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Pronounced effects of high-magnetic-field solidification on metamagnetic transition in tetragonal Cu2Sb-type Mn1.8Cu0.2Sb alloy
Journal of Magnetism and Magnetic Materials 442 (2017) 67–71
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Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Research articles
Pronounced effects of high-magnetic-field solidification on metamagnetic transition in tetragonal Cu2Sb-type Mn1.8Cu0.2Sb alloy Xiaoqian Zhou a,c, Hui Zhong a,c, Dedong Yu a,c, Zhenchao Wen b,⇑, Weibin Cui a,c,⇑, Qiang Wang a a
Key Laboratory of Electromagnetic Processing of Materials (Ministry of Education), Northeastern University, Shenyang 110819, China Institute for Materials Research (IMR) and Center for Spintronics Research Network (CSRN), Tohoku University, Sendai 980-8577, Japan c Department of Physics and Chemistry of Materials, School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China b
a r t i c l e
i n f o
Article history: Received 8 April 2017 Received in revised form 19 May 2017 Accepted 13 June 2017 Available online 15 June 2017 Keywords: Metamagnetic phase transformation Magnetocaloric materials Exchange bias High-magnetic-field solidification
a b s t r a c t The tetragonal Cu2Sb-type Mn1.8Cu0.2Sb alloy was prepared by solidification under zero field and high magnetic field of 12 T. No thermal hysteresis was observed. High-magnetic-field (HMF) solidification led into c-axis texture in Mn1.8Cu0.2Sb alloy. Moreover, the severity of metamagnetic transformation in a Mn1.8Cu0.2Sb alloy was steepened and the resultant magnetic entropy change was enhanced. The exchange bias at low temperature was observed. Our work demonstrated the pronounced influences of HMF solidification on the crystallographic orientation, metamagnetic phase transformation and the magnetic exchange couplings. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction In the past few decades, refrigeration based on the magnetocaloric effects (MCEs) with phase transformation had been proposed to be an environmentally friendly and efficient alternative compared with gas-compression-based technology. The MCE was intrinsic to all materials, especially the materials which presented the first-order magnetic transformation (FOMT), in which it was called giant MCE. Two types of FOMT, known as the magnetostructural transformation (MST) and magnetoelatic transformation (MET), had been reported [1]. In the former case, during phase transformation, the crystalline structure and magnetic structure were changed simultaneously. MST had been reported in many materials, such as MnAs [2], Gd5Si2Ge2 [3], Mn-M-Ge materials (M = Co or Ge) [4,5] and Ni-(Co)-Mn-X(X = In, Sn, Sb) alloys [6–8]. As comparison, in MET case, only the magnetic structure was changed while crystallographic symmetry was hardly changed, such as in MnFeP0.45As0.55 [9] and LaFe11.4Si1.6 [10]. Since the energy barrier during MET was much lower than MST, the materials undergoing the MET usually possessed smaller hysteresis. Usually, Mn-based intermetallics possessed large moments per Mn atom [11] and had been attractive due to the low cost and easy ⇑ Corresponding authors at: Key Laboratory of Electromagnetic Processing of Materials (Ministry of Education), Northeastern University, Shenyang 110819, China (W. Cui). E-mail addresses: [email protected] (Z. Wen), cuiweibin@epm. neu.edu.cn (W. Cui). http://dx.doi.org/10.1016/j.jmmm.2017.06.072 0304-8853/Ó 2017 Elsevier B.V. All rights reserved.
processing. Among them, tetragonal Cu2Sb-type Mn2Sb was one candidate [12–14]. In this system, there were two crystallographic non-equivalent MnI and MnII sites with 3.9 lB and 2.1 lB respectively [14], forming two sublattices. Within each sublattice the magnetic moments were parallel with each other but the moments of these two sublattices were antiparallel. Therefore there was a net moment and Mn2Sb was ferrimagnetic (FIM) up to 550 K. The exchange coupling between Mn atoms was sensitive to the interactomic spacing. The lattice expansion or constriction by temperature was not sufficient to trigger the antiferromagnetic (AFM) state in pure Mn2Sb phase. However, if the Mn was replaced by Co, Cr, V, Cu [15–18] or Sb by Sn, Ge and As [19–22], the critical interatomic spacing of Mn atoms was possibly accessible as the temperature decreased. The so-called exchange inversion [23,24] occurred leading to AFM state. Due to the different magnetic ordering in FIM and AFM state, the large entropy changes were expected. Since crystallographic structure was not changed before and after phase transformation, small thermal hysteresis was expected in Mn2Sb system, which was beneficial for applications. Exchange bias (EB) effect was firstly discovered in nanostructured Co/CoO system with Co as FM phase and CoO as AFM phase [25]. Recently, Mn-enriched Ni-Mn-based [26,27] and Fe-Mnbased [28] Heusler alloys also displayed large EB field. The origin of EB effect in Heusler alloy was argued to be the nanoscale AFM phases embedded in the FM matrix [28] or the magnetic glassy state caused by the closely-fought AFM and FM exchange coupling [26,29–31] with the isotropic microstructure in these alloy
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systems. There were seldom reports of EB phenomenon in the single-phase alloys with textured microstructure. To gain the texture, HMF solidification with a slow cooling rate was a promising approach. High density with texture was reported in the fissile Si-vacant CoMnSi alloy [32] and Ni-Mn-Ga alloy [33]. In Tb-DyFe alloys, along the preferred crystallographic direction caused by HMF solidification, enhanced magnetostriction was reported [34,35]. These results indicated that magnetic properties could be enhanced through crystallographic orientation caused by HMF solidification. On the other hand, in the phase transformation of FIM or FM state to AFM state, close-fought parallel/antiparallel coupling and magnetic moment geometry would be affected by HMF. By cooling through critical temperature under HMF, the antiparallel geometry due to AFM exchange coupling would be unstable and the magnetic disorder possibly leads into the unique magnetic behaviors. Since Cu substitution was reported to trigger the metamagnetic transition [18], in this work, Mn1.8Cu0.2Sb alloy were prepared by solidification under zero field (ZF) and HMF of 12 T. By studying their metamagnetic transition, the steepened phase transition and enhanced magnetocaloric effect had been found. The exchange bias was also observed in the HMFsolidified alloy. These results demonstrated the drastic influences of HMF solidification on metamagnetic phase transition and manipulation on exchange coupling. 2. Experimental Mn1.8Cu0.2Sb alloy were prepared by conventionally arcmelting the appropriate raw materials with purity higher than 99.9% in water-cooled crucible under argon atmosphere. The ascast alloys were then put into a sapphire crucible, heated up to 1373 K in a vacuum of 2 10 4 Pa. After the alloys were totally molten, they were later solidified into cylinder shape under zero field and high magnetic field of 12 T slowly to room temperature respectively. X-ray diffraction with Cu Ka radiation was used to analyze the phase constituent. The c-plane of the cylinder molten alloys was exposed to Cu Ka radiation. The magnetic measurements were carried out by using magnetic property measurement system (MPMS) up to 5 T. The magnetic field used in the magnetic measurement was applied along the c-axis direction of cylinder sample, which was parallel with the direction of magnetic field used in the solidification. The magnetic entropy changes (DS) were calculated by Maxwell relationship, which had been proved to be equivalent with Clausius-Claperyon equation except the spike value on DS-T curve calculated by Maxwell relationship [36–40].
Fig. 1. XRD patterns of zero-field and high-magnetic-field (HMF) solidified Mn1.8Cu0.2Sb alloys under 12 T. The inset figure shows the scheme of X-ray diffraction on HMF-solidified Mn1.8Cu0.2Sb alloy. The a-b-plane of the cylinder ingot is exposed to the X-ray radiation.
3. Results and discussions Fig. 1 compares the XRD patterns of ZF-solidified and HMFsolidified Mn1.8Cu0.2Sb alloy. In the ZF-solidified alloy, the indexed pattern indicates the standard tetragonal Cu2Sb-type crystallographic structure. By HMF solidification under 12 T, the intensities of (0 0 2) peak, (0 0 3) peak and (1 1 3) peak are enhanced and the (0 0 4) peak appears, indicating that HMF solidification lead into the c-axis texture in the Mn1.8Cu0.2Sb alloys. The lattice parameters, a and c, of ZF-solidified Mn1.8Cu0.2Sb alloy are 0.409 nm and 0.655 nm respectively, which are consistent with those reported in Ref. [18]. By solidification under 12 T, the lattice parameters of the alloy are hardly changed. Fig. 2a compares the isofield M-T curves in cooling and heating process under 0.05 T. In ZF-solidified alloy, a low magnetization is observed at 100 K. With increasing temperature, the magnetization is increased very slowly, indicating a weak transition from AFM state to FIM state. In the cooling process, the magnetization is maximized near 210 K, where the irreversibility is observed. By further
Fig. 2. Isofield thermal magnetizations at (a) 0.05 T and (b) 3 T for zero-field (ZF) and high-magnetic-field (HMF) solidified Mn1.8Cu0.2Sb alloys under 12 T. The close curves stands for the heating process and the open ones for the cooling process.
decreasing the temperature, the magnetization in the cooling process is overlapped with that in the heating process, indicating the zero thermal hysteresis. Under 3 T, a similarly slow increment on magnetization is also observed starting from 100 K as seen in
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Fig. 2b. The overlapped magnetizations are observed without thermal hysteresis during the heating/cooling process. The magnetization is also maximized at 210 K, as the case measured under 0.05 T. But the irreversibility which is observed at low field disappears. It indicates that such irreversibility is only dependent on the magnetic process instead of an intrinsic property. Since the alloy gradually transforms from AFM state to FIM state in a heating process, the remnant AFM region will plays the role of pinning sites preventing the motion of domains wall in FIM region, if the external field is small. It maybe leads into the observed irreversibility. As comparison, in the alloy solidified under 12 T, the magnetization measured under 0.05 T starts to increase at 180 K and a maximum value is obtained near 200 K, showing a narrowed transition temperature span, which is consistent with the result measured under 3 T. The isothermal M-H curves of the ZF-solidified and HMFsolidified alloys are compared in Fig. 3. In the ZF-solidified alloy (see Fig. 3a), the linear magnetization process is observed when the temperature is lower than 190 K. When the temperature is
increased to be between 198 K and 214 K, the M-H curves is slightly deviated from a linear tendency, showing the very weak step-like magnetization increment. It means that the metamagnetic transition is very sluggish and difficult to be triggered by the external field. When the temperature is higher than 214 K, the magnetization is saturated after a linear increment, showing the typical FIM state. As comparison, in the HMF-solidified alloy, when the temperature is between 190 K and 202 K, the clear step-like behaviors are observed with sharper increment of magnetization. Since obviously different metamagnetization processes are observed in the alloys prepared by different condition, the temperature-dependent DS-T under the field change of 5 T are plotted in Fig. 4. In ZF-solidified alloy, the maximum of DS, DSmax, is only 2.7 Jkg 1K 1. However, due to the solidification under 12 T, the DSmax is substantially enhanced to 4.7 Jkg 1K. 1 The temperature-dependent critical field, l0Hcri, is often used to describe the metamagnetic transition. l0Hcri is defined as the field at which the metamagnetic transition is triggered at the given temperature. The slope of l0Hcri-T curve in ZF-solidified alloy is 0.23 T/K but steepened to 0.29 T/K in the HMF-solidified alloy. According to Clausius-Claperyon equation, DS is proportional to the slope of l0Hcri-T curve. Therefore, the enhanced DSM due to HMF solidification is partially originated from the increased temperature sensitivity of l0Hcri. It is known that l0Hcri is strongly related with the competition between AFM coupling and FM coupling. The varied slope of l0HcriT curve suggests the manipulation of HMF solidification on the exchange coupling. To further study such influence, the hysteresis loop measured at 10 K is shown in Fig. 5a in the HMF-solidified alloy cooled down under an external field of 5 T through metamagnetic transition temperature. A large exchange bias field (l0Hex) of 0.045 T is observed with the vertical shift. With increasing temperature, l0Hex is gradually decreased and finally becomes zero at 120 K as shown in Fig. 5b. It suggests that the blocking temperature is around 120 K, which is lower than the corresponding metamagnetic transformation temperature. The cooling-fielddependent l0Hex is shown in Fig. 5c. A l0Hex of 0.02 T is obtained
Fig. 3. The isothermal magnetization curves of (a) zero-field solidified and (b) highmagnetic-field solidified Mn1.8Cu0.2Sb alloys under 12 T.
Fig. 4. The magnetic entropy changes (DS) under the field change of 5 T in zerofield (ZF) and high-magnetic-field (HMF) solidified Mn1.8Cu0.2Sb alloys under 12 T.
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The demagnetization process in these regions will be pinned by the AFM matrix, leading to the observed EB phenomenon. Higher cooling field induces more regions with nonzero magnetization and therefore larger l0Hex is obtained. With increasing temperature, the local magnetic glassy state becomes unfrozen due to thermal distribution, leading to the decreased l0Hex. Therefore, the observed exchange bias further evidences that HMF strongly affects the magnetic phase transformation not only on microstructure but also on the exchange couplings. 4. Conclusions In summary, Mn1.8Cu0.2Sb alloy had been prepared by solidification under zero field and high magnetic field of 12 T. The texture was obtained by solidification under 12 T. No thermal hysteresis were observed in these alloys. The sluggish metamagnetic phase transition in Mn1.8Cu0.2Sb alloy had been steepened by solidification under 12 T as observed from increased temperature sensitivity of l0Hcri. The resultant magnetocaloric effect was enhanced. The exchange bias effect is also observed in the alloy solidified under 12 T. These results demonstrated the pronounced influences of high-magnetic-field solidification on metamagnetic transition and exchange couplings in Cu2Sb-type Mn1.8Cu0.2Sb alloy. Acknowledgements This work was supported by National Natural Science Foundation of China under grant No. 51501033, National Natural Science Funds for Distinguished Young Scholar under grant No. 51425401 and Fundamental Research Funds for the Central Universities under grant No. 140901001. W. B. Cui would also like to appreciate the International Collaboration Center of Institute for Materials Research (ICC-IMR), Tohoku University for supporting the visiting scholarship. Fig. 5. (a) Enlarged hysteresis loop near zero field of Mn1.8Cu0.2Sb alloy solidified under high magnetic field (HMF) of 12 T. The hysteresis loop was measured at 10 K by cooling down the HMF-solidified alloy under external field of 5 T. The inset figure shows the whole hysteresis loop. (b) The temperature-dependent exchangebias field (l0Hex) measured by cooling the HMF-solidified alloy through metamagnetic transition temperature under 5 T. (c) The cooling-field-dependent exchangebias field (l0Hex) measured at 5 K for the HMF-solidified alloy cooled down under 5 T.
at 10 K by cooling under an external field of 0.5 T. By increasing the external cooling field, the l0Hex is also increased. A linear increment is observed when the external cooling field is higher than 1 T. Note that there have been many reports on the exchange bias in the Mn-based intermetallics, such as Mn-Ni-based Heusler alloys [26,28,29]. But exchange bias phenomenon was seldom reported in Mn2Sb system. In Mn-Ni-based Heusler alloys, the coexistent and interacting AFM/FM phases led into the observed exchange bias [26,28,29]. However, since Mn1.8Cu0.2Sb alloy crystallizes in tetragonal Cu2Sb-type structure and a linear magnetization behavior is observed in Fig. 5a. The single-phase AFM state is verified both magnetically and microstructurally. By solidification under HMF, the solid alloy is paramagnetic. It is thought that all Mn moments are not as thoroughly disordered due to HMF. After the alloy is gradually cooling down into FIM state from paramagnetic state, for the Mn atoms which are adjacent to Cu atoms, their moments are inclined to be along external field leading to the locally magnetic disorder, which is only magnetic inhomogeneity instead of forming impurity phase. At this moment the remnant matrix is still FIM state. The matrix will transit into AFM state by further cooling down through metamagnetic phase transition temperature. The local regions surrounding the Cu atoms possess the net non-zero magnetization and are parasitic in the AFM matrix.
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Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Research articles
Pronounced effects of high-magnetic-field solidification on metamagnetic transition in tetragonal Cu2Sb-type Mn1.8Cu0.2Sb alloy Xiaoqian Zhou a,c, Hui Zhong a,c, Dedong Yu a,c, Zhenchao Wen b,⇑, Weibin Cui a,c,⇑, Qiang Wang a a
Key Laboratory of Electromagnetic Processing of Materials (Ministry of Education), Northeastern University, Shenyang 110819, China Institute for Materials Research (IMR) and Center for Spintronics Research Network (CSRN), Tohoku University, Sendai 980-8577, Japan c Department of Physics and Chemistry of Materials, School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China b
a r t i c l e
i n f o
Article history: Received 8 April 2017 Received in revised form 19 May 2017 Accepted 13 June 2017 Available online 15 June 2017 Keywords: Metamagnetic phase transformation Magnetocaloric materials Exchange bias High-magnetic-field solidification
a b s t r a c t The tetragonal Cu2Sb-type Mn1.8Cu0.2Sb alloy was prepared by solidification under zero field and high magnetic field of 12 T. No thermal hysteresis was observed. High-magnetic-field (HMF) solidification led into c-axis texture in Mn1.8Cu0.2Sb alloy. Moreover, the severity of metamagnetic transformation in a Mn1.8Cu0.2Sb alloy was steepened and the resultant magnetic entropy change was enhanced. The exchange bias at low temperature was observed. Our work demonstrated the pronounced influences of HMF solidification on the crystallographic orientation, metamagnetic phase transformation and the magnetic exchange couplings. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction In the past few decades, refrigeration based on the magnetocaloric effects (MCEs) with phase transformation had been proposed to be an environmentally friendly and efficient alternative compared with gas-compression-based technology. The MCE was intrinsic to all materials, especially the materials which presented the first-order magnetic transformation (FOMT), in which it was called giant MCE. Two types of FOMT, known as the magnetostructural transformation (MST) and magnetoelatic transformation (MET), had been reported [1]. In the former case, during phase transformation, the crystalline structure and magnetic structure were changed simultaneously. MST had been reported in many materials, such as MnAs [2], Gd5Si2Ge2 [3], Mn-M-Ge materials (M = Co or Ge) [4,5] and Ni-(Co)-Mn-X(X = In, Sn, Sb) alloys [6–8]. As comparison, in MET case, only the magnetic structure was changed while crystallographic symmetry was hardly changed, such as in MnFeP0.45As0.55 [9] and LaFe11.4Si1.6 [10]. Since the energy barrier during MET was much lower than MST, the materials undergoing the MET usually possessed smaller hysteresis. Usually, Mn-based intermetallics possessed large moments per Mn atom [11] and had been attractive due to the low cost and easy ⇑ Corresponding authors at: Key Laboratory of Electromagnetic Processing of Materials (Ministry of Education), Northeastern University, Shenyang 110819, China (W. Cui). E-mail addresses: [email protected] (Z. Wen), cuiweibin@epm. neu.edu.cn (W. Cui). http://dx.doi.org/10.1016/j.jmmm.2017.06.072 0304-8853/Ó 2017 Elsevier B.V. All rights reserved.
processing. Among them, tetragonal Cu2Sb-type Mn2Sb was one candidate [12–14]. In this system, there were two crystallographic non-equivalent MnI and MnII sites with 3.9 lB and 2.1 lB respectively [14], forming two sublattices. Within each sublattice the magnetic moments were parallel with each other but the moments of these two sublattices were antiparallel. Therefore there was a net moment and Mn2Sb was ferrimagnetic (FIM) up to 550 K. The exchange coupling between Mn atoms was sensitive to the interactomic spacing. The lattice expansion or constriction by temperature was not sufficient to trigger the antiferromagnetic (AFM) state in pure Mn2Sb phase. However, if the Mn was replaced by Co, Cr, V, Cu [15–18] or Sb by Sn, Ge and As [19–22], the critical interatomic spacing of Mn atoms was possibly accessible as the temperature decreased. The so-called exchange inversion [23,24] occurred leading to AFM state. Due to the different magnetic ordering in FIM and AFM state, the large entropy changes were expected. Since crystallographic structure was not changed before and after phase transformation, small thermal hysteresis was expected in Mn2Sb system, which was beneficial for applications. Exchange bias (EB) effect was firstly discovered in nanostructured Co/CoO system with Co as FM phase and CoO as AFM phase [25]. Recently, Mn-enriched Ni-Mn-based [26,27] and Fe-Mnbased [28] Heusler alloys also displayed large EB field. The origin of EB effect in Heusler alloy was argued to be the nanoscale AFM phases embedded in the FM matrix [28] or the magnetic glassy state caused by the closely-fought AFM and FM exchange coupling [26,29–31] with the isotropic microstructure in these alloy
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systems. There were seldom reports of EB phenomenon in the single-phase alloys with textured microstructure. To gain the texture, HMF solidification with a slow cooling rate was a promising approach. High density with texture was reported in the fissile Si-vacant CoMnSi alloy [32] and Ni-Mn-Ga alloy [33]. In Tb-DyFe alloys, along the preferred crystallographic direction caused by HMF solidification, enhanced magnetostriction was reported [34,35]. These results indicated that magnetic properties could be enhanced through crystallographic orientation caused by HMF solidification. On the other hand, in the phase transformation of FIM or FM state to AFM state, close-fought parallel/antiparallel coupling and magnetic moment geometry would be affected by HMF. By cooling through critical temperature under HMF, the antiparallel geometry due to AFM exchange coupling would be unstable and the magnetic disorder possibly leads into the unique magnetic behaviors. Since Cu substitution was reported to trigger the metamagnetic transition [18], in this work, Mn1.8Cu0.2Sb alloy were prepared by solidification under zero field (ZF) and HMF of 12 T. By studying their metamagnetic transition, the steepened phase transition and enhanced magnetocaloric effect had been found. The exchange bias was also observed in the HMFsolidified alloy. These results demonstrated the drastic influences of HMF solidification on metamagnetic phase transition and manipulation on exchange coupling. 2. Experimental Mn1.8Cu0.2Sb alloy were prepared by conventionally arcmelting the appropriate raw materials with purity higher than 99.9% in water-cooled crucible under argon atmosphere. The ascast alloys were then put into a sapphire crucible, heated up to 1373 K in a vacuum of 2 10 4 Pa. After the alloys were totally molten, they were later solidified into cylinder shape under zero field and high magnetic field of 12 T slowly to room temperature respectively. X-ray diffraction with Cu Ka radiation was used to analyze the phase constituent. The c-plane of the cylinder molten alloys was exposed to Cu Ka radiation. The magnetic measurements were carried out by using magnetic property measurement system (MPMS) up to 5 T. The magnetic field used in the magnetic measurement was applied along the c-axis direction of cylinder sample, which was parallel with the direction of magnetic field used in the solidification. The magnetic entropy changes (DS) were calculated by Maxwell relationship, which had been proved to be equivalent with Clausius-Claperyon equation except the spike value on DS-T curve calculated by Maxwell relationship [36–40].
Fig. 1. XRD patterns of zero-field and high-magnetic-field (HMF) solidified Mn1.8Cu0.2Sb alloys under 12 T. The inset figure shows the scheme of X-ray diffraction on HMF-solidified Mn1.8Cu0.2Sb alloy. The a-b-plane of the cylinder ingot is exposed to the X-ray radiation.
3. Results and discussions Fig. 1 compares the XRD patterns of ZF-solidified and HMFsolidified Mn1.8Cu0.2Sb alloy. In the ZF-solidified alloy, the indexed pattern indicates the standard tetragonal Cu2Sb-type crystallographic structure. By HMF solidification under 12 T, the intensities of (0 0 2) peak, (0 0 3) peak and (1 1 3) peak are enhanced and the (0 0 4) peak appears, indicating that HMF solidification lead into the c-axis texture in the Mn1.8Cu0.2Sb alloys. The lattice parameters, a and c, of ZF-solidified Mn1.8Cu0.2Sb alloy are 0.409 nm and 0.655 nm respectively, which are consistent with those reported in Ref. [18]. By solidification under 12 T, the lattice parameters of the alloy are hardly changed. Fig. 2a compares the isofield M-T curves in cooling and heating process under 0.05 T. In ZF-solidified alloy, a low magnetization is observed at 100 K. With increasing temperature, the magnetization is increased very slowly, indicating a weak transition from AFM state to FIM state. In the cooling process, the magnetization is maximized near 210 K, where the irreversibility is observed. By further
Fig. 2. Isofield thermal magnetizations at (a) 0.05 T and (b) 3 T for zero-field (ZF) and high-magnetic-field (HMF) solidified Mn1.8Cu0.2Sb alloys under 12 T. The close curves stands for the heating process and the open ones for the cooling process.
decreasing the temperature, the magnetization in the cooling process is overlapped with that in the heating process, indicating the zero thermal hysteresis. Under 3 T, a similarly slow increment on magnetization is also observed starting from 100 K as seen in
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Fig. 2b. The overlapped magnetizations are observed without thermal hysteresis during the heating/cooling process. The magnetization is also maximized at 210 K, as the case measured under 0.05 T. But the irreversibility which is observed at low field disappears. It indicates that such irreversibility is only dependent on the magnetic process instead of an intrinsic property. Since the alloy gradually transforms from AFM state to FIM state in a heating process, the remnant AFM region will plays the role of pinning sites preventing the motion of domains wall in FIM region, if the external field is small. It maybe leads into the observed irreversibility. As comparison, in the alloy solidified under 12 T, the magnetization measured under 0.05 T starts to increase at 180 K and a maximum value is obtained near 200 K, showing a narrowed transition temperature span, which is consistent with the result measured under 3 T. The isothermal M-H curves of the ZF-solidified and HMFsolidified alloys are compared in Fig. 3. In the ZF-solidified alloy (see Fig. 3a), the linear magnetization process is observed when the temperature is lower than 190 K. When the temperature is
increased to be between 198 K and 214 K, the M-H curves is slightly deviated from a linear tendency, showing the very weak step-like magnetization increment. It means that the metamagnetic transition is very sluggish and difficult to be triggered by the external field. When the temperature is higher than 214 K, the magnetization is saturated after a linear increment, showing the typical FIM state. As comparison, in the HMF-solidified alloy, when the temperature is between 190 K and 202 K, the clear step-like behaviors are observed with sharper increment of magnetization. Since obviously different metamagnetization processes are observed in the alloys prepared by different condition, the temperature-dependent DS-T under the field change of 5 T are plotted in Fig. 4. In ZF-solidified alloy, the maximum of DS, DSmax, is only 2.7 Jkg 1K 1. However, due to the solidification under 12 T, the DSmax is substantially enhanced to 4.7 Jkg 1K. 1 The temperature-dependent critical field, l0Hcri, is often used to describe the metamagnetic transition. l0Hcri is defined as the field at which the metamagnetic transition is triggered at the given temperature. The slope of l0Hcri-T curve in ZF-solidified alloy is 0.23 T/K but steepened to 0.29 T/K in the HMF-solidified alloy. According to Clausius-Claperyon equation, DS is proportional to the slope of l0Hcri-T curve. Therefore, the enhanced DSM due to HMF solidification is partially originated from the increased temperature sensitivity of l0Hcri. It is known that l0Hcri is strongly related with the competition between AFM coupling and FM coupling. The varied slope of l0HcriT curve suggests the manipulation of HMF solidification on the exchange coupling. To further study such influence, the hysteresis loop measured at 10 K is shown in Fig. 5a in the HMF-solidified alloy cooled down under an external field of 5 T through metamagnetic transition temperature. A large exchange bias field (l0Hex) of 0.045 T is observed with the vertical shift. With increasing temperature, l0Hex is gradually decreased and finally becomes zero at 120 K as shown in Fig. 5b. It suggests that the blocking temperature is around 120 K, which is lower than the corresponding metamagnetic transformation temperature. The cooling-fielddependent l0Hex is shown in Fig. 5c. A l0Hex of 0.02 T is obtained
Fig. 3. The isothermal magnetization curves of (a) zero-field solidified and (b) highmagnetic-field solidified Mn1.8Cu0.2Sb alloys under 12 T.
Fig. 4. The magnetic entropy changes (DS) under the field change of 5 T in zerofield (ZF) and high-magnetic-field (HMF) solidified Mn1.8Cu0.2Sb alloys under 12 T.
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The demagnetization process in these regions will be pinned by the AFM matrix, leading to the observed EB phenomenon. Higher cooling field induces more regions with nonzero magnetization and therefore larger l0Hex is obtained. With increasing temperature, the local magnetic glassy state becomes unfrozen due to thermal distribution, leading to the decreased l0Hex. Therefore, the observed exchange bias further evidences that HMF strongly affects the magnetic phase transformation not only on microstructure but also on the exchange couplings. 4. Conclusions In summary, Mn1.8Cu0.2Sb alloy had been prepared by solidification under zero field and high magnetic field of 12 T. The texture was obtained by solidification under 12 T. No thermal hysteresis were observed in these alloys. The sluggish metamagnetic phase transition in Mn1.8Cu0.2Sb alloy had been steepened by solidification under 12 T as observed from increased temperature sensitivity of l0Hcri. The resultant magnetocaloric effect was enhanced. The exchange bias effect is also observed in the alloy solidified under 12 T. These results demonstrated the pronounced influences of high-magnetic-field solidification on metamagnetic transition and exchange couplings in Cu2Sb-type Mn1.8Cu0.2Sb alloy. Acknowledgements This work was supported by National Natural Science Foundation of China under grant No. 51501033, National Natural Science Funds for Distinguished Young Scholar under grant No. 51425401 and Fundamental Research Funds for the Central Universities under grant No. 140901001. W. B. Cui would also like to appreciate the International Collaboration Center of Institute for Materials Research (ICC-IMR), Tohoku University for supporting the visiting scholarship. Fig. 5. (a) Enlarged hysteresis loop near zero field of Mn1.8Cu0.2Sb alloy solidified under high magnetic field (HMF) of 12 T. The hysteresis loop was measured at 10 K by cooling down the HMF-solidified alloy under external field of 5 T. The inset figure shows the whole hysteresis loop. (b) The temperature-dependent exchangebias field (l0Hex) measured by cooling the HMF-solidified alloy through metamagnetic transition temperature under 5 T. (c) The cooling-field-dependent exchangebias field (l0Hex) measured at 5 K for the HMF-solidified alloy cooled down under 5 T.
at 10 K by cooling under an external field of 0.5 T. By increasing the external cooling field, the l0Hex is also increased. A linear increment is observed when the external cooling field is higher than 1 T. Note that there have been many reports on the exchange bias in the Mn-based intermetallics, such as Mn-Ni-based Heusler alloys [26,28,29]. But exchange bias phenomenon was seldom reported in Mn2Sb system. In Mn-Ni-based Heusler alloys, the coexistent and interacting AFM/FM phases led into the observed exchange bias [26,28,29]. However, since Mn1.8Cu0.2Sb alloy crystallizes in tetragonal Cu2Sb-type structure and a linear magnetization behavior is observed in Fig. 5a. The single-phase AFM state is verified both magnetically and microstructurally. By solidification under HMF, the solid alloy is paramagnetic. It is thought that all Mn moments are not as thoroughly disordered due to HMF. After the alloy is gradually cooling down into FIM state from paramagnetic state, for the Mn atoms which are adjacent to Cu atoms, their moments are inclined to be along external field leading to the locally magnetic disorder, which is only magnetic inhomogeneity instead of forming impurity phase. At this moment the remnant matrix is still FIM state. The matrix will transit into AFM state by further cooling down through metamagnetic phase transition temperature. The local regions surrounding the Cu atoms possess the net non-zero magnetization and are parasitic in the AFM matrix.
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