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  • 1
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: 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(ring))/NiO (34 A(ring)) multilayer the NiO antiferromagnetic intensity decreases with increasing field, applied parallel to a [110] axis in the film plane. This indicates a domain repopulation in the NiO, concomitant with the magnetization of the magnetite, where the antiferromagnetic modulation direction prefers the [111] axes that are closest to the applied field direction. A broadening of the NiO peak shows that the repopulated domains are smaller. We are pursuing a complete measurement of the domain populations by measuring a series of NiO antiferromagnetic diffraction peaks. © 1996 American Institute of Physics.
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  • 2
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Presented are studies of the magnetic and structural ordering in superlattices composed of Fe3O4 and NiO, and their study by a variety of techniques including x-ray and neutron diffraction, and SQUID magnetometry. X-ray diffraction indicates that structures with individual layer thicknesses down to 8.5 A(ring) grow as single crystals in registry with the substrate lattice with a layer-thickness-dependent tetragonal lattice distortion due to epitaxial and interfacial lattice mismatch. The lattice coherence of the Fe3O4 layers, however, is degraded by stacking faults between adjacent spinel layers. Neutron diffraction indicates that the NiO orders antiferromagnetically along the [111] direction with a magnetic coherence that extends over several superlattice bilayers, and the presence of an enhancement in the NiO Néel temperature in thin layered superlattices. These results are compared with SQUID magnetometry, which shows large anisotropy energies, but a lack of favored magnetization direction, indicating that the magnetic ordering in these systems is dominated by the exchange coupling across the interfaces.
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  • 3
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 73 (1993), S. 6457-6459 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Oriented single-crystalline thin films of Fe3O4 and superlattices of Fe3O4/NiO have been grown using molecular-beam-epitaxy techniques on polished substrates of MgO(001). The 1–2-μm-thick superlattices have their Fe3O4 layer thicknesses held constant at 68 A(ring) and the NiO thickness varied from 8.5 to 102 A(ring). We report the initial ferromagnetic resonance measurements made on these samples to investigate the effects that interfacial lattice distortion and interlayer coupling have on their magnetic ordering and anisotropy. These results are compared with structural and magnetic studies made on the same films using x-ray diffraction and magnetometry techniques. We find clear evidence in the trends of both the ferromagnetic resonance fields and the linewidths as a function of NiO thickness that coupling between the layers can change the in-plane anisotropy and the overall magnetic response. The single-layer Fe3O4-only sample behaves essentially like bulk magnetite.
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  • 4
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 64 (1994), S. 381-383 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: 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.
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  • 5
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: 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(ring)) 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(ring))||NiO (9 A(ring))]500, below the Verwey transition, directly observable in (4, 0, 1/2) neutron intensities, indicates a shift to higher temperature of the charge ordering transition from the bulk Fe3O4 TVerwey at 123 K. We also describe ongoing efforts to extract the moment distribution in these superlattices from field dependent high-angle neutron diffraction.
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  • 6
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Ferromagnetic resonance, SQUID magnetometry, and x-ray diffraction have been used to characterize a set of [Fe3O4(68 A(ring))/NiO(17 A(ring))]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 anisotropy with its easy axis along the field direction. Decreased microwave loss also occurs abruptly at the film TV.
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  • 7
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Single crystal superlattices of Fe3O4 and NiO, grown on MgO(100), provide a means to study the competition between interfacial exchange coupling effects and epitaxial strain induced by these closely, but not perfectly, lattice matched materials. Both induce profound macroscopic magnetic effects in the superlattices, especially for films with very thin (〈80 A(ring)) modulation thicknesses. The Fe3O4 Verwey conductivity transition in the bulk (∼120 K) is highly sensitive to both the exact stoichiometry of the iron oxide and the presence of strain and is accompanied by a structural and magnetic anisotropy change. X-ray diffraction measurements show that these superlattices are tetragonally distorted (up to 1%), with the increased strain associated with superlattices composed of thin bilayers. By tracking the magnetic evidence of this phase transition using SQUID magnetometry techniques, we observe attenuation of the effect in superlattice films as the iron oxide layers grow thinner and the nickel oxide layers are thickened, which suggests that the presence of strain in the iron oxide layers is highly important. The small structural distortion is observable in neutron diffraction measurements of a Fe3O4 film. Studies of the diffuse and critical scattering associated with this transition in strained iron oxide films and Fe3O4/NiO superlattices and a comparison to previous work on bulk Fe3O4 single crystals will be discussed.
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  • 8
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Neutron diffraction measurements reveal the nature of the magnetic structure in Fe3O4/NiO superlattices grown by molecular beam epitaxy. By taking advantage of differences between the spinel and rocksalt crystalline symmetries, we have separated the magnetic order parameters of the Fe3O4 and NiO bilayer components. The NiO antiferromagnetic order develops at temperatures 10–150 K greater than TNéel for bulk (520 K) due to exchange coupling of the Ni spins to the Fe3O4 (TCurie=858 K) at the superlattice interfaces. The resultant scaling of the NiO transition temperature with the bilayer composition is consistent with expectations from mean-field treatments for a modulated magnetic material. In addition, the NiO spin order propagates coherently through several superlattice bilayers, while the magnetic coherence of the Fe3O4 is limited by a random stacking of the spinel unit cell at the interfaces when the NiO interlayers are thicker than ≈15 A(ring). A model for the diffraction data, based upon a Hendricks–Teller description of the interfacial disorder, demonstrates that the observed broadening of selected crystalline and magnetic reflections originates directly from these stacking faults. Combining the high angle diffraction results with those from polarized-beam reflectivity analysis, we extract specific information about the moment distribution in these superlattices and qualitatively explain the evolution of the magnetic structure with temperature.
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  • 9
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: 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 function of growth parameters indicates that Fe vacancies in these defect spinels have a profound effect on the structural distortion related to the Verwey transition. Epitaxial strain due to lattice mismatch further contributes to structural distortion, and thus to the Verwey transition. The relative contributions of these stoichiometric variations and epitaxial strain on the Verwey transition will be discussed.
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  • 10
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Oriented single-crystalline thin films of NiO and Fe3O4 and Fe3O4/NiO superlattices have been grown on cleaved and polished substrates of MgO(001) using oxygen-plasma-assisted molecular-beam epitaxy (MBE). We report the growth mode and structural characterization of these films using in situ RHEED and ex situ scanning electron microscopy and x-ray diffraction, and their magnetic characterization using SQUID magnetometry. Also reported are preliminary results of magnetotransport measurements. MgO has a very small lattice mismatch to the cubic rocksalt structure of NiO and to the half-unit-cell dimension of the spinel structure of Fe3O4. Pseudomorphic growth of superlattices consisting of alternating layers of NiO and Fe3O4 with repeat wavelength down to 17 A(ring) and of single thick layers of either of these materials are readily obtained. The grown films exhibit cubic single-crystalline symmetry in registry with the substrate, with sharp interfaces and strongly layer-thickness-dependent strain. RHEED pattern evolution in situ during growth indicates development of the rocksalt-like NiO and spinel Fe3O4 crystalline symmetries as each of the alternating layers of the superlattices are deposited. Magnetic hysteresis curves for superlattices with repeat wavelength less than 75 A(ring) exhibit a crossover from the conventional ferrimagnetic response expected of Fe3O4 found in longer wavelength superlattices and bulklike thick films to a more linear magnetic response that correlates strongly to the layer thickness dependence of lattice distortions and strain. Large magnetoresistance (6% at 55 kOe) is also observed for these short-wavelength superlattices, with the observed change in resistance nearly linear with field and with small hysteresis.
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