Craig D. England

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.

Orientation and structure of monolayer →→ multilayer phthalocyanine thin films on layered semiconductor (MoS2 and SnS2) surfaces

Thin films of both chloroindium and copper pthalocyanines have been vacuum deposited onto metal dichalcogenide surfaces such as MoS2 and SnS2, with ordering achieved for these four‐fold symmetric molecules ranging from below monolayer to multilayers. Reflection high‐energy electron diffraction suggests that square lattice geometries are adopted for low coverages of each phthalocyanine (Pc), but with multiple domains. Low‐energy electron diffraction confirms the presence of three square lattice domains, each domain rotated by 60° with respect to the other. Basal plane defects, and especially terrace sites in the metal dichalcogenide surface, are implicated as the nucleation sites for the growth of these square lattice domains. Optical spectroscopies have been used to characterize submonolayer to multilayer deposits of chloroindium phthalocyanine on SnS2 thin films, where the packing geometries of the adjacent Pcs cause perceptible changes in the position and width of the absorbance band in the visible/near...

Magnetic properties of Pd/Co multilayers

Measurements of the magnetic anisotropy of a series of Pd/Co multilayers grown by magnetically enhanced dc triode sputter deposition revealed an interface contribution of 0.53 erg/cm2 and a net volume anisotropy of −13.2×108 erg/cm3. A characteristic reduction of perpendicular anisotropy for ultrathin Co layers (<4 A) is explained by using a pair interaction model to evaluate a thickness dependence of the interface anisotropy. For Pd‐Co interfaces with perfectly sharp composition modulation an interface anisotropy of 0.79 erg/cm2 is predicted. By modeling the magnetization behavior a domain‐wall energy density of 2±0.5 erg/cm2 is obtained for Pd/Co (2 A) multilayers with Pd thicknesses in the range of 4–20 A. For perpendicular Pd75Co25 alloy thin films the same model gives domain‐wall energy densities in the range of 1–2 erg/cm2. For ultrathin Co layers the interlayer Co–Co coupling range is ∼20 A. Temperature dependence of saturation magnetization of a Pd(11 A)/Co(7.8 A) multilayer demonstrates complete ...

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 ...

Magnetic properties of Co/Pd multilayers determined by Brillouin light scattering and SQUID magnetometry

Co/Pd multilayers with modulation wavelengths between 4 and 220 A have been prepared by magnetically enhanced dc‐triode sputtering on single‐crystal sapphire substrates. Their saturation magnetization and volume and interface anisotropies have been investigated using Brillouin light scattering from collective spin waves and by SQUID magnetometry. The saturation magnetization of Co is found to be independent of the Co layer thickness and reduced by about 20% from the Co bulk value. From the comparison of the results of the two experimental methods, clear evidence for a Pd polarization is found and the polarization depth is estimated. Samples with Co thicknesses of 2 atomic layers and Pd thicknesses ≥5 atomic layers exhibit a perpendicular magnetization due to a large negative out‐of‐plane interface anisotropy. The properties of spin waves in Co/Pd multilayers with the direction of magnetization pointing out‐of‐plane are discussed with respect to an appropriate theoretical model.

RHEED and optical characterization of ordered multilayers of phthalocyanine⧸C60 and phthalocyanine/perylene-tetracarboxylicdianhydride (PTCDA)

Ordered monolayers → multilayers of trivalent metal phthalocyanines, C60, and the perylenetetracarboxylic dianhydride (PTCDA) have been formed by molecular beam epitaxy processes on both single crystal MoS2 and SnS2, and on MBE-deposited SnS2 thin films. The bulk packing structures for the trivalent metal chloride phthalocyanines lend themselves to layered growth during deposition, as do the structures for C60 and PTCDA. RHEED data collected during the formation of [(InPc-Cl)1−4 ML/(C60)1−4 ML]1−20/SnS2/mica multilayers suggests that the structures observed during the formation of single component thin films can be sustained during multilayer formation, up to several lattice periods. Absorbance spectra of Pc/C60 and Pc/PTCDA multilayers show Pc Q-bands with maxima and full widths consistent with the formation of ordered Pc layers, provided that the Pc layer thickness is kept at 1–2 monolayers throughout the formation of the multilayer assembly.

Structural and magnetic properties of Ti/Co multilayers

We have investigated the structural and magnetic properties of sputter‐deposited Ti/Co multilayer thin films. Sample composition was determined by Rutherford backscattering spectrometry. The crystal structure and composition modulation were examined with several x‐ray diffraction techniques. X‐ray diffraction analysis showed that the Ti and Co layers grow in the hcp structure and that the films are strongly textured along the (002) direction and are structurally incoherent. Composition modulation was confirmed from low‐angle x‐ray diffraction measurements. Vibrating‐sample magnetometry (VSM) and Brillouin light scattering (BLS) were used to determine the magnetic properties of the films. VSM measurements of samples with Co layer thicknesses less than ≊22 A showed no measurable net magnetization, while those with greater Co layer thicknesses had both in‐plane and perpendicular components. Using a simple model, the measured thickness dependence of the magnetization can be explained by assuming that 11.3 A o...

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 ...

Dye aggregates and organic superlattices formed by organic-inorganic molecular beam epitaxy

Abstract We describe the formation of ordered ultrathin films of chloroindium phthalocyanine (InPcCl) on thin films of the layered semiconductor SnS 2 , both of which are deposited by a molecular beam epitaxy process to form InPcCl/ SnS 2 /mica assemblies. The resultant thin films show markedly narrowed linear absorbance spectra versus those seen previously for Pc thin films. Low energy electron diffraction and reflection high energy electron diffraction data collected for the first few monolayers of InPcCl deposited on the SnS 2 substrate suggest that three (equally populated) ordered Pc domains are formed, with the Pc rings parallel to the SnS 2 surface, aligned along the principle axes of this hcp SnS 2 substrate. The red-shift and peak shape of the absorbance spectra of these thin films (about 758 nm, full width at half maximum of about 60 nm) are consistent with the type of splitting of the exciton energies expected of Pc aggregates in which the adjacent Pc rings are cofacial, but staggered in both x and y directions by about half a molecular diameter. This is the structure determined from related trivalent metal Pcs by single crystal X-ray diffraction. Loss of the flat-lying configuration of Pc in these ultrathin films is immediately apparent from the surface electron diffraction data.

Elastic properties of CuCo multilayers

Abstract The acoustic phonon modes of sputter-deposited CuCo multilayers were studied using Brillouin scattering. These are the first such measurements to be made on an f.c.c.-f.c.c. multilayer system. The multilayer samples had a fixed atomic percentage of cobalt (53 at.%). The effective elastic constants of the multilayer samples, which were obtained by comparing the data with calculated dispersion curves for the phonon modes, were independent of the multilayer modulation wavelength.