Strain determination in ultrathin bcc Fe layers on Si(001) by x-ray diffraction

Abstract

Good quality body-centered cubic iron layers can be grown on Si(001) at room temperature using thin ${\mathrm{FeSi}}_{2}$ or ${\mathrm{CoSi}}_{2}$ silicide buffer layers. The in-plane and out-of-plane strains are measured by x-ray diffraction (XRD). Due to the difference in Fe and Si parameters of +5.6%, the thin Fe layers undergo a strong tetragonal distortion. For the samples deposited on ${\mathrm{FeSi}}_{2},$ the measured biaxial in-plane compressive strain ranges from values as large as -3.8% in the thinnest films (25 ML) to -1% in the thicker layers (>40 ML). In the samples deposited on ${\mathrm{CoSi}}_{2},$ a part of the iron layer clearly observable up to 40 ML, apparently grows in pseudomorphy with the silicon, whereas another dominant part above 15 ML partly relaxes and behaves as on ${\mathrm{FeSi}}_{2}$ templates but with systematically larger strains. The evolution of the in-plane cubic effective magnetic anisotropy constant versus Fe film thickness observed in previous work, can be explained by means of fourth-order (in spin) magnetoelastic coupling in the iron lattice with the strain components determined by XRD. Finally, the observed ratio between the perpendicular and in-plane strain components differs substantially from linear elasticity theory predictions and indicates the importance of anharmonic effects and/or defects in the description of the elastic properties of such films.