Pulsed Ferrimagnetic Microwave Generator

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  • Pulsed Ferrimagnetic Microwave GeneratorB. J. Elliott, T. Schaug-pettersen, and H. J. Shaw Citation: Journal of Applied Physics 31, S400 (1960); doi: 10.1063/1.1984763 View online: http://dx.doi.org/10.1063/1.1984763 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/31/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Highpower microwave pulse generator Rev. Sci. Instrum. 63, 3156 (1992); 10.1063/1.1142569 Picosecond microwave pulse generation Appl. Phys. Lett. 38, 470 (1981); 10.1063/1.92407 Microwave Generation in Pulsed Ferrites J. Appl. Phys. 37, 1060 (1966); 10.1063/1.1708335 Microwave Harmonic Generation by Ferrimagnetic Crystals J. Appl. Phys. 32, 1905 (1961); 10.1063/1.1728261 Harmonic Generation in a Ferrimagnetic Disk J. Appl. Phys. 31, S97 (1960); 10.1063/1.1984618

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  • 400S F. B. HAGEDORX A:-\D E . .\II. GYORGY

    magnetizing field. As a result, it is possible to interpret the switching of this low remanence ferrite from one saturated state to the other as a composite process in which about half of the flux reverses by a relatively uniform mode while the other half goes more slowly by a nonuniform mode. Possibly material nonuniformities are responsible for this division into two reversal modes.

    Using this point of view, one can also interpret the results obtained by switching the ferrite from remanence to saturation. The essentially demagnetized remanent state is probably composed of a complicated fiux con-figuration, only part of which will interact with the applied field so as to produce the demagnetizing field required for the fast process. However, some of the fiux will be oriented similarly to the half-switched state of the helical mode, and this will create a large rate of change of magnetization very soon after the external magnetic field pulse is applied. The later part of the fiux reversal can then be associated with helical mode fiux which was in a less favorable initial orientation and with the nonuniform rotational fiux. Because of the complicated initial state, detailed separation of the waveforms into uniform and nonuniform parts is not possible. However, the switching time of the uniform part has been estimated as the time required for the switched fiux to increase from 10% to 50% of its total.

    Reciprocals of these times arc plot ted as C in Fig. 1 anti this plot has a switching coefficient of 0.026 oer~ted_ microsecond.

    Unfortunately, the presence of about half of the Hux in the nonuniform rotational mode obscures a more detailed interpretation of the faster process in terms of the helical mode. Nonetheless, this ferrite is an interest-ing material in that half of the fiux is switched by a relatively fast process even in small fields. The extra-polated threshold field of the fast process is therefore approximately zero. In contrast, the fast co~ponent i~ square loop ferrites appears to be small until relatively large fields are applied, leading to threshold fields of several oersteds.~3

    In summary, we have found it possible to describe qualitatively our measurement of flux reorientation in a low remanence ferrite by an equal combination of uniform and nonuniform rotation. Xonuniformity in the ferrite has been suggested as the cause for this division. Although this division obscures the detailed interpreta-tion of the faster process in terms of the helical mode. the results given herein are additional evidence of the existence of a fiux reversal process in ferrite in which the switching is speeded up by an interaction with a de-magnetizing field.

    \Ve want to thank F. J. Schnettler for supplying the ferrite used for these measurements.

    JOURNAL OF APPLIED PHYSICS SUPPLEMEKT TO VOL. 31. NO.5 MAY. 1960

    Pulsed Ferrimagnetic Microwave Generator*

    B. J. ELLIOTT, T. SCHAUG-PETTERSEN, AND H. J. SHAW Microwal1e Laboratory, W. W. Hansen Laboratories of Physics, Stanford University, Stanford, California

    The idea of making a solid-state generator using a ferrimagnetic material to convert energy from a pulsed de magnetic field into microwave radiation has been studied by several workers. Recent theoretical studies have disclosed serious basic problems for such devices. This paper describes initial experiments on a device which avoids the principal difficulties. It uses an rf input signal together with a pulsed dc magnetic field to generate microwave pulses at a frequency higher than the input frequency. An input signal at 2.4 kMc/sec and a pulsed magnetic field of 150 gauss were used to generate a pulsed output signal at 2.8 kMc/sec.

    INTRODUCTION

    T HE first specific idea for a pulsed microwave gen-erator using ferrimagnetic material appears to have been made by R. V. Pound.l In this case, a large pulsed magnetic field was to be applied to a ferrite, at right angles to a dc saturating field. The pulsed field was to rise to its peak value in a time short compared to the ferrite relaxation time, and establish, on a transient basis, a large angle () between the total mag-netic field and the uniform magnetization in the ferrite. The magnetization would then execute a uniform precession about the magnetic field thereby radiating its stored energy into a suitably designed microwave circuit.

    The research reported in this document was supported by the U. S. Army Signal Corps. Engineering Laboratories.

    I R. V. Pound, U. S. Patent No. 2,873,370.

    Silver and LevinthaP and Heard3 reported engineering studies of this scheme with particular attention to the microwave circuit and pulsed field generation problems. More recently, Morgenthaler4 published a theoretical analysis showing that the magnetization will closely follow the changing direction of the magnetic field during pulsing, and a significant angle () will not result, unless the rise time of the magnetic field is short com-pared to the precession period, which constitutes an impracticable requirement. Morgenthaler pointed out that this problem might be avoided by orienting the pulsed field at an angle of nearly 1800 to the dc satur-ating field.

    2 S. Silver and E. C. Levinthal, Levinthal Electronics Inc., Rept. No. 106, (November, 1956).

    3 H. G. Heard, Levinthal Electronics Inc., Rept. No. 104 (1955). 4 F. R. Morgenthaler, IRE Trans., MTT-7, 6 (1959).

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  • PULSED FERRI MAGNETIC MICRO\~;AVE GENERATOR 401S

    Recent theoretical work of Schaug-Pettcrsen 5 indi-cates that when the magnetic field is inverted in this way, the magnetization of the material will rapidly break up into a random pattern such that coherent radiation into a microwave circuit cannot be expected. The calculated time constant for this breakup is typi-cally a few tenths of a millimicrosecond, which would again require pulse time scales that cannot be met in practice. It should be mentioned that the growing spin waves described by Suh1 6 have a similar effect and would constitute a further problem for the 90 pulsing technique. For 180 pulsing, however, the breakup just referred to would be expected to be predominant.

    In this paper we will describe initial experiments with a device designed to avoid the above problems. This device suggested itself by analogy with recent pulsed maser generators7 and, it is found, was also suggested by Morgenthaler.4

    PRINCIPLES OF OPERATION

    In the present device an rf input signal of frequency Wp is applied to a garnet sphere which is adjusted for gyro magnetic resonance by means of a magnetic field Hu, thus establishing a uniform precession with preces-sion angle O. A pulsed field HI' is applied along the direction of Hu so that the resonant frequency of the spins is increased to

    w=wp+'YHp. (1)

    The pulsed field exerts no forces tending to change 0, so if the rise time is short compared to the garnet relaxation time T, 0 will be preserved during the rise of H p. The energy stored in the spin system at the peak of H p will then be

    E= (Ho+Hp)M(l-cosO), (2)

    where M ts the garnet magnetization. The spins no longer absorb energy from the input signal, but instead execute uniform free precession at the higher frequency wand radiate their energy as a microwave pulse into a suitable circuit.

    By making the duration of the rf pulses short com-pared with the intrinsic relaxation time of the material, the conversion efficiency of magnetic stored energy to rf energy can be made to approach 100%. The most interesting case occurs when the output frequency is much higher than the input frequency. The energy in the output pulse is then derived mainly from the pulsed field, while the energy derived from the rf input signal can be kept small in comparison.

    To avoid spin-wave excitation 0 may be held below the critical value given by Suhl.6 Using this condition, the peak pulsed power output may be obtained by time difl'erentiation of Eq. (2) giving

    Pmax= (AH1./'YTr)JV/2 (3)

    ~~s the sample volume, AHk is the linewidth of

    5 T. Schaug-Pettersen, J. App1. Phys. 31, 382S (1960). 6 H. Suh!, Proc. Inst. Radio Engrs., 44, 1271 (1956). 1 S. Faller el aT., Phys. Rev. Letters, 3, 36 (1959).

    FIG. 1. Block diagram of ferrite generator experiment.

    the potentially unstable spin wave, and Tr is the time constant of the output pulse. Using parameters appro-priate for yttrium-iron garnet, T",1O- 7 sec, T r",10-8 sec, AH k'" 1 gauss, we find maximum peak power of 4.5 w for a sphere diameter of 1 mm and output wave-length of 1 em. On a transient basis it is possible to exceed the critical angle 0, and this should allow higher power ou tpu t.

    APPARATUS

    A preliminary experiment has been carried out using the apparatus shown in Fig. 1.

    The garnet sample is located inside a dielectric rod surrounded by three metal strips. This forms a three-phase transmission line which provides suitable coupling to the sample. The fast rising pulsed field is produced by a charged coaxial line which discharges through a spark gap into a small coil surrounding the sample. Separation of the output and input signals is affected by the polarization separator and the 2.5 kMc/sec high-pass filter. The latter is matched into a crystal detector and video system having an overall rise time of 5 m~ sec.

    EXPERIMENTAL RESULTS

    Using a 30-mil diam sample of single crystal YIG with a line width AH of 1.2 oe, 1 w of input power at 2.4 kMc/sec, and a pulsed field of 150 gauss with a rise time of 5 m~ sec, the output pulse power is about 1 mw peak at 2.8 kMc/sec. The rf output envelope rises quickly to the peak value, then decays exponentially with a time constant of 20 mM sec. A larger output signal of approximately l-w peak is obtained by using a larger sphere (diam=120 mils, AH=2 oe), but the output pulse is extremely short in this case because of stronger coupling to the circuit. Polycrystalline samples with AH=45 oe have been used successfully but with reduced power output and pulse length because of higher internal losses.

    The frequency change in this initial experiment was kept intentionally small as the object was simply to prove the principle. Experiments to study performance capability at higher frequencies are being planned. As a first step an S-band to K-band (3-kMc input, 20-kMc output) generator is being designed. It is hoped that millimeter waves may eventually be generated by this method.

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