Magnetoelasticity in the Heusler Ni2MnGa alloy

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Journal of Magnetism and Magnetic Materials 196-197 (1999) 637-638

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Magnetoelasticity in the Heusler Ni2MnGa alloy •

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A. Gonzalez-Comas, E. O b r a d 6 a, L1. Mafiosa a, A. Planes a'*, A. L a b a r t a b aDepartament d'Estructura i Constituents de la Mat~ria, Facultat de Fisica, Universitat de Barcelona, Av. Diagonal 647, 08028 Barcelona, Catalonia, Spain bDepartament de Fisica Fonamental, Facultat de Fisica, Universitat de Barcelona, Av. Diagonal 647, 08028 Barcelona, Catalonia, Spain

Abstract

In this paper we present magnetic and elastic measurements on a N i - M n - G a single crystal. It is shown that the structural properties of this alloy are strongly influenced by a magnetoelastic interaction. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Magnetoelasticity; Magnetic-field-induced strains; Martensitic transformation

The Heusler Ni2MnGa alloy is the only known BCCbased ferromagnetic alloy undergoing a martensitic transition (MT). Associated with this phase transition, this material exhibits shape-memory properties [1]. In this kind of alloys, the self-accomodation of the structural variants (twin-related) produced during the MT results in negligible macroscopic strain changes of the sample. In N i - M n - G a , however, the application of a magnetic field enables the motion of these twin boundaries [2], which can result in a large recoverable strain. Due to the technological implications of these magnetically controlled strains, Ni2MnGa has recently received a great deal of attention. A unique feature in Ni2MnGa is that prior to the MT, a weakly first order transition takes place, related to the development of a micromodulated structure [3-5]. This intermediate transition can be considered a magnetically driven effect that announces the MT by the modification of the dynamical response of the BCC parent phase. Both martensitic and intermediate transitions are influenced by the interaction between structural and magnetic degrees of freedom. The aim of this work is to provide experimental evidences for the existence of a magnetoelastic coupling.

* Corresponding author. E-mail: [email protected].

The sample investigated in this work was a single crystal with composition Ni49.sMn25.4Ga25.v Samples used are the same as in Ref. [6]. Below Tc = 381 K, the sample exhibits ferromagnetic order, with magnetic moments located at the Mn atoms. At room temperature the alloy displays a BCC structure with an L2~ atomic order (space group Fm3m). On cooling, it transforms to a tetragonal modulated phase (5R in Ramsdel notation) at TM = 175 K. The interplay between magnetic and structural degrees of freedom is apparent in the change of the magnetic properties of the alloy as it goes through both the intermediate and martensitic phase transitions. Fig. 1. shows the AC magnetic susceptibility measured as a function of temperature. A sharp decrease occurs at the martensitic transition, which is due to a higher magnetic anisotropy of the martensitic phase. A relative minimum is observed at the intermediate transition temperature Tv The application of a low DC magnetic field results in a decrease of T1 [5]. Such a dependence is a clear manifestation of the magnetically driven character of this intermediate transition. Within the experimental resolution, no significant changes of TM with magnetic field have been detected. We have performed magnetization measurements as a function of magnetic field in the three structural phases of the alloy (BCC, intermediate and martensitic). Results are shown in Fig. 2. The BCC and premartensitic phases exhibit hysteresis loops typical of soft ferromagnetic

0304-8853/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 8 85-3

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A. Gonzhlez-Comas" et at. /Journal ~/ MagJ~etism and Mc~gttelic Materials t 96-197 (1999) 637-638

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materials. In the martensitic phase the slope at the origin is lower, in agreement with susceptibility measurements. Interestingly, in this low t e m p e r a t u r e phase, the slope at intermediate fields is constant, with a lower value t h a n at the origin. This b e h a v i o u r could be indicative of a certain degree of a n t i f e r r o m a g n e t i s m in the martensitic phase which could be associated with a shorter distance between a small fraction of M n a t o m s located close to the martensitic twin boundaries. A definite p r o o f of the interaction between the magnetic a n d elastic properties in the BCC phase is given by the significant change o b t a i n e d in all elastic c o n s t a n t s

Acknowledgements

A.G. and E.O acknowledge a fellowship from D G I C y T . This work has received financial s u p p o r t from the C I C y T (Spain), project MAT95-0504 a n d from C o m i s s i o n a t d ' U n i v e r s i t a t s i Recerca (Catalonia), project SGR119.

References

[1] L. Delaey, in: P Haasen ted.), Materials Science and Technology, vol. 5: Phase Transformations in Materials, VCH, Weinheim, 1991, p. 339. [2] K. Ullakko et al., Appl. Phys. Lett. 69 (19961 1966. [3] A Zheludev et al., Phys. Rev. B 5t (19951 11310. [4] Ll. Mafiosa et al., Phys. Rev. B 55 (19971 11068. [5] A. Planes et al., Phys. Rev. LetL 79 (19971 3926. [6] E Obrad6 et al., J. Appl. Phys. 83 (19981 7300.