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Simultaneous magnetooptic observation and thermomagnetic analysis of phase transitions in shape-memory Ni–Mn–Ga alloys
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) 2035–2037
Simultaneous magnetooptic observation and thermomagnetic analysis of phase transitions in shape-memory Ni–Mn–Ga alloys Oleg M. Korpusova,*, Rostislav M. Grechishkina, Victor V. Koledovb, Victor V. Khovailoc, T. Takagic, Vladimir G. Shavrovb a
b
Tver State University, Zheliabova Str., 33, Tver 170000, Russia Institute of Radioengineering & Electronics of RAS, Mokhovaya 11, Moscow 101999, Russia c Institute of Fluid Science, Tohoku University, Sendai 980-8577, Japan
Abstract Direct observations of the austenite and martensite twin and magnetic domain structures are combined with thermomagnetic analysis (low-field AC susceptibility temperature measurements) of phase transitions in ferromagnetic shape memory alloys Ni2+xMn1 xGa. Thermal hysteresis and magnetization jumps are clearly resolved by both techniques. Large Barkhausen-like jumps associated with discontinuous phase boundary and martensite twin motion are observed during the austenite–martensite transformation in contrast to the smooth ferromagnetic–paramagnetic transition near the Curie temperature. r 2003 Elsevier B.V. All rights reserved. PACS: 75.30.Kz; 75.60.Ch; 75.60.Ej Keywords: Shape memory alloy; Magnetic domain structure; Barkhausen effect
Ferromagnetic Heusler alloys Ni2+xMn1 xGa are of interest in connection with the fact that they exhibit crystallographically reversible thermoelastic martensitic transformation which may be controlled by an external magnetic field in addition to stress and temperature [1]. The optimization of this effect may be achieved by bringing together the temperatures of structural and magnetic transitions through the choice of the alloy composition [2]. The exact mechanism by which the magnetic field induces the shape memory effect is not known but it is established that reorientation of certain martensite phase twin variants is involved. Twin martensite structure and magnetic domains form a complex hierarchy of interdependent macro- and micro*Corresponding author. Tel.: +7-0822-34-4215; fax: +70822-32-1274. E-mail address: [email protected] (O.M. Korpusov).
domains [3,4]. Interpretation of this hierarchy is a difficult problem from both theoretical and experimental point of view. To facilitate the solution of this problem in the present work we combine the direct observation of the austenite and martensite structural and magnetic domain features of Ni2+xMn1 xGa alloy system with low field AC susceptibility temperature measurements. In the experiment the sample under study placed on a thermoelectric (Peltier) heatingcooling stage was surrounded by the measuring coil of a differential AC susceptometer working at a frequency of 2 kHz in such a manner that the polished sample surface could be observed with the digital optical microscope. The temperature during cooling and heating cycles was varied at about 0.5 K/min. Differential polarized light processing algorithms were used to enhance the imaging of both martensitic (optically anisotropic) and magnetic domain structures.
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.812
ARTICLE IN PRESS 2036
O.M. Korpusov et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 2035–2037 100
µ, arb. u.
µ, arb. u.
300 200 100
80 60 40
0 290
(a)
Fig. 1. Martensite twin structure of a thin Ni2.16Mn0.84Ga microcrystal (linear dimension about 400 mm) as observed in polarized light (a) and with the aid of ferrite-garnet indicator film (b).
Bitter pattern and magnetooptic indicator film techniques [4,5] were also used due to their complementary character helpful in the separation of superposed optical effects. Shown in Fig. 1 is a comparison of the images of the martensite twin structure observed directly in polarized light and with the magnetooptic indicator film. Direct observation provides good polarization contrast of optically anisotropic martensite (Fig. 1a). An important characteristic of this type of contrast (as distinct from the magnetooptical contrast) is its dependence on the angular orientation of the sample on the rotatable metallographic microscope stage with normal light incidence [6]. However, this contrast is superposed with much weaker magnetooptic Kerr contrast from magnetic domains thus masking the latter. This difficulty is surmounted either with the aid of indicator film (Fig. 1b) or differential mode of observation in polarized light, enhancing the magnetooptic contrast. Temperature variations give rise to the processes of martensite–austenite transformation accompanied by the magnetic domain structure realignment. These processes are observed directly with the aid of the microscope and the images are stored either as a movie or a series of frames along with the AC susceptibility data. Typical m(T) curves are shown in Fig. 2 for the case of Ni2.16Mn0.84Ga sample. The shape of the observed m(T) curves is typical for ferromagnetic shape-memory alloys of the Ni–Mn–Ga family with a sequence of martensitic and intermartensitic transformations [7]. In the present work we used very small samples with linear dimensions comparable to the size of magnetic macrodomains. With this provision the non-averaged individual Barkhausen-like magnetization jumps are resolved both visually and by AC susceptibility measurements.
310
T,K
330
350
298
(b)
302
306
310
T,K
Fig. 2. Temperature dependence of the initial AC susceptibility of Ni2.16Mn0.84Ga microcrystal. (a) Full cycle of cooling and heating starting from the paramagnetic state at T > Tc=340 K to the ferromagnetic austenite state (horizontal plateau) followed by transition to the ferromagnetic martensite state (abrupt falling-off of the m value at T=312 K). (b) Details of the low-temperature part of the curves showing Barkhausenlike jumps for three successive measurement runs.
The jumps are observed in the region of martensite transformations in contrast to the smooth course of the ferromagnetic–paramagnetic transition near the Curie temperature. They are shown in more detail in the right graph of Fig. 2 for the low-temperature region of the curves for three successive measurement runs. Good reproducibility of the corresponding curves is seen with only slight deviations in minute details typical of Barkhausen effect studies. Comparison with optical observations shows that large Barkhausen jumps are associated mainly with temperature-induced jump-like motion of the martensite–austenite phase boundary and martensite twin realignment. The explanation of the Barkhausen effect and hysteresis properties of a wide class of magnetic materials is based on the analysis of the processes of magnetic domain nucleation and magnetic domain wall pinning by crystal defects [8]. The results of our study indicate that in ferromagnetic shape-memory alloys the processes of martensite phase nucleation and martensite twin boundary pinning should be taken into account as well. The work was supported by RFFI-BRFFI Grant No 02-02-81030 Bel a. The authors are grateful to S. Ilyashenko and N. Steiner for their assistance in DS studies. References [1] R.C. O’Handley, J. Appl. Phys. 83 (1998) 3263. [2] A.N. Vasil’ev, A.D. Bozhko, V.V. Khovailo, Phys. Rev. B 59 (1999) 113. [3] N.I. Vlasova, G.S. Kandaurova, N.N. Schegoleva, J. Magn. Magn. Mater. 222 (2000) 138. [4] A. Sozinov, Y. Ezer, G. Kimmel, D. Giller, Y. Wolfus, Y. Yeshurin, K. Ullakko, V.K. Lindroos, J. Phys. IV (France) 11 (2001) Pr8–311.
ARTICLE IN PRESS O.M. Korpusov et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 2035–2037 [5] R.M. Grechishkin, M.Yu. Goosev, S.E. Ilyashenko, N.S. Neustroev, J. Magn. Magn. Mater. 157/158 (1996) 305. [6] R.M. Grechishkin, O.V. Malyshkina, S.S. Soshin, Ferroelectrics 222 (1999) 215.
2037
[7] A. Sozinov, A.A. Likhachev, N. Lanska, K. Ullakko, Appl. Phys. Lett. 80 (2002) 1746. [8] R.C. O’Handley, Modern Magnetic Materials: Principles and Applications, Wiley-Interscience, New York, 1999.
Journal of Magnetism and Magnetic Materials 272–276 (2004) 2035–2037
Simultaneous magnetooptic observation and thermomagnetic analysis of phase transitions in shape-memory Ni–Mn–Ga alloys Oleg M. Korpusova,*, Rostislav M. Grechishkina, Victor V. Koledovb, Victor V. Khovailoc, T. Takagic, Vladimir G. Shavrovb a
b
Tver State University, Zheliabova Str., 33, Tver 170000, Russia Institute of Radioengineering & Electronics of RAS, Mokhovaya 11, Moscow 101999, Russia c Institute of Fluid Science, Tohoku University, Sendai 980-8577, Japan
Abstract Direct observations of the austenite and martensite twin and magnetic domain structures are combined with thermomagnetic analysis (low-field AC susceptibility temperature measurements) of phase transitions in ferromagnetic shape memory alloys Ni2+xMn1 xGa. Thermal hysteresis and magnetization jumps are clearly resolved by both techniques. Large Barkhausen-like jumps associated with discontinuous phase boundary and martensite twin motion are observed during the austenite–martensite transformation in contrast to the smooth ferromagnetic–paramagnetic transition near the Curie temperature. r 2003 Elsevier B.V. All rights reserved. PACS: 75.30.Kz; 75.60.Ch; 75.60.Ej Keywords: Shape memory alloy; Magnetic domain structure; Barkhausen effect
Ferromagnetic Heusler alloys Ni2+xMn1 xGa are of interest in connection with the fact that they exhibit crystallographically reversible thermoelastic martensitic transformation which may be controlled by an external magnetic field in addition to stress and temperature [1]. The optimization of this effect may be achieved by bringing together the temperatures of structural and magnetic transitions through the choice of the alloy composition [2]. The exact mechanism by which the magnetic field induces the shape memory effect is not known but it is established that reorientation of certain martensite phase twin variants is involved. Twin martensite structure and magnetic domains form a complex hierarchy of interdependent macro- and micro*Corresponding author. Tel.: +7-0822-34-4215; fax: +70822-32-1274. E-mail address: [email protected] (O.M. Korpusov).
domains [3,4]. Interpretation of this hierarchy is a difficult problem from both theoretical and experimental point of view. To facilitate the solution of this problem in the present work we combine the direct observation of the austenite and martensite structural and magnetic domain features of Ni2+xMn1 xGa alloy system with low field AC susceptibility temperature measurements. In the experiment the sample under study placed on a thermoelectric (Peltier) heatingcooling stage was surrounded by the measuring coil of a differential AC susceptometer working at a frequency of 2 kHz in such a manner that the polished sample surface could be observed with the digital optical microscope. The temperature during cooling and heating cycles was varied at about 0.5 K/min. Differential polarized light processing algorithms were used to enhance the imaging of both martensitic (optically anisotropic) and magnetic domain structures.
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.812
ARTICLE IN PRESS 2036
O.M. Korpusov et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 2035–2037 100
µ, arb. u.
µ, arb. u.
300 200 100
80 60 40
0 290
(a)
Fig. 1. Martensite twin structure of a thin Ni2.16Mn0.84Ga microcrystal (linear dimension about 400 mm) as observed in polarized light (a) and with the aid of ferrite-garnet indicator film (b).
Bitter pattern and magnetooptic indicator film techniques [4,5] were also used due to their complementary character helpful in the separation of superposed optical effects. Shown in Fig. 1 is a comparison of the images of the martensite twin structure observed directly in polarized light and with the magnetooptic indicator film. Direct observation provides good polarization contrast of optically anisotropic martensite (Fig. 1a). An important characteristic of this type of contrast (as distinct from the magnetooptical contrast) is its dependence on the angular orientation of the sample on the rotatable metallographic microscope stage with normal light incidence [6]. However, this contrast is superposed with much weaker magnetooptic Kerr contrast from magnetic domains thus masking the latter. This difficulty is surmounted either with the aid of indicator film (Fig. 1b) or differential mode of observation in polarized light, enhancing the magnetooptic contrast. Temperature variations give rise to the processes of martensite–austenite transformation accompanied by the magnetic domain structure realignment. These processes are observed directly with the aid of the microscope and the images are stored either as a movie or a series of frames along with the AC susceptibility data. Typical m(T) curves are shown in Fig. 2 for the case of Ni2.16Mn0.84Ga sample. The shape of the observed m(T) curves is typical for ferromagnetic shape-memory alloys of the Ni–Mn–Ga family with a sequence of martensitic and intermartensitic transformations [7]. In the present work we used very small samples with linear dimensions comparable to the size of magnetic macrodomains. With this provision the non-averaged individual Barkhausen-like magnetization jumps are resolved both visually and by AC susceptibility measurements.
310
T,K
330
350
298
(b)
302
306
310
T,K
Fig. 2. Temperature dependence of the initial AC susceptibility of Ni2.16Mn0.84Ga microcrystal. (a) Full cycle of cooling and heating starting from the paramagnetic state at T > Tc=340 K to the ferromagnetic austenite state (horizontal plateau) followed by transition to the ferromagnetic martensite state (abrupt falling-off of the m value at T=312 K). (b) Details of the low-temperature part of the curves showing Barkhausenlike jumps for three successive measurement runs.
The jumps are observed in the region of martensite transformations in contrast to the smooth course of the ferromagnetic–paramagnetic transition near the Curie temperature. They are shown in more detail in the right graph of Fig. 2 for the low-temperature region of the curves for three successive measurement runs. Good reproducibility of the corresponding curves is seen with only slight deviations in minute details typical of Barkhausen effect studies. Comparison with optical observations shows that large Barkhausen jumps are associated mainly with temperature-induced jump-like motion of the martensite–austenite phase boundary and martensite twin realignment. The explanation of the Barkhausen effect and hysteresis properties of a wide class of magnetic materials is based on the analysis of the processes of magnetic domain nucleation and magnetic domain wall pinning by crystal defects [8]. The results of our study indicate that in ferromagnetic shape-memory alloys the processes of martensite phase nucleation and martensite twin boundary pinning should be taken into account as well. The work was supported by RFFI-BRFFI Grant No 02-02-81030 Bel a. The authors are grateful to S. Ilyashenko and N. Steiner for their assistance in DS studies. References [1] R.C. O’Handley, J. Appl. Phys. 83 (1998) 3263. [2] A.N. Vasil’ev, A.D. Bozhko, V.V. Khovailo, Phys. Rev. B 59 (1999) 113. [3] N.I. Vlasova, G.S. Kandaurova, N.N. Schegoleva, J. Magn. Magn. Mater. 222 (2000) 138. [4] A. Sozinov, Y. Ezer, G. Kimmel, D. Giller, Y. Wolfus, Y. Yeshurin, K. Ullakko, V.K. Lindroos, J. Phys. IV (France) 11 (2001) Pr8–311.
ARTICLE IN PRESS O.M. Korpusov et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 2035–2037 [5] R.M. Grechishkin, M.Yu. Goosev, S.E. Ilyashenko, N.S. Neustroev, J. Magn. Magn. Mater. 157/158 (1996) 305. [6] R.M. Grechishkin, O.V. Malyshkina, S.S. Soshin, Ferroelectrics 222 (1999) 215.
2037
[7] A. Sozinov, A.A. Likhachev, N. Lanska, K. Ullakko, Appl. Phys. Lett. 80 (2002) 1746. [8] R.C. O’Handley, Modern Magnetic Materials: Principles and Applications, Wiley-Interscience, New York, 1999.