THERMOMAGNETIC ANALYSIS AND DOMAIN STRUCTURE IN THE PHASE TRANSITION REGION OF Ni-Mn-Ga AND Co-Ni-Ga SHAPE MEMORY ALLOYS

101Thermomagnetic analysis and domain structure in the phase transition region of Ni-Mn-Ga and...

© 2009 Advanced Study Center Co. Ltd.

Rev.Adv.Mater.Sci. 20(2009) 101-106

Corresponding author: Teodor Breczko, e-mail: [email protected]

THERMOMAGNETIC ANALYSIS AND DOMAINSTRUCTURE IN THE PHASE TRANSITION REGION OF

Ni-Mn-Ga AND Co-Ni-Ga SHAPE MEMORY ALLOYS

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Received: February 03, 2008

Abstract. The recently developed ferromagnetic shape memory alloys (FSMA) exhibit large fieldinduced strains thereby rendering new potentials for application in transducers, actuators andother novel devices. Magnetically controlled strain in FSMA is based on the reorientation of thetwin structure of martensite under applied magnetic field. A detailed study of both martensitic andmagnetic domain structure is presented for oriented single-crystalline bulk and texturized pow-dered samples embedded in polymer matrix, and polycrystalline bulk and rapidly quenched ribbonalloys. Optical microscopy including magneto-optical indicator film technique was employed along-side with X-ray and AFM/MFM studies for the characterization of the coexisting structural andmagnetic domains. It is shown that only 180° magnetic domains exist in twin plates because mar-tensite possesses uniaxial magnetic anisotropy having magnetization vector M oriented alongeasy c-axes at angles of ±45° with respect to the twin boundaries. Due to magnetostatic couplingthe 180° magnetic domains of neighbouring twins cooperate with each other forming continuousmacrodomains running through the whole crystallite or single crystal sample and changing thedirection of M by ±90° in a zigzag fashion at each intersection of the twin boundary.

1. INTRODUCTION

Combination of ferromagnetism and structuralphase transitions in Heusler alloys is perspectivefor the production of new devices based on themagnetic field control of the size and shape of theactuator active elements. The structural phase tran-sitions in these elements proceed by the transfor-mation of high-temperature austenite cubic phaseinto a tetragonal low-temperature martensite phase[1]. Extremely high (up to 10%) magnetically in-duced deformations were shown to exist in the fam-ily of Ni

2+xMn

1-xGa alloys [2]. Also important, though

studied in less detail, are some other groups of

Heusler alloys, in particular, those on the basis ofCo–Ni–Ga [3–5].

Detailed experimental studies of the regulari-ties of formation and realignment of both marten-sitic and magnetic domain structure (DS) are nec-essary for modelling and simulation of magneticallyinduced phenomena in these alloys. These ques-tions are studied intensively by a number of re-search groups [6-12]. However the available dataare still fragmentary and need to be extended andgeneralized. In the present work we focus our at-tention on the study of martensite and magneticDS of mono-, poly-, and nanocrystalline Ni

2+x

Mn1-x

Ga and Co2+x

Ni1-x

Ga alloys.

102 T. Breczko, S. Ilyashenko, D. Bykov, O. Korpusov, M. Bramowicz et al.

2. EXPERIMENTAL

Polycrystalline Ni2+xMn1-xGa and Co2+xNi1-xGa alloyswith x from 0.12 to 0.19 were prepared by argonarc melting and homogenised by 100 hours of an-nealing at 800 °C followed by water quenching.Single crystalline samples were grown by theBridgeman method in alumina crucibles from arcmelted buttons. The initial charge was kept invacuum of 10-4 mm Hg at 1200 °C for degassing.Next the chamber was filled with pure argon to mini-mize the evaporation of manganese and the tem-perature was raised up to 1350 °C. The crystalswere grown at the rate of 20 mm per hour.

Nanocrystalline ribbons with a thickness of 30– 40 µm were obtained by rapid quenching of themelt with a rate of the order of 105 K/sec onto arotating copper wheel. Amorphous andnanocrystalline films with a thickness of 0.5 – 5mm were prepared by magnetron sputtering.

The temperatures of martensite and magnetictransitions were obtained by the method of ther-momagnetic analysis by the temperature depen-dence of the initial magnetic susceptibility. The mar-tensite and magnetic DS were observed in polar-ized light with the aid of a digital differential opticalmicroscope. Complementary methods of magneticpowder patterns, Kerr microscopy and

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Fig. 1. Temperature dependence of the initial mag-netic susceptibility χ for polycrystallineNi2.12Mn0.88Ga samples. Curve 1 – before, 2 – afterhomogenization at 800 °C for 100 hours.

Fig. 2. Temperature dependence of the initial mag-netic susceptibility χ for rapidly quenchedNi

53Mn

23.5Ga

23.5 ribbons annealed at 800°C for 5

hours. Curve 1 – field parallel, 2 – perpendicular toa ribbon length.

magnetooptic indicator films [10,11] were used toreveal the magnetic domain structure.

3. RESULTS AND DISCUSSION

3.1. Thermomagnetic analysis (TMA)

In Fig. 1 the curves of initial magnetic susceptibil-ity temperature dependence χ(T) for polycrystal-line textured alloy Ni

2.12Mn

0.88Ga are shown before

and after homogenization at 800 °C. It is seen thatthe annealing results in some changes of thecharacteristic points of TMA curves, decrease ofthe hysteresis of these curves and sharpening ofthe magnetic order – disorder (nonhysteretic rightparts of the TMA curves) and structural austenite–martensite transitions (left hysteretic parts).

The TMA curves presented in Fig. 1 are typicalfor single- and polycrystalline ferromagneticHeusler alloys and play an important diagnosticrole, because they give a clear indication of notonly the existence of the structural phase transi-tions and its characteristic temperatures (start andfinish points of the direct and reverse martensitetransitions), but also provide information on therelative volume of the martensite phase and thematerial homogeneity.


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Fig. 3. Martensite relief of an oriented Co48

Ni22

Ga30

single crystal with a size of 3×3×6 mm (a) and respec-tive interference bands observed with the aid of plane parallel indicator film applied to the samplesurface (b).

Fig. 4. Martensite structure of polycrystallineNi2.16Mn0.84Ga revealed by polarized light micros-copy on the mechanically polished surface of thesample, ×120.

The TMA curves of rapidly quenched ribbonsare similar to those of bulk single- and polycrys-tals. The χ(T) curves vary with the change of the

heat treatment regimes and the measurement con-ditions (orientation of the measuring field with re-spect to the sample long dimension, Fig. 2). The

Fig. 5. Martensite relief on the (110) plane of Co48

Ni22

Ga30

single crystal and its relationship with themagnetic DS.


Fig. 6. 180-degree structure of magneticmacrodomains on the {100} planes ofNi

49Mn

29.7Ga

21.3 single crystal with linear dimensions

of 1.2×1.5×2.5 mm.

latter effect is due to the shape anisotropy of theribbon samples (different demagnetizing factorsalong the ribbon length and perpendicular to it). In

spite of the difference in the absolute values of themeasurement signal for these two cases the cor-responding characteristic start and finish marten-

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Fig. 7. Magnetic DS of textured Co48

Ni22

Ga30

polycrystal revealed with the aid of ferrite-garnet indicatorfilm. The texture axis is oriented parallel to the long side of the sample having dimensions of 3.3×3×9.4mm.

Fig. 8. Visualization of the martensite and magnetic DS of Co2Ni

0.85Ga polycrystal. Left – martensite

structure observed in polarized light, right – magnetic DS of the same area of the sample revealed withthe aid of magnetooptic indicator film.


[001]

[010] [100] ��

(111)

��

(221)

�

Fig. 9. Magnetic domain substructure forming by sectioning of the crystal with (hh1) planes. Cubic volumeelement (at the left) is divided into two parts by the martensite boundary lying in the (110) plane. (hh1)planes are obtained by the rotation of (001) plane with respect to the [ 110] direction. Arrows and (+) and(-) signs characterize the magnetization directions of 180° magnetic domains.

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�Fig. 10. Martensite and magnetic DS of the same area of rapidly quenched Ni

53Mn

23.5Ga

23.5 ribbon.

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Fig. 11. 180-degree structure of magnetic domainsof nanocrystalline Ni

52Mn

24Ga

24 film 5 µm thick pre-

pared by magnetron sputtering.

site and austenite transition temperatures are closeto each other.

3.2. Martensitic and magnetic domain structure

Figs. 3a and 3b show the surface relief of the mar-tensite structure on different crystallographic planesof an oriented single crystal Co

48Ni

22Ga

30. The

martensite plates pass continously through thewhole crystal thus giving an evidence of the highperfection of the sample. By contrast, in polycrys-talline samples more complex distribution of crys-tallite orientation and internal stresses results inan appearance of various intersections of differenttwin systems.

The martensite relief shown in Figs. 3a and 3bis formed after direct martensite transformation ofthe sample having a flat surface prepared by pol-ishing at high temperature corresponding to the


austenite state. Otherwise, if the sample is flat pol-ished in the martensite state, no relief is observedif the sample is not subjected to thermal cyclingthrough the phase transition points. In this casethe martensite structure is revealed with the aid ofpolarized microscopy making use of the opticalanisotropy of the martensite phase (Fig. 4).

To reveal the magnetic DS and its relation tothe martensite twins the method of ferrite-garnetmagnetooptic films was used. Fig. 5 demonstratesthe martensite relief and magnetic DS on the (110)plane of a Co

48Ni

22Ga

30 single crystal, while Fig. 6

shows the magnetic DS of the Ni49

Mn29.7

Ga21.3

singlecrystal.

In polycrystalline samples with arbitrary crys-tallographic orientation of the crystallite surfacesthe magnetic DS configuration becomes more com-plicated due to the effect of magnetostatic fields atthe grain boundaries (Figs. 7 and 8)

4. CONCLUSIONS

The presented experimental results show that themain feature of the martensitic and magnetic DSof the ferromagnetic Heusler alloys under study istheir interdependence. 180-degree magnetic do-mains are continous within the limits of the crystal-lites, which, in their turn, are divided into plane-parallel martensitic twin domains. The magnetiza-tion vector of magnetic domains becomes modu-lated by the martensite domains, because the c-axes of the latter, being the easy axes of magneti-zation, are oriented at the angles of 90° with re-spect to each other. As a result, the boundariesbetween martensite domains also play the role of90-degree magnetic domain walls free of charges(of the Bloch type with div M = 0) (Fig. 9).

The configuration of the martensite and mag-netic domain structures on different crystallographicplanes depend on their orientation. The (001) planeis free of charges, and the magnetic domain wallsare oriented symmetrically at 45° with respect tothe martensite boundaries. In the (100) and (010)planes there are domains with magnetization vec-tor alternating in the directions parallel and normalto the sample surface. For other arbitrary orienta-tions the angular relations between the lines ofdomain wall intersections with the surface becomemore complicated.

The martensite structure of annealed rapidlyquenched ribbons is qualitatively the same as for

bulk samples (Fig. 10). The average magnetic do-main width is considerably decreased to the val-ues of the order of 1 mm. Observation of the mag-netic DS of nanocrystalline films prepared by mag-netron sputtering give an indication of uniaxial tex-ture in a direction normal to the film plane (Fig. 11).

The general scheme of the coexisting marten-site and magnetic DS of the investigated ferromag-netic Heusler alloys is well described by the modeldeveloped earlier for the case of polytwinned CoPt-type alloys [13].

REFERENCES

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THERMOMAGNETIC ANALYSIS AND DOMAIN STRUCTURE IN THE PHASE TRANSITION REGION OF Ni-Mn-Ga AND Co-Ni-Ga SHAPE MEMORY ALLOYS