Michael H. Wiedmann

Magnetocrystalline and magnetoelastic anisotropy in epitaxial Co/Pd superlattices

We have used molecular‐beam epitaxy (MBE) to grow Co/Pd superlattices along the three high‐symmetry crystal axes: [001], [110], and [111]. Identical conditions were maintained for all depositions, and a series of samples of fixed Pd thickness (tPd = 10 ± 1 A) and varying Co thickness (2 A ≤ tCo ≤ 22 A) were prepared for each orientation. A variety of in situ and ex situ characterization studies were made, which confirm these superlattices are single crystalline for all growth directions. The dependence of the uniaxial magnetic anisotropy energy on the Co thickness in these superlattices showed significant systematic differences for each of the three crystal orientations. These variations result entirely from differences in the volume contribution to the anisotropy. Estimates of the magnetocrystalline and magnetoelastic contributions for the (111) and (001) samples are in good agreement with the measured anisotropy energies of these oriented superlattices.

Overlayer‐induced perpendicular anisotropy in ultrathin Co films (invited)

We have used in situ polar Kerr effect measurements to study the magnetic anisotropy of MBE‐grown X/Co/Y trilayers, where X and Y are combinations of the nonmagnetic metals Ag, Au, Cu, or Pd. The competition between the perpendicular anisotropy of the initial underlayer X/Co interface and the in‐plane shape anisotropy of the Co film allows us to adjust the total anisotropy of the uncovered Co to be in plane and of moderate strength. In this way, we can measure hard‐axis (perpendicular) polar hysteresis curves in situ as a function of overlayer Y coverage, and directly deduce the anisotropy field. Polar hysteresis curves were measured in situ for systematically varied Co and overlayer Y layer thicknesses 2 A≤tCo≤20 A and 0 A≤tY≤100 A. We find, for particular combinations, the magnitude of the X/Co/Y perpendicular anisotropy is strongly peaked at ∼1 atomic layer overlayer Y coverage.

Effect of transition-metal overlayers on the perpendicular magnetism of MBE-grown, ultra-thin Co films

We have used in situ polar Kerr ellipticity measurements to study the perpendicular magnetic behavior of MBE‐grown Pd/Co/TM sandwich structures, where TM is the nonmagnetic transition metal overlayer Pd, Cu, or Ag. These structures are epitaxially deposited on thick Pd (111) buffer layers grown on Co‐seeded GaAs (110) substrates. Hysteresis curves were measured in situ for systematically varied Co and TM layer thicknesses 2 A≤tCo≤10 A and 0 A≤tTM≤200 A. We observed perpendicular loops with a coercive field of Hc≤200 Oe for the uncovered Co films for tCo≤6 A, becoming in‐plane above this thickness. However, subsequent deposition of just one atomic layer (≊2 A) of any of the TM over the Co resulted in strongly perpendicular, square hysteresis curves with Hc≥700 Oe for all films in the Co thickness range studied. Deposition of TM overlayers causes nonmonotonic behavior in Hc as a function of coverage. We find a peak in Hc at a TM coverage of tTM∼1.5 A for all materials, with a subsequent monotonic increase a...

Influence of structure on the magnetic anisotropy of Co/Pd (001) epitaxial superlattices

Abstract We have used molecular beam epitaxy to grow Co/Pd superlattices oriented along the fcc [001] crystal direction. A series of samples of fixed Pd thickness ( t Pd = 9±1A) and varying Co thickness (2≤t C o ≤8A) was prepared. The uniaxial magnetic anisotropy in these films displayed both a large perpendicular interface contribution and a large in-plane volume contribution. Using off-axial X-ray diffraction techniques to measure the fcc (113) Bragg reflection, the in-plane lattice spacing of three samples was directly measured. These measurements are consistent with a coherent strain model with the Co layers expanded in-plane by 8.0%±0.5% over that of bulk fcc Co. An estimate of the in-plane magnetoelastic energy calculated from this strain is in very good agreement with our observed volume anisotropy in these superlattices.

Influence of transition metal overlayers on the perpendicular magnetism of MBE-grown Co films

Abstract We have used in situ polar Kerr effect measurements to probe the magnetic behavior of MBE-grown Co films with and without overlayers of various non-magnetic transition metals (TM = Ag, Cu, Pd). We find a large peak in the coercivity, H c , near 2 A overlayer coverage for Ag and Cu.

Magnetic anisotropy of metal/Co/metal and metal/Co/insulator sandwiches

In situ polar Kerr‐effect measurements have been used to study the magnetic anisotropy of MBE‐grown Au(111)/Co/X and Pd(111)/Co/X sandwiches, where X is the nonmagnetic metal Ag, Au, Cu, and Pd or the insulator MgO. For the metals it was recently found that the magnitude of the Co/X perpendicular interface anisotropy is strongly peaked at ∼1 atomic layer (1.5–2.5 A) coverage. To investigate structural influences on the anisotropy, reflection high‐energy electron diffraction (RHEED) and low‐energy electron diffraction (LEED) have been used to measure changes resulting from overlayer coverage. Analysis of digitized RHEED images captured every ∼1 A during metal overlayer coverage shows no abrupt change of the in‐plane lattice constant. The out‐of‐plane lattice spacing has also been investigated as a function of nonmagnetic metal coverage by measuring LEED I‐V curves along the (0,0) rod. In the case of Cu, where the LEED behavior is nearly kinematic, no evidence was seen of any abrupt structural changes at ∼1...

Spatial modulation of the magnetic moment in Co/Pd superlattices observed by polarized neutron reflectivity

We have measured the specular reflectivity of spin‐polarized neutrons from two (111) superlattices, [Pd 80 A‖Co 60 A]×20/Pd 300 A/Co 10 A/GaAs and [Pd 53 A‖Co 32 A]×10/Pd 30 A/Cu 354 A/Si, grown by molecular‐beam epitaxy. In a saturating field of 2300 Oe, we extract for the first superlattice a Co moment value of corresponding to 78% of the bulk Co moment. At temperatures ranging from 38 to 295 K, no excess magnetic moment from polarization of the Pd atoms is evident in the nominal Pd layer. Comparable reflectivity data from the second sample support these conclusions.

Perpendicular magnetic behavior of ultra-thin Co sandwiches

Abstract In situ polar Kerr effect measurements have been used to study the properties of MBE-grown X/Co/Y trilayers, where X and Y are combinations of the non-magnetic metals Au, Cu or Pd. Polar hysteresis curves were measured in situ for systematically-varied Co and overlayer Y layer thicknesses 2 A ≤ t Co ≤ 20 A and 0 A ≤ t Y ≤ 100 A. We recently reported that for base layers of Pd or Au that induce a large perpendicular anisotropy, the magnitude of the X/Co/Y perpendicular coercivity is strongly peaked at ≌ 1 atomic layer overlayer coverage. However, in Pd overlayers the peak in coercivity is much less pronounced than for the other materials. Here we report further measurements on Pd overlayers confirming the existence of a small peak in coercivity. In addition, measurements have been performed on a ‘reversed’ layered Cu/Co/Au sandwich where the Cu base layer induces only a weak perpendicular anisotropy. We find similar peaked behavior similar to that in our earlier measurements on Au/Co/Cu.

MBE Growth of Metal/Semiconductor Interfaces

Metal/semiconductor structures are of interest in several fields including thin-film magnetism, electronics, and x-ray optics. Well controlled and characterized interfaces are crucial for the understanding of the properties of nanostructured materials. We describe techniques used to grow and characterize three such material systems: Co/Pd on GaAs(110) (using a bcc-Co seed layer), Co on Si(111), and Mo on Si(111).