Premartensitic phenomena in the ferro- and paramagnetic phases of Ni₂MnGa

Low-energy phonons were studied in the ferromagnetic and paramagnetic phases of the Heusler alloy Ni2MnGa. The investigated sample shows a martensitic phase transformation with a transition temperature T-M approximate to 284 K, only about 80 K below the Curie temperature. Therefore, premartensitic phenomena could be studied in the ferromagnetic as well as in the paramagnetic state. The (xi xi 0) TA(2)-phonon branch shows a strong but incomplete softening at xi approximate to 1/3 in the premartensitic phase when the temperature approaches T-M. The temperature dependence of this softening changes at the Curie temperature which can be explained by an additional contribution of the magnetization to the Landau free energy. At the waste vector of the strongest phonon softening a central peak occurs. A second elastic peak, with the same temperature behavior, appears at xi approximate to 0.17. The relation of these elastic precursors to the low-temperature structure is discussed.