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Large magnetoresistance in a directionally solidified Ni44.5Co5.1Mn37.1In13.3 magnetic shape memory alloy
Journal of Magnetism and Magnetic Materials 452 (2018) 249–252
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Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Research articles
Large magnetoresistance in a directionally solidified Ni44.5Co5.1Mn37.1In13.3 magnetic shape memory alloy Zongbin Li a,⇑, Wei Hu a, Fenghua Chen b,⇑, Mingang Zhang b, Zhenzhuang Li a, Bo Yang a, Xiang Zhao a, Liang Zuo a,c a b c
Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Material Science and Engineering, Northeastern University, Shenyang 110819, China Institute of Advanced Materials, School of Materials Science and Engineering, Taiyuan University of Science and Technology, Taiyuan 030024, China Taiyuan University of Science and Technology, Taiyuan 030024, China
a r t i c l e
i n f o
Article history: Received 19 October 2017 Received in revised form 12 December 2017 Accepted 27 December 2017 Available online 27 December 2017 Keywords: Shape memory materials Magnetic materials Texture Magnetostructural transformation Magnetoresistance
a b s t r a c t Polycrystalline Ni44.5Co5.1Mn37.1In13.3 alloy with coarse columnar-shaped grains and h0 0 1iA preferred orientation was prepared by directional solidification. Due to the strong magnetostructural coupling, inverse martensitic transformation can be induced by the magnetic field, resulting in large negative magnetoresistance up to 58% under the field of 3 T. Such significant field controlled functional behaviors should be attributed to the coarse grains and strong preferred orientation in the directionally solidified alloy. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Since a 3% magnetic-field-induced strain was reported in a Ni45Co5Mn36.7In13.3 single crystal [1], Ni-Mn-In based alloys have attracted considerable attention. In these alloys, the martensitic transformation involves not only the structural changes but also the magnetization variations, i.e., from ferromagnetic austenite to weak magnetic martensite. Accordingly, the martensitic transformation temperatures of these alloys can be greatly reduced by an external magnetic field, which enables the occurrence of field induced inverse martensitic transformation [1,2]. Associated with such a field induced transformation, some remarkable field controlled functional behaviors, such as magnetic shape memory effect [1,2], magnetocaloric effect (MCE) [3–6], and magnetoresistance (MR) [7–9], were developed, which is promising for the technical applications in novel magnetic actuation, refrigeration and sensoring. Yu et al. firstly reported the MR in Ni-Mn-In alloys and a giant MR over 80% was achieved in a Ni50Mn34In16 single crystal under the magnetic field of 5 T [8]. Although the single crystal alloys can exhibit superior field controlled functional behaviours, the relatively higher cost for the fabrication of single crystals becomes ⇑ Corresponding authors. E-mail addresses: [email protected] (Z. Li), [email protected] (F. Chen). https://doi.org/10.1016/j.jmmm.2017.12.093 0304-8853/Ó 2017 Elsevier B.V. All rights reserved.
an unavoidable hindrance for practical applications. Alternatively, the preparation of polycrystalline alloys is much easier and of lower cost. Thus, more attention was paid to the polycrystalline alloys. It has been reported that large MR of 64% and 56% can be achieved under the field of 5 T in polycrystalline Ni50Mn34In16 and Ni50Mn35In15 alloy [9,10], respectively. In addition, the substitution elements were introduced in order to improve the MR in polycrystalline alloys. It is found that in Fe doped Ni50Mn37xFexIn13 (x = 2–4) polycrystalline alloys, the MR increases with the increase of Fe content and the maximum MR reaches 57% for the field of 5 T [11]. In a Ni48.4Co1.9Mn34.2In13.8Ga1.7 polycrystalline alloy, a large MR of 66% can be obtained with the application of the field of 7 T [12]. Generally speaking, the MR properties in polycrystalline alloys are relatively lower with respect to those of single crystals. This could be due to the deteriorated preferential crystallographic orientation as well as the introduction of large amounts of grain boundaries in polycrystalline alloys. Microstructure control through texturation could be a practicable strategy to realize property optimization in polycrystalline alloys. Recent investigations have illustrated that the highly textured microstructure in Ni-Mn-based polycrystalline alloys contributes a lot to the improvement of field controlled performance [13– 17]. Liu et al. obtained a 0.25% magnetostrain through field induced inverse martensitic transformation in a highly textured Ni45.2Mn36.7In13Co5.1 alloy [15]. Gaitzsch et al., reported a 1%
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magnetostrain through field induced variant reorientation in a textured Ni50Mn29Ga21 alloy prepared by directional solidification [16]. Mcleod et al., found that the magnetic entropy change (DSM) can be increased by 84% in a Ni54Mn21Ga25 alloy due to the strong texture induced by thermo-mechanical training [17]. Our recent investigations have shown that the adiabatic temperature variation (DTad) induced by magnetic field in directionally solidified Ni-Mn-Ga alloys can be significantly improved due to the formation of strong texture [13,14]. The enhanced field controlled performance in polycrystalline alloys with strong preferred orientation should be attributed to the optimization of crystallographic and magnetocrystalline anisotropy through texturation, which enables the polycrystalline alloys more close to the single crystal. It is worth mentioning that the directional solidification is an efficient way to optimize the microstructure of polycrystalline alloys, by which the alloys usually form the coarse columnar grains with highly preferential orientation along the growth direction [13,14,18,19]. In this work, a polycrystalline Ni44.5Co5.1Mn37.1In13.3 alloy with h001iA preferred orientation was prepared by directional solidification. Owing to the strong magnetostructural coupling, the field induced inverse martensitic transformation was evidenced, resulting in a large negative MR up to 58% under the field of 3 T.
2. Experimental The master Ni-Co-Mn-In polycrystalline alloy was prepared by arc-melting under argon protection. To achieve a good composition homogeneity, the as-cast alloy was melted for four times. After that, the alloy was suctioned into a copper mould to obtain a cylindrical rod with the diameter of 10 mm for the subsequent directional solidification. The directional solidification was performed in a Bridgman type apparatus under argon atmosphere, where the cylindrical rod was enveloped in a high-purity corundum tube with the inner diameter of 10.5 mm. A water-cooled cylinder containing liquid Ga-In-Sn metal is used to cool down the alloy. During the experiments, the alloy in the corundum crucible was melted by using resistance heating and then directionally solidified in the Bridgman apparatus by pulling the crucible into the liquid Ga-In-Sn metal cylinder at a growth rate of 50 lm/s. The assolidified alloy was then homogenized at 1173 K for 24 h, followed by quenching into water. Parts of homogenized alloy were ground into powder and then sealed in a vacuum quartz tube, followed by annealing at 873 K for 5 h to remove the internal stress. The composition verification was performed by energy dispersive spectrometry (EDS) and the actual composition was determined to be Ni44.5Co5.1Mn37.1In13.3 (at.%). The crystal structure was analyzed by powder X-ray diffraction (XRD) at room temperature in a PANalytical X’Pert Pro MPD diffractometer. The incomplete pole figures were measured by XRD in a Rigaku SmartLab diffractometer at room temperature on the transverse section of the directionally solidified alloy. The martensitic transformation temperatures were measured by differential scanning calorimetry (DSC) with a heating and cooling rate of 10 K/min. The thermalmagnetic (M-T) curves were measured using a vibrating sample magnetometer VersaLab system with a heating and cooling rate of 3 K/min. The martensitic transformation temperatures were determined by the conventional tangent extrapolation method in DSC and M-T curves. The electrical resistance (bar sample, 10 m m 3 mm 2 mm) under the field were measured by four probe method using VersaLab system [20], where the magnetic field was applied along the length direction (i.e., directional solidification direction). The MR is calculated by MR = [R(H) R(0)]/R(0) 100%, where R(H) and R(0) are the electrical resistance measured with and without the magnetic field, respectively.
3. Results and discussions Fig. 1 displays the powder XRD pattern for the directionally solidified alloy measured at room temperature. The pattern exhibits the characteristic of martensite and it is similar to that of six-layered modulated (6M) martensite in a Ni50Mn36In14 alloy [21], suggesting that the present Ni44.5Co5.1Mn37.1In13.3 alloy may consist of 6M martensite at room temperature. Accordingly, the lattice parameters of 6M martensite were determined to be aM = 4.352 Å, bM = 5.581 Å, cM = 12.98 Å and b = 93.8°, respectively. In addition, the {2 2 0}A diffraction of austenite was also identified in the powder XRD pattern, which may indicate that the martensitic transformation temperature is close to the room temperature. Fig. 2a shows the macroscopic microstructure of the longitudinal section for the directionally solidified alloy. The initial austenite forms coarse columnar-shaped grains with several hundreds of microns in size along the solidification direction. Such microstructural feature should be attributed to the specific thermal gradient during the directional solidification process. To analyze the preferential orientation of the directionally solidified alloy, {1 2 3}M, {1 2 3}M and {0 4 0}M incomplete pole figures of the 6M martensite were measured on the transverse section by XRD, as shown in Fig. 2b. Noted that {1 2 3}M and {1 2 3}M poles are roughly located at the polar angle of 40°, whereas {0 4 0}M poles are located at the centre of the pole figure. According to the lattice correspondence between austenite and 6 M martensite in Ni-Mn-In alloys, {1 2 3}M and {1 2 3}M are originated from {2 2 0}A of austenite and {0 4 0}M from {4 0 0}A [21]. Thus, it can be inferred that the initial austenite possesses the strong preferred orientation with h0 0 1iA parallel to the solidification direction, which is consistent with previous results obtained in directionally solidified alloys [18,19]. Fig. 3 presents the temperature dependence of magnetization (M-T curves) measured under the field of 0.1 T and 3 T. In the figure, the abrupt changes in magnetization should be attributed to the martensitic transformation from ferromagnetic austenite to weak magnetic martensite. Based on the low-field M-T curves (0.1 T), the martensitic transformation temperatures (Ms, Mf, As, Af) were determined to 324 K, 303 K, 316 K, and 337 K, respectively, which is very close to those determined from DSC measurements (inset of Fig. 3), i.e., Ms = 325 K, Mf = 300 K, As = 318 K, Af = 344 K. Recently, it was reported that the martensitic transformation in directionally solidified Ni40.6Co8.5Mn40.9Sn10 and Ni42Co8-
Fig. 1. Powder XRD pattern for the directionally solidified alloy measured at room temperature.
Z. Li et al. / Journal of Magnetism and Magnetic Materials 452 (2018) 249–252
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Fig. 2. (a) Macroscopic microstructure of the longitudinal section for the directionally solidified alloy. (b) {1 2 3}M, {1 2 3}M and {0 4 0}M incomplete pole figures of the 6M martensite measured by XRD (SD: solidification direction).
Fig. 3. M-T curves measured under the field of 0.1 T and 3 T. The inset shows the DSC curves.
Mn38In12 alloy may occur in a step-like manner and cover a broad temperature region, due to chemical segregation along the growth direction [22,23]. It should be noted that there is no step-like endothermic/exothermic peaks around the martensitic transformation temperature region in the present DSC curves and the forward/inverse martensitic transformation occurs in a much narrower temperature region in comparison with that of directionally solidified Ni40.6Co8.5Mn40.9Sn10 and Ni42Co8Mn38In12 alloy [22,23], which suggests a good composition homogeneity in the present directionally solidified alloy.
Under the field of 3 T, the martensitic transformation temperatures are shifted to the lower temperature region, where the As is decreased by 13 K, with a reduction rate of 4.5 K/T. This value is comparable to that in Ni45Co5Mn36.6In13.4 single crystal, i.e., 4.3 K/ T [1]. Such phenomena indicates that the magnetic field induced inverse martensitic transformation can be expected. The reduction of the transformation temperature can be well described by Clausius-Clapeyron equation, i.e., DT/DH DM/DS, where DS and DM stand for the entropy and magnetization difference between austenite and martensite, respectively. Here, DM reaches to 90 Am2/kg under the field of 3 T and DS is determined to be 21.6 J/kg/K from the DSC measurements. Thus, the value of DT/DH can be calculated to be 4.2 K/T for the field of 3 T according to the Clausius-Clapeyron equation, which is quite close to the experimentally observed one (4.5 K/T). Fig. 4a displays the temperature dependence of the electrical resistance (R-T curves) under the field of 0 T and 3 T on heating and cooling processes across martensitic transformation. Under the zero field, the transformation from austenite to martensite is associated with a large increase of electrical resistance, which could be due to the modification of the density of state in the vicinity of the Fermi level through martensitic transformation [8]. The characteristic martensitic transformation temperatures reflected from the electrical resistance measurements are in good agreement with those from low-field M-T curves. Under the field of 3 T, the R-T curves is similar to that of zero field, but the martensitic transformation temperatures shift to the lower temperature region, confirming the occurrence of field induced inverse transformation. Fig. 4b shows the temperature dependence of MR on heating under the field of 3 T, where the MR is calculated as [R(H)-R(0)]/R
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4. Summary Polycrystalline Ni44.5Co5.1Mn37.1In13.3 alloy with coarse columnar-shaped grains and strong h0 0 1iA preferred orientation was prepared by the directional solidification. Due to the strong magnetostructural coupling, the inverse martensitic transformation can be induced by the magnetic field, resulting in large negative MR up to 58% under the field of 3 T. The present MR value is comparable to those in some polycrystalline alloys under the field of 5 T or 7 T. The enhancement of the MR should be attributed to the coarse grains and strong preferential orientation in the directionally solidified alloy. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grants No. 51431005, 51571056, 51771048, 51601033), the 863 Program of China (Grant No. 2015AA034101), the Fundamental Research Funds for the Central Universities of China (Grant No. N160205002), Fund Program for the Scientific Activities of Selected Returned Overseas Professionals in Shanxi Province, Shanxi Scholarship Council of China (Grant No. 2016-092), and Open Project of Key Laboratory for Anisotropy and Texture of Materials in Northeastern University (Grant No. ATM20170003). References
Fig. 4. (a) Temperature dependence of the electrical resistance under 0 T and 3 T. (b) Temperature dependence of MR on heating under the field of 3 T for the directionally solidified alloy and suction casting alloy.
(0). The directionally solidified alloy exhibits large negative MR due to field induced inverse martensitic transformation, where the maximum MR reaches 58% at 318 K. For a comparison, Fig. 4b also presents the MR for the polycrystalline alloy after suction casting. The maximum MR of the suction casting alloy is 40%. Apparently, the MR is greatly enhanced after directional solidification. The present MR value for the directionally solidified alloy is comparable to those in bulk polycrystalline Ni50Mn34In16 alloy ( MR = 64%, 5 T) [9], Ni48.4Co1.9Mn34.2In13.8Ga1.7 alloy (MR = 66%, 7 T) [12], Ni41Co9Mn40Sn10 alloy (MR = 53.8%, 5 T) [24], and Ni41Co9Mn39Sb11 alloy (MR = 60%, 5 T) [25], but with a relatively lower magnetic field. The above results illustrate that the present directionally solidified alloy can demonstrate remarkable magnetoresistance, where large MR up to 58% under the field of 3 T is comparable to the those in some polycrystalline alloys under the field of 5 T or 7 T. Such large MR should be attributed to coarse grains and highly preferred orientation, which may greatly reduce the resistance of field induced inverse martensitic transformation. From a technological point of view, there is a great interest in the development of high performance polycrystalline alloys, since they are easier to produce and therefore of lower cost. It is shown that the microstructure control through directional solidification could be an efficient processing route towards the property optimization of polycrystalline NiMn-based alloys.
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Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Research articles
Large magnetoresistance in a directionally solidified Ni44.5Co5.1Mn37.1In13.3 magnetic shape memory alloy Zongbin Li a,⇑, Wei Hu a, Fenghua Chen b,⇑, Mingang Zhang b, Zhenzhuang Li a, Bo Yang a, Xiang Zhao a, Liang Zuo a,c a b c
Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), School of Material Science and Engineering, Northeastern University, Shenyang 110819, China Institute of Advanced Materials, School of Materials Science and Engineering, Taiyuan University of Science and Technology, Taiyuan 030024, China Taiyuan University of Science and Technology, Taiyuan 030024, China
a r t i c l e
i n f o
Article history: Received 19 October 2017 Received in revised form 12 December 2017 Accepted 27 December 2017 Available online 27 December 2017 Keywords: Shape memory materials Magnetic materials Texture Magnetostructural transformation Magnetoresistance
a b s t r a c t Polycrystalline Ni44.5Co5.1Mn37.1In13.3 alloy with coarse columnar-shaped grains and h0 0 1iA preferred orientation was prepared by directional solidification. Due to the strong magnetostructural coupling, inverse martensitic transformation can be induced by the magnetic field, resulting in large negative magnetoresistance up to 58% under the field of 3 T. Such significant field controlled functional behaviors should be attributed to the coarse grains and strong preferred orientation in the directionally solidified alloy. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Since a 3% magnetic-field-induced strain was reported in a Ni45Co5Mn36.7In13.3 single crystal [1], Ni-Mn-In based alloys have attracted considerable attention. In these alloys, the martensitic transformation involves not only the structural changes but also the magnetization variations, i.e., from ferromagnetic austenite to weak magnetic martensite. Accordingly, the martensitic transformation temperatures of these alloys can be greatly reduced by an external magnetic field, which enables the occurrence of field induced inverse martensitic transformation [1,2]. Associated with such a field induced transformation, some remarkable field controlled functional behaviors, such as magnetic shape memory effect [1,2], magnetocaloric effect (MCE) [3–6], and magnetoresistance (MR) [7–9], were developed, which is promising for the technical applications in novel magnetic actuation, refrigeration and sensoring. Yu et al. firstly reported the MR in Ni-Mn-In alloys and a giant MR over 80% was achieved in a Ni50Mn34In16 single crystal under the magnetic field of 5 T [8]. Although the single crystal alloys can exhibit superior field controlled functional behaviours, the relatively higher cost for the fabrication of single crystals becomes ⇑ Corresponding authors. E-mail addresses: [email protected] (Z. Li), [email protected] (F. Chen). https://doi.org/10.1016/j.jmmm.2017.12.093 0304-8853/Ó 2017 Elsevier B.V. All rights reserved.
an unavoidable hindrance for practical applications. Alternatively, the preparation of polycrystalline alloys is much easier and of lower cost. Thus, more attention was paid to the polycrystalline alloys. It has been reported that large MR of 64% and 56% can be achieved under the field of 5 T in polycrystalline Ni50Mn34In16 and Ni50Mn35In15 alloy [9,10], respectively. In addition, the substitution elements were introduced in order to improve the MR in polycrystalline alloys. It is found that in Fe doped Ni50Mn37xFexIn13 (x = 2–4) polycrystalline alloys, the MR increases with the increase of Fe content and the maximum MR reaches 57% for the field of 5 T [11]. In a Ni48.4Co1.9Mn34.2In13.8Ga1.7 polycrystalline alloy, a large MR of 66% can be obtained with the application of the field of 7 T [12]. Generally speaking, the MR properties in polycrystalline alloys are relatively lower with respect to those of single crystals. This could be due to the deteriorated preferential crystallographic orientation as well as the introduction of large amounts of grain boundaries in polycrystalline alloys. Microstructure control through texturation could be a practicable strategy to realize property optimization in polycrystalline alloys. Recent investigations have illustrated that the highly textured microstructure in Ni-Mn-based polycrystalline alloys contributes a lot to the improvement of field controlled performance [13– 17]. Liu et al. obtained a 0.25% magnetostrain through field induced inverse martensitic transformation in a highly textured Ni45.2Mn36.7In13Co5.1 alloy [15]. Gaitzsch et al., reported a 1%
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magnetostrain through field induced variant reorientation in a textured Ni50Mn29Ga21 alloy prepared by directional solidification [16]. Mcleod et al., found that the magnetic entropy change (DSM) can be increased by 84% in a Ni54Mn21Ga25 alloy due to the strong texture induced by thermo-mechanical training [17]. Our recent investigations have shown that the adiabatic temperature variation (DTad) induced by magnetic field in directionally solidified Ni-Mn-Ga alloys can be significantly improved due to the formation of strong texture [13,14]. The enhanced field controlled performance in polycrystalline alloys with strong preferred orientation should be attributed to the optimization of crystallographic and magnetocrystalline anisotropy through texturation, which enables the polycrystalline alloys more close to the single crystal. It is worth mentioning that the directional solidification is an efficient way to optimize the microstructure of polycrystalline alloys, by which the alloys usually form the coarse columnar grains with highly preferential orientation along the growth direction [13,14,18,19]. In this work, a polycrystalline Ni44.5Co5.1Mn37.1In13.3 alloy with h001iA preferred orientation was prepared by directional solidification. Owing to the strong magnetostructural coupling, the field induced inverse martensitic transformation was evidenced, resulting in a large negative MR up to 58% under the field of 3 T.
2. Experimental The master Ni-Co-Mn-In polycrystalline alloy was prepared by arc-melting under argon protection. To achieve a good composition homogeneity, the as-cast alloy was melted for four times. After that, the alloy was suctioned into a copper mould to obtain a cylindrical rod with the diameter of 10 mm for the subsequent directional solidification. The directional solidification was performed in a Bridgman type apparatus under argon atmosphere, where the cylindrical rod was enveloped in a high-purity corundum tube with the inner diameter of 10.5 mm. A water-cooled cylinder containing liquid Ga-In-Sn metal is used to cool down the alloy. During the experiments, the alloy in the corundum crucible was melted by using resistance heating and then directionally solidified in the Bridgman apparatus by pulling the crucible into the liquid Ga-In-Sn metal cylinder at a growth rate of 50 lm/s. The assolidified alloy was then homogenized at 1173 K for 24 h, followed by quenching into water. Parts of homogenized alloy were ground into powder and then sealed in a vacuum quartz tube, followed by annealing at 873 K for 5 h to remove the internal stress. The composition verification was performed by energy dispersive spectrometry (EDS) and the actual composition was determined to be Ni44.5Co5.1Mn37.1In13.3 (at.%). The crystal structure was analyzed by powder X-ray diffraction (XRD) at room temperature in a PANalytical X’Pert Pro MPD diffractometer. The incomplete pole figures were measured by XRD in a Rigaku SmartLab diffractometer at room temperature on the transverse section of the directionally solidified alloy. The martensitic transformation temperatures were measured by differential scanning calorimetry (DSC) with a heating and cooling rate of 10 K/min. The thermalmagnetic (M-T) curves were measured using a vibrating sample magnetometer VersaLab system with a heating and cooling rate of 3 K/min. The martensitic transformation temperatures were determined by the conventional tangent extrapolation method in DSC and M-T curves. The electrical resistance (bar sample, 10 m m 3 mm 2 mm) under the field were measured by four probe method using VersaLab system [20], where the magnetic field was applied along the length direction (i.e., directional solidification direction). The MR is calculated by MR = [R(H) R(0)]/R(0) 100%, where R(H) and R(0) are the electrical resistance measured with and without the magnetic field, respectively.
3. Results and discussions Fig. 1 displays the powder XRD pattern for the directionally solidified alloy measured at room temperature. The pattern exhibits the characteristic of martensite and it is similar to that of six-layered modulated (6M) martensite in a Ni50Mn36In14 alloy [21], suggesting that the present Ni44.5Co5.1Mn37.1In13.3 alloy may consist of 6M martensite at room temperature. Accordingly, the lattice parameters of 6M martensite were determined to be aM = 4.352 Å, bM = 5.581 Å, cM = 12.98 Å and b = 93.8°, respectively. In addition, the {2 2 0}A diffraction of austenite was also identified in the powder XRD pattern, which may indicate that the martensitic transformation temperature is close to the room temperature. Fig. 2a shows the macroscopic microstructure of the longitudinal section for the directionally solidified alloy. The initial austenite forms coarse columnar-shaped grains with several hundreds of microns in size along the solidification direction. Such microstructural feature should be attributed to the specific thermal gradient during the directional solidification process. To analyze the preferential orientation of the directionally solidified alloy, {1 2 3}M, {1 2 3}M and {0 4 0}M incomplete pole figures of the 6M martensite were measured on the transverse section by XRD, as shown in Fig. 2b. Noted that {1 2 3}M and {1 2 3}M poles are roughly located at the polar angle of 40°, whereas {0 4 0}M poles are located at the centre of the pole figure. According to the lattice correspondence between austenite and 6 M martensite in Ni-Mn-In alloys, {1 2 3}M and {1 2 3}M are originated from {2 2 0}A of austenite and {0 4 0}M from {4 0 0}A [21]. Thus, it can be inferred that the initial austenite possesses the strong preferred orientation with h0 0 1iA parallel to the solidification direction, which is consistent with previous results obtained in directionally solidified alloys [18,19]. Fig. 3 presents the temperature dependence of magnetization (M-T curves) measured under the field of 0.1 T and 3 T. In the figure, the abrupt changes in magnetization should be attributed to the martensitic transformation from ferromagnetic austenite to weak magnetic martensite. Based on the low-field M-T curves (0.1 T), the martensitic transformation temperatures (Ms, Mf, As, Af) were determined to 324 K, 303 K, 316 K, and 337 K, respectively, which is very close to those determined from DSC measurements (inset of Fig. 3), i.e., Ms = 325 K, Mf = 300 K, As = 318 K, Af = 344 K. Recently, it was reported that the martensitic transformation in directionally solidified Ni40.6Co8.5Mn40.9Sn10 and Ni42Co8-
Fig. 1. Powder XRD pattern for the directionally solidified alloy measured at room temperature.
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Fig. 2. (a) Macroscopic microstructure of the longitudinal section for the directionally solidified alloy. (b) {1 2 3}M, {1 2 3}M and {0 4 0}M incomplete pole figures of the 6M martensite measured by XRD (SD: solidification direction).
Fig. 3. M-T curves measured under the field of 0.1 T and 3 T. The inset shows the DSC curves.
Mn38In12 alloy may occur in a step-like manner and cover a broad temperature region, due to chemical segregation along the growth direction [22,23]. It should be noted that there is no step-like endothermic/exothermic peaks around the martensitic transformation temperature region in the present DSC curves and the forward/inverse martensitic transformation occurs in a much narrower temperature region in comparison with that of directionally solidified Ni40.6Co8.5Mn40.9Sn10 and Ni42Co8Mn38In12 alloy [22,23], which suggests a good composition homogeneity in the present directionally solidified alloy.
Under the field of 3 T, the martensitic transformation temperatures are shifted to the lower temperature region, where the As is decreased by 13 K, with a reduction rate of 4.5 K/T. This value is comparable to that in Ni45Co5Mn36.6In13.4 single crystal, i.e., 4.3 K/ T [1]. Such phenomena indicates that the magnetic field induced inverse martensitic transformation can be expected. The reduction of the transformation temperature can be well described by Clausius-Clapeyron equation, i.e., DT/DH DM/DS, where DS and DM stand for the entropy and magnetization difference between austenite and martensite, respectively. Here, DM reaches to 90 Am2/kg under the field of 3 T and DS is determined to be 21.6 J/kg/K from the DSC measurements. Thus, the value of DT/DH can be calculated to be 4.2 K/T for the field of 3 T according to the Clausius-Clapeyron equation, which is quite close to the experimentally observed one (4.5 K/T). Fig. 4a displays the temperature dependence of the electrical resistance (R-T curves) under the field of 0 T and 3 T on heating and cooling processes across martensitic transformation. Under the zero field, the transformation from austenite to martensite is associated with a large increase of electrical resistance, which could be due to the modification of the density of state in the vicinity of the Fermi level through martensitic transformation [8]. The characteristic martensitic transformation temperatures reflected from the electrical resistance measurements are in good agreement with those from low-field M-T curves. Under the field of 3 T, the R-T curves is similar to that of zero field, but the martensitic transformation temperatures shift to the lower temperature region, confirming the occurrence of field induced inverse transformation. Fig. 4b shows the temperature dependence of MR on heating under the field of 3 T, where the MR is calculated as [R(H)-R(0)]/R
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4. Summary Polycrystalline Ni44.5Co5.1Mn37.1In13.3 alloy with coarse columnar-shaped grains and strong h0 0 1iA preferred orientation was prepared by the directional solidification. Due to the strong magnetostructural coupling, the inverse martensitic transformation can be induced by the magnetic field, resulting in large negative MR up to 58% under the field of 3 T. The present MR value is comparable to those in some polycrystalline alloys under the field of 5 T or 7 T. The enhancement of the MR should be attributed to the coarse grains and strong preferential orientation in the directionally solidified alloy. Acknowledgements This work is supported by the National Natural Science Foundation of China (Grants No. 51431005, 51571056, 51771048, 51601033), the 863 Program of China (Grant No. 2015AA034101), the Fundamental Research Funds for the Central Universities of China (Grant No. N160205002), Fund Program for the Scientific Activities of Selected Returned Overseas Professionals in Shanxi Province, Shanxi Scholarship Council of China (Grant No. 2016-092), and Open Project of Key Laboratory for Anisotropy and Texture of Materials in Northeastern University (Grant No. ATM20170003). References
Fig. 4. (a) Temperature dependence of the electrical resistance under 0 T and 3 T. (b) Temperature dependence of MR on heating under the field of 3 T for the directionally solidified alloy and suction casting alloy.
(0). The directionally solidified alloy exhibits large negative MR due to field induced inverse martensitic transformation, where the maximum MR reaches 58% at 318 K. For a comparison, Fig. 4b also presents the MR for the polycrystalline alloy after suction casting. The maximum MR of the suction casting alloy is 40%. Apparently, the MR is greatly enhanced after directional solidification. The present MR value for the directionally solidified alloy is comparable to those in bulk polycrystalline Ni50Mn34In16 alloy ( MR = 64%, 5 T) [9], Ni48.4Co1.9Mn34.2In13.8Ga1.7 alloy (MR = 66%, 7 T) [12], Ni41Co9Mn40Sn10 alloy (MR = 53.8%, 5 T) [24], and Ni41Co9Mn39Sb11 alloy (MR = 60%, 5 T) [25], but with a relatively lower magnetic field. The above results illustrate that the present directionally solidified alloy can demonstrate remarkable magnetoresistance, where large MR up to 58% under the field of 3 T is comparable to the those in some polycrystalline alloys under the field of 5 T or 7 T. Such large MR should be attributed to coarse grains and highly preferred orientation, which may greatly reduce the resistance of field induced inverse martensitic transformation. From a technological point of view, there is a great interest in the development of high performance polycrystalline alloys, since they are easier to produce and therefore of lower cost. It is shown that the microstructure control through directional solidification could be an efficient processing route towards the property optimization of polycrystalline NiMn-based alloys.
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