S. D. Berry

Structural and magnetic ordering in iron oxide/nickel oxide multilayers by x‐ray and neutron diffraction (invited)

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 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 Neel 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 t...

Ferromagnetic resonance and spin anisotropy in iron oxide thin films and iron oxide/nickel oxide superlattices

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 and the NiO thickness varied from 8.5 to 102 A. 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.

Characterization of the structural and magnetic ordering of Fe3O4/NiO superlattices grown by oxygen-plasma-assisted molecular-beam epitaxy

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 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 e...

Magnetization changes with modulation period in Fe3O4/NiO superlattices

We have magnetically characterized single crystal Fe3O4/NiO superlattices grown on MgO(001), where the layer thickness has been varied over a wide range. Hysteresis for modulation wavelength Λmod = 258 A is similar to bulk Fe3O4, while loops for Λmod < 80 A are nearly linear with applied field with minimal hysteresis and remanence. The magnetization slopes of the latter are roughly proportional to Λmod, and increase from 5 to 300 K. Measurements above 300 K suggest that both exchange coupling across NiO/Fe3O4 interfaces and strain-induced variations in anisotropy, rather than paramagnetism, lead to this linear behavior.

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.

Modulated electric conductivity in Fe3O4/NiO superlattices

High quality Fe3O4/NiO multilayered structures with nominal artificial periodicity 17 A/ 17 A and 34 A/34 A have been made by molecular beam epitaxy. The conductivity of the modulated structures is orders of magnitude higher for current parallel to the layers than for the transverse direction, reaching a ratio of 106−a value which is comparable with the largest known anisotropy for any material. This strong anisotropy shows that a like modulation of electrical conductivity has been achieved over distances as small as tens of A and also suggests the possibility of voltage controlled superlattice phenomena.

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...

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...

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 ...