ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 304 (2006) e448–e450 www.elsevier.com/locate/jmmm Relation between EPR spectra and electrical conductivity of Pr1ÀxPbxMnO3 perovskites B.T Conga, S.C Yub,Ã, N.D Thoa, N Chaua, T.N Huynhb, T.L Phanb a Center for Materials Science, Hanoi University of Science, 334 NguyenTrai, Hanoi, Vietnam Department of Physics, Chungbuk National University, Cheongju 361-763, Republic of Korea b Available online 24 March 2006 Abstract The correlation between structure, Curie temperature (TC), line width of EPR spectra and electrical conductivity of Pr1ÀxPbxMnO3 (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 r 2006 Elsevier B.V All rights reserved PACS: 71.30 +h; 75.47.Lx; 76.30.Àv Keywords: Perovskite; Magnetic and electrical properties; EPR The highly conducting Pr1ÀxMxMnO3 (M ¼ Ca, Sr) systems exhibit colossal magnetoresistance, charge ordering effects [1] and have a great potential to be used as solid oxide fuel cell Recently, lead-doped A1ÀxPbxMnO3 (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 La1ÀxPbxMnO3 perovskites [2,3] The aim of this contribution is to investigate the magnetic and electrical properties of lead doping praseodymium Pr1ÀxPbxMnO3 (x ¼ 0:120:5) system and their mutual correlation The Pr1ÀxPbxMnO3 (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+2 doping results in a stronger distortion of perovskite ÃCorresponding author Tel.: +82 43 2612269; fax: +82 43 2756415 E-mail address: scyu@chungbuk.ac.kr (S.C Yu) 0304-8853/$ - see front matter r 2006 Elsevier B.V All rights reserved doi:10.1016/j.jmmm.2006.02.247 structure In Table one can see a considerable increase of lattice parameter a, unit cellP volume V, average radius of A xi r2i À hrA i2 with increasing site /rAS, variance s2 ¼ +2 Pb content The TC derived from the thermomagnetic measurements are given in Table As can be seen clearly from Table 1, TC increases with increasing lead content from x ¼ 0:1 (T C ¼ 152 K) to x ¼ 0:4 (T C ¼ 256 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 x$0.5 the hole-carrier concentration seems to reduce TC Fig 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 compositions x ¼ 0:320:5 (see inst in Fig 1) show the metal–insulator 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 ARTICLE IN PRESS B.T Cong et al / Journal of Magnetism and Magnetic Materials 304 (2006) e448–e450 e449 Table Lattice parameters and characteristic temperatures x a (A˚) b (A˚) c (A˚) V (A˚3) /rAS (A˚) s2(10À5 A˚2) TMI (K) TC (K) Tmin (K) 0.1 0.2 0.3 0.4 0.5 5.345 5.373 5.401 5.422 5.437 5.479 5.473 5.472 5.468 5.467 7.747 7.747 7.747 7.749 7.748 226.9 227.8 229.0 229.7 230.3 1.196 1.213 1.230 1.247 1.265 263 468 688 702 731 — — 210 199 170 152 172 223 256 254 161 200 268 281 278 Table The parameters of samples for three conducting models x 0.1 0.2 0.3 0.4 0.5 Temperature range (K) 152–300 172–300 210–300 199–300 170–300 BG r ¼ rN exp[EH/kT] SP r ¼ ro Texp[Wp/kT] VRH r ¼ rN exp{[To/T]1/4} EH (eV) WP (eV) To( Â 107 K) l (nm) R (nm) 0.063 0.040 0.081 0.059 0.117 0.106 0.088 0.670 0.088 2.451 1.385 0.671 0.514 1.011 0.333 0.404 0.514 2.360 2.763 2.110 2.224 2.360 Fig Resistance versus temperature for the samples Pr1ÀxPbxMnO3 (x ¼ 0:120:5) (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 in Table Table 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 WP is 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 T o ¼ 17U m V =ðkl Þ in PM region Taking the Hund splitting U m ¼ eV [5], cell volume value V (Table 1), fitting values of To, we obtained localization length l, and average hopping distance at room temperature R ¼ 0:376l (To/T)0.25 These values of R are almost times larger than the Mn–Mn separation and indicate the Fig The linewidths versus temperature for the studied samples mesured at 9.2 GHz (X-band) applicable of VRH model Fig presents 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 TC and increases with increasing temperature It is well-known that DH depends on temperature similar to conductivity one, DHðTÞ$rÀ1 ðTÞ 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 ARTICLE IN PRESS e450 B.T Cong et al / Journal of Magnetism and Magnetic Materials 304 (2006) e448–e450 T o ¼ 1408:3 K The first Ea value (0.129 eV) for SP model is almost the same compared with 0.117 eV given by resistivity fitting (see Table 2) The value T o ¼ 5:09 Â 107 K is exceeding almost two times the value given in Table 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 Fig The DH(T) fitting using VRH and SP (insert) rÀ1(T) laws for sample with x ¼ 0:3 given in Fig This fitting procedure gives Ea(To) value of SP (VRH) model as 0.129 eV (5.09 Â 107 K) in the temperature range 270 KoTo300 K In the next temperature interval 300 KoTo475 K, one has E a ¼ 0:093 eV and [1] C Martin, A Maignan, M Hervieu, B Raveau, Phys Rev B 60 (1999) 1291 [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  ... of To, we obtained localization length l, and average hopping distance at room temperature R ¼ 0:376l (To/T)0.25 These values of R are almost times larger than the Mn–Mn separation and indicate... temperature for the studied samples mesured at 9.2 GHz (X-band) applicable of VRH model Fig presents the temperature dependence of line width (DH) at 9.2 GHz (X band) measured by a Jeol JES-TE300 EPR. .. 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