Giant magnetoacoustic effect in KMnF3 due to nuclear spin waves

An explanation is proposed for the gigantic magnetoacoustic effect that we observed in KMnF3 in previous work {Kh. G. Bogdanova, V. A. Golenishchev-Ku tuzov, M. I. Kurkin &etal;, Zh. Eksp. Teor. Fiz. 112, 1830 (1997) [JETP 85, 1001 (1997)]}. The effect entails a tenfold amplitude reduction of an acou stic pulse in a magnetic field that varies over the range 0-8 kOe. It is sh own that this effect is due to the interference of two nuclear magnetoelast ic waves propagating in the sample under magnetoacoustic resonance conditio ns, if this resonance occurs in the region of strong spatial dispersion of nuclear spin waves. The effect is said to be gigantic because it exceeds in magnitude the magnetoacoustic effects observed previously in magnetically ordered materials even though it is due to nuclear magnetism, which is 10(5 ) times weaker than electronic magnetism. We observe a concomitant anomalou s dependence of the dispersion of the velocity of sound on the external mag netic field. (C) 1999 American Institute of Physics. [S1063-7761(99)01505-X ].