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Magnetostriction relative to pretransition in Ni50.5Mn24.5Ga25 single crystal
Solid State Communications 138 (2006) 234–238 www.elsevier.com/locate/ssc
Magnetostriction relative to pretransition in Ni50.5Mn24.5Ga25 single crystal Yuting Cui a,b, Xiaohong Yang a, Chunyang Kong a, Yong Ma a, Fusheng Pan b,* b
a Department of Physics, Chongqing Normal University, Chongqing 400047, People’s Republic of China Department of Materials Science and Engineering, Chongqing University, Chongqing 400044, People’s Republic of China
Received 10 February 2006; received in revised form 2 March 2006; accepted 5 March 2006 by H. Akai Available online 23 March 2006
Abstract The strain behaviors as well as the structural and magnetic changes relative to the pretransition in the Ni50.5Mn24.5Ga25 single crystals have been characterized by various methods, such as pretransition strain, magnetostriction, magnetization measurements, and TEM observations. A large magnetostriction up to 505 ppm measured in the [001] direction of the sample is obtained at the pretransition temperature with only a low magnetic field of about 1 kOe applied along the [010] direction. We found that not only the pretransition strain pronounces a more large change, but also the magnetostriction at a certain temperature exhibits a more large magnitude for field applied along the [010] direction than with field along the [001] direction. It is concluded that the magnetoelastic interaction is responsible for the premartensitic transition, and the magnetoelastic interaction in the [010] direction is stronger than that in the [001] direction. q 2006 Elsevier Ltd. All rights reserved. PACS: 75.80.Cq; 81.30.kf; 75.60.Ej Keywords: D. Magnetostriction; D. Pretransition; D. Magnetization
1. Introduction Ni–Mn–Ga alloys with composition close to the stoichiometric Heusler Ni2MnGa (L21 structure) have been attracting investigation due to their some unique precursor phenomena at temperature above the martensitic transformation. Specially interesting is the pronounced temperature softening of the (1/3,1/3,0) phonon on the TA2 branch [1–3]. Depending on the specific composition [4–7], at a given temperature (TI, i.e. pretransition temperature) above the martensitic transformation temperature (TM) this soft phonon freezes and a micromodulated structure develops. The appearance of this structure occurs via a phase transition (pretransition or intermediate transition), which has been acknowledged to be very weakly first order [4–8]. A Phenomenological model [8] suggested that the occurrence of this intermediate phase transition is governed by magnetoelastic coupling interaction. The field dependence of TI suggests that a large magnetoelastic effect exists in the intermediate state. Large magnetoelastic interaction means that a large magnetostriction could be * Corresponding author.
0038-1098/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2006.03.004
expected. In this letter, we report a large magnetostriction up to 505 ppm obtained in the Ni50.5Mn24.5Ga25 Single crystal in a low magnetic field of about 1 kOe at TI, which is five times larger than that of corresponding to the parent phase. Further, the dependence of magnetostriction on temperature and the influence of magnetic field to the pretransition strain have also been investigated in detail. 2. Experiment Single crystal of Ni50.5Mn24.5Ga25 was grown in [001] direction by the Czochralski method as reported previously [9]. The grown rates of 12–15 mm hK1 and the rotation rate of 30 rpm were adopted. The single crystal were oriented by back-reflection Laue diffraction and cut into 1!6!6 mm3 pieces with {100} faces for strain and magnetization measurements. The metal strain gauges with maximum measurement of 5% and the highly elastic epoxy resin were utilized to ensure measurement reliability and avoid the gauge debonding. The magnetizations were performed by a superconducting quantum interference magnetometer (Quantum Design MPMS). Pretransition miscrostructure was observed by the transmission electron microscopy (TEM). The temperature, alternating between cooling and heating, was varied at about 2 K/min.
Y. Cui et al. / Solid State Communications 138 (2006) 234–238
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3. Results and discussion Fig. 1 shows the temperature dependence of the strain measured along [001] and [010] directions of the parent phase in a free single crystal sample of Ni50.5Mn24.5Ga25. In the cooling run, the pretransition occurred at TIz202 K, above the martensitic transformation temperature TMz123 K, and shrunk the sample in the [001] direction about 3IZK 160 ppm, as shown by the inset in detail, noting without a external stress or a biasing field. The similar scene occurs to be in the [010] direction, but exhibiting a more small expanding deformation. This pretransition strain has been observed by Kokorin, et al. [5]. However, their observation is in a loaded sample with a compressing stress equal to above 17.6 MPa. Moreover, it is worth to indicate that in same direction, as shown in Fig. 1, the strain sign of the pretransition is completely consistent with that of the martensitic transformation. Previous observations [1,5] by the TEM along h100i directions on the Ni–Mn–Ga single crystal have shown that the diffuse streaking on the diffraction patterns increases in cooling and develops into the individual satellite spots surrounding the fundamental Bragg reflection below TI. Thus, they suggest that some randomly orientation strain embryos (i.e. the ultrafine scale local distortions) exist above TI and a micromodulated domain structure is formed below TI. In present work, the evolution of pretransition microstructure on the Ni50.5Mn24.5Ga25 single crystal has also been observed along [001] zone axis. Two typical electron diffraction patterns obtained by the TEM were presented in Fig. 2. It is found that the intense diffuse streaking along the [011] direction is more pronounce than that along its lateral direction on cooling. Below TI, the extra satellite spots along the [011] direction can clearly be observed, however, it is hardly found along the ½011 direction. Comparing to the previous observations [1,5], the present results suggest that in our sample, some strain embryos
Fig. 1. Strain vs temperature curves measured in [001] (a) and [010] (b) directions.
Fig. 2. Electron diffraction patterns taken at 220 K (a) and 160 K (b) ([001] zone axis).
are preferential orientation (i.e. local distortions of some strain embryos orientate to a certain direction) above TI and an orientated micromodulated domain structure is developed below TI. This argument is in good agreement with the macro-deformation obtained in Fig. 1 in which the direction of the pretransition strain shrink is always the [001], namely the growth direction of the single crystals. According to reports previously, the pretransition exhibits a pronounced temperature softening of the (1/3,1/3,0) phonon on the transverse TA2 branch [3–6], and leads to the appearance of a micromodulated structure preceding the martensitic transformation [5]. The existence of the micromodulaed structure can be explained as the result of the freezing of thermal vibrations of the soft TA2 mode, which become the static atomic displacements in this intermediate phase [5]. Our previous investigation [10] indicated that the orientated internal stress induced from the directional solidification during the growth of the crystals with the Czochralski method could result in a preferential orientation of martensitic variants and create a quite large macro-strain. This behavior of the internal stress is analogous to that of the uniaxial external stress on the Ni–Mn–Ga system [11]. Therefore, it is reasonable to believe that the pretransition strain detected by the strain measurements as well as orientated micromodulated domain structure observed by the TEM in the Ni50.5Mn24.5Ga25 single crystal is a result of the soft-mode condensation intervened by the residual orientated internal stress. Fig. 3 shows the magnetostriction measured in the [001] direction with the magnetic field applied along the [001] (a) and [010] (b) directions at various temperatures. One can see that the magnetostriction and saturated field show a great sensitivity for the crystallographic orientation and the operation temperature. At the parent phase (240 K), the magnetostriction of about K105 ppm with field applied along the [001] direction is only slightly larger than that of about 90 ppm with field applied along the [010] direction. And both have an approximately equal saturated field of about 0.5 kOe. However, in intermediate phase, the scene is not so. At a certain temperature, the largest magnetostriction obtained is with the field applied along the [010] direction, and a larger magnetic field needed for magnetostriction approaching to saturation is also with the field applied along this direction. For example, at TI, when the field applied in measuring direction of the [001] (Fig. 3(a)), the negative magnetostriction is obtained to be K380 ppm with a relatively lower saturated field of about
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Fig. 3. Magnetostriction curves measured in the [001] direction at different temperatures, with the magnetic field applied along the [001] (a) and [010] (b) directions, respectively. The inset shows orientation of the sample and the relative alignment of the measuring direction with the applied field.
0.7 kOe. However, when the field applied in the [010] direction, the magnetostriction becomes positive and larger, up to 505 ppm, and magnetic field approaching to saturation increases to about 1 kOe, as shown in Fig. 3(b). This indicates that the magnetoelastic response of the intermediate phase is different with that of the parent phase, even though the intermediate phase still remains the cubic symmetry unchanged. Fig. 4 shows the evolution of corresponding saturated magnetostriction measured in the [001] direction as a function of temperature over a widely temperature range. One can see that, from 250 to 202 K (TI), the strong growth of the saturated magnetostriction can be observed to be around TZTI, and the largest magnetostriction is detected to be at TZTI. Further cooling from TI, the saturated magnetostriction decreases dramatically. The largest magnetostriction of 505 ppm
Fig. 4. Saturated magnetostriction measured in the [001] direction as a function of temperature.
obtained at TI for field applied along the [010] direction is five times larger than that at 240 K in the same measuring configuration, thus showing a good applicable character. The largest magnetostriction detected at TI should be attributed to having the smallest shear elastic constant (C 0 ) and the largest magnetoelastic interaction at the pretransition point, as observed by the ultrasonic techniques [2] and suggested by the magnetic property measurements [12,13]. Presently, the directional dependence of the magnetostriction and saturated field indicates further that the magnetoelastic interaction in the [010] direction is stronger than that in the [001] direction. According to the preceding discussion, this anisotropy of magnetoelastic interaction between the 2 equiv crystallographic directions of [001] and [010] should be attributed to emerging orientated micromodulated domain structure via the pretransition in this studied material, which originates from the soft-mode condensation intervened by the residual orientated internal stress. Fig. 5 shows the dependence of pretransition strain 3I measured in the [001] direction under an external dc magnetic field H. The inset exhibits an example of pretransition deformation with the field of 0 and 2 kOe applied along the [001] direction. The measurement is performed upon cooling in field from room temperature, and sample is warmed in zerofield each time from about 77 K. One can see that the pretransition deformation can be affected greatly by the biasing magnetic field. With field applied along the [001] direction, the shrinking strain promotes from K160 to K245 ppm with the increase of the field from zero to 2.5 kOe. In this measuring configuration, the saturated field in 3I–H curve is about 2.5 kOe. Turning the field laterally applied to [010] direction of the sample, however, the shrinking strain decreases from K160 to K50 ppm with the increase of the field from zero to a relatively larger saturated field of 3.5 kOe. In fact, application of the external field will increase the alignment of the magnetic domains in the direction of the external field and simultaneously promote the magnetoelastic interaction [13]. If the
Fig. 5. Pretransition strain 3I measured in the [001] direction as a function of the magnetic field H. The inset exhibits an example of pretransition deformation with the fields of 0 and 2 kOe.
Y. Cui et al. / Solid State Communications 138 (2006) 234–238
macroscopical deformation induced by the external field is consistent with the intrinsic deformation of the sample, the pretransition strain will be enhanced, as shown in curve (a) in Fig. 5. Contrary, it will be suppressed, as shown in curve (b) in Fig. 5. With the aim of pursuing the suggested anisotropy of magnetoelastic coupling between the [001] direction and the [010] direction, we have performed magnetization measurements. Fig. 6 shows isothermal M–H curves measured at different temperatures. The inset shows the magnetization curves taken at TI. Due to the pretransition, the magnetization is relatively hard to saturate below TI, but relatively easy to saturate above TI. As shown in Fig. 6, the intermediate phase exhibits a slightly high saturation field of about 0.7 kOe, but decreasing to about 0.5 kOe in the parent phase (240 K) for field applied in the [001] direction. At TI, the saturated field of about 1 kOe for field applied along the [010] direction is larger than that of 0.7 kOe for field applied along the [001] direction, which reflects the anisotropy in existence between the [001] and [010] directions in the intermediate phase. For comparison, one can see that at same temperature and with field applied along the same direction, the saturated field determined by the magnetostriction curve in Fig. 3 is completely consistent with that detected in M–H curve at present, respectively. This indicates that the largest magnetostriction is attained when the magnetization is saturated. Further, comparing the magnetization curve along the [001] direction at 165 K with that at TI, it can be seen that although both curves exhibit approximately equal saturated field of 0.7 kOe, but the initial magnetization at TI is slightly harder than that at 165 K. This suggests that the magnetoelastic interaction at TI is stronger than that at other temperatures in the intermediate phase. Thus, at TI, the largest magnetostriction is detected. In addition, it is worth to indicate that with the field applied along the same direction, the saturated field in 3 I–H curve in Fig. 5 is larger than that in the magnetostrictive curve at TI in Fig. 3 and in M–H curve in
237
the inset in Fig. 6. Contrary, the saturated value of the strain associated with the pretransition as shown in Fig. 5 is small if compared to that of the magnetostriction around pretransition temperature as shown in Fig. 4, respectively. We argue this behavior is a result that the intrinsic dynamical response of the bcc structure is modulated by the external static magnetic field, because the static field as an independent external variable not only restricts the alignment of the magnetic domains before the pretransition, but also intervenes the pretransition during this transition process. 4. Conclusion A large magnetostriction up to 505 ppm measured in the [001] direction of parent phase in Heusler alloy Ni50.5Mn24.5Ga25 single crystal has been obtained with a small pulse magnetic field up to 1 kOe applied along the [010] direction at pretransition point, which is more five times larger than that in parent phase in the same measuring configuration. The temperature dependence of magnetostriction is studied in the temperature interval, which precedes the martensitic transformation in the compound. In the intermediate phase, we found that at a certain temperature, the magnitude of the magnetostriction measured in the [001] direction of the cubic parent phase with the field applied perpendicular to the measuring direction is larger than that with field applied along the measuring direction. Moreover, it is also found that not only the magnitude of the pretransition strain is corresponding to an external dc magnetic field, but also the pretransition strain exhibits a more large change for field applied along the [010] direction than that with field along the [001] direction. Further, the structural and magnetic changes relating to the pretransition have also been investigated in detail by the magnetization measurements and TEM observations at different temperatures. The results confirm that a magnetoelastic interaction is responsible for the pretransition, and the magnetoelastic interaction in the [010] direction is stronger than that in the [001] direction. Acknowledgements The measurements were carried out at State Key Laboratory for Magnetism, Institute of Physics, Chinese Academy of Sciences. The authors thank Professor Guangheng Wu for experimental assistance. This work was supported by Key Project of National Natural Science Foundation of China (Grant no. 50371101) and Chongqing City (Grant no. 2005BB4182). References
Fig. 6. Isothermal M–H curves measured at different temperatures. The inset shows the magnetization curves taken at the pretransition point.
[1] A. Zheludev, S.M. Shapiro, P. Wochner, A. Schwartz, M. Wall, L.E. Tanner, Phys. Rev. B 51 (1995) 11310. [2] T.E. Stenger, J. Trivisonno, Phys. Rev. B 57 (1998) 2735. [3] U. Stuhr, P. Vorderwisch, V.V. Kokorin, J. Phys.: Condens. Matter 12 (2000) 7541.
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[4] V.A. Chernenko, J. Pons, C. Seguı´, E. Cesari, Acta Mater. 50 (2002) 53. [5] V.V. Kokorin, V.A. Chernenko, E. Cesari, J. Pons, C. Segui, J. Phys. Condens. Matter 8 (1996) 6457. [6] V.V. Kokorin, V.A. Chernenko, J. Pons, C. Segu, E. Cesari, Solid State Commun. 101 (1997) 7. [7] T. Casta´n, A. Planes, A. Saxena, Phys. Rev. B 67 (2003) 134113. [8] A. Planes, E. Obrado´, A.G. Comas, L. Man˜osa, Phys. Rev. Lett. 79 (1997) 3926.
[9] G.H. Wu, C.H. Yu, L.Q. Meng, J.L. Chen, F.M. Yang, S.R. Qi, W.S. Zhan, Z. Wang, Y.F. Zheng, L.C. Zhao, Appl. Phys. Lett. 75 (1999) 2990. [10] W.H. Wang, G.H. Wu, J.L. Chen, W.S. Zhan, Z. Wang, Z.Y. Gao, L.C. Zhao, J. Phys.: Condens. Matter 12 (2000) 6287. [11] V.A. Chernenko, V.A. L’vov, Philos. Mag. A 73 (1996) 999. [12] F. Zuo, X. Su, K.H. Wu, Phys. Rev. B 58 (1998) 11127. [13] Y. Ma, S. Awaji, K. Watanable, M. Matsumoto, N. Kobayashi, Appl. Phys. Lett. 76 (2000) 37.
Magnetostriction relative to pretransition in Ni50.5Mn24.5Ga25 single crystal Yuting Cui a,b, Xiaohong Yang a, Chunyang Kong a, Yong Ma a, Fusheng Pan b,* b
a Department of Physics, Chongqing Normal University, Chongqing 400047, People’s Republic of China Department of Materials Science and Engineering, Chongqing University, Chongqing 400044, People’s Republic of China
Received 10 February 2006; received in revised form 2 March 2006; accepted 5 March 2006 by H. Akai Available online 23 March 2006
Abstract The strain behaviors as well as the structural and magnetic changes relative to the pretransition in the Ni50.5Mn24.5Ga25 single crystals have been characterized by various methods, such as pretransition strain, magnetostriction, magnetization measurements, and TEM observations. A large magnetostriction up to 505 ppm measured in the [001] direction of the sample is obtained at the pretransition temperature with only a low magnetic field of about 1 kOe applied along the [010] direction. We found that not only the pretransition strain pronounces a more large change, but also the magnetostriction at a certain temperature exhibits a more large magnitude for field applied along the [010] direction than with field along the [001] direction. It is concluded that the magnetoelastic interaction is responsible for the premartensitic transition, and the magnetoelastic interaction in the [010] direction is stronger than that in the [001] direction. q 2006 Elsevier Ltd. All rights reserved. PACS: 75.80.Cq; 81.30.kf; 75.60.Ej Keywords: D. Magnetostriction; D. Pretransition; D. Magnetization
1. Introduction Ni–Mn–Ga alloys with composition close to the stoichiometric Heusler Ni2MnGa (L21 structure) have been attracting investigation due to their some unique precursor phenomena at temperature above the martensitic transformation. Specially interesting is the pronounced temperature softening of the (1/3,1/3,0) phonon on the TA2 branch [1–3]. Depending on the specific composition [4–7], at a given temperature (TI, i.e. pretransition temperature) above the martensitic transformation temperature (TM) this soft phonon freezes and a micromodulated structure develops. The appearance of this structure occurs via a phase transition (pretransition or intermediate transition), which has been acknowledged to be very weakly first order [4–8]. A Phenomenological model [8] suggested that the occurrence of this intermediate phase transition is governed by magnetoelastic coupling interaction. The field dependence of TI suggests that a large magnetoelastic effect exists in the intermediate state. Large magnetoelastic interaction means that a large magnetostriction could be * Corresponding author.
0038-1098/$ - see front matter q 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2006.03.004
expected. In this letter, we report a large magnetostriction up to 505 ppm obtained in the Ni50.5Mn24.5Ga25 Single crystal in a low magnetic field of about 1 kOe at TI, which is five times larger than that of corresponding to the parent phase. Further, the dependence of magnetostriction on temperature and the influence of magnetic field to the pretransition strain have also been investigated in detail. 2. Experiment Single crystal of Ni50.5Mn24.5Ga25 was grown in [001] direction by the Czochralski method as reported previously [9]. The grown rates of 12–15 mm hK1 and the rotation rate of 30 rpm were adopted. The single crystal were oriented by back-reflection Laue diffraction and cut into 1!6!6 mm3 pieces with {100} faces for strain and magnetization measurements. The metal strain gauges with maximum measurement of 5% and the highly elastic epoxy resin were utilized to ensure measurement reliability and avoid the gauge debonding. The magnetizations were performed by a superconducting quantum interference magnetometer (Quantum Design MPMS). Pretransition miscrostructure was observed by the transmission electron microscopy (TEM). The temperature, alternating between cooling and heating, was varied at about 2 K/min.
Y. Cui et al. / Solid State Communications 138 (2006) 234–238
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3. Results and discussion Fig. 1 shows the temperature dependence of the strain measured along [001] and [010] directions of the parent phase in a free single crystal sample of Ni50.5Mn24.5Ga25. In the cooling run, the pretransition occurred at TIz202 K, above the martensitic transformation temperature TMz123 K, and shrunk the sample in the [001] direction about 3IZK 160 ppm, as shown by the inset in detail, noting without a external stress or a biasing field. The similar scene occurs to be in the [010] direction, but exhibiting a more small expanding deformation. This pretransition strain has been observed by Kokorin, et al. [5]. However, their observation is in a loaded sample with a compressing stress equal to above 17.6 MPa. Moreover, it is worth to indicate that in same direction, as shown in Fig. 1, the strain sign of the pretransition is completely consistent with that of the martensitic transformation. Previous observations [1,5] by the TEM along h100i directions on the Ni–Mn–Ga single crystal have shown that the diffuse streaking on the diffraction patterns increases in cooling and develops into the individual satellite spots surrounding the fundamental Bragg reflection below TI. Thus, they suggest that some randomly orientation strain embryos (i.e. the ultrafine scale local distortions) exist above TI and a micromodulated domain structure is formed below TI. In present work, the evolution of pretransition microstructure on the Ni50.5Mn24.5Ga25 single crystal has also been observed along [001] zone axis. Two typical electron diffraction patterns obtained by the TEM were presented in Fig. 2. It is found that the intense diffuse streaking along the [011] direction is more pronounce than that along its lateral direction on cooling. Below TI, the extra satellite spots along the [011] direction can clearly be observed, however, it is hardly found along the ½011 direction. Comparing to the previous observations [1,5], the present results suggest that in our sample, some strain embryos
Fig. 1. Strain vs temperature curves measured in [001] (a) and [010] (b) directions.
Fig. 2. Electron diffraction patterns taken at 220 K (a) and 160 K (b) ([001] zone axis).
are preferential orientation (i.e. local distortions of some strain embryos orientate to a certain direction) above TI and an orientated micromodulated domain structure is developed below TI. This argument is in good agreement with the macro-deformation obtained in Fig. 1 in which the direction of the pretransition strain shrink is always the [001], namely the growth direction of the single crystals. According to reports previously, the pretransition exhibits a pronounced temperature softening of the (1/3,1/3,0) phonon on the transverse TA2 branch [3–6], and leads to the appearance of a micromodulated structure preceding the martensitic transformation [5]. The existence of the micromodulaed structure can be explained as the result of the freezing of thermal vibrations of the soft TA2 mode, which become the static atomic displacements in this intermediate phase [5]. Our previous investigation [10] indicated that the orientated internal stress induced from the directional solidification during the growth of the crystals with the Czochralski method could result in a preferential orientation of martensitic variants and create a quite large macro-strain. This behavior of the internal stress is analogous to that of the uniaxial external stress on the Ni–Mn–Ga system [11]. Therefore, it is reasonable to believe that the pretransition strain detected by the strain measurements as well as orientated micromodulated domain structure observed by the TEM in the Ni50.5Mn24.5Ga25 single crystal is a result of the soft-mode condensation intervened by the residual orientated internal stress. Fig. 3 shows the magnetostriction measured in the [001] direction with the magnetic field applied along the [001] (a) and [010] (b) directions at various temperatures. One can see that the magnetostriction and saturated field show a great sensitivity for the crystallographic orientation and the operation temperature. At the parent phase (240 K), the magnetostriction of about K105 ppm with field applied along the [001] direction is only slightly larger than that of about 90 ppm with field applied along the [010] direction. And both have an approximately equal saturated field of about 0.5 kOe. However, in intermediate phase, the scene is not so. At a certain temperature, the largest magnetostriction obtained is with the field applied along the [010] direction, and a larger magnetic field needed for magnetostriction approaching to saturation is also with the field applied along this direction. For example, at TI, when the field applied in measuring direction of the [001] (Fig. 3(a)), the negative magnetostriction is obtained to be K380 ppm with a relatively lower saturated field of about
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Fig. 3. Magnetostriction curves measured in the [001] direction at different temperatures, with the magnetic field applied along the [001] (a) and [010] (b) directions, respectively. The inset shows orientation of the sample and the relative alignment of the measuring direction with the applied field.
0.7 kOe. However, when the field applied in the [010] direction, the magnetostriction becomes positive and larger, up to 505 ppm, and magnetic field approaching to saturation increases to about 1 kOe, as shown in Fig. 3(b). This indicates that the magnetoelastic response of the intermediate phase is different with that of the parent phase, even though the intermediate phase still remains the cubic symmetry unchanged. Fig. 4 shows the evolution of corresponding saturated magnetostriction measured in the [001] direction as a function of temperature over a widely temperature range. One can see that, from 250 to 202 K (TI), the strong growth of the saturated magnetostriction can be observed to be around TZTI, and the largest magnetostriction is detected to be at TZTI. Further cooling from TI, the saturated magnetostriction decreases dramatically. The largest magnetostriction of 505 ppm
Fig. 4. Saturated magnetostriction measured in the [001] direction as a function of temperature.
obtained at TI for field applied along the [010] direction is five times larger than that at 240 K in the same measuring configuration, thus showing a good applicable character. The largest magnetostriction detected at TI should be attributed to having the smallest shear elastic constant (C 0 ) and the largest magnetoelastic interaction at the pretransition point, as observed by the ultrasonic techniques [2] and suggested by the magnetic property measurements [12,13]. Presently, the directional dependence of the magnetostriction and saturated field indicates further that the magnetoelastic interaction in the [010] direction is stronger than that in the [001] direction. According to the preceding discussion, this anisotropy of magnetoelastic interaction between the 2 equiv crystallographic directions of [001] and [010] should be attributed to emerging orientated micromodulated domain structure via the pretransition in this studied material, which originates from the soft-mode condensation intervened by the residual orientated internal stress. Fig. 5 shows the dependence of pretransition strain 3I measured in the [001] direction under an external dc magnetic field H. The inset exhibits an example of pretransition deformation with the field of 0 and 2 kOe applied along the [001] direction. The measurement is performed upon cooling in field from room temperature, and sample is warmed in zerofield each time from about 77 K. One can see that the pretransition deformation can be affected greatly by the biasing magnetic field. With field applied along the [001] direction, the shrinking strain promotes from K160 to K245 ppm with the increase of the field from zero to 2.5 kOe. In this measuring configuration, the saturated field in 3I–H curve is about 2.5 kOe. Turning the field laterally applied to [010] direction of the sample, however, the shrinking strain decreases from K160 to K50 ppm with the increase of the field from zero to a relatively larger saturated field of 3.5 kOe. In fact, application of the external field will increase the alignment of the magnetic domains in the direction of the external field and simultaneously promote the magnetoelastic interaction [13]. If the
Fig. 5. Pretransition strain 3I measured in the [001] direction as a function of the magnetic field H. The inset exhibits an example of pretransition deformation with the fields of 0 and 2 kOe.
Y. Cui et al. / Solid State Communications 138 (2006) 234–238
macroscopical deformation induced by the external field is consistent with the intrinsic deformation of the sample, the pretransition strain will be enhanced, as shown in curve (a) in Fig. 5. Contrary, it will be suppressed, as shown in curve (b) in Fig. 5. With the aim of pursuing the suggested anisotropy of magnetoelastic coupling between the [001] direction and the [010] direction, we have performed magnetization measurements. Fig. 6 shows isothermal M–H curves measured at different temperatures. The inset shows the magnetization curves taken at TI. Due to the pretransition, the magnetization is relatively hard to saturate below TI, but relatively easy to saturate above TI. As shown in Fig. 6, the intermediate phase exhibits a slightly high saturation field of about 0.7 kOe, but decreasing to about 0.5 kOe in the parent phase (240 K) for field applied in the [001] direction. At TI, the saturated field of about 1 kOe for field applied along the [010] direction is larger than that of 0.7 kOe for field applied along the [001] direction, which reflects the anisotropy in existence between the [001] and [010] directions in the intermediate phase. For comparison, one can see that at same temperature and with field applied along the same direction, the saturated field determined by the magnetostriction curve in Fig. 3 is completely consistent with that detected in M–H curve at present, respectively. This indicates that the largest magnetostriction is attained when the magnetization is saturated. Further, comparing the magnetization curve along the [001] direction at 165 K with that at TI, it can be seen that although both curves exhibit approximately equal saturated field of 0.7 kOe, but the initial magnetization at TI is slightly harder than that at 165 K. This suggests that the magnetoelastic interaction at TI is stronger than that at other temperatures in the intermediate phase. Thus, at TI, the largest magnetostriction is detected. In addition, it is worth to indicate that with the field applied along the same direction, the saturated field in 3 I–H curve in Fig. 5 is larger than that in the magnetostrictive curve at TI in Fig. 3 and in M–H curve in
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the inset in Fig. 6. Contrary, the saturated value of the strain associated with the pretransition as shown in Fig. 5 is small if compared to that of the magnetostriction around pretransition temperature as shown in Fig. 4, respectively. We argue this behavior is a result that the intrinsic dynamical response of the bcc structure is modulated by the external static magnetic field, because the static field as an independent external variable not only restricts the alignment of the magnetic domains before the pretransition, but also intervenes the pretransition during this transition process. 4. Conclusion A large magnetostriction up to 505 ppm measured in the [001] direction of parent phase in Heusler alloy Ni50.5Mn24.5Ga25 single crystal has been obtained with a small pulse magnetic field up to 1 kOe applied along the [010] direction at pretransition point, which is more five times larger than that in parent phase in the same measuring configuration. The temperature dependence of magnetostriction is studied in the temperature interval, which precedes the martensitic transformation in the compound. In the intermediate phase, we found that at a certain temperature, the magnitude of the magnetostriction measured in the [001] direction of the cubic parent phase with the field applied perpendicular to the measuring direction is larger than that with field applied along the measuring direction. Moreover, it is also found that not only the magnitude of the pretransition strain is corresponding to an external dc magnetic field, but also the pretransition strain exhibits a more large change for field applied along the [010] direction than that with field along the [001] direction. Further, the structural and magnetic changes relating to the pretransition have also been investigated in detail by the magnetization measurements and TEM observations at different temperatures. The results confirm that a magnetoelastic interaction is responsible for the pretransition, and the magnetoelastic interaction in the [010] direction is stronger than that in the [001] direction. Acknowledgements The measurements were carried out at State Key Laboratory for Magnetism, Institute of Physics, Chinese Academy of Sciences. The authors thank Professor Guangheng Wu for experimental assistance. This work was supported by Key Project of National Natural Science Foundation of China (Grant no. 50371101) and Chongqing City (Grant no. 2005BB4182). References
Fig. 6. Isothermal M–H curves measured at different temperatures. The inset shows the magnetization curves taken at the pretransition point.
[1] A. Zheludev, S.M. Shapiro, P. Wochner, A. Schwartz, M. Wall, L.E. Tanner, Phys. Rev. B 51 (1995) 11310. [2] T.E. Stenger, J. Trivisonno, Phys. Rev. B 57 (1998) 2735. [3] U. Stuhr, P. Vorderwisch, V.V. Kokorin, J. Phys.: Condens. Matter 12 (2000) 7541.
238
Y. Cui et al. / Solid State Communications 138 (2006) 234–238
[4] V.A. Chernenko, J. Pons, C. Seguı´, E. Cesari, Acta Mater. 50 (2002) 53. [5] V.V. Kokorin, V.A. Chernenko, E. Cesari, J. Pons, C. Segui, J. Phys. Condens. Matter 8 (1996) 6457. [6] V.V. Kokorin, V.A. Chernenko, J. Pons, C. Segu, E. Cesari, Solid State Commun. 101 (1997) 7. [7] T. Casta´n, A. Planes, A. Saxena, Phys. Rev. B 67 (2003) 134113. [8] A. Planes, E. Obrado´, A.G. Comas, L. Man˜osa, Phys. Rev. Lett. 79 (1997) 3926.
[9] G.H. Wu, C.H. Yu, L.Q. Meng, J.L. Chen, F.M. Yang, S.R. Qi, W.S. Zhan, Z. Wang, Y.F. Zheng, L.C. Zhao, Appl. Phys. Lett. 75 (1999) 2990. [10] W.H. Wang, G.H. Wu, J.L. Chen, W.S. Zhan, Z. Wang, Z.Y. Gao, L.C. Zhao, J. Phys.: Condens. Matter 12 (2000) 6287. [11] V.A. Chernenko, V.A. L’vov, Philos. Mag. A 73 (1996) 999. [12] F. Zuo, X. Su, K.H. Wu, Phys. Rev. B 58 (1998) 11127. [13] Y. Ma, S. Awaji, K. Watanable, M. Matsumoto, N. Kobayashi, Appl. Phys. Lett. 76 (2000) 37.