X-ray diffraction patterns of studied samples.. Thermomagnetic field-cooled (FC) and zero-field-cooled (ZFC) curves of.[r]
Trang 1T 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
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
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 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>
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
fu tu re technological applications Double exchange (DE) m echanism provides a qualitative 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 field variation 7T T royanchuk e t al [4] have observed th a t th e La,.xPbxM n 0 3 perovskites (x 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 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 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)
Trang 2The 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
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
rhom bohedral stru c tu re The lattice
p aram eters w ere d eterm ined from X-ray
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
glass like s ta te exists in o u r sam ples a t low
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
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
T(K) Fig.3 Thermomagnetic field-cooled (FC) and zero-field-cooled (ZFC) curves of samples m easured a t 20 Oe
Trang 3entropy (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
La,.,,SrxCoO;, [7], for L a,„P bxM n 0 3 [6] b u t less
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
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
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Fig.4 Magnetic entropy change