Search

Structural properties of single-phase films of {kappa}-In{sub 2}Se{sub 3} and {gamma}-In{sub 2}Se{sub 3} were investigated. Both films were polycrystalline but their microstructure differed considerably. The a-lattice parameter of {kappa}-In{sub 2}Se{sub 3} has been measured. Comparison between these two materials indicates that {kappa}-In{sub 2}Se{sub 3} has a significantly larger unit cell ({Delta}c = 2.5 {+-} 0.2 % and {Delta}a = 13.5 {+-} 0.5%) and a structure more similar to the {alpha}-phase of In{sub 2}Se{sub 3}.

The log normal distribution is shown to be useful for characterizing cluster distributions produced by coalescence growth mechanisms. The Smoluchowski equation and variations thereof produce cluster size distributions very similar to those produced using nozzle beam expansions and laser chemistry of organometallics. The model provides a statistically unbiased basis for interpreting cluster size distributions produced using a wide variety of synthetic methods. It also provides a unified chemical and physical basis for discussing and rationalizing the results of a wide range of gas phase cluster experiments. Under certain conditions, size distributions can be produced in which there is an alteration in the number of odd and even clusters produced. In addition to some inferences regarding fullerene chemistry, data gleaned from the literature are rationalized on the basis of the kinematics of cluster formation, the fractal dimension of clusters composed of different atoms, the Periodic Table, and the degree to which the translational motion of the coalescing species is diffusional or ballistic.

Intracavity laser absorption spectroscopy was used to measure the number density of neutral ground state barium atoms desorbed from an operating dispenser cathode. The measurement suggests an order of magnitude value of 108 atoms/cm3 at a distance of 5 cm from the emitting surface. This is the first direct state selective measurement of the barium density near to the surface of an operating cathode which can be directly traced to conventional multipass absorption spectroscopy. The value we obtain is consistent with results reported by other workers using less direct methods and allows more detailed statements to be made concerning the chemical state of the Ba at the instant of desorption.

We have used mass selected multiphoton dissociation/excitation/collisional ionization (MPD/MPE/CI) spectroscopy to study benzene chromium tricarbonyl (BCT) in an effusive molecular beam under collisionless and collisional conditions. Under collisionless conditions, our results are entirely consistent with, but not identical to, our earlier nonmass selected published experiments under bulk gas conditions. The spectroscopic results confirm that we can produce chromium atoms in a variety of well defined Rydberg states (7S3[Ns3d5], N=30–43). Following the state selective preparation of the chromium Rydberg atoms, by MPD of BCT followed by MPE of the ground state chromium atoms, they can be allowed to collide with ground electronic state benzene chromium tricarbonyl molecules producing ground state chromium ions and other products. We observe the scaling of the cross section for this collisional ionization process as a function of the Rydberg electron principal quantum number. The cross sections are a strong f...

Multiple‐frequency (2–14 GHz) ferromagnetic resonance (FMR) has been used to directly observe the coupled resonance modes of a large set of single‐crystal Fe/Cr/Fe(001) sandwiches grown by molecular‐beam epitaxy. The FMR data reveal two resonance modes which have complicated frequency dependencies for those samples that show antialigned Fe layers in zero applied field. Such alignment is only found for samples with 12 A

Direct observation of the magnetization reversal of epitaxial NiO/NiFe bilayers grown on (001) MgO and on polycrystalline Si substrates was performed by using the magneto-optical indicator film technique. It was shown that the unidirectional-axis magnetization reversal proceeds by domain nucleation and growth. A new phenomenon, an asymmetry in the activity of the domain nucleation centers, has been revealed. Remagnetization of the bilayer is shown to be governed by defect structures in the antiferromagnetic layer.

In recent work [J. Appl. Phys. 70, 10 (1991)] the spin valve effect (SVE) was measured in CoxFe1−x/Ag/Fe(x

Soft x‐ray core‐level photoabsorption has been measured from three forms of boron nitride, namely the hexagonal (hBN), cubic (cBN), and incoherent (iBN) phases. The B 1s and N 1s photoabsorption spectra of the hBN sample show evidence of both σ*‐ and π*‐empty states, indicative of sp2 bonding, while the cBN absorption spectra exhibits only sp3 bonding in absorption features characteristic of the zincblende structure. The incoherent phase of BN in the form of a thin film on a Si substrate possesses σ* and π* states similar to hBN. The B 1s π* absorption features of the iBN film show an angular dependence which suggests that the microstructure consists of hexagonal‐phase layer planes oriented normal to the BN/Si interface.

The coercive field of thin Co films is strongly dependent on substrate temperature and film thickness, but the magnetic properties of Fe films are only weakly affected by growth conditions. Fe‐Cu‐Co sandwiches deposited on glass and silicon substrates via electron‐beam evaporation exhibit plateaus in their magnetization curves when the coercive field of the Co is made larger than that of the Fe. These plateaus correspond to an applied field region in which the Co and Fe moments are antialigned and the resistance is enhanced. The isotropic or spin‐valve part of the magnetoresistance is as large as 3.3% in some sandwiches. The spin‐valve magnetoresistance has a broad peak centered at a ferromagnetic layer thickness of 60 A.

Fe‐Cu‐Co sandwiches have been grown on glass and silicon substrates using electron beam evaporation. Because the typical coercive field of a thin Co film is higher than that of a typical Fe film, the magnetization curves of these sandwiches show clearly the separate, sequential reversal of the Fe and Co moments as a function of applied field. In the applied field region where the moments of the two magnetic layers are antialigned, the spin‐valve magnetoresistance shows a peak of over 3% in amplitude at room temperature.

Nonresonant microwave‐absorption signals have been observed in a series of molecular‐beam‐epitaxy‐grown Fe/Cr/Fe (001) sandwiches which were previously shown to exhibit an antiferromagnetic coupling of the two Fe layers. Using field‐modulation detection, one observes signals which display an unusual magnetic‐field dependence containing sharp features in the range 0–2 kOe. The magnetic‐field locations of these features are independent of the microwave frequency, but their intensity depends strongly on the field‐modulation amplitude. The features also have a strong in‐plane angular dependence. Comparison with earlier magnetization and magnetoresistance data on the same samples shows that a sharp feature occurs when the magnetization orientations in the Fe films change abruptly, and that one can explain the microwave‐absorption signals in terms of magnetoresistance effects in the sandwich. Thus, microwave absorption provides a contactless way to investigate the magnetoresistance in such samples.

Magnetoresistance measurements have been performed on a set of molecular‐beam epitaxy‐grown Fe‐Cr‐Fe sandwiches with different chromium thicknesses, 12 A

An ultrahigh-density electron-beam-based data storage medium is described that consists of a diode formed by growing an InSe/GaSe phase-change bilayer film epitaxially on silicon. Bits are recorded as amorphous regions in the InSe layer and are detected via the current induced in the diode by a scanned electron beam. This signal current is modulated by differences in the electrical properties of the amorphous and crystalline states. The success of this recording scheme results from the remarkable ability of layered III-VI materials, such as InSe, to maintain useful electrical properties at their surfaces after repeated cycles of amorphization and recrystallization.

The connection between the spin structure of antiferromagnetic NiO and the exchange anisotropy observed in NiO/NiFe bilayers is not well understood. For instance, the NiO bulk-terminated (001) surface is compensated, and therefore simple models predict no exchange bias in (001)-oriented NiFe/NiO bilayers.1 Using a newly developed ion-beam sputtering (IBS) process to deposit NiO exchange-coupled films,2 we have simultaneously grown polycrystalline and epitaxial NiO/NiFe bilayers. NiO grown on NiFe/MgO is polycrystalline, while NiO grown directly on MgO is epitaxial. The in-plane orientation of the epilayers was confirmed using (hk0) x-ray diffraction. The exchange anisotropy in epitaxial (001)-oriented bilayers is about half as large as that observed in polycrystalline bilayers. The size of the exchange anisotropy does not depend on the orientation of the bias field with respect to the in-plane NiFe/NiO crystallographic direction, indicating that the same interfacial spin structure is achieved regardless o...

The magnetic coupling of the Fe layers in Fe/Si multilayers strongly depends on the morphology of the iron‐silicide interlayer that forms during deposition. Antiferromagnetic interlayer coupling is only observed in Fe/Si multilayers with crystalline interlayers in the CsCl structure. Recently, it has been shown that single layers of Fe–Si in the CsCl structure can be grown epitaxially on Si over a range of stoichiometries. FeSi films are reported to be Kondo insulators below 50 K. We find evidence of a magnetic phase transition in antiferromagnetically coupled Fe/Si multilayers. Ms measured in a constant applied field of 50 kOe shows T3/2 behavior down to 10 K. However, M(T) at a lower constant field peaks around 50 K and decreases at lower temperature, indicating enhanced antiferromagnetic coupling or a phase transition. The remanent magnetization increases monotonically with decreasing temperature and has been explained by invoking thermally activated coupling. However, the saturation field also increas...

A novel design is presented for a cryogenically cooled multisubstrate sample holder for thin film deposition in high vacuum. The cryogen flows through tubing soldered onto a fixed copper plate, while the substrates are mounted on a separate tray that is supported by a linear/rotary‐motion feedthrough. By lowering the substrate tray into firm contact with the copper plate, substrate temperatures as low as 120 K can be achieved.

We report the first results employing a new technique for studying heteromagnetic multilayers by element specific magnetic hysteresis measurements. Dramatically different Fe and Co hysteresis curves of Fe/Cu/Co trilayers were obtained by recording the soft x‐ray magnetic dichroism1 at their respective L3 absorption edges as a function of the applied magnetic field. The complicated conventional hysteresis curves, obtained by a vibrating sample magnetometer, were resolved into linear combinations of the two individual elemental hysteresis curves, thus determining the average atomic magnetic moments of each element within the multilayer structure (Fe is 2.0±0.08 μB and Co is 1.1±0.04 μB). Furthermore, fine hysteresis features associated with one element, imperceptible in the conventional curves, were also observed, and demonstrate a new and powerful means for studying heteromagnetic multilayer systems.

Thin films of Fe with interleaved Cr have generated considerable interest because of the antiferromagnetic coupling between the Fe layers. This coupling has been examined using ferromagnetic resonance and other measurements. The results have been explained on the basis of the phenomenological expression for the magnetic free energy. We have determined the magnetic free energy for a single‐crystal Fe/Cr/Fe(001) sandwich grown by molecular beam epitaxy using torque magnetometry. At fields below the anisotropy field, both the torque and free energy show a complex switching of the antiferromagnetically coupled Fe moments (Fig. 1). Preliminary calculations show that the exchange and the angle between the antiferromagnetically coupled Fe moments can be determined directly from these curves. We shall discuss our measurements and calculations and compare our results with the earlier ferromagnetic resonance results.

Single‐crystal Fe‐Cr‐Fe sandwiches have been epitaxially grown in the (100) bcc structure on ZnSe epilayers on GaAs substrates. These sandwiches form a simple well‐defined unit for studying the coupling between the Fe layers through the Cr. These simple sandwiches show not only the antiferromagnetic coupling reported earlier in (100) superlattices,1 but also competing effects of the magnetocrystalline anisotropies which are of comparable strength. Detailed studies were made of the magnetization in the (100) plane along the [100] and [110] axes for a large set of samples for which the Fe films were held at ≂40 A, but the Cr thickness (t) was varied from 5 to 85 A. We observe a narrow window of values 10

Laser induced fluorescence excitation spectra of supersonic nozzle beam cooled methylglyoxal‐d4 reveal that a deformation occurs during the radiative electronic transition that destroys the planarity of the carbonyls. Consistent with the expectation that deuteration results in an increased number of low frequency vibrations, the number of observed vibronic lines increased and the energy of the breakoff of sharp vibronic structure above the O–O band decreased compared to the protonated molecule. As for the protonated molecule, the observed vibronic structure can be almost completely assigned on the basis of two fundamentals and a repeating pattern of lines that involves the methyl internal rotation and the cis–trans internal rotation of the carbonyls. Systematic anharmonicities involving the pattern and only one of the two fundamentals suggests which molecular motions correspond to the observed fundamentals. Quantum beat and Zeeman effect experiments unambiguously show the triplet states that are responsible for the radiationless dynamics that are known to occur in these molecules. All quantum beat data were analyzable using our previously published perturbation theory method and yield triplet densities which are intermediate compared to methylglyoxal‐h4 and biacetyl‐h6 and in good agreement with an estimate obtained by direct state coupling. The density of effectively coupled triplet states increases with the energy of the initially prepared singlet state and the intramolecular coupling is 2–15 MHz independent of the amount of vibrational‐rotational excitation present. Radiationless transitions in these highly excited molecules are evidently not subject to any overriding selection rules other than spatial symmetry and conservation of total energy, total angular momentum, and nuclear spin. Zeeman experiments indicate extensive coupling of all molecular angular momenta which initially decouple at only ∼10 G. In this low field regime complicated splittings are observed which correspond roughly to g values of about 67% that of a free electron. At larger fields of about 50–60 G, the microsecond fluorescence is nearly completely quenched, and although we cannot completely explain this effect at present, we suspect both intra‐ and intermolecular processes are possible.Laser induced fluorescence excitation spectra of supersonic nozzle beam cooled methylglyoxal‐d4 reveal that a deformation occurs during the radiative electronic transition that destroys the planarity of the carbonyls. Consistent with the expectation that deuteration results in an increased number of low frequency vibrations, the number of observed vibronic lines increased and the energy of the breakoff of sharp vibronic structure above the O–O band decreased compared to the protonated molecule. As for the protonated molecule, the observed vibronic structure can be almost completely assigned on the basis of two fundamentals and a repeating pattern of lines that involves the methyl internal rotation and the cis–trans internal rotation of the carbonyls. Systematic anharmonicities involving the pattern and only one of the two fundamentals suggests which molecular motions correspond to the observed fundamentals. Quantum beat and Zeeman effect experiments unambiguously show the triplet states that are responsib...

We have measured one color multiphoton dissociation/ionization (MPD/MPI) spectra for a series of n‐alkyl substituted arene chromium tricarbonyls (ACTs). Our data indicate that intramolecular vibrational relaxation (IVR) rates for methyl, ethyl, and propyl ACTs increase relative to the benzene analog in the ratio of 3:6:34. In contrast, the net relaxation rates of the p‐xylene (dimethyl) and mesitylene (trimethyl) analogs are two and three times faster than benzene, respectively. The behavior of these latter analogs probably also reflects differences in electronic structure relative to the benzene ACT. Taken with the results of experiments on other analogs and the independently obtained results on the uncomplexed arenes, we have found a strong correlation between the presence of low frequency vibrations and the rate of IVR. The rate of IVR determines the distribution of neutral chromium atoms formed by MPD of the n‐alkyl ACTs.

We have measured one color multiphoton dissociation/ionization spectra (MPD/MPI) across the one photon region 23 809 to 28 735 cm−1 for a structurally homologous series of organomolybdenum molecules including: (η6‐cycloheptatriene)Mo(CO)3, Mo(CO)6, and a variety of (η6‐arene)Mo(CO)3‐type molecules. The degree of observability of the 7DJ, 7P0J, and 7S3 Rydberg series is observed to be a function of: (1) the number of low frequency vibrations associated with the molecular precursor, and (2) the degree of localization of the cyclic ligand to metal bond. These Rydberg series are taken as a simple but sensitive probe of the production of ground state metal atoms. The efficiency for MPD production of 7S3 molybdenum atoms relative to (η6‐benzene)Mo(CO)3 increases in the order 1:1.9:11 with addition of a methyl substituent and extending it to an ethyl group. These observations are qualitatively and quantitatively in good agreement with previously reported relative efficiencies of MPD/MPI of organochromium molecul...

We have devised a numerical method of analyzing polyatomic quantum beats for the case when so many states are coherently excited that the properties of individual states are inaccessible. The method involves ensemble averaging singlet character distributions obtained by diagonalization of random matrices and using these distributions in simulations of observable quantum beats. By ensemble averaging the simulated quantum beats, we can correlate the average parameters which describe the observable fluorescence decays to the molecular parameters that determine the form of the random matrices. We have found that when very complicated quantum beat effects are observed, extra information is available which relates the overall width of the singlet character distribution to a third observable parameter beyond the two parameters that have traditionally been employed to characterize biexponential fluorescence. In the absence of hyperfine structure this method allows the determination of the width of the singlet cha...

The fluid‐mechanical force acting on the top layers of porous dust beds is analyzed and a new interpretation made when the beds constitute the walls in turbulent gas streams. The nonlinear differential equation governing the pressure distribution in the medium is solved numerically. When a sinusoidal pressure oscillation is applied at the surface of a porous medium, direct calculations show that there may occur a net upward force, for a duration within one cycle, on the top layers of the medium. By using the present results, a possible mode for the initiation of erosion of an erodible granular bed in a turbulent gas stream is examined. This mechanism may have significance in connection with dust explosions and soil erosion.

We have measured one color multiphoton dissociation ionization (MPD/MPI) spectra for h6‐benzene chromium tricarbonyl (h6‐BCT) and d6‐benzene chromium tricarbonyl (d6‐BCT). These spectra reveal that the vibrational properties of the ligands determine the distribution of metal atom states formed by MPD of organometallic molecules. Pressure studies demonstrate that the observed effects manifest isolated molecule dynamics and that excited triplet states are probably not involved in those dynamics. Laser power dependence studies show that the observed branching between MPD production of ground state chromium atoms vs excited state atoms is a function of multiphoton excitation rate. It seems likely that photodissociation on potential surfaces corresponding to excited electronic states can occur before intramolecular energy redistribution is complete. Two mechanisms consistent with our results are suggested. One is very unique for MPD/MPI experiments in that ladder crossing based on dissociation is not the sourc...