George A. Candela

Exchange Interactions between Pairs of Differing Magnetic Spins in Heteropoly Complexes

Coupled pairs of differing magnetic spins in tungsto‐heteropoly complexes have been investigated by studying the magnetic susceptibility over the temperature range 2–300°K. The two sites of the magnetic ions are surrounded by oxygen ions in octahedral and tetrahedral coordinations with one oxygen ion in common. The ions studied were Co2+, Co3+, and Fe3+ in a total of six combinations on the two sites, offering a unique set of symmetries and ions for examining super‐exchange effects. A closed form for the susceptibility has been obtained on assuming the spin Hamiltonian H=β (g1S1+g2S2)· H‐JS1· S2, allowing for the possibility of g1≠ g2, S1≠ S2. Fitted values of J/k range from −6 to −70°K depending on the combinations of ions. The experimental agreement with the spin Hamiltonian for all cases studied is satisfactory for this system.

Jahn–Teller Distortion: Magnetic Studies of Vanadium Tetrachloride

Paramagnetic resonance of polycrystalline VCl4 diluted with TiCl4 has been observed near 9,24, and 36 GHz below 9°K. Both g values and hyperfine A tensors are axially symmetric and temperature independent with the same principal axes: g‖ = 1.920, g⊥ = 1.899, | A‖ | = 72 G, | A⊥ | = 120 G. The average g value obtained from susceptibility data is in agreement with the resonance value. The paramagnetic relaxation time T1 varies with temperature as shown by the relation T1 = 2 × 10− 9exp(25/T) sec, indicating the presence of a potential barrier approximately 18 cm− 1 in height. The results indicate a static Jahn–Teller distortion, but with large zero‐point vibrations. This situation is in between Ballhausen–de Heers ligand field and Ball‐hausen–Liehrs crystal‐field calculations.

A method for determining the region of superparamagnetism

A simple procedure which determines the coercive force as a function of temperature has been developed for superparamagnetic materials having a particle‐size distribution. This procedure accurately determines the region where superparamagnetism occurs, and it has been used to obtain the first experimental verification of the Bean‐Livingston coercive‐force temperature relation.

Surface and bulk analysis of a deactivated Raney nickel methanation catalyst

In a joint PETC-NBS experiment, a Raney nickel methanation catalyst in a hot gas recycle (HGR) bench-scale reactor (used for catalyst lifetime testing) has been examined with a wide range of modern analytical techniques sensitive to both surface and bulk chemical properties of the catalyst. The reactor was designed to use a catalyst essentially identical to that used in previous lifetime testing and to allow both the sampling at various positions along the catalyst bed and the introduction of the samples into the various analytical instruments under inert atmospheric conditions (i.e., without exposure to oxygen, water, etc.). The purpose of this work was to explore the reasons for the premature failure of the catalyst in pilot plant lifetime tests. The results indicate that in spite of significant catalyst deactivation only low levels of carbon (a small fraction of a surface monolayer) are formed on the catalyst surface. The primary cause of catalyst deactivation in this lifetime test was determined to be the growth of the nickel crystallites and subsequent decrease in active catalyst surface area.

Influence of Paramagnetic Resonance on the Static Susceptibility. The Lattice—Bath Relaxation Time of Neodymium Ethyl Sulfate

The spin—lattice—bath relaxation process of neodymium ethyl sulfate was investigated by measuring simultaneously the change in the static susceptibility and the microwave power absorbed at electron spin resonance. This technique can be used at constant temperature to distinguish the spin—lattice process from the lattice—bath process. The relaxation time of neodymium ethyl sulfate was studied at a microwave frequency of 14.5 GHz as a function of temperature, helium‐exchange gas pressure, microwave power absorbed, two crystal orientations, and crystal size. At this microwave frequency the energy transfer from the lattice to the bath appears to be the rate‐determining process. The dominant lattice—bath relaxation time, τp, is inversely proportional to the square of the bath temperature τpT2=0.20 sec·°K2 but is apparently independent of the crystal size, the helium‐exchange gas pressure, and crystal orientation. The experimental data are in essential agreement with the data obtained by other researchers using...

Influence of Paramagnetic Resonance on the Static Susceptibility. Spin—Lattice Relaxation Time of Cupric Sulfate Pentahydrate

The static direct‐current susceptibility of cupric sulfate pentahydrate was measured as a function of microwave power absorbed at electron spin resonance. This technique was used to study the spin relaxation process of cupric sulfate pentahydrate as a function of temperature, microwave power level, and crystal size. Up to less than one‐half saturation, the relaxation time τ1 was found to be independent of the degree of saturation and inversely proportional to the temperature τ1T=0.2 sec·°K, showing that the predominant mechanism at liquid‐helium temperatures is the direct spin—lattice process. At high saturation levels there is evidence of crystal heating. These results are compared with those obtained by the cw saturation method on cupric sulfate pentahydrate, and the discrepancy in the magnitude of the relaxation time and its temperature and power dependencies are explained. This new technique is shown to be especially useful for studying power‐transfer mechanisms in concentrated spin systems.

Mössbauer and magnetic susceptibility studies of biferrocenylene(II,III) picrate

Mossbauer and magnetic susceptibility studies of biferrocenylene(II,III) picrate show that extensive donor–acceptor interactions occur in this mixed-valence molecule which result in fractional oxidation states for the iron atoms.

Nondestructive characterization of oxygen‐ion‐implanted silicon‐ on‐insulator using multiple‐angle ellipsometry

Silicon‐on‐insulator formed by high‐dose and high‐energy oxygen ion implantation in silicon, called SIMOX (separation by implanted oxygen), has been characterized nondestructively by multiple‐angle ellipsometry using a He–Ne laser at 632.8 nm. A multilayered model exhibiting two interlayers, one between the top silicon layer and the buried oxide layer and the other between the buried oxide and the substrate silicon, offers a simple representation of SIMOX. The difference between low‐temperature furnace anneal (1150 °C) and an additional high‐temperature rapid thermal anneal (1150+1350 °C) on as‐implanted wafers is shown by the better agreement between the theoretical model and the experimental results for the high‐temperature annealed SIMOX sample.

An Ellipsometry System For High Accuracy Metrology Of Thin Films

A computer-controlled spectroscopic ellipsometer of high accuracy has been designed and constructed. A theta-two-theta goniometer unit and optical rail system allows various ellipsometric methods to be used to measure the parameters A and 4). Three important methods under study for accuracy, precision, and speed of measurement are the conventional null method, the rotating analyzer method, and the principal angle method. All the goniometer angles, including the angle of incidence, can be measured to an accuracy of 0.001 deg. The present light sources are two lasers with fixed wavelengths, 632.8 nm and 441.6 nm, in addition to a monochromator that can be used to scan the wavelength range from 190 to 2600 nm. A unique sample alignment system which utilizes two quadrant detectors has been developed and a simple but very effective nulling scheme is used. This instrument is primarily used for the metrology of semiconductor materials and for the calibration of reference standards for thin film thickness and refractive index.

Magnetic properties and hyperfine interactions in the β‴ phase of potassium ferrite

The hyperfine field structure and magnetic susceptibility in the temperature range 5–295 K was measured for the β‴ phase of potassium ferrite. Results show that this new phase has an antiferromagnetic ordering similar to the β phase. The hyperfine‐interaction parameters have been determined and are compared with those found in magnetite, β‐phase potassium ferrite, and barium hexaferrite.