VNƯ Journal o f Science, Mathematics - Physics 24 (2008) 30-35 Room-temperature large magnetocaloric effect in perovskites (Lai.xNdx)o.vSro.3Mn03 N guyen H oang L u o n g ’’*, N guyen Thi M P h u o n g ’’^ P T hu H ie n ’, H oang N am N h a t‘, Luc H uy Hoang^, N guyen C h au ’, N guyen H oang H a i‘ ' C enter f o r M a teria ls Science, C ollege o f Science, Vietnam N a tio n a l U niversity, H anoi, 334 N gu yen Trai, H anoi, Vietnam -H a n o i U niversity o f Education, I3 X u a n Thuy, H anoi, Vietnam R eceived M arch 2008 Abstract Study of the effect of Nd substitution for La on magnetocaloric effect (MCE) of polycrystalline perovskites (Lai^NdOojSrojMnOj (x = 0.0, 0.2 , 0.4, 0.6, 0.8, 1.0 ) is presented Large MCE is observed in all samples The presence of Nd affects to both maximum magnetic entropy change, lASmlmax, and Curie temperature, Tc- |AS„,|nax slightly reduces with low content and increases with high content of Nd and gets maximum value of 4.83 J/kg.K for X = 0.8 Tc of the sam ples d eterm in ed from the therm om agnetic curves so m ew hat increases fro m 346 K for X = 0 to 350 K for X = 0.2, then decreases to 235 K for X = 1.0 T he sam ple w ith X = 0.4 exhibiting the largest value of 74 J/kg for the relative cooling power among the studied samples has Tc= 325 K , i.e h ig h er th an ro o m tem perature O ur studied sam ples are p rom ising m aterials for m agnetic refrig eran ts in the ro o m -tem p eratu re region K eyw ords: M ag n eto calo ric effect, perovskites, m anganites Introduction The m agnetocaloric effect (M CE) is detected as the heating or the cooling o f m agnetic materials due to a varying m agnetic field M agnetic refrigeration provides an alternative m ethod for cooling 1,2] The m aterial used to provide the M CE is called a m agnetic refrigerant The M CE has been used for m any years to obtain low tem peratures (o f the order o f m ilikelvins) through adiabatic dem agnetization o f param agnetic salts [3] A t present the m agnetic refrigeration around room tem perature is o f particular interest because o f potential im pact on energy savings as well as environm e ntal concerns (the desire to elim inate the chlorofluorocarbons present in high-temperature gas-cycle systems) M CE is represented by a adiabatic tem perature change, ATad, or isothermal magnetic entropy change, ASn„ w hich are correlated w ith magnetization, M , m agnetic field change, AH, heat capacity, c, and absolute tem perature,T, by the following equations: Coưesponding author Tel: 84-4-8589496 E-mail: luongnh@vnu.edu.vn 30 Nguyen Hoang Luong et al / VNU Journal o f Science, Mathematics - Physics 24 (2008) 30-35 31 ^T^ATAH) - - " C(T,H)„ (2 ) dT obtained from M axw ell fundamental relation In Eqs (1) and (2) is the final applied m agnetic field Eqs (1) and (2) have a fundamental importance on the understanding o f the behaviour o f the M CE in solids and serve as a guide for the search o f new m aterials w ith a large M CE [2 The high cooling efficiency o f magnetic refrigerators is only realised in high m agnetic fields of about 50 kOe or higher Therefore, research for new magnetic m aterials displaying large M CE, which can be operated in low fields o f about 20 kOe that can be generated by perm anent magnets, is very important In this direction, in last years we have studied perovskite-type m anganites The perovskite-type m anganites Lni_xA’xMn03 (Ln = rare-earth elem ent, A ’ = alkaline elem ent) are attracting considerable interest because they reveal interesting phenom ena such as magnetoresi stance, M CE, charge ordering, spin-glass behaviour, and m agnetostriction effect Particularly, the com pound L a o v S ro jM n O s has received much attention due to its interesting m agnetic and m agnetotransport properties and its promise for future technological applications (see, for instance, [4]) Chau et al [5] have studied structure, magnetic and m agnetocaloric properties o f perovskite Lao.vSrojMnOa w ith small amount o f Cu substituted for Mn They have found that these m aterials exhibit maxim um m agnetic entropy change around Curie tem perature, Tc, o f about 350 K Ndo 7Sro.3M n 03 is ĩeưom agnetic with value for T c o f about 210 K, i.e far below room tem perature [6 ] The purpose o f this w ork is study o f the structure and properties o f (Lai.xNdx)o.7Sro.3M n 03 (x = 0.0, 0.2, 0.4, 0.6, 0,8, 1.0) perovskites with the expectation that they could establish M CE at roomtem perature region Experim ental The perovskites m anganites (Lai.xNdx)o.7Sro.3M n 03 (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) were prepared by using conventional pow der solid-state reaction technique A fter have been hom ogeneously m ixed and com pletely ground, the samples were pre-sintered at 0 °c for 15 h The heated samples were cooled to room temperature, reground to fine particles, and sintered at 1200 ° c for 15 h The structure o f samples was exam ined by using a Bruker D5005 X -ray diffractom eter The microstructure w as studied in a scanning electron m icroscope (SEM ) 5410 LV Jeol M agnetic m easurements w ere perform ed with a vibrating sam ple m agnetom eter (VSM ) D igital M easurem ent Systems DMS 880 in m agnetic fields up to 13.5 kOe Results and discussion For all sam ples, the SEM images show that the crystallites are hom ogeneous w ith average size o f - ^m Fig shows the SEM image for (Lao,4Ndo.6)o.7Sro,3M n 03 as an exam ple T he X-ray 32 Nguyen Hoang Luong et al / VNU Journal o f Science, Mathematics - Physics 24 (2008) 30-35 diffraction patterns o f the sam ples are presented in Fig These patterns reveal that all the studied samples are o f single phase w ith hexagonal structure w ith R-3c space group Fig SEM im age o f the sam ple (Lao,4Ndo,6)o ySrojM nO; 20 ỵ> 40 50 70 80 90 - Thets F ig X -ray diffractio n patterns o f the (Lai.xNdx)o.7Sro.3 M n perovskites Field-cooled (FC) and zero-field-cooled (ZFC) m agnetization curves for all sam ples were m easured in the m agnetic field o f 20 Oe Results are presented in Fig From Fig we can see that the FC and ZFC m agnetization curves are separated from each other at below irreversibility temperature, Tr, and there is a cusp in ZFC curves at a so-called freezing or spin-glass-like transition temperature, Tf T hese phenom ena are specific features o f spin-glass- or cluster-glass-like state The feưom agnetic-param agnetic ừansition temperature, Tc, determ ined from these m easurem ents (see Table 1) decreases w ith increasing N d content and this dependence is sim ilar to that for system (LaN d-Ca)M n 03 [7] and (La-N d-Ba)M n 03 [8 ], The value o f Tc som ew hat increases from 346 K for X = 0.0 to 350 K for X = 0.2, then decreases to 235 K for X = 1.0 The general decrease o f T ccan be due to the decrease o f < Ĩ A > and the decrease o f ratio W hile ions are substituted for ions in the sample, the ratio keeps unchanged So the decrease o f T c with increasing X can be explained by the reduction o f the double exchange interaction due to the decrease o f < Ĩ A > Nguyen Hoang Luong et al / VNU Journal o f Science, Mathematics - Physics 24 (2008) 30-35 100 150 200 250 300 350 33 400 T(K) Fig FC and ZFC therm o m ag n etic curves o f the (Lai_^Ndx)o.7Sro,3 M n sam ples in a m ag n etic field o f 20 Oe H (Oe) Fig T h e isotherm al m agnetization curves for the sam ple (Lao,4Ndo,6)o.7 Sro,3 M n The M (H) isotherm s have been m easured for all investigated sam ples at various tem peratures in a naư ow tem perature interval around the respective Tc, in magnetic field up to 13.5 kOe Fig shows a set o f isothermal M (H) curves o f perovskite (Lao.4Ndo,6)o.7Sro,3M n 03 as an exam ple The temperature dependence o f |ASm| has been detem iined from the M (H) isotherms coưespondingly Fig presents the dependence o f |ASn,| on tem perature for all the samples investigated From this figure one can see that all the samples exhibit large M CE, at m oderately low m agnetic field change V alues o f lASmlmax for all the sam ples are shown in Table |ASm|niax reaches highest value o f 4.83 J/kg.K for the com position X = 0.8 Sample w ith X = 0.6 possesses |ASm|max o f 3.56 J/kg.K a n d T c o f 293 K 34 Nguyen Hoang Luong et al / VNU Journal o f Science, Mathematics - Physics 24 (2008) 30-35 X= 1.U X= U.Í5 Ị (Laj.xNdx)o.7Sro.3 Mn 03 x=0 400 Fig The magnetic enfropy change as a function of temperature of the (Lai.xNdx)o.7Sro.3Mn03 samples For magnetic materials, the relative cooling pow er (RCP) represents a good w ay for com paring them and is defined as R C P = A S „ „ ^ ,.5 T FWUM) (3 ) where STpwHM means the full-w idth at half-m axim um o f the m agnetic entropy change versus tem perature [9], The RCP values for all studied samples are listed in Table A prom ising m aterials for magnetic refrigeration application should have high RCP and the value for T c close to room tem perature Our studied m aterials satisfy these requirements Table Curie temperature, Tc, magnetization measured at 13.5 kOe, Mi3.5, maximum magnetic entropy change, |AS,n|n,ax, and relative cooling power (RCP) value for (Lai.xNdxVySro.sMnOj X I^Smlmax (J/kg.K) RCP (K) M i3,5 (emu/g) 0.0 346 58.6 3.84 42 0.2 350 61.8 2.86 71 0.4 325 61.7 3.20 74 0.6 293 65.3 3.56 43 0.8 263 71.7 4.83 43 1.0 235 75.1 4.78 62 Tc (J/kg) C onclusion The results show that (Lai.xNdJo.vSrojMnOs m aterials are prom ising candidates for magnetic refrigerants w orking in the room -tem perature region under a m oderate applied m agnetic field A cknow ledgm ents The authors would like to thank the V ietnam N ational Fundam ental Research Program (Project 406006) for the financial support Nguyen Hoang Luong et a l / VNU Journal o f Science, Mathematics - Physics 24 (2008) 30-35 35 R eferences [1] A.M Tishin, in Handbook o f Magnetic Materials, cd FC H J Buschow, Elsevier, Amsterdam, Vol Ỉ2 (1999) Chap [2] V.K Pecharsky, K.A Gschncidner, Jr., Magnetocaloric effcct and magnetic refrigaration, J Magn Magn Mater 200 (1 9 ) 44 [3] A.H Morrish, The Physical Principles o f Magnetism, IEEE, New York, 200Ĩ, Chap [4] C.N.R Rao B Raveau (Eds.), Colossal Magnetoresistance, Charge Ordering and Related Properties o f Manganese Oxides, World Scientific, Singapore, 1998 [5] N Chau P.Q Nicm, H.N Nhat, N.H Luong, N.D Tho, Influence ofC u substitution for Mn on the structure, magnetic, magnetocaloric and magnetoresistancc properties of Lao.7Sro,3Mn perovskites, Physica B 327 (2003) 214 [6] M Tseggai, R Mathieu, p Nordblad, R Tcllgren, L.v Bau, D.N.H Nam, N x Phuc, N v Khiem, G Andre, F Bourcc Effects of magnesium substitution on the magnetic properties of Ndo TSrojMnOs, J Solid State Chem 178 (2 0 ) 1203 [7] Z.M Wang G NÍ, Q.Y Xu, H Sang, Y.w Du, Magnetocaloric cffect in pcrovskites manganitcs Lao 7-xNdxCao,3 Mn and Lao7Cao 3MnƠ , J A p p l Phys 90 (2001) 5689 [8] w Chen, L.Y Nie, w Zhong, Y J Shi, J.J Hu, A.J Li, Y w Du, Magnetocaloric effect in Nd doped pcrovskile Lao 7xNdxBao3 Mn polycrystallinc near room temperature, J Alloys Compd 395 (2005) 23-25 [9] K.A Gschncidcr, V.K Pccharsky, Magnctocaloric materials, Annu Rev Mater Set 30 (2000) 387 ... alkaline elem ent) are attracting considerable interest because they reveal interesting phenom ena such as magnetoresi stance, M CE, charge ordering, spin-glass behaviour, and m agnetostriction effect. .. pre-sintered at 0 °c for 15 h The heated samples were cooled to room temperature, reground to fine particles, and sintered at 1200 ° c for 15 h The structure o f samples was exam ined by using... presented in Fig From Fig we can see that the FC and ZFC m agnetization curves are separated from each other at below irreversibility temperature, Tr, and there is a cusp in ZFC curves at a so-called