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Exploring the details of the martensitic phase transition and magnetocaloric effect of CoMnGe0.95Ga0.05 by synchrotron and magnetic measurements
Journal of Alloys and Compounds 540 (2012) 236–240
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Exploring the details of the martensitic phase transition and magnetocaloric effect of CoMnGe0.95Ga0.05 by synchrotron and magnetic measurements I. Dincer a,⇑, E. Yüzüak a, G. Durak a, Y. Elerman a, A.M.T. Bell b, H. Ehrenberg c a
Department of Engineering Physics, Faculty of Engineering, Ankara University, 06100 Besevler, Ankara, Turkey HASYLAB/DESY, Notkestraße 85, 22607 Hamburg, Germany c Karlsruhe Institute of Technology (KIT), Institute for Applied Materials (IAM), Hermann-von-Helmholtz-Platz 1, D-76344, Eggenstein-Leopoldshafen, Germany b
a r t i c l e
i n f o
Article history: Received 7 March 2012 Received in revised form 11 May 2012 Accepted 12 May 2012 Available online 15 June 2012 Keywords: Magnetocaloric effect Martensitic transition Synchrotron measurements
a b s t r a c t The structural, magnetic and magnetocaloric properties of CoMnGe0.95Ga0.05 have been investigated by using electron microscopy, calorimetric, synchrotron and magnetic measurements. The substitution of Ga for Ge leads to decreasing on the martensitic transition temperature from 650 K to 315 K. CoMnGe0.95Ga0.05 has hexagonal structure (space group P63/mmc) above the martensitic transition temperature and orthorhombic structure (space group Pnma) below this temperature. The magnetic field dependence of magnetization measurements are performed in the heating and cooling processes around the martensitic transition temperature to determine magnetocaloric effect. It is observed that the magnetic entropy change associated with the martensitic transition temperature can be as high as 5.2 J kg1 K1 in field of 1 T. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Recently, the global warming has become an international issues and it is needed to reduce the emission of the greenhouse gases. The use of chlorofluorocarbons (CFCs) in refrigerators is restricted because CFCs destroy the ozone layer. Instead of CFCs, hydrofluorocarbons (HFCs) are used as a refrigerant. However, last studies reveals that HFCs have large green house effect. In order to be reduced the HFCs emission, it is expected to develop an alternative new refrigerant and environmentally friendly refrigeration system. The interest in the research of magnetic refrigeration has been considerably enhanced owing to its potential impact on energy savings and environmental concerns during last two decades [1]. Magnetic refrigeration should be ideal alternative cooling technology because magnetic refrigeration does not use ozone-depleting chemicals (such as CFCs), hazardous chemicals (such as ammonia), or greenhouse gases. The other important advantage of magnetic refrigeration is the amount of energy loss incurred during the refrigeration cycle. The cooling efficiency of magnetic refrigerators working with Gd has been shown to reach 60% of the theoretical limit, compared to only about 40% in the best gascompression refrigerators [2]. The use of magnetic refrigerators with such high energy efficiency will result in a reduced consumption of fossil fuels, in this way contributing to a reduced CO2 and CO releases. ⇑ Corresponding author. Tel.: +90 5055952203; fax: +90 3122127343. E-mail address: [email protected] (I. Dincer). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.05.072
As well known, magnetic refrigeration is based on the magnetocaloric effect (MCE) which is the isothermal magnetic entropy change or the adiabatic temperature change in magnetic material when it is subjected to a change in external magnetic field. Pescharsky et al. discuss the thermodynamics of the MCE [3]. While the MCE is observed in all magnetic materials near magnetic transition temperatures, the giant MCE is observed in the materials undergoing first order magnetostructural transition [3]. Nowadays, most studies on magnetocaloric materials are focused on materials undergoing a first order magnetic phase transition because of their potential applications at room temperature [3–7]. One of materials showed the first order phase transition is the Co–Mn–Ge family. According to earlier studies on CoMnGe, this alloy shows a martensitic structural transformation from the low-temperature orthorhombic TiNiSi-type structure (space group Pnma) to the high-temperature hexagonal Ni2In-type structure (space group P63/mmc) [8–10]. This martensitic transformation occurs at the structural transition temperature at around 650 K [9]. There are so many methods to decrease the structural transition temperature to room temperature from high temperature and tune magnetic properties of CoMnGe because magnetic properties are very sensitive to interatomic distances. These are as follows: (1) Doping interstitial atoms such as CoMnGeYx (Y = B or C) [14], (2) substitution of Mn for other 3d transition element such as CoMn1xTxGe (T:3d transition elements) [15,16], (3) substitution of Ge for other elements such as CoMnGe1xZx (Z:Sn or P) [17,18], (4) vacancy on Co or Mn sublattices such as Co1xMnGe or CoMn1xGe [19–22], (5) applying hydrostatic pressure [16,22]. Since there is no any
I. Dincer et al. / Journal of Alloys and Compounds 540 (2012) 236–240
published paper about the structural properties of CoMnGa, we produced and performed structural characterization of this alloy. According to our results (unpublished data), the CoMnGa alloy crystallizes in hexagonal structure (space group P63/mmc). We think that there is a possibility that CoMnGe1xGax alloys could exhibit a combined, magnetostructural transition if structural and magnetic phase transformation temperatures can be tuned together by the Ga substitution for Ge as observed in CoMnGe1xZx (Z:Sn or P) alloys [17,18]. In this article, we present the influence of Ga substitution for Ge on the structural, magnetic and magnetocaloric properties of CoMnGe by using electron microscopy, calorimetric, synchrotron and magnetic measurements. 2. Experimental The polycrystalline CoMnGe0.95Ga0.05 of about 2 g was prepared by arc-melting appropriate amounts of the high purity constituent elements (Co 99.9%, Mn 99.9%, Ge 99.9999% and Ga 99.9999%) under Ar atmosphere in a water cooled copper crucible. The polycrystalline CoMnGe0.95Ga0.05 was then encapsulated under an Ar atmosphere of 500 mbar in quartz glass. In order to obtain a homogeneous phase, this encapsulated sample was annealed for four days at 1023 K and slowly cooled down to room temperature. The composition of the alloy was determined by energy dispersive X-ray analysis (EDX) by using the EVO 40 scanning electron microscope (SEM). Differential scanning calorimetry (DSC) measurements were carried out using TA Instruments Q2000, with an empty aluminum pan as a reference.The DSC measurements were carried out in the heating as well as cooling cycles with a scanning rate 10 K/min in the temperature range between 180 and 470 K. Beamline B2 at the Hamburger Synchrotronstrahlungslabor (Germany) was used for synchrotron powder diffraction in order to determine the crystal structure of the alloy as a function of temperature between 10 and 400 K. The synchrotron powder diffraction experiments were performed for heating and cooling sequence at about 70 different temperatures. Diffraction patterns have been measured using the STOE furnace, the in-house helium cooled cryostat and on-site readable image plate detector OBI [11,12] over the 2h-range between 5° and 70° with a step size of 0.004°. Photons with 0.688105 Å wavelength were selected by a Si(1 1 1) double crystal monochromator. Analysis of the synchrotron powder diffraction patterns was carried out using the FullProf program package [13]. The magnetization measurements were made in a physical properties measurement system PPMS with a magnetic field up to 7 T in the temperature range from 5 to 350 K. Since the M(T) measurements in the different modes give us more information about magnetic properties of the alloys, the temperature dependence of magnetization were measured in zero-field-cooled (ZFC), field-cooled (FC) and field-heated (FH) modes.
3. Results and discussion Firstly we performed EDX analysis by using SEM for CoMnGe0.95Ga0.05, because the structural and magnetic properties depend very strongly on the composition. Fig. 1 shows back-scattered electron image of CoMnGe0.95Ga0.05 after homogenization treatment. According to this image, this alloy has an uniform composition. The average composition calculated from EDX analysis, are found to be 33.5(±0.8)% for Co, 33.47(±0.9)% for Mn, 31.31(±1.1)% for Ge and 1.72(±0.2)% for Ga which corresponds to
Fig. 1. Back-scattered electron image of annealed CoMnGe0.95Ga0.05.
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the actual formula CoMnGe0.95Ga0.05. The reported compositions are an average of multiple points on the alloy and no compositional heterogeneity is observed. The results of the DSC measurements are shown in Fig. 2 for annealed CoMnGe0.95Ga0.05. The difference in the peak temperatures of the heating and cooling curves indicated by the vertical dashed lines in Fig. 2 demonstrates the presence of a hysteresis associated with a first order structural phase transition. The martensitic transformation temperatures of annealed CoMnGe0.95Ga0.05 are determine from DSC curves and seen in Fig. 2. The width of the thermal hysteresis of the martensitic transformation (the difference between TAM and TMA) is 18 K. We performed X-ray powder diffraction experiment for as cast alloy at room temperature to determine its crystal structure. According to Rietveld refinement, the as cast alloy crystallizes in hexagonal structure (space group: P63/mmc). The unit cell parameters and unit cell volume of hexagonal structure are a = b = 4.0934(3) Å, c = 5.3149(5) Å and V = 77.125(5) Å3 at room temperature. In order to find the crystal structures of annealed CoMnGe0.95Ga0.05, the temperature dependence of Synchrotron diffraction experiments were performed on cooling mode from 400 K to 10 K and then on heating mode from 10 K to 400 K. Fig. 3 exhibits the Synchrotron diffraction patterns collected at 400 K, 295 K for cooling, 295 K for heating and 10 K. While we observed the diffraction peaks of hexagonal structure at high temperatures, the orthorhombic diffraction peaks are observed at low temperatures. The crystal structures of this at high and low temperatures are determined with Rietveld refinement by using FullProf package. According to Rietveld refinement results, the crystal structure of annealed CoMnGe0.95Ga0.05 at high temperatures is Ni2In-type hexagonal structure with space group P63/mmc. Its crystal structure is TiNiSi-type orthorhombic structure with space group Pnma at low temperatures. At 400 K, the unit cell parameters and unit cell volume of hexagonal structure are a = b = 4.10066(8) Å, c = 5.34144(9) Å and V = 77.786(3) Å3, respectively. At 10 K, the unit cell parameters and unit cell volume of orthorhombic structure are a = 5.88625(8) Å, b = 3.82806(7) Å, c = 7.06786(9) Å and V = 159.261(15) Å3, respectively. Fig. 3e shows the phase fractions of annealed CoMnGe0.95Ga0.05 as a function temperature. The martensitic transformation temperatures obtained from phase fraction curves are MS = 320 K, Mf = 235 K, AS = 285 K, Af = 330 K that are in good agreement with the DSC results. The temperature dependence of the unit cell volume of hexagonal and orthorhombic structures are shown in Fig. 3f. The volume of hexagonal structure is approximately two times smaller than that of orthorhombic
Fig. 2. DSC curves of annealed CoMnGe0.95Ga0.05 at heating and cooling rates of 10 K/min. MS, martensite start temperature, Mf, martensite finish temperature, AS, austenite start temperature, Af, austenite finish temperature, TAM, transition temperature from austenite to martensite and TMA, transition temperature from martensite to austenite start temperature.
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I. Dincer et al. / Journal of Alloys and Compounds 540 (2012) 236–240
Fig. 3. The Synchrotron diffraction patterns of annealed CoMnGe0.95Ga0.05 (a) at 400 K (b) at 295 K for cooling processes (c) at 295 K for heating processes and (d) at 10 K. The stars show the diffraction peaks from hexagonal structure. (e) Temperature dependence of the phase fraction (vol.%) of the hexagonal and orthorhombic structures with increasing and decreasing temperature. (f) Variation in volume of the hexagonal and orthorhombic structures with increasing and decreasing temperature.
structure. Fig. 4 shows the Rietveld refinement result of the Synchrotron diffraction pattern at 315 K cooled from high temperatures. The R-values of this refinement are Rp = 6.31, Rwp = 8.48 and Rexp = 2.68, respectively. We performed the temperature dependence of magnetization measurements for as cast and annealed CoMnGe0.95Ga0.05 to determine the influence of the heat treatment on the magnetic and magnetocaloric properties of CoMnGe0.95Ga0.05. The MðTÞ curves, measured from 5 to 350 K in a magnetic field of 100 Oe for ZFC, FC and FH modes are displayed in Fig. 5b for the as cast CoMnGe0.95Ga0.05 and (a) for the annealed CoMnGe0.95Ga0.05. The as cast CoMnGe0.95Ga0.05 has ferromagnetic character below TC = 277 K and paramagnetic above this temperature. There is no thermal hysteresis between FC and FH curves (as seen in Fig. 5b). This means that this alloy without heat treatment has no structural transition and DSC measurement confirms this result. According to Fig. 5a, annealed CoMnGe0.95Ga0.05 orders ferromagnetically below Curie temperature and paramagnetically above this temperature. In the vicinity of Curie temperature, an thermal hysteresis between FC and FH curves is observed. Considering the DSC and synchrotron
Fig. 4. The Rietveld refinement result of the Synchrotron diffraction pattern for annealed CoMnGe0.95Ga0.05 at 315 K cooled from high temperatures. The positions of Bragg reflections (bottom lines for hexagonal structure and top line for orthorhombic structure) are marked by vertical bars.
Fig. 5. The MðTÞ curves measured in magnetic field 100 Oe (a) for annealed CoMnGe0.95Ga0.05 and (b) for as cast CoMnGe0.95Ga0.05.
diffraction results together, this thermal hysteresis is attributed to a first order structural transformation. The ZFC curves begin at a low magnetization value due to the essentially random spatial configuration while cooling through TC down to low temperatures. At higher temperatures, the ZFC and FH curves merge. The Curie temperatures of this alloy are obtained from FC and FH curves as T AM ¼ 302 K and T MA ¼ 317 K, respectively. The thermal hystereC C MA sis between T AM and T is about 15 K which are in good agreC C ment with DSC results. Isothermal M H curves of annealed CoMnGe0.95Ga0.05 were measured with using two different methods to find the isothermal magnetic field response of this sample in and around the temperature range of martensitic phase transition. In the first method, the sample was slowly cooled from 350 to 345 K under zero magnetic field. At this temperature, the magnetic field dependence of magnetization was measured from 0 to 7 T and from 7 to 0 T. After the magnetization measurements was completed, temperature was decreased to next temperature. This measurement method is called as cooling process. Fig. 6a show the MðHÞ curves measured with cooling process. In the second method, after zero magnetic field cooling from 350 K to 10 K, sample was slowly heat up to 276 K. At this temperature, the magnetic field dependence of magnetization was measured from 0 to 7 T and from 7 to 0 T. After the magnetization measurements was completed, temperature was
I. Dincer et al. / Journal of Alloys and Compounds 540 (2012) 236–240
S S Fig. 6. The MðHÞ curves of annealed CoMnGe0.95Ga0.05: (a) for cooling process from 350 K and (b) for heating process from 50 K. The temperature dependence of magnetic entropy change of annealed CoMnGe0.95Ga0.05 for different magnetic fields: (c) calculated from cooling MðHÞ curves and (d) calculated from heating MðHÞ curves. The top inset: The MðHÞ curves of as cast CoMnGe0.95Ga0.05. The bottom inset: The temperature dependence of magnetic entropy change of as cast CoMnGe0.95Ga0.05.
increased to next temperature. This measurement method is called as heating process. Fig. 6b show the MðHÞ curves measured with heating process. From 321 K down to 300 K on cooling process and from 306 K up to 324 K on heating process, a metamagnetic transition with hysteresis indicates that a magnetic field induced martensitic transformation occurs, due to the large Zeeman energy between paramagnetic austenite and ferromagnetic martensite phases. Koyama et al. observed the similar field induced martensitic phase transformation at the vicinity of martensitic transformation temperature for Mn1.07Co0.92Ge [10]. Vacancy on Mn sublattice of CoMnGe (such as Mn0.965CoGe) causes also the magnetic field induced martensitic transformation [19]. According to our magnetic field dependence of magnetization measurements up to 2 T, we could not observed metamagnetic transition and magnetic hysteresis. Fig. 7 shows the MðHÞ curves in the magnetic field change of 2 T. As a result, almost zero magnetic hysteresis on MðHÞ curves was observed between magnetization and demagnetization curves at the vicinity of martensitic transformation temperature which is meaningful for the magnetic refrigeration applications. The top inset in Fig. 6 shows the MðHÞ curves of our as cast CoMnGe0.95Ga0.05. MðHÞ curves of this as cast
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alloy confirms that this alloy has only second order phase transition around TC. Using the data in Fig. 6a and b, the magnetic field induced entropy change DS is estimated from Maxwell relation RH ðDS¼0 ð@M=@TÞH dHÞ. DS for annealed CoMnGe0.95Ga0.05 is shown in Fig. 6c and d for cooling and heating process. As shown in this figure, the giant magnetocaloric effect was observed around room temperature due to the great change in magnetization near martensitic transition temperature. The peak values of DS estimated from cooling and heating processes are approximately same. This means that the value of the magnetic entropy change do not depend on the thermo-magnetic history of the alloy, unlike Ni50Mn34In16 [23]. The peak values of the magnetic entropy change are about 5.2, 11.1 and 27.1 Jkg1 K1 for the magnetic field change of 1, 2 and 5 T, respectively. These peak values of magnetic entropy change are greater than those of other magnetocaloric materials in similar fields, e.g. Gd. The bottom inset in Fig. 6 shows DSðTÞ curves of as cast CoMnGe0.95Ga0.05. According to Fig. 6, the reason of the giant MCE in CoMnGe0.95Ga0.05 is the martensitic structural phase transition. The refrigerant capacity (RC) defined RT as RC¼T 1 2 ðDSÞH dT which indicates how much heat can be transferred from the cold end to the hot end of the refrigerator in one thermodynamic cycle. The RC values of annealed CoMnGe0.95Ga0.05 are about 36, 76 and 184 J kg1 for the magnetic field change of 1, 2 and 5 T, respectively. The RC values are calculated here with the method proposed by Gschneidner and co-workers [24]. Our RC value for the magnetic field change of 5 T is comparable with the results of other magnetocaloric materials that show giant MCE at room temperature, such as Gd5Ge2Si2 [25] and Ni50Mn34In16 [26]. 4. Conclusion In conclusion, we have observed that the martensitic phase transformation temperature depends on the heat treatment on CoMnGe0.95Ga0.05. The crystal structures of annealed CoMnGe0.95Ga0.05 have been determined by analyzing the temperature dependence of Synchrotron diffraction patterns in the temperature range between 10 and 400 K. While the crystal structure of this alloy is Ni2In-type hexagonal structure with space group P63/mmc at high temperatures, its crystal structure is TiNiSi-type orthorhombic structure with space group Pnma at low temperatures. The temperature dependent magnetization measurements confirm the results of DSC and Synchrotron diffraction experiments. According to the DSðTÞ curves of as cast and annealed CoMnGe0.95Ga0.05, the main contribution to entropy change should be from structural phase transition. Since the magnetic fields up to 2 T can be generated by permanent magnets, annealed CoMnGe0.95Ga0.05 observed giant MCE should be promising MCE material for magnetic refrigeration technology when compared with the other MCE materials such as, FeRh, Gd, Gd5Ge2Si2, MnAs and Nibased Heusler alloys. The other important advantage of annealed CoMnGe0.95Ga0.05 is the thermomagnetic history independence of the magnetic entropy change. Acknowledgements This work was supported by TUBITAK (Project Number: 109T743). The authors would like to thank DESY-HASYLAB for providing the synchrotron facilities. References
Fig. 7. The MðHÞ curves of annealed CoMnGe0.95Ga0.05 for the magnetic field change of 2 T.
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[16] L. Caron, N.T. Trung, E. Brück, Phys. Rev. B 84 (2011) 020414. [17] J.B.A. Hamer, R. Daou, S. Özcan, N.D. Mathur, D.J. Fray, K.G. Sandeman, J. Magn. Magn. Mater. 321 (2009) 3535. [18] Z. Gercsi, K. Hono, K.G. Sandeman, Phys. Rev. B 83 (2011) 174403. [19] E.K. Liu, W. Zhu, L. Feng, J.L. Chen, W.H. Wang, G.H. Wu, H.Y. Liu, F.B. Meng, H.Z. Luo, Y.X. Li, EPL 91 (2010) 17003. [20] J.T. Wang, D.S. Wang, C. Chen, O. Nashima, T. Kanomata, H. Mizuseki, Y. Kawazoe, Appl. Phys. Lett. 89 (2006) 262504. [21] Y.K. Fang, C.C. Yeh, C.W. Chang, W.C. Chang, M.G. Zhu, W. Li, Scripta Mater. 57 (2007) 453. [22] S. Niziol, A. Zieba, R. Zach, M. Baj, L. Dmowski, J. Magn. Magn. Mater. 38 (1983) 205. [23] M.K. Chattopadhyay, V.K. Sharma, S.B. Roy, Appl. Phys. Lett. 92 (2008) 022503. [24] K.A. Gschneidner Jr., V.K. Pecharsky, A.O. Pecharsky, C.B. Zimm, Mater. Sci. Forum 315-317 (1999) 69. [25] V. Provenzano, A.J. Shapiro, R.D. Shull, Nature 429 (2004) 843. [26] V.K. Sharma, M.K. Chattopadhyay, S.B. Roy, J. Phys. D: Appl. Phys. 40 (2007) 104416.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Exploring the details of the martensitic phase transition and magnetocaloric effect of CoMnGe0.95Ga0.05 by synchrotron and magnetic measurements I. Dincer a,⇑, E. Yüzüak a, G. Durak a, Y. Elerman a, A.M.T. Bell b, H. Ehrenberg c a
Department of Engineering Physics, Faculty of Engineering, Ankara University, 06100 Besevler, Ankara, Turkey HASYLAB/DESY, Notkestraße 85, 22607 Hamburg, Germany c Karlsruhe Institute of Technology (KIT), Institute for Applied Materials (IAM), Hermann-von-Helmholtz-Platz 1, D-76344, Eggenstein-Leopoldshafen, Germany b
a r t i c l e
i n f o
Article history: Received 7 March 2012 Received in revised form 11 May 2012 Accepted 12 May 2012 Available online 15 June 2012 Keywords: Magnetocaloric effect Martensitic transition Synchrotron measurements
a b s t r a c t The structural, magnetic and magnetocaloric properties of CoMnGe0.95Ga0.05 have been investigated by using electron microscopy, calorimetric, synchrotron and magnetic measurements. The substitution of Ga for Ge leads to decreasing on the martensitic transition temperature from 650 K to 315 K. CoMnGe0.95Ga0.05 has hexagonal structure (space group P63/mmc) above the martensitic transition temperature and orthorhombic structure (space group Pnma) below this temperature. The magnetic field dependence of magnetization measurements are performed in the heating and cooling processes around the martensitic transition temperature to determine magnetocaloric effect. It is observed that the magnetic entropy change associated with the martensitic transition temperature can be as high as 5.2 J kg1 K1 in field of 1 T. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Recently, the global warming has become an international issues and it is needed to reduce the emission of the greenhouse gases. The use of chlorofluorocarbons (CFCs) in refrigerators is restricted because CFCs destroy the ozone layer. Instead of CFCs, hydrofluorocarbons (HFCs) are used as a refrigerant. However, last studies reveals that HFCs have large green house effect. In order to be reduced the HFCs emission, it is expected to develop an alternative new refrigerant and environmentally friendly refrigeration system. The interest in the research of magnetic refrigeration has been considerably enhanced owing to its potential impact on energy savings and environmental concerns during last two decades [1]. Magnetic refrigeration should be ideal alternative cooling technology because magnetic refrigeration does not use ozone-depleting chemicals (such as CFCs), hazardous chemicals (such as ammonia), or greenhouse gases. The other important advantage of magnetic refrigeration is the amount of energy loss incurred during the refrigeration cycle. The cooling efficiency of magnetic refrigerators working with Gd has been shown to reach 60% of the theoretical limit, compared to only about 40% in the best gascompression refrigerators [2]. The use of magnetic refrigerators with such high energy efficiency will result in a reduced consumption of fossil fuels, in this way contributing to a reduced CO2 and CO releases. ⇑ Corresponding author. Tel.: +90 5055952203; fax: +90 3122127343. E-mail address: [email protected] (I. Dincer). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.05.072
As well known, magnetic refrigeration is based on the magnetocaloric effect (MCE) which is the isothermal magnetic entropy change or the adiabatic temperature change in magnetic material when it is subjected to a change in external magnetic field. Pescharsky et al. discuss the thermodynamics of the MCE [3]. While the MCE is observed in all magnetic materials near magnetic transition temperatures, the giant MCE is observed in the materials undergoing first order magnetostructural transition [3]. Nowadays, most studies on magnetocaloric materials are focused on materials undergoing a first order magnetic phase transition because of their potential applications at room temperature [3–7]. One of materials showed the first order phase transition is the Co–Mn–Ge family. According to earlier studies on CoMnGe, this alloy shows a martensitic structural transformation from the low-temperature orthorhombic TiNiSi-type structure (space group Pnma) to the high-temperature hexagonal Ni2In-type structure (space group P63/mmc) [8–10]. This martensitic transformation occurs at the structural transition temperature at around 650 K [9]. There are so many methods to decrease the structural transition temperature to room temperature from high temperature and tune magnetic properties of CoMnGe because magnetic properties are very sensitive to interatomic distances. These are as follows: (1) Doping interstitial atoms such as CoMnGeYx (Y = B or C) [14], (2) substitution of Mn for other 3d transition element such as CoMn1xTxGe (T:3d transition elements) [15,16], (3) substitution of Ge for other elements such as CoMnGe1xZx (Z:Sn or P) [17,18], (4) vacancy on Co or Mn sublattices such as Co1xMnGe or CoMn1xGe [19–22], (5) applying hydrostatic pressure [16,22]. Since there is no any
I. Dincer et al. / Journal of Alloys and Compounds 540 (2012) 236–240
published paper about the structural properties of CoMnGa, we produced and performed structural characterization of this alloy. According to our results (unpublished data), the CoMnGa alloy crystallizes in hexagonal structure (space group P63/mmc). We think that there is a possibility that CoMnGe1xGax alloys could exhibit a combined, magnetostructural transition if structural and magnetic phase transformation temperatures can be tuned together by the Ga substitution for Ge as observed in CoMnGe1xZx (Z:Sn or P) alloys [17,18]. In this article, we present the influence of Ga substitution for Ge on the structural, magnetic and magnetocaloric properties of CoMnGe by using electron microscopy, calorimetric, synchrotron and magnetic measurements. 2. Experimental The polycrystalline CoMnGe0.95Ga0.05 of about 2 g was prepared by arc-melting appropriate amounts of the high purity constituent elements (Co 99.9%, Mn 99.9%, Ge 99.9999% and Ga 99.9999%) under Ar atmosphere in a water cooled copper crucible. The polycrystalline CoMnGe0.95Ga0.05 was then encapsulated under an Ar atmosphere of 500 mbar in quartz glass. In order to obtain a homogeneous phase, this encapsulated sample was annealed for four days at 1023 K and slowly cooled down to room temperature. The composition of the alloy was determined by energy dispersive X-ray analysis (EDX) by using the EVO 40 scanning electron microscope (SEM). Differential scanning calorimetry (DSC) measurements were carried out using TA Instruments Q2000, with an empty aluminum pan as a reference.The DSC measurements were carried out in the heating as well as cooling cycles with a scanning rate 10 K/min in the temperature range between 180 and 470 K. Beamline B2 at the Hamburger Synchrotronstrahlungslabor (Germany) was used for synchrotron powder diffraction in order to determine the crystal structure of the alloy as a function of temperature between 10 and 400 K. The synchrotron powder diffraction experiments were performed for heating and cooling sequence at about 70 different temperatures. Diffraction patterns have been measured using the STOE furnace, the in-house helium cooled cryostat and on-site readable image plate detector OBI [11,12] over the 2h-range between 5° and 70° with a step size of 0.004°. Photons with 0.688105 Å wavelength were selected by a Si(1 1 1) double crystal monochromator. Analysis of the synchrotron powder diffraction patterns was carried out using the FullProf program package [13]. The magnetization measurements were made in a physical properties measurement system PPMS with a magnetic field up to 7 T in the temperature range from 5 to 350 K. Since the M(T) measurements in the different modes give us more information about magnetic properties of the alloys, the temperature dependence of magnetization were measured in zero-field-cooled (ZFC), field-cooled (FC) and field-heated (FH) modes.
3. Results and discussion Firstly we performed EDX analysis by using SEM for CoMnGe0.95Ga0.05, because the structural and magnetic properties depend very strongly on the composition. Fig. 1 shows back-scattered electron image of CoMnGe0.95Ga0.05 after homogenization treatment. According to this image, this alloy has an uniform composition. The average composition calculated from EDX analysis, are found to be 33.5(±0.8)% for Co, 33.47(±0.9)% for Mn, 31.31(±1.1)% for Ge and 1.72(±0.2)% for Ga which corresponds to
Fig. 1. Back-scattered electron image of annealed CoMnGe0.95Ga0.05.
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the actual formula CoMnGe0.95Ga0.05. The reported compositions are an average of multiple points on the alloy and no compositional heterogeneity is observed. The results of the DSC measurements are shown in Fig. 2 for annealed CoMnGe0.95Ga0.05. The difference in the peak temperatures of the heating and cooling curves indicated by the vertical dashed lines in Fig. 2 demonstrates the presence of a hysteresis associated with a first order structural phase transition. The martensitic transformation temperatures of annealed CoMnGe0.95Ga0.05 are determine from DSC curves and seen in Fig. 2. The width of the thermal hysteresis of the martensitic transformation (the difference between TAM and TMA) is 18 K. We performed X-ray powder diffraction experiment for as cast alloy at room temperature to determine its crystal structure. According to Rietveld refinement, the as cast alloy crystallizes in hexagonal structure (space group: P63/mmc). The unit cell parameters and unit cell volume of hexagonal structure are a = b = 4.0934(3) Å, c = 5.3149(5) Å and V = 77.125(5) Å3 at room temperature. In order to find the crystal structures of annealed CoMnGe0.95Ga0.05, the temperature dependence of Synchrotron diffraction experiments were performed on cooling mode from 400 K to 10 K and then on heating mode from 10 K to 400 K. Fig. 3 exhibits the Synchrotron diffraction patterns collected at 400 K, 295 K for cooling, 295 K for heating and 10 K. While we observed the diffraction peaks of hexagonal structure at high temperatures, the orthorhombic diffraction peaks are observed at low temperatures. The crystal structures of this at high and low temperatures are determined with Rietveld refinement by using FullProf package. According to Rietveld refinement results, the crystal structure of annealed CoMnGe0.95Ga0.05 at high temperatures is Ni2In-type hexagonal structure with space group P63/mmc. Its crystal structure is TiNiSi-type orthorhombic structure with space group Pnma at low temperatures. At 400 K, the unit cell parameters and unit cell volume of hexagonal structure are a = b = 4.10066(8) Å, c = 5.34144(9) Å and V = 77.786(3) Å3, respectively. At 10 K, the unit cell parameters and unit cell volume of orthorhombic structure are a = 5.88625(8) Å, b = 3.82806(7) Å, c = 7.06786(9) Å and V = 159.261(15) Å3, respectively. Fig. 3e shows the phase fractions of annealed CoMnGe0.95Ga0.05 as a function temperature. The martensitic transformation temperatures obtained from phase fraction curves are MS = 320 K, Mf = 235 K, AS = 285 K, Af = 330 K that are in good agreement with the DSC results. The temperature dependence of the unit cell volume of hexagonal and orthorhombic structures are shown in Fig. 3f. The volume of hexagonal structure is approximately two times smaller than that of orthorhombic
Fig. 2. DSC curves of annealed CoMnGe0.95Ga0.05 at heating and cooling rates of 10 K/min. MS, martensite start temperature, Mf, martensite finish temperature, AS, austenite start temperature, Af, austenite finish temperature, TAM, transition temperature from austenite to martensite and TMA, transition temperature from martensite to austenite start temperature.
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Fig. 3. The Synchrotron diffraction patterns of annealed CoMnGe0.95Ga0.05 (a) at 400 K (b) at 295 K for cooling processes (c) at 295 K for heating processes and (d) at 10 K. The stars show the diffraction peaks from hexagonal structure. (e) Temperature dependence of the phase fraction (vol.%) of the hexagonal and orthorhombic structures with increasing and decreasing temperature. (f) Variation in volume of the hexagonal and orthorhombic structures with increasing and decreasing temperature.
structure. Fig. 4 shows the Rietveld refinement result of the Synchrotron diffraction pattern at 315 K cooled from high temperatures. The R-values of this refinement are Rp = 6.31, Rwp = 8.48 and Rexp = 2.68, respectively. We performed the temperature dependence of magnetization measurements for as cast and annealed CoMnGe0.95Ga0.05 to determine the influence of the heat treatment on the magnetic and magnetocaloric properties of CoMnGe0.95Ga0.05. The MðTÞ curves, measured from 5 to 350 K in a magnetic field of 100 Oe for ZFC, FC and FH modes are displayed in Fig. 5b for the as cast CoMnGe0.95Ga0.05 and (a) for the annealed CoMnGe0.95Ga0.05. The as cast CoMnGe0.95Ga0.05 has ferromagnetic character below TC = 277 K and paramagnetic above this temperature. There is no thermal hysteresis between FC and FH curves (as seen in Fig. 5b). This means that this alloy without heat treatment has no structural transition and DSC measurement confirms this result. According to Fig. 5a, annealed CoMnGe0.95Ga0.05 orders ferromagnetically below Curie temperature and paramagnetically above this temperature. In the vicinity of Curie temperature, an thermal hysteresis between FC and FH curves is observed. Considering the DSC and synchrotron
Fig. 4. The Rietveld refinement result of the Synchrotron diffraction pattern for annealed CoMnGe0.95Ga0.05 at 315 K cooled from high temperatures. The positions of Bragg reflections (bottom lines for hexagonal structure and top line for orthorhombic structure) are marked by vertical bars.
Fig. 5. The MðTÞ curves measured in magnetic field 100 Oe (a) for annealed CoMnGe0.95Ga0.05 and (b) for as cast CoMnGe0.95Ga0.05.
diffraction results together, this thermal hysteresis is attributed to a first order structural transformation. The ZFC curves begin at a low magnetization value due to the essentially random spatial configuration while cooling through TC down to low temperatures. At higher temperatures, the ZFC and FH curves merge. The Curie temperatures of this alloy are obtained from FC and FH curves as T AM ¼ 302 K and T MA ¼ 317 K, respectively. The thermal hystereC C MA sis between T AM and T is about 15 K which are in good agreC C ment with DSC results. Isothermal M H curves of annealed CoMnGe0.95Ga0.05 were measured with using two different methods to find the isothermal magnetic field response of this sample in and around the temperature range of martensitic phase transition. In the first method, the sample was slowly cooled from 350 to 345 K under zero magnetic field. At this temperature, the magnetic field dependence of magnetization was measured from 0 to 7 T and from 7 to 0 T. After the magnetization measurements was completed, temperature was decreased to next temperature. This measurement method is called as cooling process. Fig. 6a show the MðHÞ curves measured with cooling process. In the second method, after zero magnetic field cooling from 350 K to 10 K, sample was slowly heat up to 276 K. At this temperature, the magnetic field dependence of magnetization was measured from 0 to 7 T and from 7 to 0 T. After the magnetization measurements was completed, temperature was
I. Dincer et al. / Journal of Alloys and Compounds 540 (2012) 236–240
S S Fig. 6. The MðHÞ curves of annealed CoMnGe0.95Ga0.05: (a) for cooling process from 350 K and (b) for heating process from 50 K. The temperature dependence of magnetic entropy change of annealed CoMnGe0.95Ga0.05 for different magnetic fields: (c) calculated from cooling MðHÞ curves and (d) calculated from heating MðHÞ curves. The top inset: The MðHÞ curves of as cast CoMnGe0.95Ga0.05. The bottom inset: The temperature dependence of magnetic entropy change of as cast CoMnGe0.95Ga0.05.
increased to next temperature. This measurement method is called as heating process. Fig. 6b show the MðHÞ curves measured with heating process. From 321 K down to 300 K on cooling process and from 306 K up to 324 K on heating process, a metamagnetic transition with hysteresis indicates that a magnetic field induced martensitic transformation occurs, due to the large Zeeman energy between paramagnetic austenite and ferromagnetic martensite phases. Koyama et al. observed the similar field induced martensitic phase transformation at the vicinity of martensitic transformation temperature for Mn1.07Co0.92Ge [10]. Vacancy on Mn sublattice of CoMnGe (such as Mn0.965CoGe) causes also the magnetic field induced martensitic transformation [19]. According to our magnetic field dependence of magnetization measurements up to 2 T, we could not observed metamagnetic transition and magnetic hysteresis. Fig. 7 shows the MðHÞ curves in the magnetic field change of 2 T. As a result, almost zero magnetic hysteresis on MðHÞ curves was observed between magnetization and demagnetization curves at the vicinity of martensitic transformation temperature which is meaningful for the magnetic refrigeration applications. The top inset in Fig. 6 shows the MðHÞ curves of our as cast CoMnGe0.95Ga0.05. MðHÞ curves of this as cast
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alloy confirms that this alloy has only second order phase transition around TC. Using the data in Fig. 6a and b, the magnetic field induced entropy change DS is estimated from Maxwell relation RH ðDS¼0 ð@M=@TÞH dHÞ. DS for annealed CoMnGe0.95Ga0.05 is shown in Fig. 6c and d for cooling and heating process. As shown in this figure, the giant magnetocaloric effect was observed around room temperature due to the great change in magnetization near martensitic transition temperature. The peak values of DS estimated from cooling and heating processes are approximately same. This means that the value of the magnetic entropy change do not depend on the thermo-magnetic history of the alloy, unlike Ni50Mn34In16 [23]. The peak values of the magnetic entropy change are about 5.2, 11.1 and 27.1 Jkg1 K1 for the magnetic field change of 1, 2 and 5 T, respectively. These peak values of magnetic entropy change are greater than those of other magnetocaloric materials in similar fields, e.g. Gd. The bottom inset in Fig. 6 shows DSðTÞ curves of as cast CoMnGe0.95Ga0.05. According to Fig. 6, the reason of the giant MCE in CoMnGe0.95Ga0.05 is the martensitic structural phase transition. The refrigerant capacity (RC) defined RT as RC¼T 1 2 ðDSÞH dT which indicates how much heat can be transferred from the cold end to the hot end of the refrigerator in one thermodynamic cycle. The RC values of annealed CoMnGe0.95Ga0.05 are about 36, 76 and 184 J kg1 for the magnetic field change of 1, 2 and 5 T, respectively. The RC values are calculated here with the method proposed by Gschneidner and co-workers [24]. Our RC value for the magnetic field change of 5 T is comparable with the results of other magnetocaloric materials that show giant MCE at room temperature, such as Gd5Ge2Si2 [25] and Ni50Mn34In16 [26]. 4. Conclusion In conclusion, we have observed that the martensitic phase transformation temperature depends on the heat treatment on CoMnGe0.95Ga0.05. The crystal structures of annealed CoMnGe0.95Ga0.05 have been determined by analyzing the temperature dependence of Synchrotron diffraction patterns in the temperature range between 10 and 400 K. While the crystal structure of this alloy is Ni2In-type hexagonal structure with space group P63/mmc at high temperatures, its crystal structure is TiNiSi-type orthorhombic structure with space group Pnma at low temperatures. The temperature dependent magnetization measurements confirm the results of DSC and Synchrotron diffraction experiments. According to the DSðTÞ curves of as cast and annealed CoMnGe0.95Ga0.05, the main contribution to entropy change should be from structural phase transition. Since the magnetic fields up to 2 T can be generated by permanent magnets, annealed CoMnGe0.95Ga0.05 observed giant MCE should be promising MCE material for magnetic refrigeration technology when compared with the other MCE materials such as, FeRh, Gd, Gd5Ge2Si2, MnAs and Nibased Heusler alloys. The other important advantage of annealed CoMnGe0.95Ga0.05 is the thermomagnetic history independence of the magnetic entropy change. Acknowledgements This work was supported by TUBITAK (Project Number: 109T743). The authors would like to thank DESY-HASYLAB for providing the synchrotron facilities. References
Fig. 7. The MðHÞ curves of annealed CoMnGe0.95Ga0.05 for the magnetic field change of 2 T.
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