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The investigation of longitudinal acoustic waves in Heusler Ni53Mn25Ga22 alloy crystal with coupled magnetostructural transition

Yuri A. Kuzavko 

The Polesie Agrarian Ecological Institute of the National Academy of Science of Belarus (PAEI), Brest 224000, Belarus

Abstract
Heusler Ni53Mn25Ga22 ferromagnetic alloy crystal (sizes 4.0 x 3.8 x 10 mm) with shape memory effects (SME) has the coupled magnetostructural phase transition (the austenite starting temperature As=116oC and finishing one At=134oC, the martensite starting temperature Ms=109oC and finishing one Mt=97oC) below the Curie temperature Tc=134o C. This characterising temperatures were defined by acoustical measurements using the longitude uldtrasound transducers with 2.8, 3.5, 5 MHz resonance frequences. Temperature was measured with accuracy of 0.025K, the minimal heating speed was 1 K / 3 min and holding one was 1 K / 10 min. The sample is placed at 2 Oe constant magnet at necessity. Sample was temperature-controlledm heated with a low-ohm warmer from a power supply. Temperature was measured by termo-couple and microvolmeter. 

This alloy has reversible thermoelastic martensite transitions. The process of a return to parent shape is coupled with a reveversible transition of a deformed martensite (D4h) phase into a high-temperature austenitic (Oh) one. For our crystall the lattice parameters are a=b=0.7753 nm, c=0.6598 nm, as for Ni2MnGa a=b=0.5920 nm, c=0.5566 nm at D4h phase, a=0.5825 nm at Oh. 

We have studied experimentally the temperature dependences of the ultrasound speed and amplitude of pulse signals my means of digital measuring equipments.

The presence of a soft transverse mode TA2 takes place for the alloy [2]. It propagates along directions [110] and related with accession to zero of the elasticity constant c`=(c11-c12)/2. The theory was designed for investigated effects consisting the linear and nonlinear magnetoacoustical ones.

 

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Presentation: Oral at E-MRS Fall Meeting 2008, Symposium C, by Yuri A. Kuzavko
See On-line Journal of E-MRS Fall Meeting 2008

Submitted: 2008-05-30 10:29
Revised:   2009-06-07 00:48