Brad N. Engel

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.

Magnetic properties of epitaxial Co/Pd superlattices

A series of Co/Pd superlattices with constant Pd layer thickness of dPd = 11 A were grown on single‐crystal GaAs(110) substrates by molecular beam epitaxy (MBE). A buffer layer of 500 A of Pd provided an atomically smooth, fcc(111) single‐crystal starting surface for the superlattice deposition. The resulting superlattices maintain this crystal symmetry and smoothness throughout their epitaxial growth. Details of the structural characterization of these samples are given elsewhere in these proceedings. The magnetic properties of these films have been measured by vibrating sample magnetometry. A perpendicular easy axis is found for all seven of the superlattices presented (dCo = 2, 4, 8, 10, 12, 17, and 20 A). An unusually high coercive field of Hc = 6.6 kOe is observed for dCo = 2 A, and decreases monotonically with increasing dCo. The saturation magnetization agrees well with a model composed of bulk Co layers with an additional contribution from polarized Pd. The uniaxial magnetic anisotropy energy can ...

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...

Preparation and structural characterization of epitaxial Co/Pd(111) superlattices

High‐quality epitaxial fcc Co/Pd(111) superlattices have been prepared on single‐crystal GaAs(110) substrates using molecular‐beam epitaxy. An ultrathin (6 A) buffer layer of bcc Co(110) was used to promote epitaxial growth on the GaAs substrate. A thick (≊ 500 A) buffer layer of fcc Pd(111) was then deposited to provide a smooth surface for superlattice growth. In addition, the Pd buffer layer isolates the superlattice from chemical interaction with the GaAs substrate. Co/Pd superlattices with ultrathin (<20 A) Co layers are found to have a perpendicular easy axis of magnetization. The source of the perpendicular magnetization in these superlattices is attributed to an interface anisotropy, which depends on the interface quality and crystal structure. The structure of the superlattices has been investigated using a wide range of techniques, including reflection high‐ energy electron diffraction, low‐energy electron diffraction, scanning tunneling microscopy, Rutherford backscattering spectroscopy, and 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...

MBE growth and magneto-optic properties of magnetic multilayers

Abstract Recent interest in the magnetic and magneto-optic properties of transition metal/transition metal multilayers has been stimulated by the discovery of perpendicular magnetism in particular systems such as Co/Pd and Co/Pt. Due to their favorable magneto-optic wavelength dependence and enhanced corrosion resistance, these materials show promise as future data storage media. However, partially due to the large variety of thin-film deposition methods and growth conditions, it has been difficult to obtain a clear understanding of the mechanisms of magnetic anisotropy in these systems. In order to create controlled and well characterized model systems, we have grown a series of epitaxial Co/Pd superlattices oriented along the three high-symmetry crystal directions [001], [110], and [111] on single-crystal GaAs substrates by molecular beam epitaxy [MBE]. Simultaneously, we have deposited polycrystalline Co/Pd multilayers on Si substrates mounted alongside the GaAs for direct comparisons of epitaxial and non-epitaxial films produced under identical conditions. The structural properties of these multilayers were determined by low-and reflection high-energy electron diffraction (LEED and RHEED), low- and high-angle X-ray diffraction, and scanning tunneling microscopy (STM). 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. A review of our work on the structural influences responsible for these differences is presented.

Superconducting properties of Nb-CuMn multilayers

In the last years, superconducting multilayers have attracted a large amount of studies due to their interesting properties. Infact, these systems can be used, for example, to investigate the coupling between superconducting layers in different situations (superconducting/insulator, superconducting/metallic, superconducting/magnetic), to analyze the properties of superconductivity in the presence of reduced dimensionality, or to better check effects also observed in high critical temperature layered oxides [1–3]. In particular, the study of interlaver superconducting coupling via magnetic separating slabs has lately gained an increased interest because the coexistence between superconductivity and magnetism seems to play an important role in many properties of high temperature compounds [4].