S. T. Purcell

Large surface anisotropies in ultrathin films of bcc and fcc Fe(001) (invited)

Large uniaxial anisotropies associated with interfaces are observed for ultrathin films (3‐28 ML) of bcc Fe(001) grown epitaxially on Ag(001) single‐crystal substrates and for epitaxial sandwiches of fcc Fe(001) grown with three layers of Fe using Cu as substrate and coverlayers. The uniaxial anisotropy is well described by a pseudosurface anisotropy term as theoretically predicted, yet that theory also predicts large in‐plane anisotropies that are not observed. Adequate treatment of spin‐orbit coupling in magnetic theories remains a challenge. Comparisons of ultrathin films of bcc Fe(001) on Ag(001) with different coverlayers of Ag or Au show subtle differences in magnetic behavior as studied by ferromagnetic resonance (FMR) and Brillouin light scattering (BLS). The FMR measurements were carried out at 9.6, 36.6, and 73 GHz microwave frequencies. The BLS measurements were performed using a six‐pass Fabry–Perot interferometer. The power of the techniques of molecular‐beam epitaxy (MBE) for producing well‐...

Development of magnetic anisotropies in ultrathin epitaxial films of Fe(001) and Ni(001)

Ultrathin films, bcc Fe(001) on Ag(001), fcc Fe(001) on Cu(001) and Fe/Ni(001) bilayers on Ag, were grown by molecular beam epitaxy. A wide range of surface science tools were employed to establish the quality of epitaxial growth. Ferromagnetic resonance and Brillouin light scattering were used to extract the magnetic properties. Emphasis was placed on the study of magnetic anisotropies. Large uniaxial anisotropies with easy axis perpendicular to the film surface were observed in all ultrathin structures studied. These anisotropies were particularly strong in fcc Fe and bcc Fe films. In sufficiently thin samples the saturation magnetization was oriented perpendicularly to the film surface in the absence of an applied field. It has been demonstrated that in bcc Fe films the uniaxial perpendicular anisotropy originates at the film interfaces. In situ measurements indentified the strength of the uniaxial perpendicular anisotropy constant at the Fe/vacuum, Fe/Ag and Fe/Au interfaces asKus = 0.96, 0.63, and 0.3 ergs/cm2 respectively. The surface anisotropies deduced for [bulk Fe/noble metal] interfaces are in good agreement with the values obtained from ultrathin films. Hence the perpendicular surface ansiotropies originate in the broken symmetry at abrupt interfaces. An observed decrease in the cubic anisotropy in bcc Fe ultrathin films has been explained by the presence of a weak 4th order in-plane surface anisotropy,K1∥S=0.012 ergs/cm2. Fe/Ni bilayers were also investigated. Ni grew in the pure bcc structure for the first 3–6 ML and then transformed to a new structure which exhibited unique magnetic properties. Transformed ultrathin bilayers possessed large inplane 4th order anisotropies far surpassing those observed in bulk Fe and Ni. The large 4th order anisotropies originate in crystallographic defects formed during the Ni lattice transformation.

Engineering magnetic materials on the atomic scale (invited)

Ultrahigh vacuum (UHV) systems and the use of atomic beams for deposition of atoms layer by layer combine to make possible the creation of new materials. The applications to metallic magnetism are gaining increasing attention. The building of sandwiches of magnetic and nonmagnetic layers should lead to increased understanding of the propagation of spin polarization through metals and the effects of finite thickness on the ground state properties and the thermodynamics of magnetic materials. The most important step in this process is in the first layer, i.e., the preparation of the substrate and the determination of the quality of the interface and of the overlayer. The techniques of surface science, e.g., residual gas analysis (RGA), reflection high energy electron diffraction (RHEED), Auger electron spectroscopy (AES), and x‐ray photoemission spectroscopy (XPS) are essential for the characterization of the interface. Illustrations of these include our own work on body‐centered‐cubic Ni deposited epitaxia...

Rheed Intensities and Oscillations During the Growth of Iron on Iron Whiskers

The homoepitaxial growth of Fe on (001) surfaces of Fe whiskers is used to study Reflection High Energy Electron Diffraction. Variations in the intensity of the specular spot are followed during growth for temperatures from 100 K to 600 K. Mean field models of growth are used for the classification of types of RHEED oscillation patterns. Simple ideas of diffraction are employed to partially account for the effects of attenuation and surface disorder on the RHEED intensities.

Epitaxial Growths and Surface Science Techniques Applied to the Case of Ni Overlayers on Single Crystal Fe(001)

The rapidly increasing interest and activity in the study of epitaxially deposited magnetic films on single crystal substrates stem both from the ability to stabilize metastable crystalline structures which do not exist otherwise in nature and from theoretical predictions of enhanced magnetic moments and crystalline anisotropics in low dimensional systems. For example, recent spectacular experimental results1,2 and theoretical calculations3 show that the crystalline anisotropy field in ultrathin Fe films is capable of overcoming the demagnetizing field perpendicular to its surface, making such films an ideal building block for multilayered permanent supermagnets. This is an example of the creation of new magnetic materials by means of atomic engineering. It should be pointed out that such recent advances and future progress in atomic engineering would not be possible without Molecular Beam Epitaxy (MBE) techniques using controlled atomic beams in Ultra High Vacuum (UHV) and using state of the art surface science techniques such as Reflection High Energy Electron Diffraction (RHEED), spin polarized or unpolarized Auger Electron Spectroscopy (AES) and X-Ray Photoelectron Spectroscopy (XPS).

Deducing 3d spin polarization from 3s XPS in ultrathin metal films grown by molecular beam epitaxy

Abstract 3s XPS spectra are used to deduce the spin polarization of 3d electrons in Mn films by comparison with all the 3d transition elements, V(Mn) and Ag(Mn). On the time scale of 4×10-15s it appears that Mn has either 5μB per atom or no moment. The probability of observing the 5μB state decreases from Mn in Ag to Mn in its several forms to Mn in V.

Increased Magnetic Moments in Transition Elements Through Epitaxy

The elements of the first transition series, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu, are of special importance for magnetism and metallurgy. The phase diagrams of these elements and their alloys with one another and elements such as C, Si and Al fill encylopedic volumes.1–5 The correlation between atom size, by various measures, and magnetic moment has long been noted.6 This correlation is readily seen by comparing the atomic concentrations of the first, second and third transition series elements as shown in Fig. 1, where the densities for the second and third series elements have been scaled for comparison with the first transition series. It seems that there is some missing density, excess volume, in Cr, Mn, Fe, Co, and Ni, all of which show ordering of magnetic moments. The main purpose of this paper is to argue that in the cases of Fe, Mn, Cr, and possibly V, artificially increasing the volume of these elements through controlled epitaxial growth may lead to higher magnetic moments and other technologically important magnetic properties. We like to call this atomic engineering, implying that we are building structures atom by atom.