Core-level binding-energy shifts at surfaces and in solids

In this paper, we review some of the key concepts in ultrathin film magnetism which underpin nanomagnetism. We survey the results of recent experimental and theoretical studies of well characterized epitaxial structures based on Fe, Co and Ni to illustrate how intrinsic fundamental properties such as the magnetic exchange interactions, magnetic moment and magnetic anisotropies change markedly in ultrathin films as compared with their bulk counterparts, and to emphasize the role of atomic scale structure, strain and crystallinity in determining the magnetic properties. After introducing the key length scales in magnetism, we describe the 2D magnetic phase transition and survey studies of the thickness dependent Curie temperature and the critical exponents which characterize the paramagnetic–ferromagnetic phase transition. We next discuss recent experimental and theoretical results on the determination of the exchange constant, followed by an overview of measurements of the magnetic moment in the elemental 3d transition metal thin films in the various crystal phases that have been successfully stabilized, thereby illustrating the sensitivity of the magnetic moment to the local symmetry and to the atomic environment. Finally, we discuss briefly the magnetic anisotropies of Fe, Co and Ni in the fcc crystalline phase, to emphasize the role of structure and the details of the interface in influencing the magnetic properties. The dramatic effect that adsorbates can have on the magnetic anisotropies of thin magnetic films is also discussed. Our survey demonstrates that the fundamental properties, namely, the magnetic moment and magnetic anisotropies of ultrathin films have dramatically different behaviour compared with those of the bulk while the comparable size of the structural and magnetic contributions to the total energy of ultrathin structures results in an exquisitely sensitive dependence of the magnetic properties on the film structure.

https://iopscience.iop.org/article/10.1088/0034-4885/71/5/056501/pdf

Magnetism in ultrathin film structures

Ion beam crystallography of surfaces and interfaces

(1993). Ultrathin metallic magnetic films: magnetic anisotropies and exchange interactions. Advances in Physics: Vol. 42, No. 5, pp. 523-639.

Ultrathin metallic magnetic films: magnetic anisotropies and exchange interactions

Adsorbed layer and thin film growth modes monitored by Auger electron spectroscopy

We have combined high-angular-resolution Auger-electron diffraction, kinematical scattering calculations, low-energy-electron diffraction (done in a pulse-counting mode), and high-energy-resolution x-ray photoelectron spectroscopy to examine the formation of the Fe/GaAs(001)-c(8\ifmmode\times\else\texttimes\fi{}2) interface. We find that clusters of bcc Fe at least three atomic layers deep grow in registry with the substrate for coverages up to \ensuremath{\sim}4 monolayer equivalents. These clusters contain Ga and As atoms which have been liberated from the GaAs substrate. Above this coverage, the clusters coalesce into a continuous bcc Fe matrix with a lattice constant equal to half that of GaAs and with principal crystallographic axes parallel to those of the substrate. This epitaxial Fe overlayer contains Ga and As in solution in the bcc lattice with the impurity atoms occupying interstitial face-center sites. The concentration of Ga and As decreases with distance from the GaAs substrate. At the same time, we find clear evidence for surface segregation of As and enrichment of the near-surface region.

Simultaneous epitaxy and substrate out-diffusion at a metal-semiconductor interface: Fe/GaAs(001)- c (8×2)

We have investigated the atomic structure of annealed Co/Si(111) interfaces prepared by the evaporation of one-, two-, and thirty-monolayer equivalent coverages of Co. Experimental and theoretical angular distributions of the Co LMM Auger intensity reveal that for all coverages investigated a ${\mathrm{CoSi}}_{2}$ phase forms which is rotated 180\ifmmode^\circ\else\textdegree\fi{} about the surface normal relative to the substrate. At low coverages, the ${\mathrm{CoSi}}_{2}$ phase is in the form of clusters which appear to be two to three ${\mathrm{CoSi}}_{2}$ layers in thickness and are terminated by a Si(111) bilayer. The Si(111) overlayer, which is in addition to the terminal plane of Si atoms associated with the outermost ${\mathrm{CoSi}}_{2}$ layer, also appears to be rotated 180\ifmmode^\circ\else\textdegree\fi{} about the surface normal relative to the Si substrate.

High-temperature nucleation and silicide formation at the Co/Si(111)-7×7 interface: A structural investigation.

We have used medium- and high-energy Auger-electron diffraction and low-energy electron diffraction to probe the structure of the Fe/Cu(001) interface, the Cu/Fe/Cu(001) sandwich, and the Fe/Cu/Fe/Cu(001) superlattice. Metastable epitaxial fcc overlayers of Fe with good structural quality grow on room-temperature Cu(001) for coverages up to five monolayers (ML). Above 5 ML, the structural quality of the overlayer diminishes. The first monolayer of Fe does not adsorb uniformly, but rather forms two-layer deep clusters. Above 1 ML, the film coalesces into a well-ordered fcc overlayer. When the substrate temperature is held at 125 \ifmmode^\circ\else\textdegree\fi{}C or higher during evaporation, Fe atoms in-diffuse and displace Cu atoms at lattice sites. The fcc Fe overlayer formed at room temperature also acts as an excellent template for subsequent Cu overlayer growth. We show that Cu depositions on 8-ML Fe/Cu(001) adsorb uniformly (no clustering) and form a fcc film with the same level of structural quality as the substrate. Further Fe depositions on the Cu overlayer maintain an fcc structure, illustrating the feasibility of generating Fe/Cu superlattices in which ultrathin layers of metastable fcc Fe can be sandwiched between Cu layers of any thickness.

Formation and structure of Fe/Cu(001) interfaces, sandwiches, and superlattices.

We present high-resolution synchrotron-radiation core-level photoemission results which show that the room-temperature deposition of Co onto GaAs(110) induces limited substrate disruption, leading to Ga and As intermixing with the metal overlayer. High-energy Auger electron diffraction and low-energy electron diffraction measurements indicate that Co orders weakly to form a metastable, epitaxial bcc matrix with principal crystallographic axes parallel to those of the substrate. Angle-dependent x-ray photoemission results show that the distribution of Ga and As in the Co overlayer varies with, and is dependent on, the Co thickness. Together, these results show that the amount of dissociated Ga and As in the overlayer decays exponentially away from the Co/GaAs interface (1/e length 3 A\r{}); that As surface segregates and its concentration profile is exponential away from the vacuum into the Co overlayer (surface concentration 5.8 at. % at 150-A\r{} Co thickness, 1/e decay length 5 A\r{}); and that the As solubility in the Co film far from either boundary is 0.26 at. %. This paper demonstrates that substrate disruption and epitaxial growth are not mutually exclusive provided that the disruption is not too severe and that conditions necessary for epitaxy exist.

S. A. Chambers

Papers

Simultaneous epitaxy and substrate out-diffusion at a metal-semiconductor interface: Fe/GaAs(001)- c (8×2)

High-temperature nucleation and silicide formation at the Co/Si(111)-7×7 interface: A structural investigation.

Formation and structure of Fe/Cu(001) interfaces, sandwiches, and superlattices.

Epitaxy, overlayer growth, and surface segregation for Co/GaAs(110) and Co/GaAs(100)-c(82).

Direct observation of elastic strain and relaxation at a metal-metal interface by Auger electron diffraction: Cu/Ni(001).