A. Chaiken

Atomic-layer engineering of cuprate superconductors

A technique for atomic layer-by-layer synthesis of cuprate superconductors and other complex oxides has been developed. Thin films with excellent transport properties and atomically flat surfaces and interfaces are obtained. The samples are engineered by stacking molecular layers of different compounds to assemble multilayers and superlattices, by adding or omitting atomic monolayers to create novel compounds, and by doping within specified atomic monolayers to fabricate, for the first time, intra-cell barriers. Apart from manufacturing trilayer Josephson junctions withIcRn>5 mV, this technique enables one to customize both the materials and the devices according to the needs of a specific experiment. A number of fundamental issues, such as the dimensionality of the HTSC state, existence of long-range proximity effects, occurrence of resonant tunneling with a specified number of hops, etc., have been addressed in this way. Synthesis of the first “artificial” metastable HTSC compounds is also reported.

Exchange anisotropy in epitaxial and polycrystalline NiO/NiFe bilayers

(001)-oriented NiO/NiFe bilayers were grown on single crystal MgO (001) substrates by ion beam sputtering in order to determine the effect that the crystalline orientation of the NiO antiferromagnetic layer has on the magnetization curve of the NiFe ferromagnetic layer. The simplest model predicts no exchange anisotropy for the (001)-oriented NiO surface, which in its bulk termination is magnetically compensated. Nonetheless exchange anisotropy is present in the epitaxial films, although it is approximately half as large as in polycrystalline films that were grown simultaneously. The surface anisotropy in the epitaxial films is found to contain cubic and unidirectional components, while that in the polycrystalline film is best described by a uniaxial plus unidirectional anisotropy. Experiments indicate that differences in exchange field and coercivity between polycrystalline and epitaxial NiFe/NiO bilayers couples arise due to variations in induced surface anisotropy. Implications of these observations for models of induced exchange anisotropy in NiO/NiFe bilayer couples will be discussed. {copyright} {ital 1998} {ital The American Physical Society}

NiO exchange bias layers grown by direct ion beam sputtering of a nickel oxide target

A new processes for fabricating NiO exchange bias layers has been developed. The process involves the direct ion beam sputtering (IBS) of a NiO target. The process is simpler than other deposition techniques for producing NiO buffer layers, and facilitates the deposition of an entire spin-valve layered structure using IBS without breaking vacuum. The layer thickness and temperature dependence of the exchange field for NiO/NiFe films produced using IBS are presented and are similar to those reported for similar films deposited using reactive magnetron sputtering. The magnetic properties of highly textured exchange couples deposited on single crystal substrates are compared to those of simultaneously deposited polycrystalline films, and both show comparable exchange fields. These results are compared to current theories describing the exchange coupling at the NiO/NiFe interface.

Direct experimental study of the magnetization reversal process in epitaxial and polycrystalline films with unidirectional anisotropy

Direct observation of the magnetization reversal of epitaxial NiO/NiFe bilayers grown on (001) MgO and on polycrystalline Si substrates was performed by using the magneto-optical indicator film technique. It was shown that the unidirectional-axis magnetization reversal proceeds by domain nucleation and growth. A new phenomenon, an asymmetry in the activity of the domain nucleation centers, has been revealed. Remagnetization of the bilayer is shown to be governed by defect structures in the antiferromagnetic layer.

Soft-x-ray fluorescence study of buried silicides in antiferromagnetically coupled Fe/Si multilayers

Multilayer films made by alternate deposition of two materials play an important role in electronic and optical devices such as quantum-well lasers and x-ray mirrors. In addition, novel phenomena like giant magnetoresistance and dimensional crossover in superconductors have emerged from studies of multilayers. While sophisticated x-ray techniques are widely used to study the morphology of multilayer films, progress in studying the electronic structure has been slower. The short mean-free path of low-energy electrons severely limits the usefulness of photoemission and related electron free path of low-energy electrons severely limit spectroscopies for multilayer studies. Soft x-ray fluorescence (SXF) is a bulk-sensitive photon-in, photon-out method to study valence band electronic states. Near-edge x-ray absorption fine-structure spectroscopy (NEXAFS) measured with partial photon yield can give complementary bulk-sensitive information about unoccupied states. Both these methods are element-specific since the incident x-ray photons excite electrons from core levels. By combining NEXAFS and SXF measurements on buried layers in multilayers and comparing these spectra to data on appropriate reference compounds, it is possible to obtain a detailed picture of the electronic structure. Results are presented for a study of a Fe/Si multilayer system.

Transmission electron study of heteroepitaxial growth in the BiSrCaCuO system

Films of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} and Bi{sub 2}Sr{sub 2}CuO{sub 6} have been grown using Atomic-Layer-by-Layer Molecular Beam Epitaxy (ALL-MBE) on lattice-matched substrates. These materials have been combined with layers of closely related metastable compounds like Bi{sub 2}Sr{sub 2}Ca{sub 7}Cu{sub 8}O{sub 20} (2278) and rare-earth-doped compounds like Bi{sub 2}Sr{sub 2}Dy{sub {ital x}}Ca{sub 1{minus}{ital x}}Cu{sub 2}O{sub 8} (Dy:2212) to form heterostructures with unique superconducting properties, including superconductor/insulator multilayers and tunnel junctions. Transmission electron microscopy (TEM) has been used to study the morphology and microstructure of these heterostructures. These TEM studies shed light on the physical properties of the films, and give insight into the growth mode of highly anisotropic solids like Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8}. {copyright} {ital 1996 Materials Research Society.}

MAGNETIC PROPERTIES OF EPITAXIAL AND POLYCRYSTALLINE FE/SI MULTILAYERS

Fe/Si multilayers with antiferromagnetic interlayer coupling have been grown via ion‐beam sputtering on both glass and single‐crystal substrates. High‐angle x‐ray diffraction measurements show that both sets of films have narrow Fe peaks, implying a large crystallite size and crystalline iron silicide spacer layers. Low‐angle x‐ray diffraction measurements show that films grown on glass have rougher interfaces than those grown on single‐crystal substrates. The multilayers grown on glass have a larger remanent magnetization than the multilayers grown on single‐crystal substrates. The observation of magnetocrystalline anisotropy in hysteresis loops and (hkl) peaks in x‐ray diffraction demonstrates that the films grown on MgO and Ge are epitaxial. The smaller remanent magnetization in Fe/Si multilayers with better layering suggests that the remanence is not an intrinsic property.

Exchange anisotropy in polycrystalline and epitaxial (001)-oriented NiO/NiFe bilayers grown by ion beam sputtering (abstract)

The connection between the spin structure of antiferromagnetic NiO and the exchange anisotropy observed in NiO/NiFe bilayers is not well understood. For instance, the NiO bulk-terminated (001) surface is compensated, and therefore simple models predict no exchange bias in (001)-oriented NiFe/NiO bilayers.1 Using a newly developed ion-beam sputtering (IBS) process to deposit NiO exchange-coupled films,2 we have simultaneously grown polycrystalline and epitaxial NiO/NiFe bilayers. NiO grown on NiFe/MgO is polycrystalline, while NiO grown directly on MgO is epitaxial. The in-plane orientation of the epilayers was confirmed using (hk0) x-ray diffraction. The exchange anisotropy in epitaxial (001)-oriented bilayers is about half as large as that observed in polycrystalline bilayers. The size of the exchange anisotropy does not depend on the orientation of the bias field with respect to the in-plane NiFe/NiO crystallographic direction, indicating that the same interfacial spin structure is achieved regardless o...

Anomalous temperature dependence of interlayer coupling in Fe/Si multilayers (abstract)

The magnetic coupling of the Fe layers in Fe/Si multilayers strongly depends on the morphology of the iron‐silicide interlayer that forms during deposition. Antiferromagnetic interlayer coupling is only observed in Fe/Si multilayers with crystalline interlayers in the CsCl structure. Recently, it has been shown that single layers of Fe–Si in the CsCl structure can be grown epitaxially on Si over a range of stoichiometries. FeSi films are reported to be Kondo insulators below 50 K. We find evidence of a magnetic phase transition in antiferromagnetically coupled Fe/Si multilayers. Ms measured in a constant applied field of 50 kOe shows T3/2 behavior down to 10 K. However, M(T) at a lower constant field peaks around 50 K and decreases at lower temperature, indicating enhanced antiferromagnetic coupling or a phase transition. The remanent magnetization increases monotonically with decreasing temperature and has been explained by invoking thermally activated coupling. However, the saturation field also increas...

Microanalytical study of defect formation in thin bismuth strontium calcium copper oxide films

Thin bismuth strontium calcium copper oxide (BSCCO) films and BSCCO/insulator/BSCCO trilayers have been prepared on SrTiO3 and MgO substrates by evaporation from elemental sources in ozone atmosphere. Accurate control of the stoichiometry is achieved through monitoring of the atomic fluxes by use of in situ atomic absorption spectroscopy, as well as by reflection high-energy electron diffraction. Nevertheless, nanometer-scale second- phase precipitates are sometimes observed. These defects and the flat regions around them have been probed by a variety of microanalytical techniques, including Rutherford backscattering spectroscopy, particle-induced x-ray emission, atomic force microscopy, microscopic secondary ion mass spectroscopy and transmission electron microscopy.