R. C. LeCraw

MAGNETO‐OPTICAL PROPERTIES OF A GREEN ROOM‐TEMPERATURE FERROMAGNET: FeBO3

Ferric fluoride (FeF3) is a transparent green uniaxial weak ferromagnet (canted antiferromagnet) with a Curie temperature of 365 °K. FeF3 is far more transparent in the visible and uv than any other known material which has a spontaneous magnetic moment at room temperature. FeF3 at room temperature has 6 absorption bands between 7000 A and the band edge which is at 2440 A. There is a local absorption minimum of α = 4.4 cm−1 in the green at 5225 A. The Faraday rotation is 180 °/cm at 5225 A and increases to 1100 °/cm at 3000 A. The birefringent phase retardation at 5200 A is about 80 000 °/cm. The two indices of refraction are about 1.54 in the visible spectrum.

Temperature Dependence of the Spin-Wave Spectrum of Yttrium Iron Garnet

The temperature dependence of the exchange constant D in the equation of the spin-wave spectrum has been measured from 4.2°K–500°K in single crystals of very pure yttrium iron garnet. The results are compared, with good agreement up to 325°K, to predictions based on an extension of the random phase approximation to a ferrimagnet.

Angular Momentum Compensation in Narrow Linewidth Ferrimagnets

Studies are described of the ferrimagnetic garnet system Eu3Fe5—xGaxO12. It is shown that a compensation point of the net angular momentum occurs at x≅1.2, at which the resonance linewidth remains narrow. In all previously observed ferrimagnetic systems with angular momentum compensation points, e.g. GdIG, strong line broadening occurs. The reasons for this are discussed together with the relaxation processes involved. Application of the resulting large g factors to millimeter wave harmonic generation is considered.

FERROMAGNETIC RESONANCE IN FeBO3, A GREEN ROOM‐TEMPERATURE FERROMAGNET

Iron borate (FeBO3) is a transparent, green, room‐temperature weak ferromagnet (a canted anti‐ferromagnet) with a Curie temperature of 348°K. Ferromagnetic resonance measurements, i.e., linewidth, anisotropy fields, and g factor vs temperature and frequency are reported for the first time on this material. A low‐temperature linewidth maximum of unknown origin is also observed. Its narrow linewidth, large magnetic anisotropy, large optical Faraday rotation, and transparency in the green (the latter two reported elsewhere) made FeBO3 of interest for devices such as electronically tunable cavities and filters up to ∼100 GHz and high‐speed magneto‐optic modulators.

Generation of Microwave Elastic Vibrations in a Disk by Ferromagnetic Resonance

A calculation is presented of the generation of microwave elastic waves by the uniform precession resonance in ferromagnets. The excitation of the thickness shear modes in a normally magnetized ferromagnetic disk is treated. For uniform precession the elastic modes are driven only on the surfaces of the disk. The effects of long wavelength spin waves magnetostrictively driven by the elastic waves can be included in the analysis by a change in the elastic modulus. The coupling out of the elastic energy by bonding the disk to a non‐magnetic delay medium is considered for arbitrary values of the elastic impedances of the two media. An equivalent electrical circuit for transducers is presented; the efficiency and bandwidth of disk transducers are calculated for a disk placed in a matched one port resonant cavity. A figure of merit for such transducers is defined, and calculated for yttrium iron garnet. Due to interference between the elastic waves generated on the two disk surfaces, the realizable bandwidth o...

Surface‐Independent Spin‐Wave Relaxation in Ferromagnetic Resonance of Yttrium Iron Garnet

In contrast to the ferromagnetic resonance tine width ΔH, which depends strongly upon surface polish, the “line width” ΔHk of a z‐directed spin wave is shown to be surface independent. This permits observation of a relaxation process which appears to be determined primarily by collisions between thermally excited spin waves rather than by a volume or surface distribution of magnetic inhomogeneities. In yttrium iron garnet ΔHk is observed to be 0.10 oe at room temperature and to increase monotonically with increasing temperature. The 0.10 oe corresponds to a relaxation time of 1.14 μsec. It is believed this is the narrowest line width yet indicated for a ferromagnetic material.

Magnetoacoustic Resonance in Yttrium Iron Garnet

Additional information is given on the conditions under which magnetoacoustic resonance is observed in polished single crystal spheres of yttrium iron garnet.

New Acoustic and Magnetic Properties of YIG and YAG with Small Mn and Ni Additions

The resonant frequency and Q of the acoustic modes of YIG and YAG have been investigated as a function of temperature at approximately 1 Mc/sec. The samples studied have either Mn or Ni additions in the range from 0 to 1% of the total weight. For the samples with Mn or Ni additions, the resonant frequency of the shear modes has a maximum as a function of temperature. This maximum does not exist with pure YIG and YAG. The Mn or Ni additions also lead to a large increase in the low‐temperature acoustic loss. Similar results have been obtained with Mn additions in the corundum and spinel structures. We tentatively attribute the observed effects to the static and dynamic Jahn—Teller distortions arising from the Mn and Ni additions.

Second‐Harmonic Generation in Large g‐Factor Ferrimagnets

Harmonic generation in a ferrimagnet is enhanced by an increase in the g factor. The physical reasons for this are examined for the case of second‐harmonic generation in sphere and disk samples. Theoretical performance characteristics for a new, narrow‐linewidth, large g‐factor material are compared with those for conventional ferrites.

Magnetoelastic Coupling Constants of Terbium and Europium Iron Garnets

The purpose of this paper is twofold: (1) to present data on the magnetoelastic constants of rare‐earth iron garnets, a subject which for various reasons has been little studied thus far, and (2) to describe a new method for obtaining these constants which is applicable to many ferromagnetic insulators and does not require the use of strain gauges. The new method was developed from a study of the coupling introduced between long wavelength acoustic modes and long wavelength spin‐wave modes by ions in which the orbital moment is not quenched, such as rare‐earth ions, Co2+, and Fe2+. The effect of the coupling caused by such ions is observed as a magnetic field dependence of the resonant frequency of an acoustic resonator made from the given material, even above dc saturation. The acoustic Q above dc saturation is also field‐dependent. A theory has been obtained which explains the observed field dependences of both v(H) and Q(H) as well as other important features of the magnetoacoustic interaction. It will...