Far-infrared spectroscopy has been used extensively for the study of antiferromagnetic resonance, that is, single-magnon excitations at the Brillouin zone center. The recent discovery of two-magnon excitations in antiferromagnets by Halley and Silvera now permits precise measurements of magnon energies at selected points on the Brillouin-zone boundary. The discovery and theoretical interpretation of the two-magnon effect is described in this review paper. Experimental results for the antiferromagnets MnF2, FeF2, and CoF2 and for the canted-spin weak ferromagnets NiF2 and MnCO3 are also presented. The experimental data are discussed in relation to the current theoretical understanding of the two-magnon effect.
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P. L. Richards (1967) studied this question.
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