D. M. Lind

Epitaxial growth of CrO2 thin films by chemical-vapor deposition from a Cr8O21 precursor

Presently, the best epitaxial thin films of CrO2 are made by chemical-vapor deposition (CVD) in a two-zone furnace with oxygen flow from a CrO3 precursor. The growth mode has previously been described as CrO3 vaporizing in the first zone, and thermally decomposing at higher temperature in the second zone onto a substrate. In the more recent works, the focus has been on the properties of the obtained layers rather than on deposition mechanisms. In the present experimental work, we attack the epitaxial growth of CrO2 by two completely different methods, namely, molecular-beam epitaxy (MBE) and CVD. We focus on the CVD process itself, and show the importance of an intermediate compound, Cr8O21, for the growth of CrO2 films. We show that it is not necessary to start the CVD from CrO3; instead, one can prepare Cr8O21 ex situ, and use it directly for the growth of high-quality CrO2 epitaxial layers, avoiding any contamination caused by the decomposition of CrO3 to Cr8O21. We discuss in parallel our failed attem...

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.

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.

Detection of field-dependent antiferromagnetic domains in exchange-biased Fe3O4/NiO superlattices

Neutron diffraction studies of Fe3O4/NiO superlattices reveal that the field dependence of domains in the antiferromagnetic NiO is correlated with the presence or absence of exchange biasing. Measurements of the full width at half maximum of the (111) NiO reflection show that after cooling in zero field, the antiferromagnetic domain size both parallel and perpendicular to the growth axis is sensitive to the strength of the applied field. In contrast, after cooling from room temperature in a field of 5 T, the domain size is generally smaller than the zero-field-cooled value and does not vary with field. These data suggest that exchange biasing originates from domain walls frozen into the antiferromagnet upon field cooling.

Polarized neutron reflectometry studies of magnetic oxidic Fe3O4/NiO and Fe3O4/CoO multilayers

The magnetic properties of [1 0 0] oriented Fe3O4/NiO and Fe3O4/CoO multilayers, MBE-grown on MgO(0 0 1) substrates, have been studied by polarized neutron reflectometry. In both samples, the Fe3O4 layer exhibits a depth-dependent magnetic profile characterized by a reduction in the magnetization near the interfaces. The possible origins of this behaviour, such as domain wall formation in the ferrimagnetic layer and deviations in stoichiometry, are discussed.

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.

Polarized neutron diffraction studies of exchange-coupled Fe3O4/NiO superlattices

In order to understand the interplay between exchange coupling and magnetic structure, we have examined the magnetic ordering of a series of epitaxial Fe3O4/NiO superlattices using polarized neutron diffraction techniques. As expected, the net ferrimagnetic moment of the Fe3O4 layers aligns parallel to an applied magnetic field. The antiferromagnetic NiO spins order into alternating antiparallel 〈111〉 planes as in bulk, but the direction of the spins in the planes are determined by field preparation. The NiO moments tend to align perpendicular to the field. In addition, the relative population of the NiO domains varies as the field is raised. The changes in the antiferromagnetic spin order relative to bulk seem to result from magnetic coupling with the Fe3O4 moments.