SPIN-WAVES AND PHONON ANOMALY IN THE HEUSLER ALLOY NI2MNGA

Magnons and phonons in Ni2MnGa were investigated with neutron spectros copy. The alloy shows a martensitic phase transition from a cubic high -temperature phase to a tetragonal one within the ferromagnetic state, about 80 K below the Curie temperature. The spin waves show no anisot ropy in the low-energy range. The corresponding magnetic exchange stif fness constant is 108 +/- 10 meV Angstrom(2). The TA(2)-phonon branch shows a strong but incomplete softening at xi similar to 1/3 when the temperature approaches the martensitic phase transition temperature.