DSpace Repository :: Browsing by Author "Willis, RF"

Browsing by Author "Willis, RF"

Now showing 1 - 20 of 22

BAND-STRUCTURE OF THE MAGNETIC FCC PSEUDOMORPHS - NI(100), CO(100), AND FE(100)
(American Physical Society, 1993-10-01) Mankey, GJ; Willis, RF; Himpsel, FJ; International Business Machines (IBM); University of Alabama Tuscaloosa
We report angle-resolved inverse-photoemission spectroscopy and ultraviolet-photoemission-spectroscopy measurements of the electronic structure of Cu(100), Ni(100), and pseudomorphic films of fcc Co and Fe on Cu(100). The unoccupied critical points at X are determined. These results are combined with recent photoemission data for use as input to an empirical combined-interpolation-scheme calculation of the energy-band dispersions for the allowed bands at k(parallel-to) = 0. The ferromagnetic exchange splittings of the DELTA5-symmetry 3d bands are estimated to be DELTAE(ex) = 0.3 eV, 1.2 eV, and 1.2 eV for Ni(100), Co(100), and Fe(100), respectively. The exchange splitting of the fcc Fe indicates that films deposited at room temperature are in the predicted low-spin magnetic phase. Annealing the Fe films produces a paramagnetic phase that exhibits no exchange splitting.
THE EFFECT OF CHEMISORPTION ON THE MAGNETIC-BEHAVIOR OF METASTABLE LAYERS OF FCC CO(001) AND FE(001)
(American Institute of Physics, 1990) Kief, MT; Mankey, GJ; Willis, RF; Pennsylvania Commonwealth System of Higher Education (PCSHE); Pennsylvania State University; University of Alabama Tuscaloosa
We have investigated the effect of chemisorption on the magnetic properties of metastable layers of fcc Co(001) and fcc Fe(001) epitaxed to Cu(001) substrates. The Curie temperature has been observed to fall rapidly with decreasing film thickness below 10 monolayers,1 so that the low-temperature chemisorptive properties can be studied. We have observed the binding energies of various adsorbates using thermal desorption kinetics as a function of magnetic film thickness. The magnetic behavior of the films was monitored in situ using the surface magneto-optic Kerr effect (SMOKE) method,2 and changes in the electronic states at the Fermi level were monitored with high-resolution photoelectron spectroscopy. The question of the basis for a so-called "magnetocatalytic effect"3 is addressed in the light of these results.
THE EFFECT OF MICROSTRUCTURE ON THE MAGNETIC-BEHAVIOR OF EPITAXIAL COBALT LAYERS
(American Institute of Physics) Mankey, GJ; Kief, MT; Willis, RF; Pennsylvania Commonwealth System of Higher Education (PCSHE); Pennsylvania State University; International Business Machines (IBM); University of Alabama Tuscaloosa
We have studied the magnetic behavior of thin cobalt films epitaxed on a Cu (001) substrate as a function of their growth temperature. At 150 K, the film texture is rough and the surface coverage is incomplete for film thicknesses < 1.2 monolayers. Smoother films are obtained as the substrate temperature is increased, but at the expense of increased copper interdiffusion. Growth of films at 450 K produces smooth continuous epitaxial layers but coated with a layer of interdiffused copper which serves to lower the surface free energy. Copper interdiffusion can be controlled at intermediate temperatures between 300 and 450 K. We report on the effect of annealing cycles on the microstructure of these films in relation to their magnetic properties, as revealed by surface magneto-optic Kerr effect hysteresis loop behavior.
EFFECT OF SUBSTRATE SYMMETRY ON THE PREFERRED MAGNETIZATION ORIENTATION OF NI FILMS ON CU
(American Institute of Physics, 1998-06-04) Wu, SZ; Mankey, GJ; Willis, RF; Pennsylvania Commonwealth System of Higher Education (PCSHE); Pennsylvania State University; Pennsylvania State University - University Park; United States Department of Energy (DOE); Lawrence Berkeley National Laboratory; University of Alabama Tuscaloosa
For ultrathin Ni films grown on Cu(100) and Cu(110), the easy axis of magnetization starts out in the plane of the films. Above a critical thickness, the preferred magnetization orientation changes to perpendicular to the films. This behavior is contrary to the spin reorientation transition reported for many thin film ferromagnetic systems where the preferred magnetization orientation starts out perpendicular to the film and then switches to in-plane above a critical thickness. We report in situ surface magneto-optic Kerr effect measurements of the magnetic anisotropy as a function of film thickness. The change in the predominant domain orientation distribution occurs at 8 monolayers (MLs) for Ni on Cu(100) and at 16 ML for Ni on Cu(110). For Ni on Cu(lll) no such change is observed down to 3 ML thickness. These results are discussed in terms of competing surface anisotropy and dipole-dipole energy terms. © 1995, American Vacuum Society. All rights reserved.
Enhanced spin polarization of conduction electrons in Ni explained by comparison with Cu
(American Physical Society, 2000-06-15) Altmann, KN; Petrovykh, DY; Mankey, GJ; Shannon, N; Gilman, N; Hochstrasser, M; Willis, RF; Himpsel, FJ; University of Wisconsin System; University of Wisconsin Madison; University of Alabama Tuscaloosa; Pennsylvania Commonwealth System of Higher Education (PCSHE); Pennsylvania State University; Pennsylvania State University - University Park
The spin-split Fermi-level crossings of the conduction band in Ni are mapped out by high-resolution photoemission and compared to the equivalent crossing in Cu. The area of the quasiparticle peak decreases rapidly below E-F in Ni, but not in Cu. Majority spins have larger spectral weight at E-F than minority spins, thereby enhancing the spin polarization beyond that expected from the density-of-states. A large part of the effect can be traced to a rapid variation of the matrix element with k at the point where the s,p band begins to hybridize with the d(z)(2) state. However, it is quite possible that che intensity drop in Ni is reinforced by a transfer of spectral weight from single-particle to many-electron excitations. The results suggest that the matrix element should he considered for explaining the enhanced spin polarization observed for Ni in spin-polarized tunneling.
FINITE-SIZE-SCALING BEHAVIOR OF FERROMAGNETIC THIN-FILMS
(American Institute of Physics, 1998-06-04) Huang, F; Mankey, GJ; Kief, MT; Willis, RF; University of Alabama Tuscaloosa
We have used molecular-beam epitaxy to grow high-quality pseudomorphic Ni and Co1Ni9 films on Cu(001). From temperature-dependent surface magneto-optic Kerr effect measurements of these films, we have determined the finite-size scaling behavior of the Curie temperature of ultrathin films for a thickness range of n=2.5-16 monolayers (ML). The film thickness dependent Curie temperature for each of these ferromagnetic thin-film systems, T(C)(n), is described by a finite-size scaling formula: [T(C)(infinity) - Tc(n)]/T(C)(n) = [(n - n')/n0]-1/nu, where T(C)(infinity) is the bulk Curie temperature, n0 = 2.5 +/- 0. 5 ML for Co films and 3.5 +/- 0.4 ML for Ni and Co1Ni9 films is the microscopic length scale, and v=0.76 +/- 0.08 is the bulk correlation length exponent. An interesting result is that Tc(n) extrapolates to zero in the single mononolayer limit, n' = 1.
Growth and magnetic properties of CoxNi1-x and FexNi1-x ultrathin films on Cu(100)
(American Physical Society, 1997-08-01) Schumann, FO; Wu, SZ; Mankey, GJ; Willis, RF; Louisiana State University System; Louisiana State University; University of Alabama Tuscaloosa
We have grown ultrathin CoxNi1-x and FexNi1-x films on Cu(100) with varying stochiometry x. We find that these alloy films grow in a fee phase on Cu(100). With the surface magneto-optic Kerr effect we measured the variation of the Curie temperature T-C as a function of the film thickness n in monolayers. Fitting an empirical scaling curve to our results we are able to extrapolate the value T-C(n=x) for samples with different stochiometry. We use this framework in order to determine T-C(n=x) for FexNi1-x alloy films, in particular for concentrations close to 65% Fe content. Bulk Fe65Ni35 shows a collapse of magnetic long-range order and a fcc-to-bce structural transition, which is the so-called Invar effect. In ultrathin Fe65Ni35 films, we observe a ''quenching'' of the Invar effect, because growth on a Cu(100) substrate forces the film to adept the Cu lattice spacing thereby suppressing the structural relaxation.
Growth and magnetic properties of FexNi1-x ultrathin films on Cu(100)
(American Institute of Physics, 1996-08-17) Schumann, FO; Wu, SZ; Mankey, GJ; Willis, RF; University of Wisconsin System; University of Wisconsin Madison; University of Alabama Tuscaloosa
We have investigated ultrathin FexNi1-x films grown epitaxially on Cu(100) with different stochiometry. With the surface magneto-optic Kerr effect (SMOKE) we measured the variation of the Curie temperature T-C as a function of the film thickness n in monolayers (ML). Using the results of our previous investigations on finite-size scaling (Huang et al.), we are able to extrapolate the value T-C(infinity) for samples with different Fe content. In particular, alloy films with Fe concentrations close to 65% remain ferromagnetic. This is in contrast to bulk Fe65Ni35, which shows a collapse of long range order, which is the so-called invar effect associated with a fcc to bcc structural transition. Growing these alloy films on a Cu(100) substrate forces them to adapt the Cu lattice spacing, thereby suppressing the structural relaxation. (C) 1996 American Institute of Physics.
HYDROGEN CHEMISORPTION ON FERROMAGNETIC THIN-FILM SURFACES
(American Institute of Physics, 1998-06-04) Mankey, GJ; Kief, MT; Huang, F; Willis, RF; International Business Machines (IBM); University of Alabama Tuscaloosa
We report measurements of hydrogen desorption and its effect on the magnetism of pseudomorphic ultrathin films of Fe, Co, and Ni on Cu (001). Hydrogen desorption is measured with thermal desorption spectroscopy (TDS) and changes in the magnetic properties with saturation hydrogen coverage are measured with the surface magneto-optic Kerr effect (SMOKE). TDS of hydrogen shows that Co films less than 2.5 monolayers thick do not completely cover the substrate, and that Co deposited at 300 K does not alloy with the Cu(001) substrate. By carefully controlling the thickness of the Ni films, we can tune the ferromagnetic Curie temperature to below the hydrogen desorption temperature. The thermal desorption of hydrogen from the paramagnetic Ni thin film surface is similar to that from bulk ferromagnetic Ni(001) indicating magnetism has no measurable effect on thermal desorption kinetics. Hydrogen is more weakly chemisorbed on ferromagnetic Co thin film surfaces and more strongly chemisorbed on ferromagnetic Fe thin film surfaces as compared to Ni(001). SMOKE measurements show hydrogen chemisorption produces a magnetically ''dead'' surface layer on Ni films, a reduction in the surface layer magnetism on Co films, and an enhancement in the surface layer magnetism on Fe films.
INTERFACIAL ANISOTROPY AND MAGNETIC TRANSITION OF COBALT FILMS ON CU(111)
(American Institute of Physics, 1998-08-17) Huang, F; Mankey, GJ; Willis, RF; University of Alabama Tuscaloosa
We measure the magnetic properties of ultrathin films of Co epitaxially grown on single crystal Cu(111) using surface magneto-optic Kerr effect. The magnetic behavior is compared with that of the same films, but covered by Cu overlayers to study the effects of film morphology on ferromagnetism. The uncapped films show mainly in-plane anisotropy, but a weak perpendicular magnetization is always present. Capping a 1.5 monolayer (ML) Co film with Cu significantly enhances perpendicular anisotropy with 1 ML of Cu overlayer eliminating the in-plane magnetization completely. The perpendicular magnetization reaches its maximum value with 3 ML of Cu overlayer, approximately 5 times higher than the uncapped films, and then decreases as more Cu is deposited. Annealing can further increase the perpendicular magnetization with the in-plane component remaining weak. In sharp contrast, this perpendicular anisotropy enhancement by Cu overlayers is not seen for Co films on Cu(100). The temperature dependence of magnetization fits to the phenomenological power law M approximately (1 - T/T(C))beta with beta=0.15+/-0.05 for the uncapped 1.5 ML Co film and shows a linear behavior after the film is covered by Cu overlayers.
Magnetic behavior of FexNi((1-x)) and CoxNi((1-x)) pseudomorphic films on Cu(100)
(American Institute of Physics, 1998-06-04) Wu, SZ; Schumann, FO; Mankey, GJ; Willis, RF; Pennsylvania Commonwealth System of Higher Education (PCSHE); Pennsylvania State University; Pennsylvania State University - University Park; United States Department of Energy (DOE); Lawrence Berkeley National Laboratory; University of Alabama Tuscaloosa
We set out to compare the ferromagnetic behavior of FexNi(1-x) and CoxNi((1-x)) films of varying stoichiometry epitaxially grown on Cu(100). The thickness chosen was 5 ML over a wide alloy composition range. Using a scaling law for the thickness vs Curie temperature, we extrapolate the bulk fee film thickness and plot the bulk fee Curie temperature as a function of composition. The results suggest (a) that the Invar effect is quenched in these ultrathin pseudomorphic fee films of FexNi(1-x) alloys and (b) the magnetic phase of Fe atoms is the low-spin ferromagnetic phase. The CoxNi(1-x) films show the expected monotonic decrease in T-c with increasing Ni content. (C) 1996 American Vacuum Society.
Magnetic x-ray dichroism in the spectroscopy of ultrathin magnetic alloy films
(American Institute of Physics, 1996-05) Tobin, JG; Goodman, KW; Mankey, GJ; Willis, RF; Denlinger, JD; Rotenberg, E; Warwick, A; Pennsylvania Commonwealth System of Higher Education (PCSHE); Pennsylvania State University; Pennsylvania State University - University Park; United States Department of Energy (DOE); Lawrence Berkeley National Laboratory; University of Alabama Tuscaloosa
The magnetic structure of nanoscale alloy films has been probed using the magnetic x-ray Linear dichroism in photoelectron spectroscopy and magnetic x-ray circular dichroism in x-ray absorption spectroscopy. FeNi and CoFe epitaxial films were grown on Cu(001), in situ and using MBE techniques. Because soft x-rays were used to probe the 2p and 3p core levels, both elemental selectivity and magnetic sensitivity were achieved simultaneously. Correlation of these magnetic techniques with compositional and structural information will be done. Ultimately, from studies such as this a complete determination of the structure-property relationships will be achievable. (C) 1996 American Vacuum Society.
Magnetic x-ray linear dichroism in the photoelectron spectroscopy of ultrathin magnetic alloy films
(American Institute of Physics, 1998-08-17) Tobin, JG; Goodman, KW; Mankey, GJ; Willis, RF; Denlinger, JD; Rotenberg, E; Warwick, A; Pennsylvania Commonwealth System of Higher Education (PCSHE); Pennsylvania State University; Pennsylvania State University - University Park; United States Department of Energy (DOE); Lawrence Berkeley National Laboratory; University of California System; University of California Berkeley; University of Alabama Tuscaloosa
The magnetic structure of nanoscale alloy films has been probed using the magnetic x-ray linear dichroism in photoelectron spectroscopy. FeNi and CoFe epitaxial films were grown on Cu(001), in situ and using molecular beam epitaxy techniques. The magnetic x-ray linear dichroism measurements were made at the Spectromicroscopy Facility of the Third Generation Advanced Light Source. Because soft x-rays were used to generate photoemission from the 3p core levels, both elemental selectivity and magnetic sensitivity were achieved simultaneously. (C) 1996 American Institute of Physics.
MAGNETIC-PROPERTIES OF PSEUDOMORPHIC FERROMAGNETIC ALLOY-FILMS ON CU(100)
(American Institute of Physics, 1995) Mankey, GJ; Wu, SZ; Schumann, FO; Huang, F; Kief, MT; Willis, RF; Pennsylvania Commonwealth System of Higher Education (PCSHE); Pennsylvania State University; Pennsylvania State University - University Park; United States Department of Energy (DOE); Lawrence Berkeley National Laboratory; University of Alabama Tuscaloosa
We report the variation of the thickness dependence of the Curie temperature for a range of ultrathin ferromagnetic alloy films. By simultaneously depositing Fe or Co and Ni on Cu(100) we produce pseudomorphic alloys with controlled stoichiometry. An analysis of the Curie temperature versus thickness curves with an empirical finite size scaling formula reveals that the Curie temperature in the thick film limit increases monotonically with Co or Fe concentration. We compare the bulk alloy Curie temperatures to those of the alloy films extrapolated from the few monolayers limit. © 1995, American Vacuum Society. All rights reserved.
MAGNETISM IN THE FEW-MONOLAYERS LIMIT - A SURFACE MAGNETOOPTIC KERR-EFFECT STUDY OF THE MAGNETIC-BEHAVIOR OF ULTRATHIN FILMS OF CO, NI, AND CO-NI ALLOYS ON CU(100) AND CU(111)
(American Physical Society, 1994-02-01) Huang, F; Kief, MT; Mankey, GJ; Willis, RF; Pennsylvania Commonwealth System of Higher Education (PCSHE); Pennsylvania State University; University of Alabama Tuscaloosa
The surface magneto-optic Kerr effect (SMOKE) was used to investigate the magnetic properties of epitaxial thin films of Co, Ni, and their alloys grown on Cu(100) and Cu(111). The Curie temperature T(C) is higher for the same films of a given thickness on Cu(111) than on Cu(100). All the films show a change in the power-law exponent beta of the magnetization density M approximately (1-T/T(C))beta with reducing film thickness. Ni films on Cu(100) undergo a particularly abrupt crossover at approximately 7 monolayers (ML) from three-dimensional Heisenberg (beta=0.37) to finite-size two-dimensional XY (beta=0.23) behavior as the film thickness is reduced. The characteristic power-law exponent beta=0.23 of these films appears to be an experimental realization of Kosterlitz-Thouless behavior over a restricted temperature range. A similar, but more gradual crossover occurs for the Ni films on Cu(111) at 8 to 12 ML. The finite-size scaling behavior in the few-monolayers-thickness range is compared with that reported for Ising thin-film behavior. In all instances T(C) extrapolates with decreasing thickness to zero at one monolayer. The dimensionality crossover and finite-size scaling behavior is discussed in the light of our current understanding of spin-wave quantization, anisotropy, and film microstructure.
MICROMAGNETIC PROPERTIES OF ULTRATHIN COBALT FILMS
(American Institute of Physics, 1991) Kief, MT; Mankey, GJ; Willis, RF; Pennsylvania Commonwealth System of Higher Education (PCSHE); Pennsylvania State University; University of Alabama Tuscaloosa
The properties of the magnetic domain structures of ultrathin fcc cobalt films epitaxially grown on Cu (001) have been examined using an ultrahigh vacuum surface magneto-optic Kerr effect instrument. The evolution of magnetic behavior is observed for film thicknesses ranging from 1.4 to 7.5 monolayers. The coercivity is sensitive to film growth temperature and thermal cycling history. The coercivity decreases with diminishing film thickness and falls to very low values for the thinnest layers. The results are discussed in terms of Neel domain-wall micromagnetics for ultrathin films.
QUANTUM-SIZE EFFECTS IN ULTRATHIN CU(100) FILMS
(American Institute of Physics, 1998-06-04) Mankey, GJ; Willis, RF; Ortega, JE; Himpsel, FJ; Autonomous University of Madrid; International Business Machines (IBM); University of Alabama Tuscaloosa
We report an angle resolved photoemission study of quantum size effects observed in the electronic structure of pseudomorphic Cu(100) films. The 3d band of a one-layer film exhibits an 80% reduction in its perpendicular dispersion and a 10% reduction in its parallel dispersion. The perpendicular dispersion of the 3 d band evolves rapidly with increasing film thickness, since the 3 d electrons are strongly localized on the ion cores. Quantum-well resonances are produced due to the discretization of the perpendicular wave vector in the s,p band. They produce oscillations in the Fermi level intensity as they disperse through it when the film thickness is increased. The observed oscillation period of approximately six layers matches that of oscillations in the magnetic exchange coupling between ferromagnetic Co films separated by Cu spacer layers. The connection of the quantum-well states to bulk electronic structure is determined by comparing oscillation amplitudes for two different photon energies.
QUANTUM-WELL STATES AND MAGNETIC COUPLING BETWEEN FERROMAGNETS THROUGH A NOBLE-METAL LAYER
(American Physical Society, 1993-01-15) Ortega, JE; Himpsel, FJ; Mankey, GJ; Willis, RF; Pennsylvania Commonwealth System of Higher Education (PCSHE); Pennsylvania State University; University of Alabama Tuscaloosa
Using inverse photoemission and photoemission we find that the bulk bands become discretized in highly perfect layer structures, such as Cu on fcc Co(100), Cu on fcc Fe(100), Ag on bcc Fe(100), Au on bcc Fe (100), fcc Co on Cu(100), and bcc Fe on Au(100). The electronic structure is analyzed in the framework of quantum-well states consisting of bulk Bloch functions modulated by an envelope function. The wavelength of the envelope function is determined from the lambda/2 interferometer fringes produced by the periodic appearance of quantum-well states with increasing film thickness. Using k conservation, one obtains an absolute measurement of the band dispersion for the s,p bands of Fe, Cu, Ag, and Au. Quantum-well states at the Fermi level are found to be closely connected with oscillatory magnetic coupling in superlattices. They are spin polarized, even in noble metals, due to the spin-dependent band structure of the confining ferromagnet. The oscillation period is half the wavelength of the envelope function. The corresponding wave vector is given by the Fermi wave vector and by the wave vector of the nearest s,p band edge via 2(k(edge) - k(F)). This turns out to be equivalent to Ruderman-Kittel-Kasuya-Yosida theory.
QUANTUM-WELL STATES AND MAGNETIC COUPLING BETWEEN NOBLE-METALS AND FERROMAGNETS
(American Institute of Physics, 1998-06-04) Ortega, JE; Himpsel, FJ; Mankey, GJ; Willis, RF; Pennsylvania Commonwealth System of Higher Education (PCSHE); Pennsylvania State University; University of Alabama Tuscaloosa
We have searched for the electronic states that mediate oscillatory magnetic coupling in superlattices, and have found strong evidence that these are quantum well states, which are created by quantizing the momentum of s,p-band states perpendicular to the interfaces. In noble metal layers on ferromagnets we find such states at the Fermi level, where they are able to influence magnetic coupling and transport. They exhibit several traits connecting them with oscillatory magnetic coupling, such as periodicity and spin polarization.
REORIENTATION PHASE-TRANSITION BEHAVIOR IN METASTABLE EPITAXIAL COBALT-COPPER ALLOYS
(American Institute of Physics, 1991) Kief, MT; Mankey, GJ; Willis, RF; Pennsylvania Commonwealth System of Higher Education (PCSHE); Pennsylvania State University; University of Alabama Tuscaloosa
We report experimental results on the ferromagnetic critical behavior of cobalt and metastable cobalt-copper alloy two-dimensional layers CuxCo1-x epitaxially grown on Cu(001). Copper alloying reduces the magnitude of the cobalt magnetic moment and introduces a magnetization component normal to the film. Reorientation of the magnetization back into the plane occurs via a second-order phase transition at a temperature which is lower than the film Curie temperature. We discuss these results in light of the recent predictions of Pescia and Pokrovsky.

Browsing by Author "Willis, RF"

Results Per Page

Sort Options