PREMARTENSITIC PHENOMENA IN THE FERROMAGNETIC AND PARAMAGNETIC PHASESOF NI2MNGA

Low-energy phonons were studied in the ferromagnetic and paramagnetic phases of the Heusler alloy Ni2MnGa. The investigated sample shows a m artensitic phase transformation with a transition temperature T-M appr oximate to 284 K, only about 80 K below the Curie temperature. Therefo re, premartensitic phenomena could be studied in the ferromagnetic as well as in the paramagnetic state. The (xi xi 0) TA(2)-phonon branch s hows a strong but incomplete softening at xi approximate to 1/3 in the premartensitic phase when the temperature approaches T-M. The tempera ture dependence of this softening changes at the Curie temperature whi ch can be explained by an additional contribution of the magnetization to the Landau free energy. At the waste vector of the strongest phono n softening a central peak occurs. A second elastic peak, with the sam e temperature behavior, appears at xi approximate to 0.17. The relatio n of these elastic precursors to the low-temperature structure is disc ussed.