B. J. Daniels

In situ x‐ray measurements of the initial epitaxy of Fe(001) films on MgO(001)

Small‐ and large‐angle x‐ray scattering from epitaxial Fe(001) on an MgO(001) surface has been measured as a function of film thickness, using grazing incidence x‐ray scattering. Small‐angle scattering shows that for Fe thicknesses less than 15 monolayers, the Fe is islanded with Fe[110]∥MgO[100]. For deposition at 360 °C, the Fe lattice parameter increases toward the MgO surface net spacing with increasing thickness in the 1–10 monolayer coverage regime, and then relaxes back toward the bulk Fe lattice parameter at greater thicknesses. Agglomeration of the islands results in changes in the Fe lattice parameter and in the high‐angle peak widths. Prior to agglomeration, the measured in‐plane lattice parameter versus thickness is described by a pairwise site interaction between the island and the substrate interface nets. Strain relaxation subsequent to agglomeration is described by continuum elasticity theory.

Enhanced mechanical hardness in compositionally modulated Fe(001)Pt(001) and Fe(001)Cr(001) epitaxial thin films

The hardnesses and elastic moduli of sputter-deposited epitaxial Fe(001)Pt(001) and Fe(001)Cr(001) multilayers grown on MgO(001) were evaluated as a function of composition wavelength Λ. X-ray diffraction was used to characterize the structure of these multilayers, allowing for the examination of the structural underpinnings of the mechanical properties in these systems. For both Fe/Pt and Fe/Cr multilayers, nanoindentation results reveal no appreciable enhancement in the elastic modulus (the so-called supermodulus effect) over a broad range of Λ. A reduced modulus is observed at small Λ in the Fe/Pt multilayer films, which can be attributed to interfacial bonds which are weaker than those in the bulk. Nanoindentation data reveal that for Fe/Pt multilayers, the hardness is enhanced over that expected from a simple rule of mixtures by a factor of approximately 2.5. This enhancement in hardness occurs over a considerable range in Λ (2–10 nm) and is not a function of Λ in this wavelength regime. Preliminary results indicate that the hardness of Fe/Cr multilayers obtained via nanoindentation is also enhanced over the rule of mixtures value by a slightly smaller amount than observed in the Fe/Pt system. The high hardness may arise from structure modulated strengthening (f.c.c./b.c.c.) in the Fe/Pt system. The structural difference between Fe and Pt is a barrier to dislocation motion between the two materials, and this contributes to the hardness of these multilayer films. However, since a large enhancement is seen in the Fe/Cr system, where no structure modulated strengthening occurs, this structural effect may be minor. The dominant mechanism responsible for the hardness enhancement in Fe(001)Pt(001) and Fe(001)Cr(001) multilayers has not yet been identified. Experiments are underway to determine whether the strength enhancement arises from the number of interfaces, the stress state, the shear modulus discontinuity, or other effects in these multilayer thin films.

Structural evolution during deposition of epitaxial Fe/Pt(001) multilayers

We have investigated the structure of epitaxial Fe/Pt(001) multilayers deposited by direct current magnetron sputtering. In these multilayers, the structure of the Fe layers depends on their thickness: Thick (tFe>22 A) Fe layers are body-centered cubic (bcc), while thin (tFe<12 A) Fe layers are face-centered cubic (fcc). Ex situ x-ray diffraction reveals that the unstrained lattice parameter of bcc Fe in epitaxial multilayers is significantly greater than that of bulk bcc Fe, possibly due to alloying with Pt. This suggests that the observed “fcc Fe” is actually an intermixed fcc Fe–Pt interfacial layer. To investigate this possibility, we have performed grazing-incidence x-ray scattering in situ during deposition of epitaxial Fe/Pt(001) multilayers. The structure of Fe(001) layers as thin as 10 A is bcc, strained due to epitaxial mismatch with the Pt(001) underlayer. Additional Fe deposition results in relaxation of the bcc Fe lattice parameter toward its bulk value. Deposition of Pt onto a 50 A thick bcc...

Effect of coherency stresses on the hardness of epitaxial Fe(001)/Pt(001) multilayers

The effect of coherency stresses on the hardness of epitaxial, sputter‐deposited Fe(001)/Pt(001) multilayers was investigated. Coherency stresses were over 2 GPa for films with a bilayer period, Λ, of 44 A and relaxed by more than a factor of 10 for films with Λ=76 and 121 A. Since the hardness of these films was constant at approximately 9 GPa over this range of Λ, we conclude that the contribution of coherency stresses to the enhanced hardness is small for this system.

Enhanced Mechanical Hardness in Compositionally Modulated Fe/Pt and Fe/Cr Epitaxial Thin Films

The hardnesses and elastic moduli of sputter-deposited Fe/Pt and Fe/Cr multilayers grown on MgO(001) are evaluated as a function of composition wavelength, Λ. Structural determination by x-ray diffraction showed these films to be oriented in the plane as well as out of the plane. The mechanical behavior of these films was evaluated by nanoindentation. The combination of nanoindentation and x-ray diffraction is an attempt to determine the structural underpinnings of the mechanical behavior of these metal multilayer systems. For both systems there is no observed enhancement in the elastic modulus (the so-called supermodulus effect) across a wide range of bilayer spacings. Nanoindentation results show that for Fe/Pt multilayers, the hardness is enhanced over that expected from a simple rule of mixtures by a factor of approximately 2.5, with a maximum enhancement of 2.8 times this value at a wavelength of 25 A. This enhancement in hardness occurs for bilayer spacings from 20 A to 100 A and is not a strong function of Λ over this range. Results for Fe/Cr multilayers show a hardness enhancement over a similar wavelength range of approximately two times the rule of mixtures value, with a maximum enhancement of 2.2 times this value at a wavelength of 40 A. The larger hardness enhancement in the Fe/Pt system may be due to the structural barrier (FCC/BCC) to dislocation motion between the two materials. The dominant mechanism responsible for the hardness enhancement in Fe/Pt and Fe/Cr multilayers is not yet known, however three models for dislocation interactions which could account for the hardness enhancement in these multilayers are discussed.

Effect of Cr doping on the magnetoresistance and saturation field of epitaxial Fe1−xCrx(001)/Cr(001) multilayers

The effect of Cr doping of the Fe layer on the magnetoresistance and saturation field of epitaxial, sputter‐deposited Fe1−xCrx(001)/Cr(001) multilayers was examined. Films with a composition of 100 A Cr/[14 A Fe1−xCrx/8 A Cr]50, where x was varied from 0 to 0.5, were deposited onto single crystal MgO(001). The room temperature magnetoresistance was constant at approximately 31% for x≤0.2 and decreased with higher Cr concentrations. The saturation field decreased linearly with increasing Cr concentration over the entire range. Doping the Fe layer with Cr results in an increase of the spin‐dependent scattering (Δρ) for 0.1≤x≤0.2 and an increase in the sensitivity of these films for all Cr concentrations.

A structure investigation of epitaxial Fe–Pt multilayers

Abstract Fe/Pt epitaxial multilayers have been studied by cross-sectional transmission electron microscopy and high-resolution transmission electron microscopy (HRTEM). The Fe/Pt multilayers were prepared by sputter deposition on MgO substrates. All samples had the (002)Fe/Pt, plane parallel to the surface of MgO. The nominal bilayer period Λ was varied from 2.5 to 12.1 nm. The multilayers with Λ ≥ 4.4 nm exhibited a Bain orientation relationship between fcc Pt and the tetragonally distorted bcc Fe. Examination of the multilayers with Λ = 12.1 nm revealed the least distorted bcc Fe structure and the best-defined interface between the bcc Fe and fcc Pt layers. The HRTEM images taken from the areas adjacent to the interface revealed that (200)bccFe is rotated 3° away from (200)Pt. This rotation appears to arise in order to accommodate the elastic misfit between the layers. The ratio of the interplanar distance of the (002)bccFe planes relative to that of (110)bcCFe, that is d(oo2)Fe/d(110)Fe, was found to be 1.37 for Λ = 7.6 nm and 1.28 for Λ = 4.4 nm, which are smaller than the value of 1.414 expected for an undistorted bcc structure. The variation in the ratio could be related to a relaxation of the internal stresses developed in the multilayers. For the Fe/Pt multilayers with a nominal value of Λ = 2.5 nm, both the selected area diffraction patterns and the HRTEM images show epitaxial growth of fcc Fe and Pt with a cube-on-cube orientation relationship, that is [010]Fe//[010]pt, (002)Fe//(002)pt, with about 1° deviation. A tetragonal distortion occurred in the Pt layers with the largest lattice constant in the plane of the film. Diffuse diffraction spots and streaking along the film growth direction were observed in all multilayers, probably owing to a combination of both the small dimensions of the layer structure and the formation of a Fe/Pt solid solution close to the interfaces.

The Search for the Supermodulus Effect

The supermodulus effect has been reported as an anomalous increase of as much as several hundred percent in the elastic properties of certain compositionally-modulated thin metal films as the wavelength of the composition modulation decreases near 2 nm. Although elastic property variations have been detected by a variety of methods, including mechanical deflection experiments and acoustic measurements, the sign, magnitude and physical basis for such an effect remain in dispute. We report on the mechanical properties of compositionally-modulated Au-Ni and Ag-Pd thin films as determined by three different mechanical deflection experiments: nanoindentation, microbeam deflection and bulge testing. Au-Ni films were fabricated by alternately sputtering Au and Ni onto oxidized Si substrates and were tested by nanoindentation and microbeam deflection techniques. The nanoindentation experiments reveal a decrease of about 15% in the indentation modulus at a composition wavelength near 1.6 nm. The microbeam deflection experiments showed only small variations in in-plane stiffness but did reveal strongly wavelength-dependent substrate interaction stresses in these films. A simple analysis of the bulge test indicates that these stresses can reproduce the major features of the supermodulus effect, as reported from bulge test results, as artifacts of the analysis method. A more thorough evaluation of the bulge test shows that this technique can be used to obtain accurate and reproducible results if the initial and boundary conditions are properly accounted for. Ag-Pd films were also prepared by sputtering and were tested using improved sample preparation, bulge testing and data analysis techniques. Variations in the biaxial modulus were small and in agreement with the beam deflection results.

The Effect of Impurity Doping of The Magnetic Layer on the Magnetoresistance and Saturation Field of FeCr/Cr and CoCu/Cu Multilayers

The effect of doping the magnetic layer on the magnetoresistance and saturation field of epitaxial, sputter-deposited FeCr/Cr(001) and CoCu/Cu(001) multilayers was examined. FeCr/Cr films with a composition of [14 A Fe 1−x Cr x (001)/8 A Cr(001)] 50 where x was varied from 0 to 0.5 and CoCu/Cu films with a composition of [8 A Co 1−x Cu x (001)/21 A Cu(001)] 40 where x was varied from 0 to 0.2 were deposited onto single crystal MgO(001). The bilayer period and epitaxial orientation of these films were determined by x-ray diffraction. In the FeCr/Cr system the room temperature magnetoresistance was constant at approximately 31% for x≤0.2 and decreased for larger Cr concentrations. The spin-dependent scattering (Δ p ) is increased over the x =0 value for x =0.1 and x =0.2. The magnetic field required to saturate these multilayers decreases linearly with increasing Cr concentration. The net result is that the sensitivity of these films is increased by Cr doping of the Fe layer. In the CoCu/Cu system the room temperature magnetoresistance, saturation field, and saturation magnetization decrease with the addition of Cu to the Co layer. In contrast to the FeCr/Cr system Δ p does not increase with doping of the ferromagnetic layer. However, as in the FeCr/Cr films the sensitivity of these multilayers is increased with respect to that of the x =0 CoCu/Cu multilayer.