Magnetically driven shape-memory effect and colossal magnetostriction in Ni2MnGa-based Heusler alloys

1. Objectives

     The objective of the project is the development of general approach to handle with the single- and polycrystalline materials revealing colossal deformation under magnetic field. This major task can be divided into a number of specific problems that form the content of the present project. These tasks include:
1. The preparation of polycrystalline ingots and single crystals of stoichiometric Ni2MnGa and non-stoichiometric Ni2±x±yMn1±xGa1±y ternary Heusler alloys.
2. The investigation of phase diagrams of ferromagnetic Ni2MnGa-based alloys in magnetic field - temperature - mechanical stress - chemical composition coordinates. To carry out a complex theoretical investigation of the system using both phenomenological theory and first principles calculations. To obtain phase diagrams of the ground state. To take into account the external mechanical stress and magnetic field.
3. The theoretical and experimental study of the influence of magnetoelastic coupling on the kinetics and nucleation of martensitic and magnetic domains. To investigate the influence of chemical composition on the physical properties different compositions of Ni2±x±yMn1±xGa1±y ternary Heusler alloys.
4. The investigation of mechanical properties (stress-strain curves) of chosen Heusler alloys, including the study on possibility of multiple mechanical and thermal cycling. The training of ferromagnetic alloys for two-way shape memory by combination of the stress, thermal cycling and magnetic field.
5. The investigation of rearrangement of conjugated ferromagnetic and martensitic domains structures in tetragonal phase of Ni2±x±yMn1±xGa1±y under moderate magnetic field.
6. The achievement of colossal magnetically driven deformations through the redistribution of martensitic variants on polycrystalline samples comparable by the order of magnitude with those obtained on single crystalline samples.
7. The realization of the effects of two-way shape memory, superelasticity and superplasticity in Ni2MnGa-based alloys through the magnetically driven shift of austenite-martensite transformation temperature.
8. The development of the prototype devices based on magnetically induced giant deformation on the basis of the above mentioned research, particularly the sensors and actuators.

2. Background & Justification for Undertaking the Project

     There are several classes of intelligent materials, which can significantly change their shape and dimensions under application of external fields. Among them are the magnetostrictive alloys which can be deformed by magnetic field (up to 0.1 per cent) and shape memory alloys, which can be deformed by mechanical stress (up to 10 per cent) in martensitic state.
The colossal deformations (up to 6 per cent) under action of the magnetic field can be obtained in materials, which undergo the thermoelastic transformation into ferromagnetic martensitic phase. Colossal magnetically induced strains can be achieved either through rearrangement of the martensitic twins by magnetic field [1], or through magnetically induced shift of the martensite - austenite phase transition.
     The former approach was used in recent studies carried out in 1996-2001 (see [2] for references). It is based on the fact that due to the magnetocrystalline anisotropy, structural and magnetic domains in these materials are closely related to each other and cannot be changed independently. Application of external field leads to simultaneous conjugated redistribution of martensitic and magnetic structures. The reconstruction of martensitic domains is possible only in the case when their boundaries are mobile, i.e. if the martensite transformation is a thermoelastic one. Thus, to obtain the colossal deformation under magnetic field in the framework of the first approach the following conditions must be fulfilled:
          - the transition into martensitic phase must be thermoelastic;
          - the martensitic phase must be ferromagnetic;
          - the martensitic phase should possess sufficiently large magnetocrystalline anisotropy.
     These requirements are fully met only in few materials, namely binary alloys FePt and FePd and Ni2MnGa-based Heusler alloys. Most promising among these alloys are the Ni2MnGa-based alloys which have shown up to 6 % deformation under magnetic fields less than 1 T. The Heusler alloy Ni2MnGa of stoichiometric composition undergoes the ferromagnetic transition at 376 K and the thermoelastic martensitic transformation at 202 K [3]. The martensitic transformation takes place from high temperature cubic phase into low temperature tetragonal phase. The c/a ratio in the tetragonal phase is 0.94, which corresponds to the maximum magnetically induced deformations (about 6 %) in these alloys. The magnetic anisotropy of the cubic phase is negligibly small, but in the tetragonal phase it is reasonably large and the easy axis of magnetization is oriented along c axis [1].
     The alternative approach has been developed by the authors of the project [4-6]. They have shown that properties of Ni2MnGa-based alloys are quite sensitive to composition. With deviation from stoichiometry in Ni2+xMn1-xGa series the Curie temperature decreases, while the martensitic transition temperature increases until both temperatures merge in the composition range x = 0.18 - 0.20 [4]. For the alloys of this composition range the transition from a paramagnetic high temperature phase to a ferromagnetic low temperature phase occurs. (The temperature of this transition appears to be higher than the room temperature, which is important for practical applications). For the samples with merging martensitic and magnetic phase transitions an increase in the phase transition temperature is proportional to the magnetic field, with the coefficient of proportionality being of the order of 1 K/T. The magnetic field induced reversible martensite transition at fixed temperature and pressure was observed in [5]. Recently [6], colossal magnetically induced strains (up to 3 %) and magnetic field induced one-way shape memory effect, (i. e. the restoration of the shape of the initially deformed sample by magnetic field at fixed temperature and pressure), has been observed in polycrystalline samples. See video film, demonstrating the effect observed via INTERNET http://jre.cplire.ru/jre/may01/1/text.html.

References for the part 2.
1. R.Tickle, R.D.James. Magnetic and magnetomechanical properties of Ni2MnGa. JMMM 195, 627 (1999).
2. R.C. O'Handley, S.J. Murray, M. Marioni, H. Nembach, S.M. Allen. Phenomenology of giant magnetic-field-induced strain in ferromagnetic shape-memory materials. J. Appl. Phys. 87, No. 9, 4712 (2000).
3. P.J. Webster, K.R.A.Ziebeck, S.L. Town, M.S. Peak. Magnetic order and phase transformation in Ni2MnGa. Philos. Mag. B. 49, No. 3, 295 (1984).
4. A.N.Vasil'ev , A.D.Bozko, I.E. Dikshtein, V.V.Khovailo, V.D. Buchel'nikov, M. Matsumoto, S. Suzuki, V.G. Shavrov, T. Takagi, J. Tani. Structural and magnetic phase transitions in shape memory alloys Ni2+XMnXGa. Phys. Rev. B59(2), 1113 (1999).
5. A.A. Cherechukin, I.E. Dikshtein, D.I. Ermakov, V.V. Koledov, L.V. Koledov, T.Takagi, A.A. Tulaikova, V.G. Shavrov. Reversible structural phase transition in Ni-Mn-Ga alloys in a magnetic field. JETP Lett., 72, No. 7, 373 (2000).
6. V.G. Shavrov, A.A. Cherechukin, I.E. Dikshtein, A.A. Glebov, V.V. Koledov, D.V. Kosolapov, E.P. Krasnoperov, A.A. Tulaikova. Observation of the one-way shape memory effect in Ni-Mn-Fe-Ga Heusler alloy due to the magnetic field induced martensite - austenite transition. Radioelectronics, No 5, (2001), http://jre.cplire.ru .

3. Scientific/Technical Description

Task 1. The preparation of polycrystalline ingots and single crystals of stoichiometric Ni2MnGa and non-stoichiometric Ni2±x±yMn1±xGa1±y ternary Heusler alloys.
     Objectives: The objective and the output of this task is the preparation of high quality samples for physical measurements.
     Method used: Arc-melting and Chochralski technique. The characterization of uniformity and crystal lattice parameters of the samples will be performed by X-rays technique and various physical methods.
     Schedule: The preparation of polycrystalline samples and single crystals growth will be carried out during the lifetime of project.
     Criteria of success: The samples should be of size and quality acceptable for the proposed research.

Task 2. The development of the phenomenological models for the behavior of Ni2MnGa-based alloys in magnetic field taking into account the influence of magnetoelastic coupling on the structural and magnetic phase transitions. The investigation of phase diagrams of ferromagnetic Ni2MnGa-based alloys in magnetic field - temperature - mechanical stress - concentration coordinates.
     Objectives: The analytical and numerical calculation of phase diagrams of Ni2MnGa-based alloys taking into account the magnetic order parameter, deformation order parameter and modulation order parameter. The analytical and numerical calculation of phase diagrams of Ni2MnGa-based alloys taking into account the influence of magnetic field and mechanical stress. The calculation of "chemical composition - critical temperatures" phase diagrams. The experimental determination of phase diagrams of Ni2MnGa-based alloys by various physical methods.
     Inputs: The program packages for numerical calculation of phase diagram of Ni2MnGa-based alloys. The polycrystalline and single crystals of Ni2MnGa-based alloys for experimental determination of phase diagrams.
     Outputs: The theoretical and experimental phase diagrams of Ni2MnGa-based alloys
     Schedule: During the lifetime of the project.
     Method used: The numerical methods of calculations and various experimental techniques.
     Criteria of success: The phase diagrams obtained will be compared with known phase diagrams for some alloys.

Task 3. The theoretical and experimental study of the influence of magnetoelastic coupling on the kinetics and nucleation of martensitic and magnetic domains. The establishment of the interrelations between the chemical composition of Ni2MnGa-based alloys and their physical properties.
     Objectives: The objectives of this task is characterization of kinetics and nucleation of martensitic and magnetic domains in Ni2MnGa-based alloys and determination of interrelations between the chemical composition of Ni2MnGa-based alloys and their physical properties.
     Inputs: The objects under study will be polycrystalline and single crystals grown during the lifetime of the project.
     Outputs: The experimental and theoretical determination of main parameters responsible for the behavior of Ni2MnGa-based alloys in magnetic field and mechanical stress.
     Schedule: This task will be carried out during the lifetime of project.
     Method used: To characterize physical properties of Ni2MnGa-based alloys the transport, magnetic, thermal, optical and other techniques and various theoretical calculations will be involved.
     Criteria of success: Choice of the best samples for subsequence studies will be done on basis on these measurements and calculations.

Task 4. The investigation of mechanical properties (stress-strain curves) of chosen Heusler alloys, including the study on possibility of multiple mechanical and thermal cycling. The training of ferromagnetic alloys for two-way shape memory by combination of the stress, thermal cycling and magnetic field.
     Objectives: The objective of this task is the investigation of static and dynamic mechanical properties of Ni2MnGa-based alloys including the investigation of materials reaction on loading and magnetic field.
     Inputs: Well-annealed soft polycrystalline samples and single crystals will be used for the training for this materials.
     Outputs: Stress-strain curves will be obtained for these materials. Maximal accessible deformations will be estimated. The information on the crystallographic domain structures will be obtained on this stage.
     Schedule: During the lifetime of the project.
     Method used: The methods to be employed at this stage will be specially designed loading devices and optical equipment to detect martensitic and magnetic domain structures.
     Criteria of success: The achievement of large superplastical deformation will be considered as success of this stage.

Task 5. The investigation of rearrangement of conjugated ferromagnetic and martensitic domains structures in tetragonal phase of Ni2±x±yMn1±xGa1±y under moderate magnetic field.
     Objectives: A strategy will be developed at this stage for inducing colossal magnetostriction in ferromagnetic thermoelastic martensite on monocrystalline samples.
     Inputs: Certain relations between material constants that promote effect of colossal magnetostriction are expected to be obtaining on previous stages.
     Outputs: A theoretical analysis and experimental verification of domain redistribution caused by a magnetic field will be given.
     Schedule: Second year of the project.
     Method used: Optical measurements and theoretical models of martensitic domain redistribution will be employed.
     Criteria of success: The realization of monodomain martensitic state under action of magnetic field.

Task 6. The achievement of colossal magnetically driven deformations through the redistribution of martensitic variants on polycrystalline samples comparable by the order of magnitude with those obtained on single crystalline samples.
     Objectives: The objective of this stage is the realization of colossal magnetostriction through the redistribution of martensitic variants.
     Inputs: Well-annealed soft polycrystalline samples and single crystals will be used to realize the effect of colossal magnetostriction.
     Schedule: Second year of the project.
     Method used: A preliminary prestressed monodomain sample will be placed into magnetic field perpendicular to the easy axis of magnetisation. In case of sufficiently large magnetic anisotropy and low value of effective elastic modulus the redistribution of martensitic variants can result in significant up to 6% reversible magnetostriction of Ni2MnGa-based alloys.

Task 7. The realization of the effects of two-way shape memory, superelasticity and superplasticity in Ni2MnGa-based alloys through the magnetically driven shift of austenite-martensite transformation temperature.
     Objectives: At fixed temperature and under fixed stress to demonstrate major thermoelastic effects in Ni2MnGa-based alloys due to the action of magnetic field. These effects include the one way shape memory effect, the two-way shape memory effect, the superelasticity, the superplasticity and the giant mechanical dumping.
     Inputs: The results of the previous investigation, including study of phase diagrams of Ni2MnGa-based alloys, the knowledge of mechanical and thermoelastic properties will be the starting point of this stage of investigation.
     Outputs: The number of magnetoelastic effects will be investigated and proposals for their application will be made.
     Schedule: Second year of the project.
     Method used: The training of the samples in magnetic fields and under mechanical stress will be employed.

Task 8. The development of the prototype devices based on magnetically induced giant deformation on the basis of the above mentioned research, particularly the sensors and actuators.
     Objectives: The general approach is to be developed to create magnetically driven sensors and actuators on the basis of previous studies.
     Outputs: The polycrystalline samples and single crystals of Ni2MnGa-based alloys will be used as driving elements of microactuators and other devices designed for practical application.
     Schedule: Second year of the project.
     Criteria of success: The realization of any practically useful device can be considered as successful output of fundamental research.

4. Management.

     All the scientific tasks mentioned will be performed simultaneously by Russian and Japanese teams involved into the project, providing complementary information to each other. The teams will be closely interconnected by using samples of same origin, by intensive exchange of information over e-mail, and by exchange of scientists. The appearance of new objects of investigation will require coordinated studies of their properties. Meeting of the project participants are planned at the various conferences on magnetism and magnetic materials, shape memory effect and at the institutes of the various teams.

5. Summary

     The unique feature of ternary Heusler Ni2Mn1Ga1-based alloys is that they experience the martensitic transformation at about room temperature while being in a ferromagnetic state. The study of physical properties of these alloys will provide new information on interaction of electronic, magnetic and elastic subsystems in metals. Magnetically driven two-way shape memory in Ni2Mn1Ga1-based alloys can be achieved through the shift of martensite-austenite transition, and the giant magnetostriction could be obtained through the redistribution of martensitic variants in moderate magnetic field. The realization of magnetically driven shape memory effect and superelasticity will make grounds for development of new technologies in medicine and industry.

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