Journal of Magnetism and Magnetic Materials 304 2006 e448–e450Relation between EPR spectra and electrical conductivity of B.T.. Phanb a Center for Materials Science, Hanoi University of
Trang 1Journal of Magnetism and Magnetic Materials 304 (2006) e448–e450
Relation between EPR spectra and electrical conductivity of
B.T Conga, S.C Yub, , N.D Thoa, N Chaua, T.N Huynhb, T.L Phanb
a Center for Materials Science, Hanoi University of Science, 334 NguyenTrai, Hanoi, Vietnam
b Department of Physics, Chungbuk National University, Cheongju 361-763, Republic of Korea
Available online 24 March 2006
Abstract
The correlation between structure, Curie temperature (TC), line width of EPR spectra and electrical conductivity of Pr1xPbxMnO3 (x ¼ 0:1, 0.2, 0.3, 0.4, and 0.5) perovskites is discussed It was shown that both adiabatic small polaron and variable range hopping models are good for description of conductivity in paramagnetic region but the first one is more suitable for interpretation of temperature dependence of EPR line width in temperature range 1.2 TCoTo1.3 TC
r2006 Elsevier B.V All rights reserved
PACS: 71.30 +h; 75.47.Lx; 76.30.v
Keywords: Perovskite; Magnetic and electrical properties; EPR
The highly conducting Pr1xMxMnO3 (M ¼ Ca, Sr)
systems exhibit colossal magnetoresistance, charge
order-ing effects [1] and have a great potential to be used as
solid oxide fuel cell Recently, lead-doped A1xPbxMnO3
(A: rare-earth ions) compounds have attracted much
attention because their interesting physical properties
occurring near room temperature There are several works
on lead doping lanthanum La1xPbxMnO3 perovskites
[2,3] The aim of this contribution is to investigate the
magnetic and electrical properties of lead doping
praseo-dymium Pr1xPbxMnO3 (x ¼ 0:120:5) system and their
mutual correlation
The Pr1xPbxMnO3 (x ¼ 0:1, 0.2, 0.3, 0.4, and 0.5)
perovskites compound were synthesized by solid-state
reaction method similar to that described in Ref [3] The
XRD patterns recorded by Bruker D5005 confirm that all
samples are of single phase with orthorhombic structure
Because Pr3+ion radius significantly smaller than radii of
La+3 and Pb+2 ions (ionic radii of these ions are 1.179,
1.216, and 1.35 A˚ respectively, see Ref [4]) the effect of
Pb+2doping results in a stronger distortion of perovskite
structure InTable 1one can see a considerable increase of lattice parameter a, unit cell volume V, average radius of A site /rAS, variance s2¼P
xir2
i hrAi2 with increasing
Pb+2 content The TC derived from the thermomagnetic measurements are given inTable 1 As can be seen clearly from Table 1, TC increases with increasing lead content from x ¼ 0:1 (TC¼152 K) to x ¼ 0:4 (TC¼256 K) and seems slightly reduced in sample x ¼ 0:5 Increasing of /rAS due to lead doping is equivalent to increasing internal pressure, extending the Mn–O–Mn angle and eg electron bandwidth, which enhances strength of double exchange and then TC enhances For low doping level x, the A-ion size effect dominates but for higher value x0.5 the hole-carrier concentration seems to reduce TC
Fig 1 plotted the temperature dependence of sample’s resistivity The obtained results show that the conducting mechanism changes strongly with lead-doping fraction The compounds x ¼ 0:1 and 0.2 exhibit semiconducting behavior in the whole temperature range, while composi-tions x ¼ 0:320:5 (see inst inFig 1) show the metal–insu-lator transitions This behavior is understood because enhancement of FM double exchange strength near x ¼ 0:3 making samples to have metallic conducting below TC We used the following models for r(T) curve fitting: band gap
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0304-8853/$ - see front matter r 2006 Elsevier B.V All rights reserved.
doi:10.1016/j.jmmm.2006.02.247
Corresponding author Tel.: +82 43 2612269; fax: +82 43 2756415.
E-mail address: scyu@chungbuk.ac.kr (S.C Yu).
Trang 2(BG) for x ¼ 0:1, 0.2 samples, the small polaron (SP) and
Mott–Viret’s variable range hopping (VRH) [5] for all
samples in temperature regions given inTable 2 Table 2
shows the temperature dependence of resistivity laws, BG
energy EH, an activation polaron hopping energy WP,
characterized temperature To obtained by linear fitting
The energy WPis largest for x ¼ 0:3 sample According to
Viret [5], electrical carriers in magnetic perovskite are
hopping between localized states due to random magnetic
potential, and To¼17UmV =ðkl3Þ in PM region Taking
the Hund splitting Um¼2 eV [5], cell volume value V
(Table 1), fitting values of To, we obtained localization
length l, and average hopping distance at room
tempera-ture R ¼ 0:376l (To/T)0.25 These values of R are almost 4
times larger than the Mn–Mn separation and indicate the
applicable of VRH model.Fig 2presents the temperature dependence of line width (DH) at 9.2 GHz (X band) measured by a Jeol JES-TE300 EPR spectrometer DH has minimum at Tmin (see Table 1), which is slightly higher than TCand increases with increasing temperature
It is well-known that DH depends on temperature similar
to conductivity one, DHðT Þr1ðTÞ Some of us have used the adiabatic SP conductivity law for interpretation of EPR experiment data[6] Because samples x ¼ 0:3, 0.4, 0.5 have similar DH(T) dependence, we applied both SP, and Mott–Viret’s VRH conductivity laws for fitting only DH(T) curve of x ¼ 0:3 sample and the result is
Table 1
Lattice parameters and characteristic temperatures
x a (A˚) b (A˚) c (A˚) V (A˚3) /r A S (A˚) s2(105A˚2) T MI (K) T C (K) T min (K)
Fig 1 Resistance versus temperature for the samples Pr 1x Pb x MnO 3
(x ¼ 0:120:5).
Fig 2 The linewidths versus temperature for the studied samples mesured
at 9.2 GHz (X-band).
Table 2
The parameters of samples for three conducting models
x Temperature range (K) BG r ¼ r N exp[E H /kT] SP r ¼ r o Texp[W p /kT] VRH r ¼ r N exp{[T o /T]1/4}
Trang 3given in Fig 3 This fitting procedure gives Ea(To) value
of SP (VRH) model as 0.129 eV (5.09 107K) in the
temperature range 270 KoTo300 K In the next
tempera-ture interval 300 KoTo475 K, one has Ea¼0:093 eV and
To¼1408:3 K The first Eavalue (0.129 eV) for SP model
is almost the same compared with 0.117 eV given by resistivity fitting (see Table 2) The value To¼5:09
107K is exceeding almost two times the value given in
Table 2 A possible origin of this discrepancy is VRH model applied with fluctuating random magnetic potential near TC but EPR spectra is better registered in full PM state far TC
This work was supported by Laboratory of Magnetism, Chungbuk National University, and the ASIA research center (VNU)
References
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[2] S.S Manoharan, N.Y Vasanthacharya, M.S Hegde, K.M Satyalakshmi, V Prasad, S.V Subramanyam, J Appl Phys 76 (1994) 3923.
[3] N Chau, H.N Nhat, N.H Luong, D.L Minh, N.D Tho, N.N Chau, Physica B 327 (2003) 270.
[4] R.D Shannon, Acta Cryst A 32 (1976) 751.
[5] M Viret, L Lanno, J.M.D Coey, Phys Rev B 55 (1997) 8067 [6] A.N Ulianov, G.G Levchenko, Seong-Cho Yu, Solid Stat Commun.
123 (2002) 383.
Fig 3 The DH(T) fitting using VRH and SP (insert) r 1 (T) laws for
sample with x ¼ 0:3.