- Email: [email protected]
Magnetic transitions and structure of a NiMnGa ferromagnetic shape memory alloy prepared by melt spinning technique
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 320 (2008) L116–L120 www.elsevier.com/locate/jmmm
Letter to the Editor
Magnetic transitions and structure of a NiMnGa ferromagnetic shape memory alloy prepared by melt spinning technique A.K. Panda, M. Ghosh, Arvind Kumar, A. Mitra National Metallurgical Laboratory, Materials Science and Technology Division, PO: Burmamines, Jamshedpur, Jharkhand 831007, India Received 13 July 2007; received in revised form 7 March 2008 Available online 6 April 2008
Abstract A ferromagnetic shape memory alloy with nomial composition Ni52.5Mn24.5Ga23 (at%) was developed by a melt spinning technique. The as-spun ribbon showed dominant L21 austenitic (cubic) structure with a splitting of the primary peak in the X-ray diffractogram indicating the existence of a martensitic feature. The quenched-in martensitic plates were revealed in transmission electron microscopy. An increase of magnetization at low temperature indicated a martensite to austenite transformation and its reverse with a drop in magnetization during the cooling cycle. Higher magnetic fields propel martensite–austenite transformation spontaneously. r 2008 Elsevier B.V. All rights reserved. PACS: 75.20.En; 75.50.Cc Keywords: Shape memory alloy; Melt-spun ribbons; Martensitic transformation; Magnetization
1. Introduction Recently, a lot of attention has been paid to improve the response time of actuators by using magnetic-field-induced shape memory effects. The first in the series of materials that potentially revealed this phenomenon belonged to the NiMnGa alloy system. These actuator materials called as ferromagnetic shape memory alloys (FSMAs) underwent reorientation of twin structures under the application of a magnetic field [1]. The ferromagnetic shape memory effect refers to either the reversible field-induced austenite to martensite transformation or the rearrangement of martensite variants by an applied field leading to an overall change of shape. Such transformations are preferred to take place above the room temperature for application in actuators [2]. In the past, many of the reported stoichiometric compositions of NiMnGa alloys prepared through single-crystal growth techniques exhibited low transformaCorresponding author. Tel.: +91 657 2271709x2216; fax: +91 657 2270527. E-mail addresses: [email protected], [email protected] (A.K. Panda).
0304-8853/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.03.055
tion temperatures [3]. The limitations underlying such processing routes as well as the low transformation temperatures restricted the efficient use of FSMAs for practical applications. Although, single-crystal growth technique effectively forms preferred orientations but the material is not only brittle but also prone to the formation of defects during crystal growth. There has been paradigm shift in the development of ferromagnetic shape memory alloys using different preparation techniques. The rapid solidification route through melt spinning technique has been explored to prepare NiMnGa shape memory alloys in the form of ribbons. This technique, which is usually applicable to metallic glass preparations, has been found to be effective in freezing the Heusler Ni2MnGa stoichiometry to L21 austenitic (cubic) structure [4]. The magnetic shape memory effect (MSME) has been found to be actuating between the austenitic L21 Heusler structure and the quenched-in martensite. The present work attempts to investigate the reversible transformation in NiMnGa alloy prepared by melt spinning technique. The composition was selected from the perspective of applications bearing transformations close to room temperature and above.
ARTICLE IN PRESS A.K. Panda et al. / Journal of Magnetism and Magnetic Materials 320 (2008) L116–L120
from the proportionally decaying intensities as proceeding from (2 2 0) to (4 4 0) reflections. Such ordered L21 reflections have been reported for NiFeGa ribbons [7]. The lattice constant ‘a’ derived from the primary reflection d220 has been found to be 5.8039 A. It is interesting to note that the (2 2 0) reflection showed a splitting about its original position. Such a splitting is attributed to the existence of some tetragonal feature (martensitic), which is small enough in volume fraction to suppress the cubic L21 structure [8]. This can be explained in terms of higher Ga concentration in the matrix (Table 1) as compared to the grain boundary. The dense clustering of Ga atoms in the matrix is also responsible for such splitting of the X-ray primary reflection [9]. To investigate the phase transition and its recovery during shape memory effect, differential thermal analysis (DTA) was carried out at a slow scan rate of 2 K/min for two consecutive heating cycles, and is shown in Fig. 2. The first and the second heating cycles showed endothermic transformations. These endothermic peaks indicated reverse martensitic transformation (martensite to austenite). As evaluated here, the austenitic start temperature AS1 and AS2 are 305 and 307 K, respectively, during the first and second heating cycles. It is noteworthy, that DTA scan of the sample up to 330 K during the first heating cycle did not distort the structural recovery. Hence, the shape of the endothermic plot in the second heating cycle was identical. This was further supported from their nearly equal enthalpy of transformation obtained as 1.11
2. Experimental The master alloy with a nominal composition Ni52.5Mn24.5Ga23 (at%) was prepared by vacuum arc melting. The master alloy was induction melted and then rapidly quenched into ribbons by melt spinning technique. The phases of the as-quenched ribbon were identified using an X-ray diffractometer (Philips D500) with a Cu-Ka radiation. The microstructure of the as-quenched ribbon and the elemental distribution within the phases was studied using scanning electron microscope (SEM, Jeol 400) and energy dispersive spectroscope (EDS) analysis. The structure of the as-spun ribbons was also observed using transmission electron microscopy (Philips CM-200). The magnetization studies were carried out using a vibrating sample magnetometer (VSM, Lakeshore: 7404). 3. Result and discussion 3.1. Phase transformation and structure Microstructural characterization and identification of the phases in the as-spun ribbons were carried out using scanning electron microscopy (SEM) and X-ray diffractometry (XRD), respectively. The SEM micrograph and the diffractogram are shown in Fig. 1a and b, respectively. The material exhibits polyhedral grains having distinct grain boundaries. Some pitting is attributed to over etching. Quantitative analysis of grain matrix and grain boundaries was carried out. The elemental distribution of nickel, manganese and gallium is indicated in Table 1. It was observed that there was a relative increase in nickel and manganese concentration at the grain boundary. However, the gallium concentration decreased drastically at the grain boundary, which is at par with the earlier reports on Ni56Mn18.8Ga24.5Gd0.7 alloy [5]. X-ray diffractogram of the as-spun ribbon shown in Fig. 1b revealed an ordered L21 (cubic) austenite structure [6]. The highly ordered L21 cubic structure was evident
Table 1 Elemental distribution within the microstructure of the as-spun ribbons Elements
Composition (at%) Grain boundary
67.45 10.35 22.2
79.5 11.26 9.24
(220)
Matrix
Intensity (A.U.)
42
44 46 48 Degrees (2θ)
(422)
(400)
40
80
60
(440)
(220)
Nickel Manganese Gallium
Intensity (A.U.) 40
L117
100
Degrees (2θ) Fig. 1. (a) Scanning electron micrograph of the as-spun ribbon. (b) X-ray diffractogram of the as-spun ribbon. Inset shows splitting of primary peak.
ARTICLE IN PRESS A.K. Panda et al. / Journal of Magnetism and Magnetic Materials 320 (2008) L116–L120
L118
and 1.01 J/g for the first and second heating cycles, respectively. The onset of the transformation in the second heating cycle shifted only by 2 K. This is in contrast to the distortions in the endothermic peak occurring during hightemperature heating cycles [10]. The low enthalpy of transformation in the present case as compared with 2–10 J/g for the reported NiMnGa alloys [10,11] was a subject of curiosity. It was observed from XRD (Fig. 1b) that the as-spun ribbon had an austenitic L21 (cubic) structure as a dominant phase. The transmission electron micrograph revealed granular structure with a pronounced austenitic matrix as shown in Fig. 3a. Magnified image of grain boundary region (Fig. 3b) showed well-defined stripelike morphology of a martensitic phase [2]. The phase boundary between the martensite plates was more or less straight and distinct. TEM examination illustrated a low-volume fraction of textured martensite phase within the austenite L21 structure. This finding endorsed the inference of XRD study (Fig. 1b).
3.2. Magnetic transformation The thermal variation of magnetization of the as-spun ribbon at a low magnetizing field of 2.0 kA/m was obtained using vibrating sample magnetometer and is shown in Fig. 4. It was observed that during the heating cycle, the magnetization initially increased to reach a maximum value around 300 K. This initial rise in magnetisation is attributed to the change of martensitic state along with some nonferromagnetic components (possibly of antiferromagnetic origin) to an austenitic state with long-range ferromagnetic ordering [12]. However, investigations are also being carried out to explore whether such rise in magnetization is due to the lowering of anisotropy constant during martensite to austenite phase transformation when both the phases are ferromagnetic in nature [13,14]. After the initial rise, the magnetization started decreasing beyond 300 K indicating ferromagnetic to paramagnetic transition. From the derivative of the drop in magnetization, the Curie 2.5
As-Spun Ribbon Magnetising field =2.0 kA/m
Magnetisation, Tesla (x 10-3)
Endo thermic Heat flow (W/g)
0.36
2nd Heating Cycle 0.34
AS2 = 307 K Reverse Transformation (Martensite to Austenite)
0.26 1st Heating Cycle
2.0
1.5 Heating Cycle
1.0
AS1 = 305 K
Cooling Cycle TC = 302 K
0.24 310
320 Temperature (°C)
330
Fig. 2. Differential thermal analysis plots of the as-spun ribbon. Heating rate 2 K/min.
300
320 Temperature (K)
340
Fig. 4. Thermal variation of magnetisation of the as-spun ribbon at a magnetizing field of 2 kA/m.
Fig. 3. TEM micrograph showing (a) grains interface and (b) grain boundary region (blow-up of 3a).
ARTICLE IN PRESS A.K. Panda et al. / Journal of Magnetism and Magnetic Materials 320 (2008) L116–L120
Magnetising field = 400 kA/m 150 100
Magnetisation, Tesla (x 10-3)
50 0 = 40 kA/m
60 40 20 0
L119
in the ferromagnetic domains as the structure changes from a highly anisotropic martensite to a low anisotropic L21 austenitic phase. Additionally, the long-range ferromagnetic ordering in austenite is also the driving force behind such initial rise in magnetisation. At higher magnetic fields, the intensity of the field is high enough to make the domain transitions between these two ferromagnetic states spontaneous. Thus, at higher magnetizing fields, though such transition occurred but was spontaneous enough to reveal any rise in the initial magnetisation. Subsequently, the magnetisation of the ferromagnetic L21 austenitic phase dropped down to its own premartensitic paramagnetic [16] state around its Curie temperature. 4. Conclusion
= 2.0 kA/m 2
1
300
320 Temperature (K)
340
Fig. 5. Effect of magnetizing field on the thermal variation of magnetisation of the as-spun ribbon.
A ferromagnetic shape memory alloy with a nominal composition Ni52.5Mn24.5Ga23 (atomic %) was prepared in the form of ribbons by melt spinning technique. The asspun ribbon revealed granular morphology with L21 austenitic (cubic) structure in combination with martensitic phase. Martensite–austenite transformation and its reversibility was observed from thermal and magnetisation studies. Magnetic field intensity is critically important to make the martensite to austenite transformation spontaneous. Acknowledgment
temperature obtained during the heating cycle was 302 K. During the cooling cycle, the magnetic transition was found to be reversible with the Curie temperature remaining unchanged. Below 300 K, cooling cycle represented a more pronounced effect of forward martensitic transformation (austenite-martensite). This distinct change in the cooling cycle may be attributed to some sort of atomic rearrangements in the lattice sites during the heating cycle [10]. The increasing intensity of magnetic field on the martensite to austenite transformation showed interesting behaviour as indicated in Fig. 5. It was observed, that as the magnetizing field was increased from 2.0 to 400 kA/m, the distinct initial rise in magnetization above 295 K became suppressed. Thus, at higher magnetic field the spontaneous martensite to austenite transformation indicates the predominant role of magnetic field in the actuation process [14]. It needs further clarification on field-induced magnetic transitions that is responsible for the shape memory effect (SME). This phenomenon occurs when magnetic transition takes place between the martensitic and the L21 austenitic state, whereby both are essentially ferromagnetic and the Curie temperature is sufficiently high for the martensitic transformation [15]. In this condition, at low magnetic fields, the distinct initial rise in magnetization owes to the drastic lowering of anisotropy
The authors express their sincere gratitude to the Director, National Metallurgical Laboratory, Jamshedpur, India for kindly giving necessary permission to publish this work. References [1] K. Ullako, J.K. Huang, V.V. Kokorin, R.C. O’Handley, Appl. Phys. Lett. 69 (1996) 1133. [2] H.B. Xu, Y. Li, C.B. Jiang, Mater. Sci. Eng. A 438-440 (2006) 1065. [3] S.Y. Chu, R. Gallagher, M. De Graef, M.E. McHenry, IEEE Trans. Magn. 37 (2001) 2666. [4] Z.H. Liu, M. Zhang, Y.T. Cui, Y.Q. Zhou, W.H. Wang, G.H. Wu, X.X. Zhang, G. Xiao, Appl. Phys. Lett. 82 (2003) 424. [5] Z. Zeyu, L. Yi, D. Jingfang, H. Peng, W. Guangheng, C. Yongqin, J. Rare Earths 24 (2006) 579. [6] C. Jiang, Y. Muhammad, l. Deng, W. Wu, H. Xu, Acta Mater. 52 (2004) 2779. [7] Z.H. Liu, H. Liu, X.X. Zhang, M. Zhang, X.F. Dai, H.N. Hu, J.L. Chen, G.H. Wu, Phys. Lett. A 329 (2004) 214. [8] P.J. Webster, K.R.A. Ziebeck, S.L. Town, M.S. Peak, Philos. Mag. B 49 (1984) 295. [9] G.D. Liu, Z.H. Liu, X.F. Dai, S.Y. Yu, J.L. Chen, G.H. Wu, Sci. Technol. Adv. Mater. 6 (2005) 772. [10] J. Pons, C. Segui, V.A. Chernenko, E. Cesari, P. Ochin, R. Portier, Mater. Sci. Eng. A 273–275 (1999) 315. [11] J.I. Perez-Landazabal, C. Gomez-Polo, V. Recarte, C. Segui, E. Cesari, P. Ochin, J. Magn. Magn. Mater. 290-291 (2005) 826. [12] T. Krenke, E. Duman, M. Acet, E.F. Wassermann, X. Moya, L. Manosa, A. Planes, Nat. Mater. 4 (2005) 450.
ARTICLE IN PRESS L120
A.K. Panda et al. / Journal of Magnetism and Magnetic Materials 320 (2008) L116–L120
[13] V. Recarte, J.I. Perez-Landazabal, C. Gomez-Polo, C. Segui, E. Cesari, P. Ochin, Mater. Sci. Eng. A 438–440 (2006) 937. [14] J. Gutierrez, J.M. Barandiaran, p. Lazpita, C. Segui, E. Cesari, Sensors Actuators A 129 (2006) 163.
[15] L. Manosa, A. Planes, M. Acet, E. Duman, E.F. Wassermann, J. Magn. Magn. Mater. 272 (2004) 2090. [16] L. Manosa, A. G-Comas, E. Obrado, A. Planes, Mater. Sci. Eng. A 273 (1999) 329.
Journal of Magnetism and Magnetic Materials 320 (2008) L116–L120 www.elsevier.com/locate/jmmm
Letter to the Editor
Magnetic transitions and structure of a NiMnGa ferromagnetic shape memory alloy prepared by melt spinning technique A.K. Panda, M. Ghosh, Arvind Kumar, A. Mitra National Metallurgical Laboratory, Materials Science and Technology Division, PO: Burmamines, Jamshedpur, Jharkhand 831007, India Received 13 July 2007; received in revised form 7 March 2008 Available online 6 April 2008
Abstract A ferromagnetic shape memory alloy with nomial composition Ni52.5Mn24.5Ga23 (at%) was developed by a melt spinning technique. The as-spun ribbon showed dominant L21 austenitic (cubic) structure with a splitting of the primary peak in the X-ray diffractogram indicating the existence of a martensitic feature. The quenched-in martensitic plates were revealed in transmission electron microscopy. An increase of magnetization at low temperature indicated a martensite to austenite transformation and its reverse with a drop in magnetization during the cooling cycle. Higher magnetic fields propel martensite–austenite transformation spontaneously. r 2008 Elsevier B.V. All rights reserved. PACS: 75.20.En; 75.50.Cc Keywords: Shape memory alloy; Melt-spun ribbons; Martensitic transformation; Magnetization
1. Introduction Recently, a lot of attention has been paid to improve the response time of actuators by using magnetic-field-induced shape memory effects. The first in the series of materials that potentially revealed this phenomenon belonged to the NiMnGa alloy system. These actuator materials called as ferromagnetic shape memory alloys (FSMAs) underwent reorientation of twin structures under the application of a magnetic field [1]. The ferromagnetic shape memory effect refers to either the reversible field-induced austenite to martensite transformation or the rearrangement of martensite variants by an applied field leading to an overall change of shape. Such transformations are preferred to take place above the room temperature for application in actuators [2]. In the past, many of the reported stoichiometric compositions of NiMnGa alloys prepared through single-crystal growth techniques exhibited low transformaCorresponding author. Tel.: +91 657 2271709x2216; fax: +91 657 2270527. E-mail addresses: [email protected], [email protected] (A.K. Panda).
0304-8853/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.03.055
tion temperatures [3]. The limitations underlying such processing routes as well as the low transformation temperatures restricted the efficient use of FSMAs for practical applications. Although, single-crystal growth technique effectively forms preferred orientations but the material is not only brittle but also prone to the formation of defects during crystal growth. There has been paradigm shift in the development of ferromagnetic shape memory alloys using different preparation techniques. The rapid solidification route through melt spinning technique has been explored to prepare NiMnGa shape memory alloys in the form of ribbons. This technique, which is usually applicable to metallic glass preparations, has been found to be effective in freezing the Heusler Ni2MnGa stoichiometry to L21 austenitic (cubic) structure [4]. The magnetic shape memory effect (MSME) has been found to be actuating between the austenitic L21 Heusler structure and the quenched-in martensite. The present work attempts to investigate the reversible transformation in NiMnGa alloy prepared by melt spinning technique. The composition was selected from the perspective of applications bearing transformations close to room temperature and above.
ARTICLE IN PRESS A.K. Panda et al. / Journal of Magnetism and Magnetic Materials 320 (2008) L116–L120
from the proportionally decaying intensities as proceeding from (2 2 0) to (4 4 0) reflections. Such ordered L21 reflections have been reported for NiFeGa ribbons [7]. The lattice constant ‘a’ derived from the primary reflection d220 has been found to be 5.8039 A. It is interesting to note that the (2 2 0) reflection showed a splitting about its original position. Such a splitting is attributed to the existence of some tetragonal feature (martensitic), which is small enough in volume fraction to suppress the cubic L21 structure [8]. This can be explained in terms of higher Ga concentration in the matrix (Table 1) as compared to the grain boundary. The dense clustering of Ga atoms in the matrix is also responsible for such splitting of the X-ray primary reflection [9]. To investigate the phase transition and its recovery during shape memory effect, differential thermal analysis (DTA) was carried out at a slow scan rate of 2 K/min for two consecutive heating cycles, and is shown in Fig. 2. The first and the second heating cycles showed endothermic transformations. These endothermic peaks indicated reverse martensitic transformation (martensite to austenite). As evaluated here, the austenitic start temperature AS1 and AS2 are 305 and 307 K, respectively, during the first and second heating cycles. It is noteworthy, that DTA scan of the sample up to 330 K during the first heating cycle did not distort the structural recovery. Hence, the shape of the endothermic plot in the second heating cycle was identical. This was further supported from their nearly equal enthalpy of transformation obtained as 1.11
2. Experimental The master alloy with a nominal composition Ni52.5Mn24.5Ga23 (at%) was prepared by vacuum arc melting. The master alloy was induction melted and then rapidly quenched into ribbons by melt spinning technique. The phases of the as-quenched ribbon were identified using an X-ray diffractometer (Philips D500) with a Cu-Ka radiation. The microstructure of the as-quenched ribbon and the elemental distribution within the phases was studied using scanning electron microscope (SEM, Jeol 400) and energy dispersive spectroscope (EDS) analysis. The structure of the as-spun ribbons was also observed using transmission electron microscopy (Philips CM-200). The magnetization studies were carried out using a vibrating sample magnetometer (VSM, Lakeshore: 7404). 3. Result and discussion 3.1. Phase transformation and structure Microstructural characterization and identification of the phases in the as-spun ribbons were carried out using scanning electron microscopy (SEM) and X-ray diffractometry (XRD), respectively. The SEM micrograph and the diffractogram are shown in Fig. 1a and b, respectively. The material exhibits polyhedral grains having distinct grain boundaries. Some pitting is attributed to over etching. Quantitative analysis of grain matrix and grain boundaries was carried out. The elemental distribution of nickel, manganese and gallium is indicated in Table 1. It was observed that there was a relative increase in nickel and manganese concentration at the grain boundary. However, the gallium concentration decreased drastically at the grain boundary, which is at par with the earlier reports on Ni56Mn18.8Ga24.5Gd0.7 alloy [5]. X-ray diffractogram of the as-spun ribbon shown in Fig. 1b revealed an ordered L21 (cubic) austenite structure [6]. The highly ordered L21 cubic structure was evident
Table 1 Elemental distribution within the microstructure of the as-spun ribbons Elements
Composition (at%) Grain boundary
67.45 10.35 22.2
79.5 11.26 9.24
(220)
Matrix
Intensity (A.U.)
42
44 46 48 Degrees (2θ)
(422)
(400)
40
80
60
(440)
(220)
Nickel Manganese Gallium
Intensity (A.U.) 40
L117
100
Degrees (2θ) Fig. 1. (a) Scanning electron micrograph of the as-spun ribbon. (b) X-ray diffractogram of the as-spun ribbon. Inset shows splitting of primary peak.
ARTICLE IN PRESS A.K. Panda et al. / Journal of Magnetism and Magnetic Materials 320 (2008) L116–L120
L118
and 1.01 J/g for the first and second heating cycles, respectively. The onset of the transformation in the second heating cycle shifted only by 2 K. This is in contrast to the distortions in the endothermic peak occurring during hightemperature heating cycles [10]. The low enthalpy of transformation in the present case as compared with 2–10 J/g for the reported NiMnGa alloys [10,11] was a subject of curiosity. It was observed from XRD (Fig. 1b) that the as-spun ribbon had an austenitic L21 (cubic) structure as a dominant phase. The transmission electron micrograph revealed granular structure with a pronounced austenitic matrix as shown in Fig. 3a. Magnified image of grain boundary region (Fig. 3b) showed well-defined stripelike morphology of a martensitic phase [2]. The phase boundary between the martensite plates was more or less straight and distinct. TEM examination illustrated a low-volume fraction of textured martensite phase within the austenite L21 structure. This finding endorsed the inference of XRD study (Fig. 1b).
3.2. Magnetic transformation The thermal variation of magnetization of the as-spun ribbon at a low magnetizing field of 2.0 kA/m was obtained using vibrating sample magnetometer and is shown in Fig. 4. It was observed that during the heating cycle, the magnetization initially increased to reach a maximum value around 300 K. This initial rise in magnetisation is attributed to the change of martensitic state along with some nonferromagnetic components (possibly of antiferromagnetic origin) to an austenitic state with long-range ferromagnetic ordering [12]. However, investigations are also being carried out to explore whether such rise in magnetization is due to the lowering of anisotropy constant during martensite to austenite phase transformation when both the phases are ferromagnetic in nature [13,14]. After the initial rise, the magnetization started decreasing beyond 300 K indicating ferromagnetic to paramagnetic transition. From the derivative of the drop in magnetization, the Curie 2.5
As-Spun Ribbon Magnetising field =2.0 kA/m
Magnetisation, Tesla (x 10-3)
Endo thermic Heat flow (W/g)
0.36
2nd Heating Cycle 0.34
AS2 = 307 K Reverse Transformation (Martensite to Austenite)
0.26 1st Heating Cycle
2.0
1.5 Heating Cycle
1.0
AS1 = 305 K
Cooling Cycle TC = 302 K
0.24 310
320 Temperature (°C)
330
Fig. 2. Differential thermal analysis plots of the as-spun ribbon. Heating rate 2 K/min.
300
320 Temperature (K)
340
Fig. 4. Thermal variation of magnetisation of the as-spun ribbon at a magnetizing field of 2 kA/m.
Fig. 3. TEM micrograph showing (a) grains interface and (b) grain boundary region (blow-up of 3a).
ARTICLE IN PRESS A.K. Panda et al. / Journal of Magnetism and Magnetic Materials 320 (2008) L116–L120
Magnetising field = 400 kA/m 150 100
Magnetisation, Tesla (x 10-3)
50 0 = 40 kA/m
60 40 20 0
L119
in the ferromagnetic domains as the structure changes from a highly anisotropic martensite to a low anisotropic L21 austenitic phase. Additionally, the long-range ferromagnetic ordering in austenite is also the driving force behind such initial rise in magnetisation. At higher magnetic fields, the intensity of the field is high enough to make the domain transitions between these two ferromagnetic states spontaneous. Thus, at higher magnetizing fields, though such transition occurred but was spontaneous enough to reveal any rise in the initial magnetisation. Subsequently, the magnetisation of the ferromagnetic L21 austenitic phase dropped down to its own premartensitic paramagnetic [16] state around its Curie temperature. 4. Conclusion
= 2.0 kA/m 2
1
300
320 Temperature (K)
340
Fig. 5. Effect of magnetizing field on the thermal variation of magnetisation of the as-spun ribbon.
A ferromagnetic shape memory alloy with a nominal composition Ni52.5Mn24.5Ga23 (atomic %) was prepared in the form of ribbons by melt spinning technique. The asspun ribbon revealed granular morphology with L21 austenitic (cubic) structure in combination with martensitic phase. Martensite–austenite transformation and its reversibility was observed from thermal and magnetisation studies. Magnetic field intensity is critically important to make the martensite to austenite transformation spontaneous. Acknowledgment
temperature obtained during the heating cycle was 302 K. During the cooling cycle, the magnetic transition was found to be reversible with the Curie temperature remaining unchanged. Below 300 K, cooling cycle represented a more pronounced effect of forward martensitic transformation (austenite-martensite). This distinct change in the cooling cycle may be attributed to some sort of atomic rearrangements in the lattice sites during the heating cycle [10]. The increasing intensity of magnetic field on the martensite to austenite transformation showed interesting behaviour as indicated in Fig. 5. It was observed, that as the magnetizing field was increased from 2.0 to 400 kA/m, the distinct initial rise in magnetization above 295 K became suppressed. Thus, at higher magnetic field the spontaneous martensite to austenite transformation indicates the predominant role of magnetic field in the actuation process [14]. It needs further clarification on field-induced magnetic transitions that is responsible for the shape memory effect (SME). This phenomenon occurs when magnetic transition takes place between the martensitic and the L21 austenitic state, whereby both are essentially ferromagnetic and the Curie temperature is sufficiently high for the martensitic transformation [15]. In this condition, at low magnetic fields, the distinct initial rise in magnetization owes to the drastic lowering of anisotropy
The authors express their sincere gratitude to the Director, National Metallurgical Laboratory, Jamshedpur, India for kindly giving necessary permission to publish this work. References [1] K. Ullako, J.K. Huang, V.V. Kokorin, R.C. O’Handley, Appl. Phys. Lett. 69 (1996) 1133. [2] H.B. Xu, Y. Li, C.B. Jiang, Mater. Sci. Eng. A 438-440 (2006) 1065. [3] S.Y. Chu, R. Gallagher, M. De Graef, M.E. McHenry, IEEE Trans. Magn. 37 (2001) 2666. [4] Z.H. Liu, M. Zhang, Y.T. Cui, Y.Q. Zhou, W.H. Wang, G.H. Wu, X.X. Zhang, G. Xiao, Appl. Phys. Lett. 82 (2003) 424. [5] Z. Zeyu, L. Yi, D. Jingfang, H. Peng, W. Guangheng, C. Yongqin, J. Rare Earths 24 (2006) 579. [6] C. Jiang, Y. Muhammad, l. Deng, W. Wu, H. Xu, Acta Mater. 52 (2004) 2779. [7] Z.H. Liu, H. Liu, X.X. Zhang, M. Zhang, X.F. Dai, H.N. Hu, J.L. Chen, G.H. Wu, Phys. Lett. A 329 (2004) 214. [8] P.J. Webster, K.R.A. Ziebeck, S.L. Town, M.S. Peak, Philos. Mag. B 49 (1984) 295. [9] G.D. Liu, Z.H. Liu, X.F. Dai, S.Y. Yu, J.L. Chen, G.H. Wu, Sci. Technol. Adv. Mater. 6 (2005) 772. [10] J. Pons, C. Segui, V.A. Chernenko, E. Cesari, P. Ochin, R. Portier, Mater. Sci. Eng. A 273–275 (1999) 315. [11] J.I. Perez-Landazabal, C. Gomez-Polo, V. Recarte, C. Segui, E. Cesari, P. Ochin, J. Magn. Magn. Mater. 290-291 (2005) 826. [12] T. Krenke, E. Duman, M. Acet, E.F. Wassermann, X. Moya, L. Manosa, A. Planes, Nat. Mater. 4 (2005) 450.
ARTICLE IN PRESS L120
A.K. Panda et al. / Journal of Magnetism and Magnetic Materials 320 (2008) L116–L120
[13] V. Recarte, J.I. Perez-Landazabal, C. Gomez-Polo, C. Segui, E. Cesari, P. Ochin, Mater. Sci. Eng. A 438–440 (2006) 937. [14] J. Gutierrez, J.M. Barandiaran, p. Lazpita, C. Segui, E. Cesari, Sensors Actuators A 129 (2006) 163.
[15] L. Manosa, A. Planes, M. Acet, E. Duman, E.F. Wassermann, J. Magn. Magn. Mater. 272 (2004) 2090. [16] L. Manosa, A. G-Comas, E. Obrado, A. Planes, Mater. Sci. Eng. A 273 (1999) 329.