P. R. Emtage

The physics of zinc oxide varistors

This paper presents a physical description of the action of ZnO varistors, which are complex ceramic bodies of ZnO grains sintered in an oxide flux; their conductivity is very low at low voltage, but becomes high after a certain breakdown voltage is reached. It is found that depletion layers in the ZnO adjacent to the intergranular layers of oxide flux are the principal barrier to conduction at low voltage. These depletion layers are formed because electron traps in the intergranular layer absorb electrons from the ZnO, and the oxide flux itself is found to be more conductive than the neighboring barriers; conduction within the flux is observed at low temperatures. Electrical breakdown is associated with the completion of trap filling in thin regions of the intergranular layer. This model is supported by a wide range of physical phenomena.

Interaction of magnetostatic waves with a current

It is shown that the interaction of magnetostatic waves with a current can be characterized by a coupling constant analogous to, but much greater than, the piezoelectric coupling constant. The theory is applied both to surface and forward‐traveling volume magnetostatic waves, and the problems of excitation, reflection, and absorption by a single microstrip and by a narrow‐band interdigital transducer are treated. It is found that the coupling constant is close to 0.5 for any reciprocal wave, and is greater for any nonreciprocal wave, when the transducer is in contact with the magnetic medium. These values are too large to allow effective signal processing, since the emitted waves react strongly on the transducer; narrow‐band transducers must be lifted above the surface so as to weaken the coupling.

Statistics and grain size in zinc oxide varistors

A ZnO varistor is a ceramic with grains of various sizes. Therefore, some of the electrical breakdown characteristics of the ceramic reflect variations in the voltage impressed upon a grain, rather than the breakdown characteristics of an average grain boundary. It is found that the mean breakdown voltage per grain in the ceramic is less than that of an isolated grain boundary because there are chains of long grains through the ceramic; the density of such chains is finite even in a thick sample. The maximum value of the coefficient of nonlinearity αmax, which measures the rapidity of breakdown, is found to be characteristic of the grain boundaries when αmax is moderate, that is, if αmax <50. The high values of αmax observed at low temperatures appear to be limited by statistical effects rather than being representative of the boundaries.

Magnetostatic volume wave propagation in a ferrimagnetic double layer

Using the technique of ’’surface permeabilities,’’ an expression is derived for the dispersion of magnetostatic waves propagating in two close proximity ferrimagnetic films. Two modes of propagation for volume waves are identified respectively as the symmetric and antisymmetric modes from the symmetry of the rf magnetization. Useful group delay behavior is shown to result from films of equal thickness. Some measurements are reported using two yttrium iron garnet films sandwiching simple single finger transducers. Difficulties in exciting the symmetric forward volume mode are explained in terms of the coupling coefficients for these double film structures.

Interaction of Signals in Ferromagnetic Microwave Limiters

A weak secondary signal is partially absorbed in a ferromagnetic microwave limiter that is saturated by a strong primary signal; the absorption is greatest when the two signals are close in frequency. The width of this absorption is determined here, and is found to be proportional to the spin wave linewidth and to the square root of the excess power in the primary signal. The theory of this effect is presented and is found to agree well with experiment.

Generation of magnetostatic surface waves by a microstrip

The distribution of current across the width of a microstrip line can be severely distorted by a neighboring magnetic medium. The strength of the emission of magnetostatic surface waves by a narrow microstrip is calculated, taking approximate account of this distortion. Theory and observation agree that the effect is large (more than a factor of 2) in the upper part of the band of surface wave frequencies.

Brillouin light scattering on yttrium iron garnet films in a magnetostatic wave device structure

Brillouin light scattering (BLS) has been used for the direct detection of magnetostatic wave (MSW) excitations in a MSW microwave device structure. The results are for a signal‐to‐noise enhancer device which consists of a yttrium iron garnet (YIG) film on a gadolinium gallium garnet substrate. The YIG film is in contact with a 30‐μm‐wide stripline and the static magnetic field is parallel to the stripline. MSW excitations were observed over the frequency range 2– 4 GHz. At low power, magnetostatic surface waves (MSSW) were excited with propagation perpendicular to the stripline and field direction. At high power, parametric half‐frequency magnons were also observed. The measured magnon dispersion for the MSSW excitations, determined using the wave vector selective capabilities of the BLS spectrometer, is in good agreement with theory. The measured intensity profiles for MSW excitations at low‐power levels show strong MSSW excitations over the entire surface wave band. At high power, the scattering intens...

Acoustic radiation by interdigitated grids on LiNbO3

Measurements are presented which show the acoustic power radiated as a function of frequency by an interdigitated transducer on the surface of a lithium‐niobate boule. Several peaks are seen which are identified as either surface waves or bulk longitudinal waves travelling close to the boule surface. The transducer is seen to cause radiation at all frequencies above that for fundamental surface‐wave generation. Additionally, the signal radiated to a nearby transducer is measured. This signal comprises just surface waves and a weaker bulk longitudinal wave. These are associated with the peaks in the launched power spectrum if reflections are suppressed. In finite regular‐shaped samples reflections cannot be entirely suppressed and cause many anomalous signals. Over‐all the results show that the average strength of bulk‐wave generation is at least an order of magnitude more than that of surface‐wave generation.

Nonlinear microwave signal-processing devices using thin ferrimagnetic films

The signal-processing characteristics of signal-to-noise enhancers and frequency-selective limiters are described. Performance of several practical devices is discussed. The theory of below-threshold operation is reviewed. New theoretical results are presented for calculation of threshold power and above-threshold power loss. Frequency selectivity and the consequences of linear losses are calculated for the limiter.

Dispersive delay at gigahertz frequencies using magnetostatic waves

Magnetostatic waves are intrinsically suited for use in microwave dispersive delay lines, because they are readily generated at high frequencies and their velocity is frequency dependent. Here we review progress toward meeting the systems requirement that the delay time be linear in frequency. Magnetostatic waves can approach this condition if a ground plane is nearby or if two or more ferrite layers are present; with forward volume waves one may also use a dispersive reflecting array, as is done with surface acoustic waves. The use of a properly shaped ground plane gives the lowest phase errors so far measured (±16° over 0.6 GHz); multiple ferrite layers show promise of allowing the greatest delay.