Koledov V., Kostiuk D., Kuzavko Y., Shavrov V.

Crystals of Heysler Ni2+X+YMn1-XGa1-Y alloy possess effect of shape memory (SME) on temperature and magnetic field that promises various practical use. The only obstacle for mass usage is necessity of high fields (™ 80 kOe) for magnetocontrolled SME. Fields decrease up to 78 kOe due to applying pulses of low-frequency intensive ultrasound to Ni-Mn-Ga sample was experimentally shown earlier by the authors. The hypothesis was also proposed, that applying ultrasound when producing Heysler alloy essentially decreases the operating fields, possibly up to 20 kOe - ones provided by constant magnets. In Ni2MnGa crystal pre-martensitic (modulated) phase transition (PT) precedes the PT from cubic ausetenite phase into tetragonal martensite one. Structural PTs are caused by elastic anharmoinzm; in wide area of temperatures interaction of crystal’s elastic and magnetic subsystems is extremal and extremely strong magneto-mechanical effects are observed. For Ni2MnGa experimental data of linear acoustic properties shows that parameter of acoustic anisotropy A surpasses approximately on one order the A=4, measured for the light-plane antiferromagnet - α-Fe2O3 hematite in the area of its orientational PT (known as strongly nonlinear acoustic material). If C(2) elasticity modules decrease, C(3) elasticity modules and above increase at the approach to PT. Nonlinear acoustic effects are connected with the last ones, e.g. generation of the maximum harmonics. Strong acoustic nonlinearity and anisotropy is mostly connected with growth of electron-phonon interaction, which is effectively observed at occurrence of nestings of a Fermi surface, i.e. at coincidence of phonon wave vectors with electrons quasi-pulse. Authors are grateful to BRFFI and RFFI (æ04ò-080 and 04-02-81058 grants) and ME of RB (05-505 grant).