D.J. Keavney

Effects of interfacial roughness on site-probed multilayers of Fe(100)/Ag(100)

Abstract Mossbauer spectroscopy was performed on three [Fe 9 (100)/Ag 40 (100)] 25 multilayers grown by molecular beam epitaxy (MBE). All of the Fe bilayer components in each sample were selectively loaded with 2 monolayers (ML) of 57 Fe: at the Fe-on-Ag interfaces of one sample, at the Ag-on-Fe interfaces of another, and at the Fe bilayer centers of the third. The hyperfine fields, quadrupole splittings, and isomer shifts of these multilayers were measured at 16, 100 and 300 K. We observed widely different values and trends as the temperature increased, leading us to repeat an earlier claim that three rather than two distinct sites are needed to correctly describe the Fe bilayer components in Fe(100)/Ag(100) multilayered systems.

Growth of Fe(110)/bcc Ni(110) superlattices by molecular beam epitaxy

Abstract We have grown a series of Fe(110)/bcc Ni(110) superlattices by MBE. RHEED and X-ray analysis indicated that even for Fe thicknesses of only 2 ML, the subsequent Ni layer could be grown in a pseudomorphic phase with a structure resembling that of the Fe template up to a thickness of 8 ML. Magnetometry determined that this distorted bcc phase is ferromagnetic at room temperature. Transmission Mossbauer spectroscopy implied the existence of two magnetic Fe sites, one we associate with Fe atoms that have diffused into the Ni layers and the other with Fe atoms that mainly coordinate other Fe atoms. The diffused site showed no magnetic splitting at 300 K in an Fe(110)/Ag(111) film that was grown for comparison to the Fe(110)/Ni films, implying that the distorted bcc Ni(110) is magnetic.

Interlayer coupling in epitaxial Fe(110)/Ag(111) multilayers

Abstract We have detected a possible oscillatory interlayer coupling in Fe(110)/Ag(111) multilayers grown by molecular beam epitaxy. The oscillation period is between 5 and 9 ML. The coupling was examined using a site-specific Mossbauer spectroscopy technique to determine the spin-wave spectrum at the first two monolayers of the Fe(110) surface. As the interlayer exchange decreases, the spin-wave spectrum softens, resulting in a faster decrease in the nuclear hyperfine field with temperature. At an interlayer thickness of 6 ML, the coupling is so strong that the spin-wave spectrum appears almost bulk-like.

Oscillatory interlayer coupling through (111) oriented noble metal spacers

We use a surface‐sensitive Mossbauer spectroscopy technique to examine the spin‐wave spectrum, at the Fe(110)/NM(111) interface only, of a multilayer structure with a noble metal (NM) interlayer. We find that the temperature dependence of the hyperfine field follows a Bloch law (1−BT3/2), and use spin‐wave calculations to connect the surface spin‐wave stiffness parameter, B, to the interlayer exchange coupling. Films grown with Ag(111) interlayers show clear oscillations with a period of 6 ML, in good agreement with recent predictions.

Measurements of magnetic interaction through silver in epitaxial Fe(110)/Ag(111) superlattices by Mössbauer spectroscopy

Magnetic interactions between Fe layers in Fe(110)/Ag(111) superlattices grown by molecular beam epitaxy have been observed using Mössbauer spectroscopy. By measuring the temperature dependence of the hyperfine field at the Fe layers, characteristics of the spin-wave spectrum can be deduced. As the Ag thickness between layers is increased, the magnetic interaction between Fe layers decreases, and the spin-wave spectrum undergoes a transformation from three-dimensional to quasi-two-dimensional.

Magnetic and Structural Analysis of Ultra-Thin Magnetic Films, Multilayers, and Superlattices by Mössbauer Spectroscopy

In the study of thin films and surfaces as well as the examination of superlattices, the careful characterization of these systems including their structural, Magnetic, transport, and other properties has been absolutely crucial to the advancement of the field. As Means of sample preparation have progressed, techniques for evaluating the flatness, continuity, crystallinity, etc. of thin films and surfaces have become ever more necessary to understand the resulting magnetic and electronic properties. Because iron is often a constituent of Magnetic thin films and because the isotope 57 Fe shows a strong Mossbauer Effect over a wide temperature range the technique of Mossbauer Spectroscopy offers much to the study of surfaces, thin films, and superlattices.

Oscillatory exchange coupling in MBE‐grown Fe(110)/Ag(111) multilayers (abstract)

The nature of the interlayer coupling in epitaxial Fe(110)/Ag(111) multilayer structures was investigated using a highly surface‐sensitive Mossbauer spectroscopy technique. The films were grown by molecular beam epitaxy and analyzed with x‐ray diffraction and in situ RHEED to verify their crystallinity and orientation. All the films took the general form [56Fe3057Fe2Agx]15, where x=3 to 22 monolayers (ML). The entire film is invisible to the Mossbauer effect except for the 2 ML 57Fe layers, which therefore act as a probe of the magnetic environment of the Fe surfaces. Information regarding the surface spin‐wave spectrum can then be obtained by measuring the temperature dependence of the hyperfine field at the surface. In the limit of no Ag interlayer, the 57Fe probe layers would be in direct contact with 56Fe on both sides and therefore should display a (1−BT3/2) hyperfine field temperature dependence, with B=5.2×10−6 K−3/2. As the Ag interlayer thickness is increased, we expect the hyperfine field to fol...

Mössbauer study of different magnetic properties between opposite interfaces of Fe(100)/Ag(100) multilayers

Abstract A series of Fe x (100)/Ag 40 (100) multilayers with x = 3, 6 and 9 monolayers (ML) were grown by MBE. Their crystallinity was verified by in situ RHEED. Their magnetic properties as well as their structural properties were analyzed by 57 Fe Mossbauer spectroscopy. The 3 ML film showed two Mossbauer components with parameters very different from those of bulk Fe. The 6 and 9 ML films exhibited 3 components, the additional one with very bulk-like parameters (and growing in intensity with Fe thickness). These results have led us to assign the two other components to the two interface regions, Fe/Ag and Ag/Fe. To confirm the assignment and distinguish the two interfaces, we grew three more 9 ML films, placing 2 ML 57 Fe in the bulk and at each interface, with the remaining 7 ML 50 Fe. The Mossbauer hyperfine parameters of these sites were distinguished in a way consistent with the above assignment. In addition, we also investigated the temperature dependence of the magnetic hyperfine field for the three Fe sites. We conclude that the differing tetragonal distortions in the two interface regions are responsible for the appearance of the two interface sites.

Magnetic and structural properties of Fe(110)/Ag(111) and Fe(100)/Ag(100) multilayers

A comparison of the magnetic and structural properties and growth characteristics between Fe(110)/Ag(111) and Fe(100)/Ag(100) multilayers is presented. The two types of multilayers were made of the same constituent materials but with different oricutations, allowing us to examine the interesting interplay between structure and magnetism. We found fundamentally different magnetic properties including magnetocrystalline anisotropy and surface/interface and thin film magnetism for the two types of multilayers, and their origins were discussed.

A Comparison of Growth and Magnetic Properties of FE(ll0)/AG(lll) and FE(100)/AG(100) Multilayers

Two series of Fe/Ag multilayers were grown in a Perkin-Elmer 430B MBE system, one of the Fe(110)/Ag(lll) orientation and another of the Fe(100)/Ag(100) orientation. Vastly different techniques were developed by this group and others to achieve epitaxial growth of both of these systems. Using RHEED, it was inferred that the optimal growth of Fe(110) on Ag(lll) occurred at a substrate temperature of 180° C. In contrast, the growth of Fe(lO0)/Ag(100) proceeded with the sharpest RHEED streaks at a reduced substrate temperature. We believe that these fundamentally different growth parameters are the result of physically different growth modes, conjectured to be: edge growth (Fe 110), and a more nucleated growth (Fe 100). Accordingly, dissimilar magnetic interfacial properties are also strongly in evidence, accounted for by the structural differences associated with the different Fe planes. Furthermore, Fe(110) layers as thin as 3 ML were grown on Ag(lll) and showed no superparamagnetism and a genuine 2-dimensional behavior of M(T). However, the Fe(100) on Ag(100) multilayers in a similar thickness range exhibited strong relaxation and a comparatively reduced Curie temperature.