through the ∼119 K Verwey transition. The behavior of the Verwey transition as a ...