IOP PUBLISHING
JOURNAL OF PHYSICS D: APPLIED PHYSICS
J. Phys. D: Appl. Phys. 40 (2007) 7287–7290
doi:10.1088/0022-3727/40/23/005
Giant low-field magnetic entropy changes in Ni45Mn44−xCrxSn11 ferromagnetic shape memory alloys C L Zhang, W Q Zou, H C Xuan, Z D Han, D H Wang1 , B X Gu and Y W Du National Laboratory of Solid State Microstructures, Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China E-mail:
[email protected]
Received 20 June 2007, in final form 11 September 2007 Published 16 November 2007 Online at stacks.iop.org/JPhysD/40/7287 Abstract A series of Ni45 Mn44−x Crx Sn11 (x = 0, 1, 2) ferromagnetic shape memory alloys were prepared. With slight doping of Cr, the martensitic transition temperatures decrease rapidly. The magnetic entropy changes at a low magnetic field were investigated in these alloys. The maximum value of SM is 23.4 J kg−1 K−1 , which was observed in Ni45 Mn43 CrSn11 alloys. The origin of the magnetic entropy changes in these alloys has been discussed. The giant low-field magnetic entropy changes and low cost make Ni45 Mn44−x Crx Sn11 alloys a promising candidate for magnetic refrigeration.
1. Introduction Compared with conventional vapour-cycle refrigeration, magnetic refrigeration, which is based on the magnetocaloric effect (MCE) of magnetic materials, has many advantages, such as the absence of harmful gas, less noise, low energy cost and high efficiency. As the ‘heart’ of magnetic refrigerators, magnetic materials with large magnetic entropy Due to the changes (SM ) are of great importance. rapid change in magnetization in the vicinity of the phase transition temperatures, magnetic materials undergoing firstorder transition (FOT), such as LaFe13−x Six [1], Gd5 Si4−x Gex [2] and MnFeP0.45 As0.55 [3] alloys, often exhibit large SM . Ferromagnetic shape memory alloys (FSMAs), which show the shape memory effect and magnetism simultaneously, undergo a first-order martensitic transition (MT) from a high symmetry phase (austenite) to a low symmetry phase (martensite) with decreasing temperature [4]. Among them, the Ni–Mn–X (X = Ga, In, Sn, Sb) alloys are the most heavily studied FSMAs [5–7]. Since the MT is often accompanied with abrupt changes in magnetization and electronic resistivity, large MCE or magnetoresistance effects can be observed near their MT temperatures [8–12]. It is reported that, in Ni–Mn based FSMAs, the MT temperature can be adjusted by two methods. One is tuning the chemical composition. 1
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0022-3727/07/237287+04$30.00
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Krenke et al reported that in Ni50 Mn50−x Snx alloys, the MT temperatures varied remarkably by changing the proportion of Mn and Sn [7]. The other method is atomic substitution by other elements. For example, in the Ni2 MnGa system [13], when Mn sites were partially substituted with other transition elements, such as magnetic Co or nonmagnetic Cu, the MT temperatures increased rapidly with increasing Co or Cu concentration. To date, the most commonly substituted transition elements have been Fe, Co and Cu. But there are few reports about other transition elements, such as Cr, which are used to dope in Ni–Mn-based FSMAs. In this work, we prepared the Ni45 Mn44−x Crx Sn11 alloys and investigated the effect of Cr doping on MT and MCE in Ni–Mn–Sn alloys.
2. Experiment Polycrystalline Ni45 Mn44−x Crx Sn11 (x = 0, 1 and 2) alloys were prepared by arc-melting the high purity raw materials under an argon atmosphere. For the sake of homogeneity, the ingots were turned over and re-melted several times. The samples were sealed in vacuumed quartz tubes and annealed at 1173 K for 48 h, then quenched in cool water. Magnetic properties were measured by a vibrating sample meter (VSM, LakeShore 7300) under a magnetic field up to 10 kOe. The temperature dependence of magnetization curves M(T ) for the Ni45 Mn44−x Crx Sn11 alloys in 100 Oe was
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7
M (emu/g)
6
Table 1. The values of the martensitic start temperature (Ms ), the martensitic finish temperature (Mf ), the austenitic start temperature (As ), the austenitic finish temperature (Af ), the e/a value and magnetic entropy change (SM ) for Ni45 Mn44−x Crx Sn11 alloys (x = 0, 1 and 2).
Ni45Mn44Sn11 Ni45Mn43CrSn11
8
Ni45Mn42Cr2Sn11 at 100 Oe
5 4 Ms
3 2 Mf
Af
180
200
220
240
260
280
300
320
T (K)
Figure 1. The temperature dependence of magnetization for Ni45 Mn44−x Crx Sn11 alloys (x = 0, 1 and 2) under a magnetic field of 100 Oe on cooling and heating.
measured in the heating and cooling processes. Then the isothermal magnetization curves M(H ) for the samples were measured near their MT temperatures, which are defined from the M(T ) curves. During the measurement, the samples were first heated up to the Curie temperature of austenite. This treatment leads to the samples being in the paramagnetic state. Then the samples were cooled down in the zero field below the martensitic finish temperature (Mf ) and kept at a constant temperature for several minutes. After that, the M(H ) curves measurements were carried out in a series of constant temperatures arranged in a heating sequence. The samples were heated in a slow heating velocity and then kept at this temperature for a few minutes, which leads to the process being assumed as quasi-static.
3. Results and discussion Figure 1 shows the temperature dependence of magnetization for the Ni45 Mn44−x Crx Sn11 alloys in 100 Oe on heating and cooling. With the change in the temperature, all the alloys show similar MT behaviours. For each sample on heating, a weak ferromagnetic phase is first observed in the martensite state, then the magnetization decreases with increasing temperature. Further heating leads to a sudden jump of magnetization, which corresponds to the reverse MT. Then the magnetization remains almost constant in the ferromagnetic austenitic phase. Finally, the magnetization decreases again, and a paramagnetic transition occurs. Between the cooling and the heating processes, thermal hysteresis around the MT temperatures, which is a signature of the FOT, can be observed in all the samples. For comparison, no obvious thermal hysteresis can be found near the Curie temperature of austenite. The characteristic temperatures of MT are the austenitic start temperature (As ), the austenitic finish temperature (Af ), the martensitic start temperature (Ms ) and Mf . All these temperatures of Ni45 Mn44−x Crx Sn11 alloys are listed in table 1. In Ni–Mn based FSMAs, it is reported that the characteristic temperatures of MT are very sensitive to the valence electron concentration e/a (electrons per atom) [14,15]. Here the valence electrons are calculated as the sum of 3d 7288
Ms (K)
Mf (K)
As (K)
Af (K)
e/a
SM (J kg−1 K−1 )
0 1 2
270 235 220
255 230 206
270 240 215
285 246 240
8.02 8.01 8.00
6.8 23.4 9.9
As
1 0
x
and 4s electrons for Ni, Mn and Cr and 5s and 5p electrons for Sn. Since Cr has fewer 3d electrons than Mn, the values of e/a of Cr-doped Ni45 Mn44 Sn11 alloys are less than that of the undoped one. From figure 1, it is obvious that the characteristic temperatures decrease rapidly with increasing content of Cr. Thus, we can tune the phase transition temperatures by doping, which is meaningful for the application of FSMAs as the working substance for magnetic refrigeration [1–3]. Figure 2 shows isothermal magnetization curves M(H ) for the Ni45 Mn44−x Crx Sn11 alloys in two field variation directions. It is reported that the austenite is ferromagnetic and the martensite is anti-ferromagnetic-like in Ni–Mn–Sn alloys [16]. From figure 2, we can observe that, with increasing temperature, the magnetization increases from the weakly magnetic martensite to the ferromagnetic austenite. For x = 1 and 2, typical metamagnetic behaviours, due to the field-induced reverse MT, were clearly observed around their MT temperatures, in the field-increasing M(H ) curves. Accordingly, no field-induced MT can be seen in the fielddecreasing M(H ) curves due to the hysteresis. The critical fields (Hc ) are 6 kOe and 7 kOe, for x = 1 and 2, respectively. But from figure 2(a), no obvious field-induced metamagnetism can be observed in the Ni45 Mn44 Sn11 alloy. This means that Hc in this alloy is larger than 10 kOe. It is worth noting that, with Cr doping in the Ni–Mn–Sn alloys, the magnetization of the austenite increases remarkably. This should be ascribed to the decrease in the MT temperature. As we know, the Arrott plots, which are obtained by plotting the values of M and H as a function of M 2 versus H /M, are an effective tool for identifying the order of the phase transition [17,18]. The Arrott curves for x = 0 and 1 are shown in figure 3. Although no obvious metamagnetic behaviour can be found in the M(H ) curves of the Ni45 Mn44 Sn11 alloy, a curved line can still be obviously observed around the MT temperature, which suggests the occurrence of a first-order MT [18]. SM in the magnetic field of 10 kOe as a function of temperature for the Ni45 Mn44−x Crx Sn11 alloys were calculated using the Maxwell relation: H ∂M(H, T ) dH . (1) SM = ∂T 0 H As shown in figure 4, the SM of the samples peak near their MT temperatures and the maximum values are 6.9 J kg−1 K−1 , 23.4 J kg−1 K−1 and 9.9 J kg−1 K−1 , for x = 0, 1 and 2, respectively. The low-field SM of Cr doped Ni–Mn–Sn alloys are giant and larger than those of Ni50 Mn37 Sn13 (3 J kg−1 K−1 at (8 J kg−1 K−1 at H = 10 kOe), Ni45.4 Mn41.5 In13.1
Ni45 Mn44−x Crx Sn11 ferromagnetic shape memory alloys
(a) X=0
1500
2
20
Ni45Mn44Sn11
900 600 300
10
0 0.0
0
260K 265K 269K 271K 273K 277K
(a)
1200 M (emu2/g2)
277 K 275 K 273 K 271 K 269 K 265 K 260 K 250 K
30 32 emu/g
M (emu/g)
40
0
2
4
6 8 H (kOe)
10
12
0.5
1.0
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2.5
14 10000
235K 238K 240K 241K 242K
(b)
241K 240K 239K 238K 237K 236K
40
M2 (emu2/g2)
60
58 emu/g
M (emu/g)
Ni45Mn43CrSn11
8000
(b) X=1
80
6000 4000 2000 0
20
0.0
0
4
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10
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231K 230K 229K 228K 227K 226K 225K 224K 222K 220K
60
40
20
0.1 0.2 0.3 0.4 H/M (kOe/emu.g-1)
0.5
0.6
Figure 3. The Arrott plots for Ni45 Mn44−x Crx Sn11 alloys (x = 0, 1).
(c) X=2
46 emu/g
M (emu/g)
80
2
25
Ni45Mn44Sn11 Ni45Mn43CrSn11
20 ∆SM (J/kg.K)
0
3.0
H/M (kOe/emu.g-1)
Ni45Mn42Cr2Sn11
15 10 5 0
0
0
2
4
6 8 H (kOe)
10
12
14
Figure 2. The isothermal magnetization curves for Ni45 Mn44−x Crx Sn11 alloys (x = 0, 1 and 2) around the MT temperatures.
H = 10 kOe) and Ni51.5 Mn22.7 Ga25.8 (4.1 J kg−1 K−1 at H = 9 kOe) [8, 12, 19]. The giant SM in the Ni45 Mn44−x Crx Sn11 alloys is attributed to the abrupt change in magnetization around the MT temperatures. The large magnetization jump comes from two aspects: (a) it is well known that FSMAs have different magnetic structures between the martensite and the austenite. As shown in figure 1, under a constant magnetic field, the variation of the temperature can lead to the MT, which is accompanied by a sharp change in the magnetization; (b) at some temperature around the MT temperatures, the applied magnetic field can also induce the MT, which is embodied in the appearance of metamagnetic
220
240
260 T (K)
280
300
Figure 4. The temperature dependence of SM in the magnetic field of 10 kOe for Ni45 Mn44−x Crx Sn11 alloys (x = 0, 1 and 2).
behaviour. It gives an additional change in magnetization, which should be helpful for obtaining large MCE. In addition, from figure 4, it is obvious that the SM is enhanced with Cr doping. According to the Maxwell relation, it is found that the key to gaining large SM is to increase the change in magnetization between Ms and Mf (Msf ). As shown in figure 2, the values of Msf are 32 emu g−1 , 58 emu g−1 and 46 emu g−1 , for x = 0, 1 and 2, respectively. On the other hand, as discussed above, field-induced metamagnetism plays an important role in acquiring large SM . From figure 2, we can find that the metamagnetic behaviour can be observed in the Ni45 Mn44−x Crx Sn11 (x = 1 and 2) alloys, while in the Ni45 Mn44 Sn11 alloy, Hc is larger than 10 kOe. Therefore, Cr doping in Ni45 Mn44 Sn11 alloy can lead to the increase in Msf and decrease in Hc and results in giant SM . 7289
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It is worth noting that the Ni45 Mn44−x Crx Sn11 alloys have several advantages as magnetic refrigerant materials: (a) giant SM can be obtained in a low field, which is important for practical application, (b) most raw materials (Ni, Mn and Cr) of Ni45 Mn44−x Crx Sn11 alloys are common transition metals, which are relatively inexpensive and (c) the working temperature ranges can be adjusted by tuning the chemical composition or atomic substitution. These good properties give potential background for applications.
4. Conclusion The effect of Cr doping on the MT and the MCE effect in Ni–Mn–Sn alloys has been investigated. The characteristic temperatures of MT decrease remarkably with increasing Cr content. Giant SM can be observed in the Ni45 Mn44−x Crx Sn11 (x = 1, 2) alloys in a low field. The magnetic phase transition near the MT temperature and the field-induced metamagnetism are responsible for the giant SM .
Acknowledgment This work was supported by the National Key Project for Basic Research (2005CB623605).
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