Yttrium Iron Garnet Sphere Research Articles

Simple procedure for aligning YIG spheres to reduce anisotropy effects

Simple procedure for aligning YIG spheres to reduce anisotropy effects

Transient Growth of Spin Waves under Parallel Pumping

We have experimentally observed the transient absorption during the application of a microwave pulse in a high-purity single-crystal YIG sphere at X-band and dc fields on the order of 900 Oe when the rf power level exceeds the threshold only slightly (less than 1 dB). Detailed calculations of the transient absorption coefficient based upon a previously proposed theory of Schlömann and taking into account the reaction of the sample upon the field in the resonant cavity agree reasonably well with the experimental data.

Surface Dependence of Magnetostatic-Mode Linewidths in Yttrium Iron Garnet

The observed linewidths of the magnetostatic modes of roughened spheres of yttrium iron garnet do not follow the expected simple dependence on the density of degenerate spinwave states. A theoretical explanation is presented of the apparently erratic differences between the linewidths of the low-order magnetostatic modes. Semiclassical spinwave theory is used to calculate the coupling between the resonant mode and the degenerate spinwaves. Linewidths dependent on surface condition are calculated on the basis of typical values for the parameters to describe rough surfaces, and fit to experimental linewdiths. The calculated and observed linewidths are presented; the agreement between them is very good. The results are consistent with the general assumptions of the previous treatments, but in addition it is shown that it is necessary to include the field configuration of the magnetostatic modes in the calculation of surface-dependent linewidths.

5-10 Ghz YIG-tuned tunnel-diode oscillator

A previously reported YIG-tuned tunnel diode oscillator was limited in its upper frequency of oscillation by the resonance of the diode capacitance with the inductance of the YIG coupling loop. The oscillator described here was designed to operate above this resonance, which is one of the causes of the curvature at the low frequency end of the tuning curve. The tunnel diode used is a Microwave Associates MA 4655 Gallium-Arsenide diode, having an operating (large signal) self-resonance at 10.3 GHz. It has a peak current of 25 mA and a resistive cutoff frequency of 25 GHz. The YIG sphere has a diameter of 0.040 inch and a saturation magnetization of 1750 gauss. It was oriented with one of the easy axes parallel to the applied magnetic field. The magnetic field was varied from 1120 to 3360 gauss in order to tune the oscillator from 5.1 to 10.2 GHz.

New Excitation Mechanism for Magnetoacoustic Resonance in YIG

Past experiments have shown that standing elastic waves can be excited in an yttrium iron garnet sphere through a parametric interaction with the magnetostatic modes. Lowest thresholds were obtained when the dc magnetic field was adjusted so that the difference between the microwave pump frequency and the acoustic frequency corresponded to the uniform precession frequency. The experiment reported in this paper utilizes a dielectric resonator to couple a high transverse rf magnetic field to the YIG sphere. This system enabled the observation of magnetoacoustic resonance using dc fields which are considerably lower than would be required to bias the idler frequency at the uniform precession. An explanation is offered which invokes a two-step parametric process making use of Suhl's theory of the subsidiary absorption.

YIG Filters as Envelope Limiters

Envelope limiters are used in such applications as FM demodulation and power leveling. Recently, the envelope-limiting properties of yttrium-iron-garnet (YIG) filters were reported for the special cases of unmodulated and pulsed input signals. Measured data is presented hereon the response of a YIG limiter to AM carriers having modulation index of the order of 50 percent. Sinusoidal, square-wave, and low-pass noise modulating signals were used in the measurements. It was found that a YIG filter will give good envelope Iimiting for modulating frequencies in the submegacycle range. At these low frequencies the carrier and the side frequencies are not limited selectively. At higher modulating frequencies where the limiting is frequency selective, the YIG filter will not remove the variations. In fact, in the particular filter tested, the modulation index was increased, rather than decreased, at modulating frequencies greater than about 750 kc/s. A graph is given showing the measured factor of reduction (or increase) of modulation index, as a function of modulating frequency. The response of the limiter as a function of carrier frequency, modulating frequency, and input power is shown by oscilloscope displays produced by sweeping the carrier frequency or input power. In addition, selected photographs of output envelope waveforms are given.

Contribution of the Two-Magnon Process to Magnetostatic-Mode Relaxation

The linewidths of the magnetostatic modes have been measured at $X$ band with the static magnetic field along the [111] direction in several highly polished spheres of single-crystal pure yttrium iron garnet (YIG). The large variations of the magnetostatic-mode linewidths previously found in larger linewidth spheres were not observed in the highly polished YIG spheres. However, there is a residual variation of mode linewidths at both 300\ifmmode^\circ\else\textdegree\fi{}K and 4.2\ifmmode^\circ\else\textdegree\fi{}K, the variation being somewhat larger at 4.2\ifmmode^\circ\else\textdegree\fi{}K. It was found that the essential features of the variation of mode linewidths in a highly polished YIG sphere can be explained by the surface-pit-scattering theory of Sparks, Loudon, and Kittel when the linewidth predicted by this theory is multiplied by an enhancement factor ${F}_{\mathrm{nmr}}$ to include the effect of the spatial variation of magnetization intensity for the individual magnetostatic modes. The results of calculations based on the Sparks-Loudon-Kittel theory for the linewidths due to scattering by fractional-micron-size pits are presented, as well as computed values of the enhancement factor ${F}_{\mathrm{nmr}}$ for most of the low-order modes. From the measurements it is estimated that at both temperatures the contribution of the two-magnon process to the uniform-precession linewidth is 0.10 Oe or less. This indicates that the major relaxation channel for the uniform-precession mode is not via degenerate spin-wave interactions. However, at 4.2\ifmmode^\circ\else\textdegree\fi{}K this may be due to rare-earth impurities broadening the linewidth. The conclusions reached regarding the contribution of the two-magnon process to magnetic resonance relaxation of the uniform-precession mode in highly polished YIG spheres are in substantial agreement with the modulation experiments of Fletcher, LeCraw, and Spencer.

Nonreciprocal Coupling with Single-Crystal Ferrites

A calculation of nonreciprocal coupling in microwave circuits with small ferrite samples tuned to ferromagnetic resonance is presented. It is shown that this coupling may be applied to the construction of simple resonant isolators, gyrators and circulators. Experimental results for the coupling in rectangular and ridge guides, applying YIG spheres, are presented. The construction of a simple X-band waveguide junction, acting as a 4-port resonant circulator, is described. Such a filter circulator, which may act as a switch or a frequency selective power divider, can be made tunable over the waveguide frequency range, with a bandwidth in the order of 10 Mc, and with values of insertion loss and isolation, which are comparable to those of conventional circulators.

Precise Ferromagnetic Resonance Magnetometers for Differential Measurements in Fields Characterized by Large Gradients

Design considerations, construction details, and measured performance of two magnetometer systems utilizing microwave ferromagnetic resonance in small single-crystal spheres of stoichiometric yttrium iron garnet (YIG) are presented. One system is for general use at ambient temperatures, while the second is designed for high precision measurements at liquid helium temperatures. These instruments are capable of measuring magnetic field strength to a precision of approximately one part in 104 even when the field is characterized by gradients as large as 1000 G/in. Since the YIG spheres employed are only 0.015 in. in diameter, these magnetometers come close to being ideal point probes.

The satellite ferrimagnetic power limiter : L. J. Varnerin, R. L. Comstock, W. A. Dean and R. W. Kordon, Bell Syst. Tech. J.XLII, 1817 (July 1963)

To limit the 4080-mc local oscillator signal power input to the beat oscillator modulator of the Telstar satellite communications repeater, a subsidiary absorption limiter was used which consisted of an optically polished sphere of single-crystal yttrium iron garnet (YIG), placed in a resonant transmission cavity between the amplified 4080-mc output of the traveling-wave tube and the BO modulator input. The limiter holds the output power nearly constant above a given input threshold; below this threshold the YIG is linear and introduces only a small loss. The threshold is determined, for a given sample at a given frequency, by the external magnetic bias field. Temperature compensation over the desired range was obtained by orienting the crystal with the dc magnetic field along a [100] or “hard” axis. The total weight of the limiter package, including the bias magnet and cavity, is 13 ounces

Magnetoacoustic Resonance in Lithium Ferrite

The parametric coupling of magnetostatic and elastic modes (magnetoacoustic resonance) has been observed in a single-crystal lithium ferrite sphere. Observation of this effect was made possible by the development of highly ordered stoichiometric lithium ferrite which has the narrowest ferromagnetic resonance linewidth of the spinel ferrimagnets [ΔH(300°K)=1.7 Oe, ΔH(134°K)=0.9 Oe at 5.4 G/sec]. The Q of the oblate-prolate elastic mode for this sphere was found to be 175 000 at 300°K. An estimate of one of the two magnetoelastic coupling constants resulted in a value B1=65×106 erg cm−3. This estimate was made by comparing the magnetoacoustic threshold for a sphere of this material with the threshold for a sphere of yttrium iron garnet (YIG), for which the coupling constants are known. A figure of merit for magnetoelastic materials is presented on the basis of which it appears that lithium ferrite may be superior to YIG in some transducer applications.

YIG filters in the 50–500 Mc range

YIG filters in the 50–500 Mc range

Temperature Stabilization of Gyromagnetic Couplers (Correspondence)

A gyromagnetic coupler using a single-crystal YIG sphere as a coupling element suffers from two significant sources of temperature instability. One of these is anisotropy drift, a characteristic that is internal to the coupling element, since it stems directly from temperature induced variations in crystalline anisotropy. The other is appropriately characterized as external; it derives from temperature induced variations in the magnetic biasing source. Either or both of these variations will result in a change in the resonant frequeucy of a gyromagnetic coupler. The 3-db bandwidth of a low loss YIG coupler may be of the order of 40 Mc, hence a change in resonant frequency of as little as 5 Mc will be detected as an increase in insertion loss at the original frequency. It is therefore quite desirable that the variations which contribute to this instability be reduced to a minimum. Means have been developed for eliminating both of these instabilities, thus rendering the gyromagnetic coupler a much more practical device under a variety of environmental conditions.

The Satellite Ferrimagnetic Power Limiter

To limit the 4080-mc local oscillator signal power input to the beat oscillator modulator of the Telstar satellite communications repeater, a subsidiary absorption limiter was used which consisted of an optically polished sphere of single-crystal yttrium iron garnet (YIG), placed in a resonant transmission cavity between the amplified 4080-mc output of the traveling-wave tube and the BO modulator input. The limiter holds the output power nearly constant above a given input threshold; below this threshold the YIG is linear and introduces only a small loss. The threshold is determined, for a given sample at a given frequency, by the external magnetic bias field. Temperature compensation over the desired range was obtained by orienting the crystal with the dc magnetic field along a [100] or “hard” axis. The total weight of the limiter package, including the bias magnet and cavity, is 13 ounces

Parametric Coupling of the Magnetization and Strain in a Ferrimagnet. I. Parametric Excitation of Magnetostatic and Elastic Modes

A theoretical treatment is presented of the parametric coupling between the magnetization and elastic strain in a ferrimagnet produced by magnetic pumping. In this paper the parametric coupling between resonant magnetostatic and elastic modes is discussed. The case of a disk vibrating in a longitudinal mode coupled to the uniform precession mode is discussed in detail. The oscillation threshold for this simple geometry is calculated and shows clearly the dependence of the threshold on the properties of the modes and the magnetoelastic coupling coefficients. It has been found that the threshold calculated for the disk geometry agrees to within an order of magnitude with measurements of the threshold for this effect using single-crystal YIG spheres.

Extension of Coincidence Limiting Frequency Range in Ferrimagnets

Power limiting using the first-order spin-wave instability has been observed in a ferrimagnetically coupled filter-limiter at higher frequencies than had previously been believed possible. For example, a filter-limiter, using orthogonal strip-line resonators coupled by the dipolar fields of a YIG sphere, was constructed at 4000 Mc. Output power was limited with the sharp break at the threshold and flat limiting curve characteristic of coincidence limiters. This result is in contrast with the operation of similar limiters at this frequency which use the second-order spin-wave instability in which the limiting characteristic has a much more gradual slope transition above the threshold. Coincidence limiters have previously been believed to have a maximum operating frequency ωc=2ωMNT (3.27 kMc for a YIG sphere) since, with an isolated sample, coincidence of the uniform precession resonance and the subsidiary absorption occurs only for ω<ωc. These results are observed for a coupled mode resonance in these ferrimagnetically coupled circuits which occurs for dc magnetic field values in the subsidiary absorption region below the uncoupled uniform precession value (ω/γ). It is found that a minimum value of the circuit-sample coupling is required to observe this extension of coincidence type limiting. Limiting thresholds as low as −10 dBm have been observed with the strip-line circuit and lower levels are believed to be possible. Wide low-level bandwidths (>10%) are also obtained with these circuits, but with an increase in the limiting threshold. The above results are explained by computing the normal modes of the microwave-ferrimagnetic resonator circuit. It is found for tight enough coupling that one of the branches of the normal mode spectrum is in coincidence with the subsidiary absorption at a higher frequency than the uniform precession resonance. The usual coincidence limiter operation (ω<ωc) is thus the small coupling case of the more general type of operation.

The Microwave Measurement of Narrow Line Width Ferromagnets by Radiated Fields

A simple waveguide method for measuring narrow line widths of ferromagnets is discussed. The precision reported from the measurement of a single crystal YIG sphere is ?H=0.55±0.02 oersted. The equipment used is capable of modification for more precise measurements.

Acoustic Losses in Ferromagnetic Insulators

Studies of acoustic losses in single crystal garnets and ferrites using magnetostrictively driven spherical resonators are described. Resonant modes have been observed in spheres of yttrium iron garnet (YIG) at ∼9 Mc and room temperature with Q′s of ∼107. This is approximately six times greater than any other known material at the same frequency and temperature. These high Q′s have now been observed in both shear and compressional modes. [R. C. LeCraw, E. G. Spencer, and E. I. Gordon, Phys. Rev. Letters 6, 620 (1961). In this reference only a certain compressional mode showed the unusually high Q.] It should be noted that the above Q in YIG may be only a lower bound. Contact losses have been eliminated by levitation and by using higher order modes. However, due to complicating factors such as the relatively large and variable dislocation density (revealed by etching) and surface defects, we are not yet certain whether the measured Q′s are characteristic of perfect YIG. The intrinsic Q could be higher than the above figure and still be consistent with the present results. The dependence of the internal friction Q−1 in pure YIG on frequency and temperature are given. The frequency dependence at 300°K is shown to be Q−1∼f in the megacycle range with possibly a slower dependence on f in the microwave range. Data are given on Q−1 at ∼20 Mc from 300° to 20.3°K. Q−1 is relatively independent of temperature down to ∼50°K. Below 50°K, Q−1 increases rapidly down to 20.3°K, indicating a large absorption maximum below 20.3°K. The origin of this maximum is at present unknown. Data are also presented on Co1.1Fei.9O4 at ∼20 Mc from 300° to 20.3°K. This material has a very large maximum (Q−1∼10−2) at 290°K. Below 50°K, however, it has quite remarkable properties, with Q−1 being lower than YIG. Instead of Q−1 increasing rapidly below 50°K, as in YIG, it decreases rapidly from 50° to 20.3°K, indicating very low losses in the helium range. Investigation of all the above effects is continuing in detail to obtain an understanding of the many questions raised by these results about acoustic losses in ferromagnetic insulators.

Ferromagnetic Resonance Magnon Distribution in Yttrium Iron Garnet

The presence of excited magnons results in a shift of the ferromagnetic resonance frequency. Calculations of this frequency shift have been made by retaining terms in the Hamiltonian up to fourth order in the spin wave variables. The shift can be expressed in terms of the number of excited k=0 magnons and k≠0 magnons. Determination of this shift can then be used to study the magnon distribution during resonance. This frequency shift has been measured in four single-crystal yttrium iron garnet spheres at 9 Gc. The shifts are observed to be directly proportional to the rf power absorbed by the sample, positive when the dc magnetic field is in [100] direction, and negative when the dc magnetic field is in the [111] direction. These results require a distribution of k≠0 magnons dominated by magnons with 48°<Θk<60°. If it is assumed that the k≠0 magnons are degenerate with the k=0 magnons, the total number of k≠0 magnons is approximately equal to the number of k=0 magnons, and magnons in the region centered about k=1−2×105 cm−1 dominate. This result is in agreement with a prediction of the surface pit scattering model; namely, the relaxation of the k=0 magnons results in the production of k≠0 magnons with wavelength approximately equal to the pit size. The results are in disagreement with any model requiring a uniform distribution of degenerate modes.

Magnetodynamic Mode Ferrite Amplifier

A longitudinally pumped ferrite amplifier has been operated using two magnetodynamic modes for the signal and idler resonant circuits. This is in contrast to the more usual electromagnetic cavity-type resonance or the ferrite magnetostatic modes. These magnetodynamic modes result from the coupling of a cavity-type resonance with a magnetostatic resonance. The coupling or mixing is strongest when the two unperturbed resonant frequencies are the same. For the amplifier described in this paper a single crystal yttrium-iron garnet sphere was used. The magnetic bias field applied was 17 kMc/γ oe. The resulting magnetodynamic modes were resonant at 21 and 14 kMc, respectively, for the two branches of the mode tuning curve. Advantages of these modes for ferrite amplification are ease of coupling and lack of interference from other resonant modes.