Volume 36, Issue 3, 01 March 1965
Index of content:
- MICROWAVE DEVICES AND MAGNETO‐OPTICS
36(1965); http://dx.doi.org/10.1063/1.1714188View Description Hide Description
Room‐temperature magnetoelastic waves have been observed in yttriumirongarnet from 1.6 to 8.5 Gc/sec. The minimum observed insertion loss, which includes conversion, transmission, and reconversion loss, was 15 dB at 1.6 Gc/sec; at 8.5 Gc/sec the smallest insertion loss was 46 dB. The transmission attenuation increases approximately linearly with frequency from about 6 to 20 dB/μsec over this frequency range. The experimentally observed conversion loss from electromagnetic to magnetoelastic energy ranges from 3 to 13 dB and is compared with theoretical values.
36(1965); http://dx.doi.org/10.1063/1.1714189View Description Hide Description
Experiments on electronically variable time delay of microwave pulses by transmission through single‐crystal YIG rods are discussed. One type of delay, recently discussed by Olson and Buchmiller and by Kaufman and Soohoo and attributed to magnetostatic waves, occurs with unpolished ends, has rapidly increasing delay with increasing H dc, and has severe pulse dispersion. The second, found here, requires parallel and polished ends, has a slowly decreasing delay with increasing H dc, and usually has at least two pulses spaced by less than the round‐trip time through the rod. Characteristics are similar to those described by Strauss, except that his experiments were of pulse reflection.
The two aspects of magnetostatic wave pulse delay to be discussed are: (1) Additional experimental verification of Schlömann's theory of wave excitation by nonuniform demagnetizing fields, by grinding a rod into ellipsoidal shape, and by measure coupling. (2) A nonreciprocal microwave time delay has been constructed utilizing magnetostatic waves in a YIG rod to couple two rectangular waveguides. The coupling is between the positions in the waveguide where the magnetic field is circularly polarized. The directionality of the coupling was found to be greater than 15 dB with delays of 4 μsec at 2500 Mc/sec.
36(1965); http://dx.doi.org/10.1063/1.1714190View Description Hide Description
A single‐pole, double‐throw, cutoff‐type ferrite switch has been developed for operation at C‐ and S‐band frequencies. The C‐band unit is designed for a peak power level of 1 MW with an insertion loss less than 1 dB. Both units display a switching time of 150 μsec. The device consists essentially of a three‐port waveguide junction with a section of reduced size ferrite‐loaded waveguide at two of the ports. The characteristics of the reduced area sections are such that microwaveenergy in the frequency range of interest is propagated if no magnetic biasing field is applied. Upon application of the biasing field, the effective permeability of the ferrite is reduced, causing the loaded waveguide to be cut off. The structure is matched so that, if the magnetic field is applied to one section and not the other, a minimum input VSWR is obtained, and there is a maximum transfer of energy to the unbiased port. Switching is accomplished by applying the magnetic biasing field to one ferrite‐loaded section, while at the same time removing the magnetic field from the other. The technique of using cutoff waveguide to create the ``off'' condition provides isolation greater than that obtained by other means, e.g., switching circulators. It is believed that this is the first device of this type to be operated at a truly high power level. High‐power operation has been achieved by the novel approach of operating above the onset threshold for subsidiary resonance absorption. This technique appears promising for use in the development of various high‐power devices in the future.
36(1965); http://dx.doi.org/10.1063/1.1714191View Description Hide Description
As part of an effort to understand the nature of the transition responsible for the very strong absorption band edge and the large magneto‐opticaleffects in ferric ion containing ferro‐ and ferrimagnets, the optical and magneto‐optical properties of erbium orthoferrites (ErFeO3, ΘN = 620°K for Fe+3 spins) have been examined by reflection techniques. Ellipsometer and Kerr rotation measurements were made on growth surfaces (c plane) at room temperature over the visible spectrum. The real and imaginary parts of the dielectric constant show two bands at 22 000 and 26 000 cm−1, with oscillator strengths of 0.03 and 0.4, respectively. The normal incidence polar Kerr rotation has a sharp peak of 11.5 min at 23 500 cm−1. There are several weaker structures between 15 000 and 22 000 cm−1 which are believed to be due to low‐lying branches of the crystal‐field spectrum of the Fe+3 ion. By combining the Kerr rotation and dielectric constant data, the off‐diagonal element of the dielectricpermittivitytensor was calculated. The results suggest that the main 26 000 cm−1 band and the unresolved 23 500 cm−1 band represent the σ and π processes of the electron transfer transition. They further suggest that the π process component of the off‐diagonal element has opposite sign relative to that for the σ process and the first allowed internal transition.
36(1965); http://dx.doi.org/10.1063/1.1714192View Description Hide Description
Faraday rotation and absorption measurements were made on a bulk single crystal and on evaporated polycrystalline films of EuSe. Bulk EuSe is ferromagnetic, transparent in the red, and exhibits an unusually large Verdet constant at room temperature: −9.7 min/Oe‐cm at 6660 Å where the absorption is 173 cm−1. Measurements at 77°K show that the Verdet constant varies approximately inversely with temperature and thereby correlates with the magnetic susceptibility. These properties are due to the Eu+ + ion and are comparable to those we have observed in other divalent europium compounds such as Eu2SiO4.
The evaporated films of EuSe exhibit a broad but well‐defined absorption band at 4600 Å which is essentially duplicated in similarly prepared films of EuS and EuO at 5200 and 5800 Å, respectively. The wavelength dependence of the Faraday rotation of EuSe exhibits a rather complicated structure with two sign reversals in the region of the absorption band. At 6660 Å the Verdet constant measured at room temperature is −6.3 min/Oe‐cm agreeing in sign and approximately in magnitude with the bulk value, whereas the maximum value of Verdet constant, +25 min/Oe‐cm, occurs at 4330 Å.
36(1965); http://dx.doi.org/10.1063/1.1714193View Description Hide Description
Circulators which can be operated at cryogenic temperatures are discussed. Both symmetrical Y‐junction circulators using polycrystallineferrimagneticgarnets and hybrid‐gyrator circulators using single crystals are evaluated. The temperature dependence of the ferromagnetic resonance losses of polycrystalline aluminum‐doped and undoped yttriumirongarnet is reported. Device performance is evaluated in terms of the measured material properties.
36(1965); http://dx.doi.org/10.1063/1.1714194View Description Hide Description
CrI3, noteworthy as one of the few insulating ferromagnets, is one of a series of presumably isomorphous compounds (CrCl3, CrBr3, and CrI3) which show similar or closely related magnetic ordering. CrI3 samples containing some single crystals (plates a few square millimeters in area) were made by reacting chromium metal and iodine in a sealed quartz tube. Magnetization measurements of the powder were made by Williams and Sherwood at several fields up to 15 300 Oe over the range 1.5° to 300°K. In addition, M (H) was plotted at 4.2°K up to 70 000 Oe. The ferromagnetic Curie temperature is 68°K, and the paramagnetic Curie temperature is 70°K. The low‐temperature saturation magnetization is 3.10 μB/Cr+ + +, corresponding to 4πM = 2690 G. The field for ferromagnetic resonance was measured at 86, 91, and 99 Gc/sec. The spectroscopic splitting factor g = 2.07, and the anisotropy field 2K/M = 28.6 kOe at 1.5°K.
Optical transmission measurements on CrI3single crystals showed a very strong absorption band down to a band edge near 10 000 cm−1. The specific magnetic rotations associated with the band edge are almost as large as those observed in the tribromide. Evidence of domain diffraction was seen.
36(1965); http://dx.doi.org/10.1063/1.1714195View Description Hide Description
Hall coefficients and resistivity have been measured over the temperature range 77° to 300°K on polycrystalline MnBi films prepared with the c axis of the crystallites normal as well as parallel to the plane of the film. The Hall resistivity follows the well‐known relation ρH = R 0 H+MR 1, where R 0 and R 1 are the ordinary and extraordinary Hall coefficients and M is the magnetization. At room temperature,R 0 is 6×10−3 cm3/C for both types of films and R 1 is 7×10−2 and 2.5×10−1 cm3/C for the perpendicular and parallel films, respectively.
Measurements of the magneto‐optic Faraday rotation of perpendicular films indicate that the Faraday rotation is directly proportional to the magnetization. Over the visible wavelength region, the Faraday rotation was found to increase as temperature decreases from 300° to 87°K, by a factor less than 25%. The specific rotation for a typical magnetically saturated film was 1.4×105 deg/cm at the 6328‐Å He–Ne laser line at room temperature.
Our measurements are in agreement with Goodenough's suggestion of metallic conductivity resulting from a d band formed from the transition metal cations with the assistance of large numbers of interstitial cations. An effective hole density of 1021 cm−3 and Hall mobility of approximately 10 cm2/V‐sec characterize these films.
36(1965); http://dx.doi.org/10.1063/1.1714196View Description Hide Description
Materials from the garnet family 3[(1‐x) Y2O3·xGd2O3]· 5[wAl2O3· (1‐w) Fe2O3] have been investigated for use in latching‐type digital phase shifters. At room temperature, as x and w varied from 0 to 0.30, the electrical properties of these materials varied in the following manner: saturation magnetization (4πM s)−1780 to 500 G, resonancelinewidth (ΔH)−33 to 200 Oe, remanence ratio R r(B r/B s)−0.90 to 0.65, coercive force (H c)−0.50 to 2.00 Oe, switching constant (S w)−0.50 to 1.00 μsec‐Oe. By proper choice of x and w, some of these properties can be made quite insensitive to temperature and power variations over wide ranges. It was found that, although H c and R r tend to undergo large unfavorable changes with x and w, they could be controlled by increasing the average size of the crystallites. A five‐bit (180°, 90°, 45°, 22½°, and 11¼°) phase shifter was developed using material from the above family in a square toroid geometry. This device operated across the frequency band 5.4 to 5.9 Gc/sec with a switching rate of 5 kc/sec and a maximum switching time of 1 μsec. Phase‐shift stability over the temperature range 50° to 125°F was better than 5%, losses were less than 0.80 dB and VSWR less than 1.25 across the frequency range. The unit performed equally well at peak power levels up to 10 kW and average power levels up to 300 W. The total energy required for continuous switching of the 180° bit at the above rates was slightly over 200 μJ.
36(1965); http://dx.doi.org/10.1063/1.1714197View Description Hide Description
If the rf and dcmagnetic fields are parallel, the absorption coefficient μ″ at low power levels is theoretically expected (for YIG at room temperature, a frequency of 10 Gc/sec, and a dc field of 1 kOe) to be of the order of 2×10−4. We have calculated how the steady‐state absorption varies with dc and rf field strength below the instability threshold. Experiments on a high‐purity YIG single crystal are in reasonably good agreement with the theoretical expectations.
36(1965); http://dx.doi.org/10.1063/1.1714198View Description Hide Description
A new magnetoelasticresonanceeffect is described. A dc magnetic field was applied in the long direction of YIG slabs (1 cm long, 0.5 cm wide, and 0.1 or 0.07 cm high), each end of which was inserted in a stripline cavity. Under appropriate conditions a pulse of microwave power applied to one cavity resulted in a pulse in the second cavity delayed by several microseconds. Small variations were observed in the amplitude of the delayed pulse as the dc field was varied. Similar variations with the same periodicity (∼1 Oe) were also observed in the undelayed reflection from the input cavity. The amplitude variation is attributed to the excitation of magnetoelasticresonances in the nonuniform magnetic field near the endface of the sample. Because of the strong coupling between spin waves and elastic waves, the turning point (at which k spin waves=0) and the endface of the sample define a magnetoelastic quasicavity whose resonances give rise to the effect noted. According to this interpretation the separation of adjacent peaks should be δH = |H′ dem|λ/2, where H′ dem is the gradient of the demagnetizing field evaluated at the crossover point (k spin wave = k phonon) and λ the wavelength of transverse elastic waves. Measurements have been made showing the dependence on dc field, slab thickness, and frequency. Agreement with theory is satisfactory.
36(1965); http://dx.doi.org/10.1063/1.1714199View Description Hide Description
Direct spectroscopic evidence has been obtained of microwave sidebands imposed on a monochromatic light beam by transmission through a single crystal of chromium tribromide at ferromagnetic resonance.Microwave frequencies above 13.5 Gc/sec may be used, and the most favorable optical wavelengths for this effect lie between 4750 and 5000 Å. This work has demonstrated modulation of a 4810‐Å beam by a 23.5‐Gc/sec microwave signal. This paper discusses the conditions under which the modulation may be observed, and how some of these may be optimized.