Antiferromagnetic Materials for Millimeter and Submillimeter Devices

Abstract

Owing to their high internal fields, antiferromagnetic materials have natural resonant frequencies in the millimeter and submillimeter wavelength region of the electromagnetic spectrum. Since these normal modes are circularly polarized and can be tuned by an applied magnetic field, devices similar to the usual ferrite devicesare possible at these high frequencies with the application of relatively low fields. The dynamics of a simple antiferromagnetic system are briefly reviewed and the important quantities which characterize antiferromagnetic devices are discussed. The figures of merit for antiferromagnetic resonance isolators and phase shift devices are derived. Experimental data on resonant frequency and linewidth, as well as a typical nonreciprocal resonance absorption trace showing a reverse to forward loss of 10 to 1 in chromic oxide at 140 kMc and at 77°K are presented.

Antiferromagnetic and Ferrimagnetic Resonance

“…Various experiments with spin-flop magnetic measurements [6], spin-flop resonance, and angular dependence are in good agreement with the molecular field approxima tion if experimentally determined susceptibilities are used. Experiments by Heller et al [36] agree with the lower-frequency results. Experiments by Heller et al [36] agree with the lower-frequency results.…”

Microwave Devices

“…Therefore, / / anis H ex and i / | nis may be neglected in Eq. For example, in Cr20 3 [73] it is about 60,000 oe giving a zero applied field resonance at about 2 mm and for M n F 2 about 94,000 oe corresponding to zero field resonance in the 1 mm region of the spectrum. It is important to note that the original equations of motion are valid only when H < (2 i/ex#anis + #!nis)1/2· This results from the fact that, when H becomes as large as the quantity under the square root, it is energetically more favorable for the sublattices to line up perpendicular to the applied dc magnetic field and then, of course, the original equations of motion are not valid.…”

“…The principles and application of antiferromagnetic isolators are illus trated in the following derivation [73] of the critical field. * Since each sublattice can be considered as being acted upon by the two effective fields H ex , and i / ams , we write the equations of motion as The anisotropy field is usually of the order of a few hundred to a few thousand oersteds.…”

“…(3.10). The performance characteristics of two isolators, one operating on each branch, are shown [76] in Fig. 21a.…”

“…Heller [73] has derived the expressions for the elements of the magnetic susceptibility tensor for antiferromagnetic resonance which are needed to evaluate the figure of merit for an isolator, R = α_/α+ = χ "/ χ 1 , where χ" is the dissipative part of the total susceptibility. Heller [73] has derived the expressions for the elements of the magnetic susceptibility tensor for antiferromagnetic resonance which are needed to evaluate the figure of merit for an isolator, R = α_/α+ = χ "/ χ 1 , where χ" is the dissipative part of the total susceptibility.…”