Gerald F. Dionne

Origin of secondary‐electron‐emission yield‐curve parameters

From an analysis of the one‐dimensional constant‐loss theory of secondary electron emission, maximum yield (δm), primary electron energy at maximum yield (Eom), and both crossover energies EIoc and EIIoc are shown to depend on the surface and bulk properties of the emitting material through simple relations. In particular, the results strongly suggest that the first crossover energy can be very dependent on surface properties, whereas the energy at maximum yield is entirely controlled by bulk properties. Refinement of the low‐energy part of the reduced yield curve by means of the results of the more realistic three‐dimensional theory leads to the development of the expression EIoc=0.51Eomδm−1.32. Comparison between theory and experiment for several secondary‐emitting materials is presented to demonstrate the accuracy of this useful relation. Finally, the implications of these results for different classes of materials are discussed in terms of basic physical properties, such as density, electrical conduct...

Room temperature ferromagnetic n-type semiconductor in (In1−xFex)2O3−σ

The thin film synthesis and characterization of room temperature ferromagnetic semiconductor (In1−xFex)2O3−σ are reported. The high thermodynamic solubility, up to 20%, of Fe ions in the In2O3 is demonstrated by a combinatorial phase mapping study where the lattice constant decreases almost linearly as Fe doping concentration increases. Extensive structural, magnetic and magneto-transport including anomalous Hall effect studies on thin film samples consistently point to a source of magnetism within the host lattice rather than from an impurity phase.

Molecular Field Coefficients of Substituted Yttrium Iron Garnets

By fitting the Neel theory of ferrimagnetism to previously reported magnetic moment‐temperature data for several {Y3}[RxFe2−x](QyFe3−y)O12 compositions, where R and Q represent diamagnetic octahedral and tetrahedral substitutions, i.e., Sc3+, In3+, Ga3+, and Al3+, the molecular field coefficients were determined to have the following linear relations with the levels of substitution: Ndd=−30.4(1−043x),Naa=−65.0(1−042y),Nad=97.0(1−0.125x−0.127y) mole/cm3, for x≤0.70 and y≤1.95. Since both intrasublattice coefficients are affected only by substitutions in the opposite sublattice, a strong similarity to Gellers random canting concepts is apparent. Below the antiferromagnetic transitions it is demonstrated that a clear correlation exists between the decrease in sublattice moment from canting and the reduction in magnitude of the molecular field constant. For both sublattices, the antiferromagnetic transition occurs when Ndd, Naa∼−20 mole/cm3. This observation lends further credence to the notion that canting ...

Bulk synthesis and high-temperature ferromagnetism of (In1−xFex)2O3−σ with Cu co-doping

The synthesis and magnetic properties of (In1−xFex)2O3−σ bulk ceramics with Cu co-doping are reported. Magnetic Fe ions are found to have high thermodynamic solubility (up to 20%) in the In2O3 host compound. The lattice constant decreases almost linearly as Fe doping concentration increases indicating the incorporation of Fe ions into the host lattice. The samples with high Fe concentration annealed under Ar reduced atmosphere were found to be ferromagnetic, and the Curie temperature is around 750K. The extensive structural and magnetic studies rule out the possibility that the observed magnetism is derived from magnetic impurity phases.

Effects of secondary electron scattering on secondary emission yield curves

The power‐law and constant‐loss theories of secondary electron emission have been modified to include the effects of secondary scattering within the solid. For power‐law exponent values of 1.35, 1.66, and 2.00, the scattering effects reduced penetration depths at maximum yield by almost a factor of 2 and decreased the maximum yields by about 70% in each case. Reduced yield curves were also computed for the different cases and the results indicate that scattering has only a small effect on the shape of the curves. Where reduced yield curves are involved, it may be concluded that the simpler theories could be sufficiently accurate in many cases. It is also shown that the exponent value of 1.35 provides reasonable agreement with experiment for lower primary energies, while 1.66 is more suitable at higher energies, in general accord with the energy ranges employed in the electron transmission measurements from which the above exponent values were originally determined.

Faraday rotation, ferromagnetism, and optical properties in Fe-doped BaTiO3

The structural, magneto-optical, optical, and magnetic properties of BaTi1−xFexO3 films grown by pulsed laser deposition onto MgO (100) have been investigated for Fe content, x=0.15, 0.2, 0.3, and 0.5. Saturation Faraday rotation varies from 0.033deg∕μm for x=0.2 to 0.109deg∕μm for x=0.5 at a 1.55-μm wavelength, with corresponding saturation magnetizations of 11.3emu∕cm3 and 18.3emu∕cm3. The optical absorption drops with a reduction in iron content and the BaTi0.85Fe0.15O3 sample has an optical absorption of 1.9dB∕mm. The magneto-optic figure of merit, which is the ratio of Faraday rotation to absorption, peaks at x=0.2 among the samples considered. The magnetic properties are interpreted in terms of the electronic structure of Fe4+ ions, and their exchange coupling with Fe3+ ions associated with oxygen vacancies.

Magnetically tunable superconducting resonators and filters

Compact, low-loss, tunable filters are needed for overload protection in the front end of many microwave-frequency systems. We have demonstrated magnetically tunable superconducting resonators and filters comprising microwave circuits coupled to ferrite substrates in monolithic structures using niobium at 4 K and hybrid resonator structures using YBCO at 77 K. A three-pole 1% bandwidth filter with 10-GHz center frequency and 1-dB insertion loss is described. In these devices the tunability results from changes in the magnetization of the ferrite rather than changes in the ferrimagnetic resonance frequency as in conventional YIG filters. Tunability data plotted us a function of magnetic field are fitted to the hysteresis theory developed previously and indicate that a tuning range of 13% is achievable. We have demonstrated switching times of less than one microsecond in structures incorporating closed magnetic paths in the form of a circular toroid.

Ferrite-superconductor devices for advanced microwave applications

Microwave devices comprising magnetized ferrite in contact with superconductor circuits designed to eliminate magnetic field penetration of the superconductor have demonstrated phase shift without significant conduction losses. The device structures are adaptable to low- or high-T/sub c/ superconductors. A nonoptimized design of a ferrite phase shifter that employs niobium or YBCO meanderlines has produced over 1000 degrees of differential phase shift with a figure of merit exceeding 1000 degrees/dB at X band. By combining superconductor meanderline sections with alternating T junctions on a ferrite substrate in a configuration with three-fold symmetry, a low-loss three-port switching circulator has been demonstrated.

Magnetic and dielectric properties of the spinel ferrite system Ni0.65Zn0.35Fe2−xMnxO4

The recent development of a high‐magnetization nickel–zinc ferrite with stress‐insensitive square hysteresis loop achieved through substitution of Mn3+ ions has prompted a study of the family Fe3+0.65Zn2+0.35 [Ni2+0.65Fe3+1.35−xMn3+x]O4. The magnetic and dielectric properties of compositions with x ranging from 0 to 0.4 were measured to determine the effects of the manganese substitutions over this concentration range. As anticipated, the saturation magnetizations decreased gradually with increasing x because of reduced magnetic moments in the B sublattice. Of greater importance, however, was the observation that maximum hysteresis loop squareness and minimum stress sensitivity occurred with x∼0.2. A single‐ion magnetostriction model is used to interpret this result. Since the electrical resistivity remained above 106 Ω cm and the dielectric loss tangent stayed below 10−3 for the entire series, Fe2+ ion formation is probably negligible.

Molecular‐field coefficients of Ti4+‐ and Zn2+‐ substituted lithium ferrites

By empirical methods similar to those outlined previously for the iron garnets, molecular‐field coefficients are reported for the complicated LiTiZn ferrite system. As in the case of the garnets, the intrasublattice coefficients NAA and NBB are linearly dependent on the Ti4+ and Zn2+ concentrations, but the intersublattice coefficient NAB has a quadratic dependence on the Ti4+ substitutions. Where possible, the computed results are compared with experiment, and the agreement is found to be good over the ranges 0≤t≤0.9 and 0≤z≤0.4.