Contains reports on five research projects.United States Army Signal Corps (Contract DA36-039-sc-74895

Douglass, D. H., Jr.

Kyhl, Robert L.

Mattuck, R. D.

Strandberg, Malcom W. P.

Wolga, G. J.

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VI. MICROWAVE SPECTROSCOPYProf. M. W. P. Strandberg F. F. Huppe D. W. OliverProf. R. L. Kyhl J. G. Ingersoll R. J. PapaDr. B. V. Gokhale J. D. Kierstead D. J. M. Park, Jr.Dr. G. J. Wolga W. J. Kossler, Jr. S. O. SchwabeP. H. Carr M. S. Lipsett J. D. StettlerD. H. Douglass, Jr. J. R. Marko C. S. SwiftB. W. Faughnan R. D. Mattuck S. H. WempleK. J. NomlandA. RUBY LINEWIDTHThe theory of the linewidth of paramagnetic ions in crystals is sufficiently wellunderstood to permit quantitative description of the observed resonance spectrum.Factors contributing to the paramagnetic-resonance linewidth are the spin-spin interac-tion with identical paramagnetic spins in the lattice, the dipole interaction with othernonsimilar magnetic moments in the lattice, and the statistical variation of the internalcrystalline field splitting that results from crystalline imperfections.It has been our observation that the linewidth of the chromium ion in corundum isunduly large for simple description by way of these effects. The linewidth in "pink"ruby is between 15 and 17 gauss. From measurements of the magnetic Q of this mate-rial we conclude that the lattice includes approximately 1 part in 104 chromium whichis substituted for the diamagnetic aluminum ion. Such a density would lead to a spin-spin linewidth from identical particle interactions of approximately 2 gauss. The widthcaused by the magnetic field from the neighboring aluminum ion of mass 27 is of thesame order, as has been observed in the nuclear magnetic resonance spectrum of thataluminum isotope. The remaining factor, crystalline imperfections giving rise to vari-ations in the zero-field splitting of the chromium paramagnetic levels, is thus the onlymechanism by which one can explain the observed ruby linewidth. Unfortunately, asDavis and Strandberg (1) pointed out, one would expect to find a great variation in line-width among the observed ruby lines because of the variation in line frequency depend-ence on the zero-field splitting parameter. This strong variation has not been observed;in fact, Frank Huppe, of this laboratory, observed the spectrum with a paramagneticresonance apparatus operating at 200 mc, and found that the resonant line at low mag-netic field, which should be almost totally independent of the zero-field splitting, hasthe same typical pink ruby linewidth.These facts are very interesting and exciting. They suggest that ruby is indeed verypoor for maser material because it has a low ratio for the number of ions per cubic cen-timeter to the intrinsic linewidth. If chromium had the theoretically expected ratio forthe quantity, it should be possible to increase present gain-bandwidth products for ruby* This work was supported in part by Signal Corps Contract DA36-039-sc-74895.(VI. MICROWAVE SPECTROSCOPY)amplifiers by a factor that is in excess of 10. For example, this would mean that anX-band amplifier operating at 1. 2 K could have a gain-bandwidth product in excess of1200 mc in a simple regenerative cavity mode of operation. Indeed, the simple, orunloaded, type of waveguide traveling-wave amplifier would be quite feasible with atheoretically understandable paramagnetic crystal. Unfortunately, ruby falls short ofthis mark by a factor greater than 10.We have also observed the ruby transition in the 55* orientation at a particular oper-ating point of approximately 2. 2 kilogauss and at 5740 mc, where the line frequency isessentially independent of the magnetic field. In this case the only line broadeningarising from other factors than the chromium spin-spin relaxation time results fromthe variation from point to point in the crystal of the angle 0 that measures the incli-nation of the static magnetic field with respect to the crystalline axis. The actual fre-quency width of the line is given as8v 8vAv - A + -- AH (1)where 0 is the angle between the applied magnetic field H and the crystalline c-axis,and AH is the random (non-spin-spin) magnetic field, caused, for example, by thenuclear magnetic moments of the neighboring aluminum A127 nuclei. The operatingpoint is so chosen that the partial derivative of the frequency with respect to H is zero,and therefore the magnetic field inhomogeneity resulting from the surrounding nucleican only change the resonant frequency by changing the direction of the applied mag-netic field at the position of the paramagnetic ion. This amounts to an effective Ae termgiven bynHAOH - H (radians) (2)where AHAl is the random field attributed to the surrounding neighboring nuclei, andHo is the operating field. The partial derivative of the frequency with respect to theangle can be computed as approximately 8400 mc/radian. The expected value of AOfrom the magnetic field inhomogeneity would yield a linewidth of approximately 8 mcfor this transition. We observed the transition at room temperature in a cavity thathas a Q of approximately 1000 at C-band. The resonant absorption was observed asconsiderably broader than the cavity width.The next step was to use a ruby crystal at low temperatures in a coaxial transmis-sion structure of greater bandwidth. The absorption could be observed directly and,again, was broad compared with the width of the klystron mode which, in this case, was10 mc. Indeed, the resonance was so broad, compared with 10 mc, that the only obser-vations that could be made were of the angular sensitivity of the absorption. One-sixtieth(VI. MICROWAVE SPECTROSCOPY)of a radian, measured crudely, was necessary to pass through the resonance, whichwould give a width greater than 100 mc.From these considerations, we conclude that the actual spin-spin width for the reso-nance is the limiting factor; and, contrary to expectation from the average concentrationof the chromium ions, the spin-spin relaxation time is much shorter, by a factor of 10or more, than we anticipated. Since the spin-spin relaxation time is a function of thedistance between ions, we are forced to conclude that the ions have a specific concen-tration that is at least 10 times greater than the mean concentration determined fromthe magnetic susceptibility measurements. This can only be true if clustering of theions occurs.We are now at the point at which we are trying to understand the causes of this clus-tering and how it can be modified. Although it must be said that the ruby crystal that isconventionally used in solid-state amplifiers is rather poor material, as compared witha perfect, nonclustered ruby, the observation is still heartening because there are pos-sibilities for improvement. Probably, other ions that fit better into the corundum struc-ture will not be clustered, or, by annealing, it may be possible to distribute the ionsmore uniformly over the lattice. A ruby crystal with a linewidth of 17 gauss has beenannealed at a temperature of 1500' C by Dr. Perry Miles, of the Laboratory for Insula-tion Research, M.I.T. We observed a reduction of the linewidth to 14 gauss after thisannealing process. A higher temperature oven is now being sought in order to extendthis annealing process to its ultimate limit.M. W. P. StrandbergReferences1. C. F. Davis, Jr. and M. W. P. Strandberg, Paramagnetic-resonance spectrumof ammonium chromium alum, Phys. Rev. 105, 447-455 (1947); Technical Report 242,Research Laboratory of Electronics, M.I. T., Dec. 9, 1955.B. PARAMAGNETIC AMPLIFIERSA nonreciprocal microwave structure with a circularly polarized cavity has beendesigned, constructed, and operated satisfactorily at X-band. We intend to continuethis work, using the structure at low temperatures as a unilateral, nonreciprocal para-magnetic amplifier. The operation will be that of a three-level amplifier with the useof a pump at KA-band. An analogous structure has been designed and constructed tooperate at L-band (1420 mc), the frequency of the hydrogen line used in radioastronomicobservations. This structure will have adequate gain-bandwidth product for observation*This work was supported in part by Signal Corps Contract DA36-039-sc-74895.(VI. MICROWAVE SPECTROSCOPY)of the hydrogen-line spectrum, and being unilateral and nonreciprocal, it will requireno circulator at L-band. Although circulators probably will be available at L-band,they are cumbersome devices, and we prefer the operating simplicity of our own device.A paper that discusses these amplifier constructions and their properties is being pre-pared.The analysis shows, for example, that not only do these nonreciprocal, circularlypolarized structures have the advantage that an additional circulator is not required,but they actually have uniquely favorable characteristics in that they are less dependenton input and output VSWR.One interesting use that has been made of the X-band circularly polarized structureis that a casual examination of the ruby spectrum disclosed absorptions other than theone intended for the actual amplifier levels. Subsequent computations indicate that mostof the transitions in the ruby spectrum preserve a high degree of circular polarizationsensitivity for a wide range of the angle 0 (which measures the angle between the rubyc-axis and the applied magnetic field) of 0 -80'.The work on circularly polarized structures has been confined to regenerative cav-ity design, although the cavity Q can be of any value; for example, a value approachingthat of slow wave structures of current design. The general disadvantages of a single-port cavity structure usually arise from the regenerative nature of the amplifier itself.In work directed toward making circularly polarized nonreciprocal amplifiers, theregenerative character of the amplifier simplifies the problems of matching and con-struction of the device. This is probably the first time that the regenerative effect isan advantage rather than a disadvantage.M. W. P. StrandbergC. PHONON-SPIN ABSORPTION IN PARAMAGNETIC CRYSTALSJAn experiment was made in an attempt to detect ultrasonic saturation of spinpopulations in irradiated fused quartz. The result was negative.Other experiments were performed on a crystal of irradiated Brazilian quartzin which the saturation effect has already been observed. The effect has thepeculiar property that in the range considered (12 mc to 19 mc) the degree ofsaturation is independent of the difference between the spin resonance and phonon-generator frequencies. This will be investigated further by increasing the spin-resonance frequency to 200 mc.R. D. Mattuck*This work was supported in part by Signal Corps Contract DA36-039-sc-74895.(VI. MICROWAVE SPECTROSCOPY)D. SPECTROSCOPY OF FREE ATOMS*An accurate gj-value has been obtained for the ground state of free atomic C135 (seeQuarterly Progress Report No. 52, p. 28). The value obtained is g = 1.33379±0.00007 inwhich the uncertainty given is the mean-square deviation. This is to be compared withthe value gj = 1. 3341, obtained by assuming L-S coupling in the P 3/2 state, withgL = 1.0000 and gs =2.0023. The result is an average over seven different AM =±1 andAM I = 0 hyperfine transitions at 8993 mc.The measurements of resonant frequency and field for each transition were inter-preted with numerical solutions of the secular equations. The calculations employnumerical values for a and b, the magnetic dipole and electric quadrupole interactionconstants for C1 3 5 , obtained from atomic-beam experiments (1). The mean-squaredeviation of gj is within the uncertainty of the atomic-beam values.The transition frequencies were measured by observing an audio beat less than 10 kcwide between the klystron signal and an X-band signal obtained by harmonic generationfrom a 6-mec signal monitored with a frequency counter.The transition field was measured by measuring the resonant frequency for protonsthat are a short distance away in the magnet gap. A correction for the field differencebetween the position of the protons and the chlorine was made. The correction wasobtained by repeating the experiment with atomic hydrogen. The free atoms and protonsin this experiment occupy the same positions in the field as in the chlorine experiment.A field correction for the hydrogen run was obtained by comparing the calculated andobserved transition frequencies and deducing the field correction required to reconcilethe data. A connection at the chlorine field strength was made by a linear interpolationof the hydrogen field correction.G. J. WolgaReferences1. J. H. Holloway, B. B. Aubrey, and J. G. King, Quarterly Progress Report,Research Laboratory of Electronics, M.I.T., April 15, 1956, pp. 35-36.E. ANTIFERROMAGNETIC RESONANCE IN MANGANOUS CHLORIDEExperimental data has been taken at KA-band and is being analyzed. It hasbeen found that the deviations from paramagnetic behavior above the Neel temper-ature can be quantitatively explained as arising from demagnetization effects thatresult from the shape of the sample. This is very interesting because demagnetiza-tion effects are usually unimportant in paramagnetic and antiferromagnetic materials.*This work was supported in part by Signal Corps Contract DA36-039-sc-74895.(VI. MICROWAVE SPECTROSCOPY)Below the N6el temperature, the analysis of the data is complicated by thesesame demagnetization effects. A complete analysis will be presented in thenext quarterly progress report.D. H. Douglass, Jr.

Microwave Spectroscopy

Contains research objectives and reports on three research projects.Signal Corps Contract DA36-039-sc-7489

VII. MICROWAVE SPECTROSCOPYProf. M. W. P. Strandberg W. J. C. Grant D. W. OliverProf. R. L. Kyhl F. F. Huppe R. J. PapaDr. B. V. Gokhale J. G. Ingersoll D. J. M. Park, Jr.Dr. G. J. Wolga J. D. Kierstead W. J. SchwabeJ. M. Andrews, Jr. W. J. Kossler, Jr. S. T. ScottP. H. Carr M. S. Lipsett M. H. SpringD. H. Douglass, Jr. J. R. Marko J. D. StettlerB. W. Faughnan R. D. Mattuck J. YacusK. J. NomlandRESEARCH OBJECTIVESThe term microwave spectroscopy has been generalized considerably since its firstuse by this group. As we have said previously in these reports, our group is interestedin the study of matter, or at least those aspects of matter that can be examined by meansof radiofrequency or microwave spectroscopy. Our present interest is mainly in para-magnetic phenomena, and we are concentrating on the problem of understanding thecoupling between paramagnetic spins in crystals with the surrounding lattice. We arealso interested in the coupling between these spins and electromagnetic fields for severalreasons. As our understanding of paramagnetic phenomena increases we are able todesign and build better quantum-mechanical amplifiers, and an understanding of thecoupling between the spins and the electromagnetic field allows us to understand quanti-tatively the spectra that we observe.These researches are leading us far afield. For example, we are now faced withthe problem of understanding how paramagnetic impurity ions become distributed in thehost lattice; there is no doubt that here, again, the applied aspects of this understandingwill be immediately evident in the help it will give in the search for better solid-stateamplifier materials. Work is being continued in other solid-state phenomena, such asthe phenomena that are encountered in the field of antiferromagnetic resonance.We have been greatly encouraged with the results that we are now obtaining fromwork with paramagnetic atoms and the interpretation of their spectra. In the past wehave been able to use the paramagnetic spectrum of oxygen atoms for studying both thevolume and surface recombination rates. These techniques are being extended to otheratoms, and we hope to be able to measure the parameters that specify atoms, such asthe electronic and nuclear g-factors and nuclear electric quadrupole moments, with aremarkably simple and universal experimental arrangement and with high precision.Work is also being continued on beam masers. We hope to use them as researchtools to study interactions at very low particle density. These efforts have led us, some-what tangentially, into the consideration of time and frequency standards, and a smallpart of our effort is devoted to gaining an understanding of the inherent theoretical limi-tations of these devices and to considering, for example, solid-state oscillators as stablefrequency sources.M. W. P. Strandberg, R. L. KyhlA. PHONON-SPIN ABSORPTION COEFFICIENTS IN PARAMAGNETIC CRYSTALSSeveral experiments were run in an attempt to detect the phonon-spin effect in potas-sium chromicyanide. At the temperatures of 70 0 K, 4 0 K, and 1. 5 K, at which the experi-ments were run, the results were negative.Since the cyanide crystal was badly fractured because of the low temperatures thatThis work was supported in part by Signal Corps Contract DA36-039-sc-74895.(VII. MICROWAVE SPECTROSCOPY)were used, we believe that the probable source of difficulty was poor phonon conduction.We decided, therefore, to use ruby as the paramagnetic specimen in further work becauseit usually has nearly perfect lattice structure and consequent high acoustic conductivity.A ruby cylinder, 1. 5 inches long and 3/8 inch in diameter, was cut from a syntheticboule. Similar experiments were performed but no acoustic effect was observed.We then made a theoretical analysis. It indicated that our previous estimate of therequired phonon-energy density of 10- 8 ergs/cm 3 may have been too low by a factor of10 . Since the sound-energy density in the experiment was probably in the vicinity of10- 3 ergs/cm3 , steps are being taken to increase the acoustic input by using a matchingnetwork between signal generator and quartz oscillator and possibly by introducing apower amplifier. This should increase the phonon density by a factor of approximately102 and make the effect observable.R. D. MattuckB. SPECTROSCOPY OF FREE ATOMSThe hyperfine structure of free atomic chlorine was observed at X-band by themethod of paramagnetic resonance. Gaseous chlorine at a pressure of a few millimetersof mercury was dissociated in an S-band microwave discharge and flowed through thedetection cavity of the X-band spectrometer. The observed transitions correspondedto AM. = ±1 and AM. = 0 for both stable chlorine isotopes. The spectra were obtainedj iwith a signal-to-noise ratio of 10-100; the ratio depends upon the operating pressure.Further experiments are in progress to improve the resolution and to obtain accurategj values for both isotopes.G. J. WolgaC. ANTIFERROMAGNETIC RESONANCE IN ANHYDROUS MANGANESECHLORIDE (MnC12)The spectrum of single-crystal MnC1Z at K-A band (35 kmc) is very nearly complete.It is indicated in Fig. VII-1. There appears to be no anisotropy for rotations about theC-axis. For rotations about an axis normal to the C-axis, there is considerable anisot-ropy. (See Fig. VII-2.) Points A and B in both figures represent the same temperatureand orientation of the crystal.Our data cannot be explained by the simple theory of antiferromagnetic resonance(i. e., the simple theory predicts no deviation from g = 2 above the critical tem-perature Tc ). Qualitatively, the deviations in the data are attributable to the2c(VII. MICROWAVE SPECTROSCOPY)120a a6I-zz 4cncr 2Fig. VII-1. Resonant field (H) versustemperature for H perpen-dicular to the C-axis.Fig. VII-2. Resonant fieldangle (betweenaxis).(H) versusH and C-short-range order of the system (1). The spins are strongly constrained to lienormal to the C-axis, and within this plane the spins are nearly free to rotate.have not yet been explained quantitatively.in a planeThe dataD. H. Douglass, Jr.References1. D. H. Douglass, Jr., Antiferromagnetic resonance, Quarterly Progress ReportNo. 51, Research Laboratory of Electronics, M.I.T., Oct. 15, 1958, p. 47.12.0 _-10.0 -o 80-60 I4.0LINE FOR g = 2Tc TcI 2 3TEMPERATURE (oK)

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Microwave Spectroscopy

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