VNU. JOURNAL OF SCIENCE. Mathematics - Physics. T.xx. N03AP, 2004
T H E E L E C T R IC , M A G N ETIC AND M A G N ETO C A LO R IC P R O P E R T IE S O F P E R O V S K IT E S L a06(P b 04.xC a x)M nO 3 (x = 0 .0 ,0 .2 )
N g u y e n D u e T h o , D u o n g T h i H a n h , N g u y e n C h a u D epartm ent o f Physics, College o f Science, V N U
A b s tr a c t: In th e p re v io u s p a p e r w e in ve stig a te d th e s tru ctu re , m ag ne tic, m a g n e to c a lo ric a n d m a g n e to re s is ta n c e p ro p e rtie s of p e ro vskite s L a ,.„P b „M n 0 3.
In th is w o rk w e rep o rt th e in flu e n c e o f th e s im u lta n e o u s s u b s titu tio n o f P b and C a o n th e p ro p e rtie s o f m a te ria ls. The stu d ie d pe ro vskite s w e re s in g le ph ase w ith rh o m b o h e d ra l s tru ctu re . T h e m ic ro s tru c tu re o f m a n g a n ite s w e re e xa m in ed b y S E M a n d th e re su lts sh o w th a t th e s a m p le N°1 ha s h o m o g e n e o u s grain s tru c tu re w ith th e s ize o f 0 .3 nm w h ile th e sa m p le N°2 ha s in h o m o g e n e o u s grain s tru c tu re w ith th e s iz e ran g in g fro m 0.4 to 1.2 nm .
T h e FC a n d Z F C m e a s u re m e n ts pe rfo rm e d a t low fie ld in d ic a te d th a t th e re is spin glass like state occurring in both compositions. The substitution of Ca for Pb (ra d iu s o f C a 2' io n s is less th a n o f Pb*’ ) in th e sa m p le le a d in g to re d u c e o f <rA>
a n d y ie ld in g d e cre a se o f T c , clo sin g to roo m te m p e ra tu re . T h e s h a rp c h a n g e of M (T ) a ro u n d F M -P M tra n sitio n le a d in g to s ig n ifica n tly in cre a se o f (ASJma,.
1. I n t r o d u c t i o n
R are e a rth m an ganites Ln^A jM nO a (Ln = ra re e a rth , A = alk a lin e earth) have a subject of inten siv e stu d y due to th e ir in teresting behaviors, such a s colossal m agnetoresistance (CMR) [1,2], charge ordering, phase sep a ratio n an d th e ir prom ised for fu tu re technological applications. Double exchange (DE) m echanism provides a qualitative explanation for th e m agnetoresistance properties of these m aterials. By su b stitu tin g Ca and Pb for La in La2/3(PbC a)1,3M n03 single crystal. Young et al [3] found th a t th e maximum m agnetic entropy change |A S J a 7.5 J/kg.K a t C urie tem p eratu re (Tc) 290K and m agnetic field variation 7T. T royanchuk e t al. [4] have observed th a t th e La,.xPbxM n 0 3 perovskites (x
= 0.4 -r 0.6) have a rhom bohedral (slightly distorted) cubic stru ctu re . H wang e t al. [5] have studied th e cry sta l s tru c tu re an d the m agnetic scaling behavior of La|.,Pb„M n03 perovskites (x = 0.0 + 0.5) and have show th a t all th e sam ples crystallize in the rhom bohedral stru c tu re . In o u r previous paper, overall investigation of properties of Lai.
xPbxM n 0 3 (x = 0.0 T 0.5) h a s been perform ed [6]. The re su lts show th a t th e sym m etry decreases from cubic (x = 0.5) to rhom bohedral (x = 0.4) an d triclinic (x = 0.3, 0.2, 0.1), moreover th e C urie te m p e ra tu re increases from 235 K for X = 0.1 to 310 K for X = 0.2 and then rem ained alm ost con stan t w ith fu rth e r increasing X.
In th is work, w e re p o rt on o u r study of structure , m agnetic an d magnetocaloric properties of La0 6(Pb0..(.jjCaJMnOj (x = 0.0, 0.2) m anganites.
2. E x p e r i m e n t a l
T he sam p les La0 6(Pb0 4.xCax)M n03 (x = 0.0, 0.2) were pre p are d by m ethod of standard solid-state reaction technique. P resin terin g are tw ice perform ed a t 800°c an d 900°c. The sam ples are s in te re d a t 920°c (sam ple Npl) an d 950°c (sam ple N°2).
1 3 3
134 N g u y e n D uc T h o , D u o n g T h i H a n h , N g u y e n C h a u
The s tru c tu re of th e sam ples w as exam ined in a B ru k er D5005 X-ray diffractom eter.
The m icrostructure an d chem ical composition w ere studied in a 5410 LV Jeol scanning electron microscope (SEM. M agnetic m easurem ents w ere perform ed in a vib ratin g sam ple m agnetom eter (VSM) DMS 880 in m agnetic field up to 13.5 kOe.
3. R e s u lts a n d d is c u s s io n
The SEM an a ly sis indicates th a t the crystallites of th e sam ple N('l h as homogeneous grain s tru c tu re (F ig.l) w ith the size of 0.3 nm w hile the sam ple Nọ2 h a s inhomogeneous grain s tru c tu re with th e g ra in size ranging from 0.4 to 1.2 nm.
Fig.l. SEM photograph of the surface of sample N°l.
Fig. 2. X-ray diffraction patterns of studied samples.
Fig.2 pre sen ts the X-ray diffraction p a tte rn s of studied sam ples. It’s clearly th a t all sam ples are of single p hase with rhom bohedral stru c tu re . The lattice p aram eters w ere d eterm ined from X-ray data. The re su lts show th a t th e lattice p aram eters decrease lightly in sam ple Ne2 with sm aller am ount of Pb.
Zero-field-cooled (ZFC) a n d field cooled (FC) m agnetization m easurem ents w ere perform ed in a m agnetic field of 20 Oe.
Fig. 3 show s t h a t th e FC an d ZFC curves of sam ples a re s ep a rate d from each o th er a t low tem peratures. It is suggested th a t spin- glass like s ta te exists in o u r sam ples a t low tem peratures. The com petition betw een ferrom agnetic an d antiferrom agnetic phase
plays an im p o rtan t role in th is case' The C urie tem p eratu re (Tc) of two sam ples were determ ined from th ese therm om agnetiç curves. T he T r decrease w ith decreasing Pb content in sam ple N"2 from X = 0 <TC = 360 K) to X = 0.2 (Tc = 305 K). T he substitution o f Ca for Pb (radius of Ca2* ion is less th a n of Pb2*) in the sam ple N°2 leading to reduce of <rA> and yielding decrease of Tc, closing to room tem perature.
Isotherm al m agnetization curves M(H) have been m easured a t various tem p eratu re around the C urie tem p eratu re, in m agnetic field up to 13.Õ kOe. In o rd e r to ev a lu ate the magnetocaloric effect of the studied sam ples, we calculated th e changes of th e magnetic
T(K)
Fig.3. Thermomagnetic field-cooled (FC) and zero-field-cooled (ZFC) curves of
samples m easured a t 20 Oe.
T h e e le c tr ic , m a g n e tic a n d m a g n e to c a lo r ic p ro p e r tie s ... 1 3 5
entropy (ASm) caused by th e application of in tern a l m agnetic fields by using th e following expression (1):
H-/«
AS(T, h ) = S(T.O) - S(T, H) = J JcM(T,H)/3T}h dH (1) W here. S<T,0) and S(T.H) re p rese n t the
entropy w ithout and w ith applied m agnetic field, respectively.
Fig.4 show s the m agnetic entropy change of sam ples as a function of tem perature. C learly th a t a sh a rp peak in I ASni I is occurred aro u n d Tc an d the m axim um value is 0.87 J/kg.K and 1.79 J/kg.K for sam ple N91 an d N°2, respectively.
The lASml mox o f sam ple N°2 is h igher th an those investigated in o ur previous re p o rts for La,.,,SrxCoO;, [7], for L a,„P bxM n 0 3 [6] b u t less th an value for La()7Sr„3M n 0 3 [8] w ith small s ubstitution of Cu for M n and for L a0.7S r03MnO3 with sm all su b stitu tio n of Ni for M n [9],
4. C o n c lu s io n s
M agnetic an d m agnetocaloric properties of rhom bohedral m anganites La06(Pb04.
sCax)M n 0 3 (x = 0.0, 0.2) w ere studied. Spin-glass like s ta te exists in both o u r sam ples a t low tem p eratu re. Sam ple Nfi2 with 1.79 J/kg.K m ay be considered a s m agnetic re frigerant m aterials o perating a t room tem perature.
A cknow ledgm ents:
This work w as supported by th e N ational F undam ental R esearch Program for N a tu ral Science (Project 421004).
R e fe re n c e s
1. R.Von. Helmolt, J. Wecker, D. Holzapfel, L. Schultz, K. Samwer, Phys. Rev. Lett. 71(1993) 2331.
2. J.M .D. Coey. M. Viret. s . M. Molnar, Adv. Phys. 48(1999) 167.
3. Young Sun M.B. Salamon, S.H. Chun, J. Appl. Phys. 92(2002) 3235.
4. 1.0. Troyanchuk, D.D. Khalyavin. H. Szymczak, Mater. Sci. Bull. 32(1997) 1637.
5. T .s. Hwang, C.H. Chen. M.F. Tai, Mater. Res. Soc. Symp. Proc. 674(2001) u 3.4.1.
6. Nguyen Chau, Hoang Nam N hat, Nguyen Hoang Luong, Đang Le Minh, Nguyen Duc Tho, Nguyen Ngoe Chau, Physica B 327(2003) 270.
7. N.H. Luong, N. Chau, P.M. Huong, D.L. Minh, N.N. Chau, B.T. Cong, M. Kurisu, J. Magn.
Magn. Mater. 242-245(2002) 760.
8. Nguyen Chau, Pham Quang Niem, Hoang Nam Nhat, Nguyen Hoang Luong, Nguyen Due Tho, Physica B 327(2003) 214.
9. Md.A. Choudhury, J.A. Akhter, D.L. Minh, N.D. Tho, N. Chau, J. Magn. Magn. Mater. 272- 276(2004) 1295.
Fig.4. Magnetic entropy change
