A.P. Payne

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.

In-situ observation of anisotropic strain relaxation in epitaxial Fe (110) films on Mo (110)

Abstract In-situ grazing incidence X-ray diffraction was used to measure the lattice parameter as a function of thickness for epitaxial Fe (110) films on Mo (110). A large, thickness-dependent, anisotropic in-plane strain was observed. This strain behavior is explained with an equilibrium misfit dislocation model incorporating anisotropic elasticity. The magnetostrictive anisotropy expected from this strain has nearly linear behavior in reciprocal film thickness and thus could be mistaken for a surface anisotropy.

The effect of roughness on the magnetoresistance of Fe/Cr multilayers

Abstract We investigated magnetoresistance in wedged Fe/Cr multilayers grown at several sputtering pressures. Low angle X-ray scattering showed that interface roughness increased with higher sputtering pressure. The increased roughness was accompanied by an increased in saturation resistance, which results in a decrease in fractional magnetoresistance. The maximum magnetoresistance (measured at room temperature) was 7.3 and 4.0% for the films grown at 1.4 and 10 mT Ar, respectively, both at the region with 13 Aring; Cr layers.

An apparatus for studying sputter deposition with x rays

While electron‐based structural probes have been extensively applied to low‐pressure film growth techniques such as evaporation, similar studies on sputter deposition have been hindered by the relatively high gas pressure (1–10 mT) inherent to the technique. Because x rays scatter more weakly than electrons the absorption in the ambient is not significant and diffraction from the growing film can be observed if a sufficiently bright source of x rays is used. We have developed a chamber which combines features of an ultrahigh‐vacuum sputter‐deposition system with the goniometric adjustments and the x‐ray windows of a scattering chamber. A description of its design and recent results are presented.

Metastable copper-chromium alloy films

Due to the large positive heat of mixing associated with the Cu–Cr binary system, solid solutions exist only as nonequilibrium states. In this study, a series of metastable Cu–Cr alloys ranging in composition from 14.1 to 75.4% copper was fabricated by sputter deposition. Symmetric, asymmetric, and grazing incidence x-ray diffraction geometries were used to trace the phase transition from bcc to fcc crystal structures with increasing Cu fraction. It is shown that the transition takes place not by a two-phase region suggested by equilibrium thermodynamics, but rather through gradual disordering of the bcc lattice as copper atoms are substitutionally accommodated. At a critical saturation near 71% Cu, the bcc structure becomes unstable relative to the fcc and a phase transition occurs. The free energies of the kinetically constrained Cu–Cr system are modeled and the results are found to agree well with observed behavior.

Observation of a rapid amorphization reaction

We have observed a rapid amorphization reaction at ambient temperature in the Gd/Co system by employing grazing incidence x-ray scattering. We find that a 135 A crystalline Gd film is amporhized in less than 30 min by deposition of Co. We postulate that the rapidity of the reaction is due to surface diffusion of Co atoms after deposition to fast diffusion sites such as grain boundaries in the Gd film. Once the interfacial region has been amorphized these fast diffusion paths are sealed off from the surface, rapid diffusion of Co into the Gd crystalline layer is prevented, and the amorphization reaction stops.

Structure of sputter-deposited Pt/Fe and Cr/Fe multilayers

Conversion electron Mössbauer spectroscopy (CEMS) and X-ray diffraction (XRD) have been used to investigate the structure of Pt/Fe and Cr/Fe multilayers deposited by magnetron sputtering. The Cr/Fe samples consisted of four samples prepared under Ar sputtering pressures of 1.3, 3.0, 5.0, and 10.0 mT, all with the same multilayer structure of 3.5 nm Cr/2.5 nm Fe, repeated 35 times onto c-Si wafer substrates. The quality of the interfaces between Cr and Fe is clearly degraded with increasing sputter pressure, as seen by changes in the relative intensities of four magnetic subspectra in the CEMS and the gradual appearance of a single-line resonance similar to Fe in solution in Cr. The low-angle XRD superlattice peaks also disappear with increasing sputter pressure, while the high-angle XRD shows a tendency for loss of the preferred (110) texture. Two films of Pt/Fe were deposited epitaxially onto MgO single crystals with bilayer periods of 1.3 nm and 2.6 nm and total thickness of 300 nm each. A transition from fcc-PtFe with near-perpendicular magnetic anisotropy to a bcc-Fe/fcc-PtFe mixture with in-plane magnetic texture is observed by CEMS for the factor of two increase in bilayer period.

Cu-Cr Multilayers and Metastable Alloy Films

A notable aspect of the copper-chromium phase diagram is the strong chemical aversion between the two elements. At room temperature, the solubility of chromium in copper is limited to .02%, while that of copper in chromium is believed to be even less [1]. The elements are even immiscible in the liquid state, exhibiting a miscibility gap which persists to an undetermined temperature. Cu-Cr alloys are of technological interest in applications ranging from electrical switches [2] to catalysis [3]. Fabrication of Cu-Cr alloys is hindered, however, by the large positive heat of mixing exhibited by the system. Cu-Cr multilayers are also of technological interest for application as soft x-ray mirrors [4]. Here the limited mutual solubility of the constituents is advantageous since interfacial mixing is reduced. In this study, we show that vapor deposition can be used to achieve complete solubility in the Cu-Cr system. On the other hand we present evidence suggesting that Cu-Cr multilayers possess sharp composition modulations with little interface mixing.