Journal of Magnetism and Magnetic Materials 374 (2015) 372–375 Contents lists available at ScienceDirect Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm Magnetic properties and magnetocaloric effect in Ni–Mn–Sn alloys N.H Dan a,n, N.H Duc a, N.H Yen a, P.T Thanh a, L.V Bau b, N.M An b, D.T.K Anh c, N.A Bang c, N.T Mai c, P.K Anh d, T.D Thanh a,e, T.L Phan e, S.C Yu e,n a Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Hanoi, Vietnam Hong Duc University, 565 Quang Trung, Dong Ve, Thanh Hoa, Vietnam c Faculty of Physics, VNU University of Science, 334 Nguyen Trai, Hanoi, Vietnam d Vietnam Academy of Military Science, 322 Le Trong Tan, Thanh Xuan, Hanoi, Vietnam e Department of Physics, Chungbuk National University, Cheongju 361-763, South Korea b art ic l e i nf o a b s t r a c t Article history: Received April 2013 Received in revised form 27 April 2014 Available online 27 August 2014 Magnetic and magnetocaloric properties in Ni50Mn50 À xSnx alloys with wide range of the Snconcentration (x ¼ 0–40) were investigated The alloys were prepared by arc-melting and subsequently annealing at 850 1C for h The X-ray diffraction analyses manifest the formation of the crystalline phases (Ni2MnSn, NiMn, Ni3Sn2, Mn3Sn, and MnSn2) in the alloys with various compositions and fabrication conditions With increasing x, the saturation magnetization first increases from near zero (at x ¼10) to above 40 emu/g (at x ¼ 20) and then decreases to below 10 emu/g (at x ¼40) for both the asmelted and annealed cases The martensitic–austenitic transition was observed in the alloys with a narrow range of x (13–15) The magnetic transitions in the alloy can be controlled by changing Snconcentration The alloy reveals both the positive and negative entropy changes with quite large magnitude (ΔSm 41 J/kg K with ΔH¼ 12 kOe) with appropriate compositions and annealing conditions & 2014 Elsevier B.V All rights reserved Keywords: Magnetic transition Giant magnetocaloric effect Heusler alloy Magnetic refrigeration Introduction Ni–Mn–Sn Heusler alloys have been attracting a lot of scientists by virtue of their giant magnetocaloric effect (GMCE) and application potential for magnetic refrigeration at room temperature [1–8] Both the positive (inverse) and negative (normal) GMCEs could be observed in these alloys by changing composition and fabrication conditions The positive GMCE is believed to relate to a transformation between martensite and austenite phases The coexistence of ferromagnetic (FM) and antiferromagnetic (AFM) orders was also observed in the alloys Adding other elements such as Cu, Co, Al etc [9–16] and changing fabrication conditions [17–20] are common ways to understand the magnetic mechanism and achieve the desired GMCEs for the alloys The magnetic orders and the magnetocaloric effects in Ni–Mn–Si alloys were found to be very sensitive to their composition and fabrication conditions Therefore, systematic studies on these alloys are still needed The compositions of the alloys in most of published papers are rather limited In this work, we investigated magnetic and magnetocaloric properties in the Ni50Mn50 À xSnx alloys with a wide range of the Sn-concentration (x ¼0–40) to make more clear the variation n Corresponding authors E-mail addresses: dannh@ims.vast.ac.vn (N.H Dan), scyu@chungbuk.ac.kr (S.C Yu) http://dx.doi.org/10.1016/j.jmmm.2014.08.061 0304-8853/& 2014 Elsevier B.V All rights reserved trend versus composition of magnetic and magnetocaloric properties of the material Experiment Alloys with nominal compositions of Ni50Mn50 À xSnx (x¼ 0, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 30 and 40) were prepared from pure metals (4 N) of Ni, Mn and Sn by using an arc-melting method A part of each sample was annealed at 850 1C for h in a vacuum of 10 À Torr and then quenched to room temperature by Ar-flow The quenching rate was about 100 1C per minute The structure of the samples was examined by means of powder X-ray diffraction (XRD) on a Siemens D5000 X-Ray diffractometer with CuKα radiation The magnetic and magnetocaloric properties of the samples were characterized by magnetization measurements on pulsed field and vibrating sample magnetometers Results and discussion Fig shows room temperature powder-XRD patterns of the asmelted and annealed Ni0.5Mn0.5 À xSnx alloys in the range of 30– 651 Crystalline phases of Ni2MnSn, NiMn, Ni3Sn2, Mn3Sn and MnSn2 are identified from these patterns The number and relative intensity of diffraction peaks, i.e crystalline structure, are varied N.H Dan et al / Journal of Magnetism and Magnetic Materials 374 (2015) 372–375 373 Fig XRD patterns of as-melted (a) and annealed (b) Ni50Mn50 À xSnx alloys with varying the Sn-concentration (x) for both the as-melted and annealed cases The Mn3Sn phase is a main phase with low values of x, while the Ni3Sn2 phase is dominated at high values of x The Ni2MnSn phase is not observed in any alloy with x r11 After annealing, the structure of some samples is changed quite clearly, especially for the samples with x ¼13–15 The annealed alloys are almost single phase of Ni2MnSn with x ¼13–20 Fig presents the room temperature magnetization at magnetic field of 50 kOe, M50 kOe, of the as-melted and annealed Ni50Mn50 À xSnx alloys with various Sn-concentrations We can see that the variation trend of M50 kOe of both the as-melted and annealed alloys is similar as x varies When x is increased, M50 kOe first increases rapidly from near zero (at x¼ 10) to above 30 emu/g (at x ¼15) then gradually reaches the maximum value of about 40 emu/g (at x ¼20) After that, M50 kOe slowly decreases to $37 emu/g (at x ¼30) and finally goes down quickly to below 10 emu/g (at x¼40) We can also see that M50 kOe of the annealed alloys is higher than that of the as-melted ones with xo 20 but lower with x4 20 The strong variation of M50 kOe of the alloys can be explained by the change of the exchange interactions of atoms in the alloys when the Sn-concentration is changed The results in published reports showed that the Mn atoms in the alloys could be coupled ferromagnetically or antiferromagnetically depending on their composition [10,11] The various exchange interactions could make the alloys become weak or strong ferromagnets with Fig Magnetization at magnetic field of 50 kOe of the as-melted and annealed Ni50Mn50 À xSnx alloys with various Sn-concentration (x) The inset shows hysteresis loops of the two annealed alloys with x¼ 13 and 20 different saturation magnetization and Curie temperatures The various magnetic orders in the alloys can also be realized via hysteresis loops of the alloys In the inset of Fig 2, the different shapes of hysteresis loops of the two annealed alloys with x¼13 and x¼ 20 are clearly observed The former commonly characterizes for non-colinear magnetic orders and the latter is for colinear magnetic 374 N.H Dan et al / Journal of Magnetism and Magnetic Materials 374 (2015) 372–375 orders The magnetization of the sample with x¼13 continuously increases with increasing the applied magnetic field up to 50 kOe, while the magnetization of the sample with x¼20 is almost saturated This hard-saturated feature probably is due to the coexistence of various magnetic orders in the alloy As shown in Fig 3, the annealed alloy with x¼ 13 undergoes a process of phase transition in room temperature region The short-range and longrange orders are believed to coexist in this process The short-range magnetic orders normally cause the hard-saturated feature for the alloy, which can be compared with that of some amorphous alloys A quite detailed investigation on magnetic properties for the alloys with x¼ 20 and 30 was reported in our previous publication [8] The magnetic orders or magnetic phases in the alloys are not only influenced by the Sn-concentration but also by the annealing process, especially for the alloys with xo15 M50 kOe of the alloys with xo15 is increased quite strongly after annealing The effect of the annealing process (at 850 1C for h) on the magnetic properties of the alloys is more clearly observed by the thermomagnetization measurements Fig shows representative thermomagnetization curves in an applied field of 100 Oe for the alloys In the as-melted state, the alloys almost behave as single ferromagnetic phase with all the Sn-concentrations (see Fig 3a) The Curie temperature (TC) of the as-melted alloys is monotonically increased from $240 K (at x¼ 0) to $ 360 K (at x¼40) by increasing the Sn-concentration (inset of Fig 3a) Nevertheless, the increasing rate of TC is lower with higher Sn-concentrations TC could be affected quite strongly by adding other elements, especially adding Co [10–12] After annealing, the shape of the thermomagnetization curves of the alloys with x¼13, 14 and 15 is very different from that of the as-melted ones (Fig 3b) This can be explained by the structural transformation at certain temperatures leading to the transition of the magnetic phases in the annealed alloys [1–3] The structural transformation in the alloys was also found to be clearly dependent on fabricating methods, for example, arc-melting or melt-spinning [17–20], and annealing process [18] It is supposed that both the martensite and austenite phases can exist in the Ni–Mn–Sn based alloys with appropriate Snconcentration and annealing conditions The austenite phase is ferromagnetic, while the martensite phase relates to weakferromagnetic or antiferromagnetic orders [9–12] In general, by heating up the multi-phase Ni–Mn–Sn alloys from low temperature, the magnetization decreases to a minimum at the temperature T M f (martensite finishes) then start to increase fast at the temperature T As (austenite starts) The fast increase stops at the temperature T Af (austenite finishes) The last magnetic transition of the ferromagnetic to paramagnetic phases occurs at the temperature T AC (Curie temperature of austenite) It should be noted that, the range of Sn-concentration for the existence of the martensite in the alloys is rather narrow (in few percents) and the volume fraction of the martensite strongly depends on the fabrication conditions The magnetic phase transitions greatly influence on the magnetocaloric effect as presented below The magnetic entropy change (ΔSm) for the alloys was calculated from data of the magnetization versus magnetic field (M–H) curves at various temperatures by using the relation Z Sm T; Hị ẳ H ∂M ∂T dH ð1Þ H In this work, the magnetic field change (ΔH) for determining the magnitude of the GMCEs of the alloys is 12 kOe The negative magnetic entropy change versus temperature ΔSm(T) for the annealed Ni50Mn50À xSnx alloys with various Sn-concentrations is presented Fig The inset of Fig shows both the positive and negative magnetic entropy changes versus temperature for the annealed alloy with x¼13 We can see that the maximum negative magnetic entropy change (|ΔSm|max) of the alloys is larger than J/kg K and slightly increases with decreasing x from 30 to 13 The ΔSm(T) curves of the annealed alloys with x¼20 and 30 are similar to Fig Thermomagnetization curves in an applied field of 100 Oe for the as-melted (a) and annealed (b) Ni50Mn50 À xSnx alloys The inset shows Sn-concentration dependence of Curie temperature of the as-melted alloys Fig Negative magnetic entropy change (ΔH¼ 12 kOe) versus temperature for annealed Ni50Mn50 À xSnx alloys The inset shows both the positive and negative magnetic entropy changes versus temperature for the annealed alloy with x ¼13 N.H Dan et al / Journal of Magnetism and Magnetic Materials 374 (2015) 372–375 those of the as-melted ones [8] This means the annealing process does not influence on the negative GMCE but on the positive GMCE of the alloys It is worth noting that the temperature of the maximum negative magnetic entropy change (T|ΔSm|max), which depends on the martensite–austenite transition, is also shifted to room temperature region with decreasing the Sn-concentration The maximum positive magnetic entropy change of the annealed alloy with x¼ 13 is larger than its negative one (1.9 J/kg K) and occurs near 300 K Thus, both the positive and negative GMCE of this alloy are possible for application of magnetic refrigeration at room temperature Conclusion The Ni50Mn50 À xSnx alloys with a wide range of Snconcentration (x ¼0–40) were fabricated by arc-melting and subsequently annealing The influence of composition and fabrication conditions on structure, magnetic properties and magnetocaloric effect was investigated systematically The formation of crystalline phases of Ni2MnSn, NiMn, Ni3Sn2, Mn3Sn, MnSn2 was observed The coexistence of various magnetic orders was revealed by both the magnetic hysteresis and thermomagnetization measurements The magnetization at high magnetic field of 50 kOe was determined The martensitic–austenitic transition only took place in the alloys with a narrow range of Sn-concentration (x ¼13–15) With appropriate compositions and annealing conditions, the alloy shows both the positive and negative entropy changes with quite large magnitude, (ΔSm 41 J/kg K with ΔH ¼12 kOe) The working temperature of the alloys can be controlled by changing Snconcentration, making them possible for application in magnetic refrigeration in room temperature Acknowledgment This work was supported by the National Foundation for Science and Technology Development (NAFOSTED) of Vietnam under Grant numbers of 103.02–2011.23 and 103.02–2010.28 and the Converging 375 Research Center Program funded by the Ministry of Science, ICT and Future Planning, Korea (2014048835) A part of the work was done in the Key Laboratory for Electronic Materials and Devices, and Laboratory of Magnetism and Superconductivity, Institute of Materials Science, VAST, Vietnam References [1] T Krenke, E Duman, M Acet, E.F Wassermann, X Moya, L Mañosa, A 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in uence of composition and fabrication conditions on structure, magnetic properties and magnetocaloric effect was investigated systematically... arc-melting or melt-spinning [17–20], and annealing process [18] It is supposed that both the martensite and austenite phases can exist in the Ni–Mn–Sn based alloys with appropriate Snconcentration and. .. investigation on magnetic properties for the alloys with x¼ 20 and 30 was reported in our previous publication [8] The magnetic orders or magnetic phases in the alloys are not only in uenced by the