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We studied the transition between the antiferromagnetic and the surface spin-flop phases of a uniaxial antiferromagnetic [Fe(14 \AA)/Cr(11 \AA]$_{\rm x20}$ superlattice. For external fields applied parallel to the in-plane easy axis, the layer-by-layer configuration, calculated in the framework of a mean-field one-dimensional model, was benchmarked against published polarized neutron reflectivity data. For an in-plane field $H$ applied at an angle $\psi \ne 0$ with the easy axis, magnetometry shows that the magnetization $M$ vanishes at H=0, then increases slowly with increasing $H$. At a critical value of $H$, a finite jump in $M(H)$ is observed for $\psi5^{\rm o}$. A dramatic increase in the full width at half maximum of the magnetic susceptibility is observed for $\psi \ge 5^{\rm o}$. The phase diagram obtained from micromagnetic calculations displays a first-order transition to a surface spin-flop phase for low $\psi$ values, while the transition becomes continuous for $\psi$ greater than a critical angle, $\psi_{\rm max} \approx 4.75^{\rm o}$. This is in fair agreement with the experimentally observed results., Comment: 24 pages, 7 figures

In exchange-spring magnets with temperature-induced interfacial intermixing, differences in the extent of diffusion of constituent elements typically lead to local compositional changes. The authors demonstrate that adding an artificially created intermixed layer in Sm–Co∕Fe exchange-spring permanent magnets enhances the exchange coupling effectiveness without modifying the local composition of the Sm–Co layer. Element- and depth-resolved magnetization measurements verify that the diffusion extent of Sm and Co is similar across the interface.

Open recoil loops are often interpreted as a consequence of a breakdown in exchange coupling and attributed to the decoupled soft phase in exchange-coupled permanent magnets. However, in element-specific recoil loop measurements on Sm–Co∕Fe exchange spring magnets, the authors found that the open recoil loops were present not only in the soft (Fe) layer but also in the hard (Sm–Co) layer, and were not a consequence of exchange coupling breakdown between the soft and hard layers. Comparison between the experimental results and micromagnetic calculations reveals that the observed open recoil loops originate from the anisotropy variations in the Sm–Co layer.

Using element-specific, x-ray probes of magnetism the authors observe a net ferromagnetic moment from Mn in SrRuO3∕SrMnO3 (SRO/SMO) superlattice films. It is found that the magnetic behavior of the SRO and SMO layers is significantly modified by their exchange interaction. Bulk magnetometry shows a two-step, easy-axis magnetization reversal process and x-ray measurements confirm that the reversal with higher coercivity involves the magnetization in the SMO layers. The results provide strong evidence for the presence of pinned SRO magnetization at the SRO/SMO interface. Angle-dependent measurements reveal that the net Mn moment is due to a canted antiferromagnetic spin configuration in the SMO layers.

Digital superlattices of (LaMnO3)m∕(SrMnO3)2m (m⩽3) having the same nominal composition as La1∕3Sr2∕3MnO3, an antiferromagnetic insulator, have been synthesized by means of molecular beam epitaxy. All superlattices show a sharp resistive maximum at temperatures near to or higher than the Neel temperature of bulk La1∕3Sr2∕3MnO3. No discernible signatures of magnetic ordering are observed near the resistivity peak for m⩽2, but m=3 has a susceptibility peak consistent with antiferromagnet ordering, which is canted when cooled in a small magnetic field. Analogies are drawn to dx2−y2 orbitally ordered, layered antiferromagnetic manganites to explain the observations.

Cr-dosed FePt films with total thicknesses of 1.8 and 3.6nm were grown in an ultrahigh vacuum at room temperature and studied by means of the surface magneto-optic Kerr effect, scanning electron microscopy, and x-ray magnetic circular dichroism. After annealing at 550°C, the L10 and A1 phases are spatially separated. In the L10 phase, round nanoparticles form that are 15±4.8nm in diameter and possess out-of-plane magnetic easy axes. An addition of

The dynamics of individual magnetic vortices and vortex pairs confined in lithographically defined ferromagnetic ellipses were measured using a microwave reflection technique. Resonance frequencies were detected in the subgigahertz range for Ni80Fe20 (Permalloy) ellipses ranging in size from 3 by 1.5μm to 1 by 0.5μm, 40nm in thickness. Micromagnetic simulations indicate that the single mode observed for one vortex is a translational mode and that the two eigenmodes associated with a vortex pair correspond to coupled translational modes of vortices with parallel or antiparallel core polarizations. The single-vortex resonance scales inversely with the ellipse size, in agreement with the simulations and analytical theory.

Nonlocal spin injection is demonstrated in Co∕Cu∕Co lateral spin valves. Measurement of a series of structures with varying Cu length between the Co electrodes permits a direct determination of both the Cu spin diffusion length λs and the spin injection polarization P at the Co∕Cu interface. The value of λs is 200±20nm at 10K and ⩾110nm at 300K, which is shorter than previously reported, due to the possible presence of impurities in the Cu. However, the value of P is >7%, which is improved compared with previous reports of 2%, which is attributed to a higher interfacial quality.

We present an electron-beam heating device for a scanning tunneling microscopy (STM) that can be used for heating both the STM tip and the sample to 2200K. Mounted on a linear bellows with electrical feedthroughs, the device can be readily installed into the transfer path of a vacuum load lock. We demonstrate the heating capability of the device by flash cleaning Ru(0001) and Fe(001) crystals, and W tips. The flash-cleaned W tips are coated with Fe and further used for spin-polarized imaging of the Mn∕Fe(001) system.

The exchange-spring behavior found in Sm–Co (20nm)∕Fe epitaxial bilayer films was investigated by analyzing major hysteresis and recoil curves as a function of anneal conditions. The hard layer consists of nanocrystalline intermetallic Sm–Co hexagonal phases (majority phase Sm2Co7 with SmCo3 and SmCo5). Recoil curves, obtained from the successive removal to remanence and reapplication of an increasingly negative field from the major demagnetization curve, reveal the reversible and irreversible components of the magnetization. The Sm–Co thickness was fixed at 20nm while the Fe thicknesses of 10 and 20nm were studied, with ex situ annealing carried out in evacuated, sealed silica tubes at different temperatures. The peak in the recoil curve area is associated with the coercivity of the hard phase. The development of the soft component magnetization is revealed by the departure of the recoil area from zero with application of a reverse field. These two features together confirm that annealing stabilizes the ...

The spin dynamics are calculated for a model system consisting of magnetically soft, layered nanomagnets, in which two ferromagnetic (F) cylindrical dots, each with a magnetic vortex ground state, are separated by a non-magnetic spacer (N). This permits a study of the effects of interlayer magnetostatic interactions on the vortex dynamics. The system was explored by applying the equations of motion for the vortex core positions. The restoring force was calculated taking into account the magnetostatic interactions assuming a realistic surface charge free spin distribution. For tri-layer F/N/F dots with opposite chiralities and the same core polarizations (lowest energy state), two eigenmodes are predicted analytically and confirmed via micromagnetic simulations. One mode is in the sub-GHz range for submicron dot diameters and corresponds to quasi-circular rotation of the cores about the dot center. A second mode is in the MHz range corresponding to a small amplitude rotation of the mean core position. The eigenfrequencies depend strongly on the geometrical parameters of the system, suggesting that magnetostatic effects play a dominant role in determining the vortex dynamics., Comment: One PDF file including text and 4 figures

The high-frequency dynamic properties of patterned ferromagnetic microstripe arrays were studied using ferromagnetic resonance (FMR). The samples were fabricated by means of electron-beam lithography and magnetron sputtering followed by liftoff. The 20nm thick and 1μm wide stripes were arranged into a regular array with a period of 2μm. The resonance spectra were taken at a fixed frequency of 9.38GHz for different sample orientations with respect to the applied field. In the case of perpendicularly magnetized stripes, a set of multiple resonance peaks is observed below the main FMR peak. A quantitative description of the observed spectra is given using a dipolar-exchange spin-wave dispersion equation. The calculations are in agreement with the experimental data.

It is demonstrated here that an already ideal exchange–spring magnet can be further improved by intermixing the interface. This is counter-intuitive to the general expectation that optimal exchange–spring magnet behavior requires an ideal, atomically coherent soft–hard interface. Epitaxial Sm–Co/Fe thin-film exchange–spring bilayers are thermally processed, by annealing or high-temperature deposition, to induce interdiffusion. With increasing processing temperature, the hysteresis loop becomes more single-phase-like, yet the magnetization remains fully reversible. The interface is characterized via synchrotron x-ray scattering and electron microscopy elemental mapping. The magnetization behavior is modeled by assuming a graded interface where the material parameters vary continuously. The simulations produce demagnetization curves similar to experimental observations.

The magnetization reversal process for micron and submicron disk-shaped dots is controlled by successive nucleation, displacement, and annihilation of a magnetic vortex. Here the reversal process for a system involving two ferromagnetic disks separated by a nonmagnetic spacer is investigated experimentally, analytically, and numerically. Permalloy (Ni80Fe20 or Py) dots with thicknesses of up to 40nm and diameters of 0.5–2.5μm separated by a copper spacer (1–45nm thick) were considered. Micromagnetic simulations indicate that the disks will each support oppositely directed vortices at remanence and also show the hysteresis of the coupled structures. The calculations are compared to hysteresis loops and x-ray photoemission electron microscopy images of Py∕Cu∕Py dots produced by electron-beam lithography and magnetron sputtering.

We have investigated the effect of substrate inclination on the in-plane magnetic anisotropy of ultrathin Fe films grown on Al2O3(0001). For Fe films grown on flat Al2O3 substrates, no preferred direction of magnetization exists in the film plane due to the three equivalent epitaxial orientations of Fe(110) that form. However, for Fe grown on an inclined Al2O3 substrate, a uniaxial anisotropy appears that is parallel to the step edges. The anisotropy increases in magnitude with decreasing Fe thickness and growth temperature, and as the surface morphology changes from being rough to being smooth. We attribute the uniaxial anisotropy to the effective demagnetization field caused by the surface corrugation.

Magnetic nanoparticles coated with biochemical ligands are enabling many biological and medical applications. In particular biomagnetic sensors have potential advantages of simplicity and rapidity. We demonstrate a substrate-free biomagnetic sensing approach using the magnetic ac susceptibility of ferromagnetic particles suspended in a liquid. The magnetic relaxation of these particles is mainly due to Brownian rotational diffusion, which can be modified by binding the particles to the intended target. This scheme has several advantages: (i) it requires only one binding event; (ii) there is an inherent check of integrity; and (iii) the signal contains additional information about the target size.

The in-plane Fe spin spiral structure for $\mathrm{F}\mathrm{e}/\mathrm{S}\mathrm{m}\ensuremath{-}\mathrm{C}\mathrm{o}(11\ifmmode\bar\else\textasciimacron\fi{}00)$ exchange-spring magnetic bilayers was investigated by ${}^{57}\mathrm{Fe}$ conversion electron M\"ossbauer spectroscopy during the magnetization reversal process, and correlated with hysteresis loops measured via the longitudinal magneto-optic Kerr effect. A direct experimental proof for a relaxed spiral angular spin distribution in the Fe layer is presented. Under negative applied fields the interfacial Fe spins rotate by up to $40\ifmmode^\circ\else\textdegree\fi{}$ with respect to the easy axis of the Sm-Co hard-magnet layer.

We have studied superparamagnetic behavior of ultrathin Fe films grown on an Al2O3(0001) substrate at various growth temperatures. It is demonstrated that 1-nm-thick Fe films are in the superparamagnetic state, and the blocking temperature is strongly dependent on the growth temperature. The blocking temperature has a minimum value of 30 K for a growth temperature of 473 K, while it is ∼70 K at other growth temperatures. In order to clarify the behavior, we consider the Fe growth mechanism and the magnetic interactions between Fe particles. Fe grows as three-dimensional islands at all temperatures studied and forms particles. The volume of the particles is observed via atomic force microscopy to increase with increasing growth temperature. In the case of growth at 323 and 373 K, Fe forms small particles that are close together and that interact with each other. For growth at 673 and 773 K, Fe forms relatively large particles and the magnetic properties are dominated by the individual particles.

A method for the analysis of temperature and field dependent magnetization data near the Curie temperature TC is devised, which allows the reliable determination of critical exponents even in strongly inhomogeneous ferromagnets. It is shown, both experimentally and theoretically, that the field-induced broadening of the ferromagnetic phase transition follows a power law with a characteristic critical exponent β⋅δ, where β and δ are the conventional temperature and field dependent critical exponents of the magnetization. Experimental results on inhomogeneous La2/3Ca1/3MnO3 films yield critical exponents β=0.3682±0.0033 and δ=4.76±0.32, even though the intrinsic TC distribution width is larger than the field-induced effect.

(Received 26 November 199 1) In a recent letter’ an elementary formula was derived for the complex magneto-optical polar Kerr rotation 0k from a model superlattice. The calculations apply, at nor- mal incidence only, to a semi-infinite two component su- perlattice in the specific case where the refractive indices nl and nz of the two media (labeled 1 and 2) are equal. An approximate formula is also given for flk when n1 and n2 differ by a small amount A given by A= (n2-nl)/nl. (1) The only conditions for the applicability of the formula are that 2r[nlldl4 and 27-r 1 n2 d24 (2) where

We present the effects of antiferromagnetic (AF) order of the Cr spacers in Fe/Cr(001) superlattices on the interlayer coupling of the Fe layers. AF order of the Cr spacers is suppressed for layer thicknesses less than 42 A. For ≳42 A of Cr, the Neel temperature (TN) increases rapidly and asymptotically approaches the bulk value for thick Cr spacers as characterized by a transition‐temperature shift exponent λ=1.4±0.3. Neutron diffraction confirms both the AF order of the Cr layers in superlattices with 62, 100, and 200 A thick Cr layers, and the existence of the incommensurate, transverse spin‐density‐wave magnetic structure whose nesting wave vector is equal to that of bulk Cr. The AF ordering of the Cr results in anomalies in a variety of magnetic properties, including the interlayer coupling, remanent magnetization, coercivity, and magnetoresistance. Most strikingly, the 90° or ‘‘biquadratic’’ coupling of the Fe layers observed for T≳TN is suppressed below TN as confirmed by polarized neutron reflecti...

Angle‐ and spin‐resolved photoemission was utilized to investigate the 5d bulk bands and the surface state of Gd(0001) in the temperature range of 130–350 K. The bulk bands at 1–2 eV below the Fermi energy EF show Stoner‐like behavior, while the temperature dependence of the surface state near EF indicates spin‐mixing behavior due to fluctuating local 5d moments. The secondary electron spectra of the Gd surfaces both before and after initial oxygen adsorption show a polarization dip at low kinetic energies due to the extra scattering channel for minority electrons via the unoccupied 4f level. The temperature dependencies of the surface and bulk magnetization are separated using the spin polarization of the surface state and the bulk exchange splitting.

We report giant magnetoresistance (GMR) as high as 150% at 4.2 K in epitaxial Fe/Cr superlattices. Samples prepared by magnetron sputtering on MgO(100) produced the expected Fe/Cr(100) orientation; however, deposition on MgO(110) produced a novel (211) low‐miller‐index orientation. The crystallographic orientations and superlattice nature of the samples were confirmed by both low‐ and high‐angle x‐ray diffraction. The Fe/Cr(211) superlattices have an in‐plane uniaxial anisotropy (KU=9×105 ergs/cm3) along the Fe[011] which give the sample a rich magnetic behavior. The GMR with the current along the [011] is ≊20% higher than that along the [111] direction of the (211) plane. The GMR for the (211) superlattices shows four oscillations with Cr thickness. Both orientations shows the expected long‐period oscillation of 18 A Cr and a GMR increase by a factor a 4 when cooled from room temperature to 4.2 K.

An analysis of the bonding and charge transfer as a function of transition-metal concentration is presented for the bct superconducting system Y(Rh/sub 1-x/Ru/sub x/)/sub 4/B/sub 4/. A sharp drop in the superconducting critical temperature T/sub c/ from 9.5 K to below 1.0 K near a critical concentration is not reflected in the smooth, linear variation of the single-crystal lattice parameters and B-B interatomic distances. Analysis of the boron KVV Auger data indicates the boron p-like states near the Fermi energy are increasingly populated in a continuous manner with increasing x. We find no evidence of any abrupt changes in the electronic structure near x/sub cr/.

Low-energy electron-diffraction, Auger-electron, He I photoemission, and electron-energy-loss spectroscopies were used to characterize the epitaxy and electronic structure of Cr overlayers deposited on single-crystal Au(100). Epitaxy was achieved for substrate-deposition temperatures above approx.100--150 /sup 0/C. The observed strain-free p(1 x 1) Cr/Au(100) epitaxy is due to the fortuitous matchings of the primitive two-dimensional square nets of Cr(100) and Au(100) bulk terminations. Surface states near the Fermi energy of pure Cr(100) are found to persist at the Cr-Au interface, in agreement with recent theoretical findings. The electronic structure of Cr multilayers on Au(100) is closely related to that of ordinary bcc Cr. The results are discussed in terms of the recently reported superconducting behavior of Au/Cr/Au(100) sandwiches attributed to fcc Cr possessing the 41%-expanded lattice constant of Au. An alternate hypothesis for the sandwich superconductivity is suggested in terms of disordered Cr.

Longitudinal Kerr-effect measurements using both s- and p-polarized light are presented as a function of thickness for bcc Fe films grown epitaxially on Au(100) in ultrahigh vacuum (UHV). The motivation is to explore the origins of the magnetooptic response at Fe thicknesses comparable to the depth penetration of light. Special efforts were taken to optically compensate for the birefringence of the UHV window. The compensated measurements yield the magnitude of the complex rotation phi/sub m/ = (phi'/sup 2/+phi''/sup 2/)/sup 1/2/, where phi' and phi'' are the real magnetooptic rotation and the ellipticity, respectively. The results show linear initial increase of phi/sub m/ with thickness (up to the optical penetration depth) followed by a shallow peak and leveling off to a saturation value of the thick-film Kerr rotation. A review of the relevant theory includes the Faraday and Kerr contributions to the magnetooptic response. The Faraday contribution arises from metallic reflection from the substrate and passage back through the iron overlayer. We argue that the Faraday effect dominates the response in the ultrathin limit, while the Kerr effect controls the thick-film regime.

The surface magneto-optic Kerr effect (SMOKE) technique was used to search for ferromagnetism in monolayer-range films of Cr and Fe grown on Au(100) and Cu(100). The growth modes were characterized using low energy electron diffraction (LEED) and Auger electron spectroscopy. The fcc structure of Cr could not be stabilized on Cu(100). Ferromagnetism was not observed for the Cr/Au(100) films at temperature above 100 K. Ferromagnetism also was not observed for fcc Fe/Cu(100) grown at room temperature; but for growth at >150°C, a ferromagnetic, metastable state was observed for the top layer of the Fe film, in the absence of bulk ferromagnetism. The ferromagnetic Fe/Au(100) system was used to establish the sensitivity of the approach.

Two closely lying (in energy) states of fcc iron grown on copper (100) have been identified by magneto-optic and photoemission experiments. The as-grown state at 460 K exhibits in-plane surface ferromagnetism in magneto-optic measurements. Ferromagnetism parallel to the plane is not observed for the room-temperature state; its absence is supported by a comparison of our calculated and measured work functions and by the photoemission results. LEED shows expanded interplanar spacing at the surface. The temperature behavior and effect of fresh iron deposition indicate that the thermal transition between the two states is first order.

Local electronic density of states (DOS) at the boron site for Y(Rh/sub 1-x/Ru/sub x/)/sub 4/B/sub 4/ superconductors, with x = 0.15, 0.50, and 0.85, have been characterized by means of B KVV Auger line-shape analyses, with supporting data from x-ray- and ultraviolet-photoemission experiments and electron-energy-loss experiments. The motivation was to test Johnston's model, in which the unusually precipitous drop of the superconducting transition temperature for x> or approx. =0.5 is attributed to an abrupt charge transfer to the boron site. Comparisons of the experimental Auger line shapes are made to those we have generated from band-structure calculations by Jarlborg et al. for the related compound YRh/sub 4/B/sub 4/. The Auger data reveal a continuous evolution of the B local DOS with x, which does not support the Johnston model. The analyses indicate that there is a continuous modification of the hybridization of the B p-like and the transition-metal d-like electronic states with x. We suggest that these changes in the pd hybridization may be related to a previously reported lattice distortion for this system, and that these effects directly influence the electron-phonon interaction strength.

Calculated Si ${L}_{23}\mathrm{VV}$ Auger line shapes are presented for five realistic structural models of hydrogenated amorphous silicon ($a$-Si: H), based on the local-electronic-structure calculations of Ching, Lam, and Lin. The calculated Auger spectra show peak shifts of the main Si ${L}_{23}{M}_{23}{M}_{23}$ ($\mathrm{pp}$) transition up to 6 eV, and distinct line shapes dependent on the H-bonding configuration. Comparison of the calculated spectra to the experimental work of Allie et al. on sputtered $a$-Si: H clearly shows an expected increased prominence of the ${L}_{23}{M}_{1}{M}_{23}$ ($\mathrm{sp}$) term relative to the ${L}_{23}{M}_{23}{M}_{23}$ ($\mathrm{pp}$) term, due to the Si $3s$ localization upon hydrogenation. The results indicate that valuable structural information is available from Auger spectroscopy for this interesting system.

The magnetic field dependence (hysteresis curve) and the temperature dependence of the magnetization at a Ni(110) surface was measured by polarized low-energy electron diffraction. The diffracted intensities are spin dependent by a few percent. The temperature dependence of the surface magnetization measured in the range $0.5l\frac{T}{{T}_{c}}l0.8$, is significantly different from that of the bulk.

The heat capacities of V/sub 3/Si, V/sub 3/Ga, V/sub 3/Sn and V/sub 3/Sn/sub 0.5/Ga/sub 0.5/ have been measured in the temperature range 2-400K. It is found that V/sub 3/Ga and V/sub 3/Si (which have high T/sub c/'s) have lower high temperature heat capacities than the other (lower T/sub c/) materials. The reason for this correlation is discussed. It is concluded that the higher T/sub c/ materials have unusually large anharmonicities. A model relating the high temperature anharmonic heat capacity to T/sub c/ is developed. The assumptions of this model are critically discussed. 24 references.

We have studied the electronic structure of fcc iron grown epitaxially on (100) copper with ultraviolet photoemission (UPS), surface magneto‐optic Kerr effect (SMOKE), low‐energy electron diffraction (LEED), and self‐consistent film linearized muffin‐tin‐orbital (FLMTO) calculations. This study identifies and explains novel characteristic UPS features of fcc iron and a substantial change in the work function compared to bcc iron. The SMOKE measurements show that fcc iron grown on copper has two metastable states lying close in energy. One of these is not ferromagnetic and occurs at room temperature, while the other is ferromagnetic and occurs at 190 °C. As shown by LEED, the ferromagnetic state has an expanded surface interplanar lattice; and UPS shows exchange splittings in that state.

Heat capacity measurements between 0.07 and 20 K and 0 and 38 kOe are reported for the system ( La Ce)Al2. Normal state measurements show that the crystal-field ground state of the Ce ion is a doublet and reveal a characteristic Kondo anomaly in zero magnetic field. In the superconducting state there is evidence for an impurity band at low energies within the gap, and for a second order transition at the lower critical temperature Tc2.

Born--von Karman (BvK) lattice-dynamical calculations were performed for the rhombohedral Chevrel-phase superconductors such as PbMo/sub 6/S/sub 8/. The dynamical matrix was obtained using Lennard-Jones (LJ) potentials, which describe interactions between Mo-Mo, Mo-S, S-S, and Pb-S pairs of atoms. The parameters for these potentials were adjusted using the comparison of the moments of the phonon spectrum F (..omega..) with their values obtained from heat-capacity data. Dispersion curves along the three-fold axis show a splitting of the three Pb-atom-dominated modes into a low-lying transverse doublet and a longitudinal singlet. These branches hybridize with the acoustic branches. F (..omega..) is calculated for binary and ternary Chevrel-phase sulfides and selenides by making appropriate mass changes. Moments and site-dependent projections of F (..omega..) are calculated and their associated Debye temperatures are tabulated. Torsional character is found for certain modes in agreement with previous identifications made using the molecular-crystal model. The heat capacity, entropy, inelastic neutron-scattering spectra, Moessbauer recoil-free fraction for /sup 119/Sn, anharmonicity, the apparent T/sup 2/ resistivity below 40 K for sputtered films, and the form of the superconducting tunnelling spectrum ..cap alpha../sup 2/F are discussed. The applicability of the BvK approach using LJ potentials indicates that short-range interactions are important. Hence, the previouslymore » introduced molecular-crystal model is a reasonable, though not rigorous, simplification of the lattice dynamics of this important class of superconductors.« less

Autoionization emission associated with direct recombination of the resonant 3p..-->..3d transitions have been examined for all the 3d transition-metal elements from /sub 21/Sc to /sub 29/Cu. Two characteristic trends with atomic number Z are observed which correlate with those previously reported in the 3p-electron energy-loss spectra. Firstly, the intensity of this emission decreases with increasing Z. Secondly, the separation of the emission peak from the 3p threshold is large (approx.10 eV) toward the beginning of the series (/sub 21/Sc through /sub 24/Cr) and abruptly drops to a few electron volts near the middle of the series (/sub 25/Mn, /sub 26/Fe). This latter reduction in peak separation causes the autoionization and (M/sub 2,3/VV) Auger emission to merge into a single spectral feature. We show for /sub 25/Mn (and /sub 22/Ti) that the two contributions can be separately identified by means of oxygen-dosing experiments. Oxidation dramatically shifts the Auger emission to lower energy leaving the autoionization emission as a separate peak pinned above the 3p threshold. These measurements shed light on the spectral details in the 3p line shapes of the autoionization and the electron-energy-loss spectra. We report angle-resolved measurements for Cr which show different angular dependences of the emission and absorptionmore » processes.« less

We report new measurements of the normal-state electrical resistances of several high-${T}_{c}$ (\ensuremath{\sim} 20 K) $A\ensuremath{-}15$ structure superconductors. It is found that the resistances are linear functions of ${T}^{2}$ from ${T}_{c}$ to 40 K. Analysis of available lattice heat-capacity data on the same materials shows that it also varies as ${T}^{2}$ over the same temperature interval. These observations suggest that the ${T}^{2}$ resistivity is due to electron-phonon scattering. This interpretation is supported by the results of a model calculation for the temperature dependence of the resistivity and the lattice-heat capacity of ${\mathrm{Nb}}_{3}$Sn.

The heat capacities of ${\mathrm{Nb}}_{3}$Al, ${\mathrm{Nb}}_{3}$Sn, and ${\mathrm{Nb}}_{3}$Sb were measured between 2 and 400 K. Their superconducting transition temperatures ${T}_{c}$ are 18.5, 17.9, and 0.2 K, respectively. The higher-temperature entropy was analyzed, and it was found that phonon-mode softening occurs on cooling for the two high-${T}_{c}$ compounds, but not for ${\mathrm{Nb}}_{3}$Sb. Geometric mean phonon frequencies ${\ensuremath{\omega}}_{g}$ are also evaluated for these compounds and for ${\mathrm{V}}_{3}X$, where $X=\mathrm{Si},\mathrm{Ga},{\mathrm{Ga}}_{0.5}{\mathrm{Sn}}_{0.5}, \mathrm{and} \mathrm{Sn}$. As is shown, ${\ensuremath{\omega}}_{g}$ is an appropriate phonon average for evaluating McMillan's expression for the electron-phonon mass enhancement. Average phonon properties cannot reliably be used to calculate ${T}_{c}$ values for these $A\ensuremath{-}15$ materials. It is suggested that, for the higher electronic density-of-states materials, select phonon modes strongly couple to the electronic system and influence the magnitude of ${T}_{c}$ to a greater extent than average-phonon correlations would indicate. This view is supported by the observation that the higher-density-of-states materials exhibit the more pronounced phonon-mode softening on cooling.

We report ultraviolet and x-ray-photoelectron spectroscopy and electron-energy-loss spectroscopy results for clean LaCoO/sub 3/ and LaCoO/sub 3/ surfaces covered with chemisorbed oxygen at room temperature. Also the temperature dependence of the HeII spectrum (h..nu..=40.8 eV) for clean LaCoO/sub 3/ was monitored between approx.140 and 1050 K. We found that chemisorbed oxygen caused significant changes in the surface electronic structure of LaCoO/sub 3/ near the Fermi energy, as did changing temperature. Temperature-dependent electronic properties are expected, based on the known bulk low-spin--high-spin conversion of the trivalent d/sup 6/ Co ions, as has been successfully addressed by Goodenough. The temperature dependences we observed, while consistent with Goodenough's bulk model, could also be understood if the ground state of the surface Co ions were reversed from that of the bulk. We suggest that such a reversal could be driven by a small positional relaxation (expansion) of the top atomic layers of the material. Finally we discuss the implications of the chemisorption results and the postulated surface relaxation in light of the Voorhoeve picture of the high catalytic activity of LaCoO/sub 3/ for CO oxidation.

A relatively simple technique for studying surface magnetism is employed using the magneto-optic Kerr effect. Magnetic hysteresis data are presented for clean, monolayer-range Fe films epitaxially-deposited on Au(100) in the (1×1) surface structure. The Curie temperature TC is reduced to values near half that for bulk (bcc) Fe for coverages below one layer.

The surface magneto-optic Kerr effect (SMOKE) is used to study the magnetism of monolayer-range films of Fe grown on Au(100). The Fe grows epitaxially, in the [100] direction, in a layer-by-layer fashion for the first few layers, and raises a layer of Au above it to lower the surface free energy. Hysteresis curves have been measured for submonolayer and few-layer-thick films as a function of temperature. The Curie temperature for a 0.56 monolayer film is substantial (150°C) but markedly reduced from the Curie temperature for bulk Fe. The overall shape of the hysteresis curve and the coercivity give information on the morphology of the films corollary to LEED and Auger studies.

The surface magneto‐optic Kerr effect (SMOKE) is used to explore the magnetism of ultra thin Fe films extending into the monolayer regime. Both bcc α‐Fe and fcc γ‐Fe single‐crystalline, multilayer films are prepared on the bulk‐terminated (1×1) structures of Au(100) and Cu(100), respectively. The characterizations of epitaxy and growth mode are performed using low‐energy electron diffraction and Auger electron spectroscopy. Monolayer range Fe/Au(100) is ferromagnetic with a lower Curie temperature than bulk α‐Fe. The controversial γ‐Fe/Cu(100) system exhibits a striking, metastable, surface magnetic phase at temperatures above room temperature, but does not exhibit bulk ferromagnetism.

Heat-capacity (2-400 K) and inelastic-neutron-scattering studies of the phonon spectra are reported for superconducting, high-critical-field, Chevrel-phase, lead and tin molybdenum sulfides (${T}_{c}\ensuremath{\sim}11\ensuremath{-}15$ K). Nine phonon modes per unit cell are identified whose frequencies are in the immediate region Bergmann and Rainer indicate is necessary to optimize ${T}_{c}$. Acoustic-phonon softening on cooling is observed. A molecular-crystal model is proposed for the lattice dynamics of these materials.

Total energies as a function of carbon-metal-layer separation and carbon-metal perpendicular stretch frequencies are calculated for small clusters configured to be analogs for the chemisorption system CO on Cu/Ru(0001). The clusters (with 7--10 metal atoms) include analogs of CO/Cu(111) and CO/Ru(0001) with on-top and threefold binding sites. The latter are more stable in both cases. Mixed-metal clusters are also studied (i) with CO bound to Cu in proximity to Ru and (ii) with CO bound to Ru in proximity to Cu. The former, case (i), show enhanced CO binding with respect to CO on pure Cu, and the latter, case (ii), show weakened CO binding with respect to CO on pure Ru. Possible precursor intermediate states are described for commensurate and incommensurate epitaxy. Bonding to Ru sublayers through a Cu adlayer is also described. The results are discussed in conjunction with previous experimental findings, and anticipate future experimental vibrational characterizations.

The heat capacities of five samples of $\ensuremath{\alpha}$-uranium, including one single crystal, have been measured between approximately 0.1 and 2 K, at zero pressure. The four polycrystalline samples showed broad bulk superconducting transitions. The single crystal, for which susceptibility measurements showed a transition near 0.3 K, was not completely superconducting at the lowest temperature of our measurements, and we estimate ${\overline{T}}_{c}\ensuremath{\le}0.1$ K. The shapes of the heat-capacity anomalies associated with the transitions to the superconducting state were those of broadened BCS transitions, thus showing that local moments and pair-breaking mechanisms are not involved in limiting the values of ${T}_{c}$. The values of $\ensuremath{\gamma}$, the coefficient of the electronic heat capacity, were significantly higher for the polycrystalline samples than for the single crystal, and there is evidence from other work of a similar trend in the lattice heat capacity. It is suggested that these trends are related to the pressure dependence of the same parameters, and that these pressure dependences and that of ${T}_{c}$ are all produced by an unusual and strong pressure dependence of the phonon spectrum.