Materials in which the reversible form and size control by magnetic field is possible, attracts much attention now. Highest effect is registered for Ni_2+x+yMn_1-xGa_y alloys. Crystal size there can be changed up to 6% by magnetic field (the theoretical deformation limit at transformation from austenite to martensite). With different stechiometric composition the temperature of martensite transition T may be close to the room one and lower than the Curie point of material. Sometimes martensite phase transition (PT) is preceded by premartensite one (PM PT) with anomalies of speed and absorption of longitudinal and transverse acoustic waves (LA and TA). The transverse mode TA₂ is softened near the PM PT with the k = (ξ,ξ,0) wave vector at ξ =0.33 and e = (l 1 0) polarization vector. Its speed changes from 740 m/s (T=300 K) to 614 m/s (TPM=255 K) and 903 m/s (TM =215 K) at f=3.7 MHz frequency.

Acoustic anisotropy of a crystal reaches the value of A=28 at PM PT. Structural PT usually takes place in a Ni₂MnGa crystal, and after that the orientational PT can be realized at temperature decrease and in H||[001] field.

We examined LA and TA inclined fall in (100) plane, reflection from the crystall free border (110) and from its border with liquid or dielectric. Spread directions and amplitudes of reflected waves in (001) plane of a crystal were determined, and are not pure modes but quasi-longitudinal and quasi-transverse. The colossal acoustic anisotropy of a crystal in its PT area brings allows to control the angles of reflection and transformation of wave types by temperature and magnetic field. Starting from a critical fall angle of TA, the LA, appeared at reflection, turns into the accompanying surface oscillation, and close to PT point it reflects into the volume.

Authors are grateful to BRFFI and RFFI (F04R-080, 04-02-81058, 03-02-17443 grants) and Ministry of Education of Belarus (05-550 grant) for their financial support of carried out research.

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