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15.04.2007

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the magnetocaloric effect » materials

Below, we describe the families of materials most promising for applications in magnetic cooling devices to work around room temperature (RT). 

 

1. Family R-G (R: rare-earths, G: metalloid).

Since the discovery of the huge MCE in Gd5Si2Ge2 (35 J/kgK@280 K), in 1997[1], about 140 papers have been published. That value is due to a coupled magneto-structural first-order transition, driven by temperature[2], magnetic field[3] and pressure[4]. Several series have been studied from this parent compound, namely: (i) Gd5(Si1-xGex)4 alloys, with magnetic entropy change (MEnCh) ranging from 61 J/kgK@90 K to 8.6 J/kgK@340 K[5]; (ii) (Gd,Pr/Tb)5Si4[6], without any significant change on the magnetocaloric properties; (iii) R5(Si1-xGex)4 alloys, where R means a rare-earth, like Nd[7], Tb[8], Dy[9] and Ho[10]; and (iv) many others[5]. Also interest is the work of Morellon et al[4] that could merge the structural and magnetic transition of Tb5Si2Ge2 compounds by applying 8.6 kbar of hydrostatic pressure; increasing therefore the MCE.

 

2. Family R-M-G (M: transition metal).

Another series with magneto-structural coupling is the La(FexSi1-x)13[11]. For instance, MEnCh for x=0.88 reaches 26 J/kgK@188 K. These compounds under a negative pressure (expansion of the unit cell), via insertion of Hydrogen, could shift the transition temperature up to RT[12]; while positive pressure (compression of the unit cell), via hydrostatic pressure, increase the MEnCh value, decreasing, however, the transition temperature[13]. Substitution of 50% of La for Pr has also been done [14], increasing the value of MEnCh for 30 J/kgK@185 K.

 

3. Family Mn-M-G

MnAs is a compound that presents a coupled magneto-structural transition of first order, resulting therefore in a huge MEnCh: 32 J/kgK@318 K [15]. Wada et al.[15], studied Mn(As,Sb) and found that both, transition temperature and MEnCh decrease by adding Sb; disqualifying therefore this series comparing to the parent compound. More recently, Gama et al. [16] applied 2.2 kbar in MnAs compound and could increase the MEnCh up to 267 J/kgK@280 K.

Other compounds with Mn are those of the serie MnFe(P,As). Tegus et al.[17] pointed out the highest MEnCh value of that serie:  32 J/kgK@220 K for the compound MnFeP0.65As0.35.

Concerning the Heusler alloys Ni2MnGa, this material has a decoupled magneto-structural transition of about 100 K. Hu et al.[18] was the first to report the magnetocaloric properties of this compound, and, more recently, Zhou et al.[19] reported a coupled magneto-structural transition for the out of stoichiometry Ni55Mn19Ga26, with a huge, but narrow, MEnCh: 20 J/kgK@317 K.

 

4. Manganites family (RMnO3)

The MCE of mixed-valency manganites was first measured by Morelli et al. in 1996, for thick films of La2/3(Ca/Sr/Ba)1/3MnO3 [20]. The magneto-structural coupling [21] and the numerous possibilities of exchanging elements in synthesis make manganites a rich field in MCE study.

The substitution of La by other rare-earth ion in La2/3Ca1/3MnO3 manganite can tune the transition temperature and enhance magnetocaloric properties [22]. Some manganites systems show also a magneto-structural coupling, due to charge/orbital ordering [5,23]. For instance, Chen and co-workers found a MEnCh value of 7.1 J/kg.K near the charge/orbital ordering temperature (161K) of the Pr0.5Sr0.5MnO3 manganite [24].

  

5. Intermetallics family R-M

There are several sub-families, namely the Laves Phase compounds, as RCo2[25], RAl2[26] and RNi2[27], but all of these are not suitable for applications around room temperature due to their low values of MEnCh. On the contrary, those systems are the reason of several theoretical models, due to its interesting, from the academic point of view, magnetocaloric properties[27]. Other compounds can also be cited: Nd2Fe17 (5.9 J/kgK@325 K)[28], Gd7Pd3 (6.5 J/kgK@323 K)[29], Gd4Bi3 (2.7 J/kgK@332 K)[30] and Gd2In (4.5 J/kgK@194 K)[31].

All of those values of MEnCh above reported are related to 5 T of magnetic field change.

 

Conclusions

All of those families above described, and their corresponding compounds, have problems that avoid their immediate application in a magnetic cooling device. We can mention those systems with (i) first order magnetic transition and a consequent thermal hysteresis, producing than energy losses during the thermo-magnetic cycle; (ii) high pure rare-earth metals, making them economically unviable; (iii) elements that need special handling, for instance the poison Arsenic; (iv) narrow MEnCh curve, avoiding therefore a wide thermo-magnetic cycle and, finally, (v) low values of MEnCh, decreasing the cooling power of the device.

This brief overview of the actually know materials for applications of the MCE around room temperature ratifies the claim of the scientific community: we (researchers) still need to seek for new materials and/or optimize the magnetic features of those know materials.

 

References

[1] Pecharsky, Phys. Rev. Lett. 78 (1997) 4494

[2] Choe, Phys. Rev. Lett. 84 (2000) 4617

[3] Pecharsky, Phys. Rev. Lett. 91 (2003) 197204

[4] Morellon, Phys. Rev. Lett. 93 (2004) 137201

[5] Gschneidner, Rep. Prog. Phys. 68 (2005) 1479

[6] Spichkin, J. Appl. Phys. 89 (2001) 1738

[7] Thuy, Proc. 8th Asia-Pacific Physics Conf. (Singapore: World Scientific) (2001) 354

[8] Morellon, Appl. Phys. Lett. 79 (2001) 1318

[9] Ivtchenko, Adv. Cryog. Eng. 46 (2000) 405

[10] Thuy, J. Magn. Magn. Mater. 262 (2003) 432

[11] Hu, J. Phys: Condens. Matter 12 (2000) 2691

[12] Fujita, Phys. Rev. B 67 (2003) 104416

[13] Fujita, Phys. Rev. B 65 (2001) 014410

[14] Fujieda, Proceedings of the First IIR International Conf. on Magnetic Refrigeration at Room Temperature, Montreux-Switzerland (2005) 193

[15] Wada, Appl. Phys. Lett. 79 (2001) 3302

[16] Gama, Phys. Rev. Lett. 93 (2004) 237202

[17] Tegus, Physica B 319 (2002) 174

[18] Hu, Appl. Phys. Lett., 76 (2000) 3460

[19] Zhou, J. Phys.: Condens. Matter 16 (2004) L39

[20] Morelli, J. App. Phys. 79 (1996) 373

[21] Amaral, J. Magn. Magn. Mater. 272 (2004) 2104

[22] Chen, J. Magn. Magn. Mater. 257 (2003) 254

[23] Reis, Phys. Rev. B 71 (2005) 144413

[24] Chen, Europhys. Lett. 52 (2000) 589

[25] Wang, Phys. Lett. A 297 (2002) 247

[26] de Oliveira, Phys. Rev. B 69 (2004) 064421

[27] von Ranke, Phys. Rev. B 63 (2001) 184406

[28] Dan’kov, Adv. Cryog. Eng. 46 (2000) 397

[29] Canepa, Intermetallics 10 (2002) 731

[30] Niu, J. Magn. Magn. Mater. 234 (2001) 193

[31] Ilyn, Cryocoolers 11 ed R G Ross Jr (New York: Kluwer/Plenum) (2001) 457