DSpace at VNU: Spin dynamics, electrical and magnetic properties of (La0.5Pr0.5)(0.7)Pb0.3Mn1-xCuxO3 (x=0, 0.02) perovskites

ARTICLE IN PRESS Physica B 371 (2006) 317–322 www.elsevier.com/locate/physb Spin dynamics, electrical and magnetic properties of (La0.5Pr0.5)0.7Pb0.3Mn1ÀxCuxO3 (x ¼ 0, 0.02) perovskites T.L Phana,Ã, N.D Thob, M.H Phanc, N.D Had, N Chaub, S.C Yua a Department of Physics, Chungbuk National University, 361-763 Cheongju, Korea Center for Materials Science, University of Science, 334 Nguyen Trai, Hanoi, Vietnam c Department of Aerospace Engineering, Bristol University, BS8 1TR, UK d Department of Materials Engineering, Chungnam National University, 305-764, Korea b Received 21 July 2005; received in revised form 25 September 2005; accepted 21 October 2005 Abstract Magnetization, resistivity, and electron spin resonance (ESR) measurements were carried out to investigate spin dynamics, electrical and magnetic properties of (La0.5Pr0.5)0.7Pb0.3Mn1ÀxCuxO3 (x ¼ 0, 0.02) perovskites It was found that with Cu addition the Curie temperature decreased from 326 K for x ¼ to 300 K for x ¼ 0:02 composition The x ¼ sample underwent a metallic–insulator transition at $130 K, whereas the x ¼ 0:02 sample with higher resistivity exhibited an insulating behavior as a whole It was furthermore found that asymmetrical ESR signals at low temperatures became Lorentzian at high temperatures above Tmin The temperature dependence of the linewidth, DHðTÞ, at T4T ts well to the one-phonon model, DHTị ẳ AT þ B The activation energy Ea, determined from the ESR intensity with respect to temperature, are 0.16 and 0.14 eV for x ¼ and 0.02 compositions, respectively In terms of our experimental results, it is reasonable to conclude that the Cu addition led to a suppression of the ferromagnetism and conductivity of the parent compound (x ¼ 0) r 2005 Elsevier B.V All rights reserved PACS: 71.30.+h; 72.15.Gd; 75.30.Kz Keywords: Perovskite manganites; Magnetic and electrical properties; ESR Introduction Parent lanthanum-manganese oxides (the so-called manganites), LaMnO3 and CaMnO3, with a perovskite structure were first studied by Jonker and Santen [1] Both these precursor compounds are an antiferromagnetic (AFM) insulator, where the orientation of nearest-neighbor spins is opposite [1–3] Recent progresses in manufacturing thin film materials have led to the discovery of the colossal magnetoresistance (CMR) effect, observed around the Curie temperature, TC, in several hole-doped manganites of R1Àx A0x MnO3 (where R ¼ La, Pr, Nd, and A0 ¼ Ca, Sr, Ba, Pb, etc.) It has been found that ÃCorresponding author Department of Physics, University of Bristol, Bristol BS8 1TL, UK Tel.: +44(0) 117 928 8750; fax: +82 43 274 7811 E-mail address: ptlong2512@yahoo.com (T.L Phan) 0921-4526/$ - see front matter r 2005 Elsevier B.V All rights reserved doi:10.1016/j.physb.2005.10.134 the TC value of the manganese perovskites strongly depends on the doping concentration and the effectiveaverage radius of doped ions (A0 ) In order to make such a manganese perovskite possible for practical uses, its TC is often controlled to a value near room temperature In view of existing CMR materials, a family of Ca-doped La1ÀxCaxMnO3 (0.2pxp0.5) manganites has attracted the widest interest in the aspects of experimental and theoretical researches [4–11], because they exhibited the largest CMR effect However, the TC value of this material family is about 270 K, still far away from room temperature [10,11] Fortunately, two other systems with higher TC were found to be Sr- and Pb-doped manganites of La1Àx(Sr,Pb)xMnO3 [11–15] In particular, La1ÀxPbxMnO3 (0.1pxp0.5) compounds with TC around room temperature are promising candidates for room temperature ARTICLE IN PRESS T.L Phan et al / Physica B 371 (2006) 317–322 applications such as magnetic sensors, magnetoresistive read heads, and magnetic refrigerators [13,16] In accessing the physical mechanism of the CMR effect in the doped manganites, a double-exchange (DE) interaction model was proposed by Zener [17] This model considers electronic-exchange processes between Mn3+ and Mn4+ ions via the Mn–O bond and the Mn–O–Mn bond angle [17,18] However, Millis et al [19,20] recently indicated that the only DE model is not sufficient enough to elucidate the entire physical picture of the CMR effect in the manganites and that addition into the DE model the Jahn–Teller effect, which arises from a strong electronphonon coupling, is extremely important This has been experimentally verified by several studies of Raman scattering, neutron powder diffraction, and electron spin resonance (ESR) [5,18] Despite a number of previous studies [18–20], the understanding of several physical phenomena such as phase-separation and magneto-transport mechanism in the manganites still remains controversial in part due to the complex nature of the problem [21] With the hope of gaining some more insight into the nature of the electrical transport and magnetic properties as well as internal dynamics of such a doped manganite, in the present work, a thorough study of the electrical and magnetic properties of polycrystalline (La0.5Pr0.5)0.7Pb0.3 Mn1ÀxCuxO3 (x ¼ 0, 0.02) perovskites were carried out by means of magnetization, resistivity, and electron spin resonance (ESR) measurements The experimental results reveal that the partial substitution of Mn by Cu resulted in the weakening of double-exchange ferromagnetic interactions in the Cu-doped sample No DE process took place between Mn3+ and Cu2+ ions The internal dynamic properties of the samples were exposed by ESR spectra Results and discussion Fig shows the X-ray diffraction patterns of (La0.5Pr0.5)0.7Pb0.3Mn1ÀxCuxO3 (x ¼ 0, 0.02) samples The samples are single-phase with an orthorhombic structure; the lattice parameters (a, b, and c) are summarized in Table As one can see clearly from the table, with addition of Cu, the lattice constants of the Cudoped sample (x ¼ 0:02) slightly decreased comparing to the Cu-free sample (x ¼ 0) This is understandable because of the fact that the radius of the Cu2+ ion is smaller than that of the Mn3+ ion Similar trend was reported by other authors [21] MZFC(T) and MFC(T) curves measured at 20 Oe are displayed in Fig The ferromagnetic (FM)-to-paramagnetic (PM) phase transition temperature, TC, determined from these magnetization curves, is about 326 and 300 K for x ¼ and 0.02 compositions, respectively This indicates a considerable reduction of TC in samples with Cu substitution for Mn However, the TC of the x ¼ 0:02 sample is still of 300 K, which may be of interest in the development of magnetic refrigerants for room-temperature magnetic refrigeration applications [16,22] A reduction in magnetization of the x ¼ 0:02 sample compared to the x ¼ sample could be ascribed to the decrease in FM interactions due to the Cu doping effect [21] It is worth noting that, at temperatures below TC, a separation of 30 25 Intensity (a u) 318 Experimental details (La0.5Pr0.5)0.7Pb0.3Mn1ÀxCuxO3 (x ¼ 0, 0.02) polycrystalline samples were prepared by conventional solid-state reaction Powder precursors of La2O3, Pr2O3, PbO, CuO and MnCO3 with a high purity were well-mixed stoichiometrically and then pressed into two pellets The pellets were pre-sintered in turn at 850 and 900 1C for 15 h, after several intermediate grinding and pressing Finally, they were annealed at 1000 1C for 15 h, and then slowly cooled down to room temperature The whole processes were carried out in the normal air condition The quality of the final samples was checked by powder X-ray diffraction patterns (D5005-Brucker) The temperature dependence of the DC resistivity was measured by the four-probe technique using a closed cycle-helium refrigerator Zerofield-cooled (ZFC) and field-cooled (FC) magnetizations were performed on a vibrating sample magnetometer (VSM), DMS-880, with a maximum field value of 1.5 T ESR measurements were carried out with a JEOL-JES-TE300 ESR spectrometer operating at 9.2 GHz (X-band) 20 15 10 x = 0.02 x=0 20 30 40 50 60 70 2Θ (°) Fig The X-ray diffraction patterns of (La0.5Pr0.5)0.7Pb0.3Mn1ÀxCuxO3 (x ¼ 0, 0.02) samples Table The lattice parameters of (La0.5Pr0.5)0.7Pb0.3Mn1ÀxCuxO3 (x ¼ 0, 0.02) samples x a (A˚) b (A˚) c (A˚) V (A˚3) 0.02 5.499 5.478 5.464 5.482 7.736 7.705 233.44 231.38 ARTICLE IN PRESS T.L Phan et al / Physica B 371 (2006) 317–322 FC (a) 30 15 80 x=0 x = 0.02 160 240 320 400 T (K) ZFC (b) x=0 x = 0.02 45 ρ (Ω.cm) M (T) M (emu/g) 1.5 1.0 100 Hex = kOe 60 10 Ln(ρ) (Ω.cm) 75 2.0 319 Hex = 20 Oe 0.5 x=0 x = 0.02 0.0 60 100 150 200 250 300 T (K) 350 400 450 500 Fig Temperature dependence of field-cooled (FC) and zero-fieldcooled (ZFC) magnetizations taken at 20 Oe for (La0.5Pr0.5)0.7Pb0.3Mn1ÀxCuxO3 (x ¼ 0, 0.02) samples The inset shows the temperature dependence of FC and ZFC magnetization for (La0.5Pr0.5)0.7Pb0.3Mn1ÀxCuxO3 (x ¼ 0) measured at 1000 Oe MFC(T) from MZFC(T) for both compositions was observed This is typically symptomatic of ferromagnets with a strong anisotropy field arising from FM clusters that are usually observed in unconventional ferromagnets With the presence of the FM clusters, magnetic moments of spins inside the cluster could be frozen in directions energetically favored by their local anisotropy or an external magnetic field as the system is cooled down from a high temperature in zero or non-zero magnetic field, respectively [23] This feature seems to be very widespread in manganese perovskites as FC magnetization is performed at low applied fields When the applied magnetic field, Hex, is high enough, such feature will disappear and, MZFC(T) and MFC(T) curves become coincide with each other because of the fact that the sufficiently high field could suppress entirely anisotropy fields in the system (see the inset of Fig 2, magnetization of the two samples measured at kOe) Fig shows the temperature dependence of the DC resistivity of the x ¼ and 0.02 samples As one can see from Fig 3, with respect to the increase of temperature the x ¼ sample exhibited a metallic-to-insulator transition at $130 K while only insulating behavior was observed in the x ¼ 0:02 sample in the whole temperature range investigated In the present case, we did not measure resistivity values at lower temperatures, maybe, there was the metallic behavior [24] As shown earlier in Refs [7,24] on a (La0.5Pr0.5)0.7Ca0.3MnO3 expitaxy film and Pr0.65Ca0.35Àx SrxMnO3 compounds, a metallic-to-insulator transition followed by a broadening band around the phase transition temperature was observed This coincides with what we observed here on (La0.5Pr0.5)0.7Pb0.3MnO3 (x ¼ 0), and can be explained due to the presence of charge-ordering states [24] It should be noted that the magnitude of the resistivity of the x ¼ 0:02 sample at a given temperature is larger than 120 180 T (K) 240 300 0.24 0.27 1/4 T 0.30 0.33 -1/4 (K ) Fig (a) The temperature dependence of resistivity, r(T), for (La0.5Pr0.5)0.7Pb0.3Mn1ÀxCuxO3 (x ¼ 0, 0.02) samples; (b) the logarithm of resistivity Ln(r) versus TÀ1/4, solid lines are fitting curves according to Mott’s variable-range-hopping model that of the x ¼ sample In connection with the magnetization data, it is reasonable to conclude that the Cu addition leads to a decrease of the ferromagnetic interaction and conductivity of the parent compound (x ¼ 0) This might be related to changes in the exchange mechanism occurring among eg electrons of Mn and Cu ions [21] Here, an emerging question is why the resistivity of the (La0.5Pr0.5)0.7Pb0.3MnO3 (x ¼ 0) sample becomes higher with Cu addition? In order to address this question, the model of Mott’s variable-range-hopping (VHR) insulating behavior r(T) ¼ r0 exp[(T0/T)1/4] (where r0 is a pre-exponential factor and T0 is a constant) has been used to analyze the resistivity data of the presently investigated samples Determined values of T0 are 4.55  107 and 16.12  107 (K) for x ¼ and 0.02 compositions, respectively In the present work, we have taken into account the density of states of the system at the Fermi level N(EF)-which is obtained using the equation of T0 ¼ 18a3/kBN(EF), where kB is the Boltzmann factor and a is the electron wave-function decay constant Using a ¼ 2:22 nmÀ1 [24,25], N(EF) is deduced to be 1.04  1025 and 0.63  1020 eVÀ1 cmÀ1 for x ¼ and 0.02 compositions, respectively This enables us to state that the decrease of the density of states on the Fermi level in the Cu-doped sample is the origin leading to the decrease of the electrical conductivity of the sample, as compared to the Cu-free sample As can be seen in Fig 3(b), the VHR model can describe the r(T) data for the x ¼ 0:02 sample in a large temperature range, rather than for the x ¼ sample This is in good agreement with what was reported in Ref [21] with respect to an increase of Cu content To understand internal spin dynamics of the presently investigated samples, we recorded ESR spectra at different temperatures above from TC, as shown in Fig It can be seen that asymmetrical ESR signals at low temperatures ARTICLE IN PRESS T.L Phan et al / Physica B 371 (2006) 317–322 320 x = 0.02 x=0 340 x3 308 K 363 353 428 x4 x=0 300 280 Hr (mT) ESR signal (a u) 320 328 K 423 260 340 320 x = 0.02 300 200 400 600 800 200 DC field (mT) 400 600 800 280 Fig X-band ESR spectra for (La0.5Pr0.5)0.7Pb0.3Mn1ÀxCuxO3 (x ¼ 0, 0.02) samples at selected temperatures around Tmin Table Experimental parameters obtained for (La0.5Pr0.5)0.7Pb0.3Mn1ÀxCuxO3 (x ¼ 0, 0.02) samples x TC (K) T0  107 Tmin (K) (K) Y (K) 4.55 16.12 341 327 B (Oe/K) DHmin (Oe) Ea (eV) 7.02 6.64 0.16 0.14 260 300 320 340 360 380 400 T (K) 420 440 460 480 Fig The plot of the resonance position of high and low field lines for x ¼ and 0.02 compositions with respect to temperature Solid lines are guides to the eye Two lines at ToT due to FM and PM correlations become a sole line at T4T x=0 1.4 x = 0.02 0.02 326 300 363 351 559 716 became symmetrical-Lorentzian at temperatures T4Tmin (Tmin is, in Table 2, the temperature corresponding to the narrowest ESR linewidth) With decreasing temperature in the region T C oToT , the ESR signals were splitted into two lines, in which the resonant line at a lower field changed in its position with temperature [21,26], see Fig In particular, these two lines strongly competed in amplitude at temperatures around Tmin This may be considered a signature of coexistence of two competing phases, namely, FM and PM phases At temperatures TpT C , the FM phase is dominant and it strongly suppresses ESR signals of the PM phase As a matter of fact, it was found in the ferromagnetic-low-temperature region appearing several resonance lines [27–29] due to the presence of FM correlations and spins in FM microregions (or FM clusters) These spins were strongly influenced by an anisotropy field (arising from itself FM clusters) added to the external magnetic field On the other hand, recent reports also pointed out experimental evidences on the presence of phase separation, this led to the additional appearance of resonance lines of mixed phases [27,28], introducing to the broadening of the ESR linewidth at low temperatures However, at temperatures above Tmin, single ESR signals in the Lorentzian shape for the samples were observed; where the samples are completely in the PM state This differs from the results of Ref [21], where for the Cu-doped samples the two lines arising from the coexistence of double-exchange FM and ∆H (kOe) 1.2 1.0 0.8 0.6 340 360 380 400 420 T (K) 440 460 480 Fig The temperature dependence of the ESR linewidth, DHTị for (La0.5Pr0.5)0.7Pb0.3Mn1xCuxO3 (x ẳ 0, 0.02) samples; the solid lines fit to a function of DHTị ẳ AT ỵ B super-exchange antiferromagnetic (AFM) interactions were observed even above Tmin This differential can be understood, because of the fact that the Cu-doping level in our system is so small that formation of AFM clusters due to Cu2+–Mn3+ and/or Cu2+–Mn4+ interactions is trivial only It is furthermore suggested that the origin of the observed ESR signals comes from the participation of electron spins of Mn3+, Mn4+, and Cu2+ ions in correlation to crystal fields caused by the neighboring ions [4–6,9,26] Fig shows the temperature dependence of the ESR linewidth, DHTị, for the x ẳ and 0.02 samples DHðTÞ reached a minimum value DHmin at Tmin (Table 2), and its ARTICLE IN PRESS T.L Phan et al / Physica B 371 (2006) 317–322 1/l (a u) 15 Intensity (a u) appearance is probably related to the exchange narrowing and decay values of the correlation function around the phase transition [9] In the temperature range studied, DHðTÞ of the x ¼ 0:02 sample was higher than that of the x ¼ sample; this could be related to difference in the concentration of Mn3+ and Mn4+ ions in the samples In general, the ESR linewidth for manganites containing either Mn3+ or Mn4+ ions individually is usually larger than that for manganites containing both these ions [4,9] Accordingly, in our system the Mn substitution by Cu resulted in an increase in the concentration of Mn4+ ions Paying attention to the variation of DHðTÞ at temperatures (lower Tmin), it increases because of the development of FM correlations and forming FM clusters [27–31] However, the increase of DHðTÞ in the region T4T is different, it relates to correlation of FM clusters existing on a large range of temperature [14,26]; this is concerned via the activation energy values as being presented later Based upon the relation between the resistivity rðTÞ and DHðTÞ data at temperatures above Tmin, on the other hand, it is stated that the hopping rate of charge carriers could limit the lifetime of the spin state, thereby resulting in a broadening of EPR spectra with increasing temperature [31,32] Regarding the interaction mechanism in the PM region, one see that the Lande factor g of the samples is close to 2.00, and it is temperature independent This means that the spin–spin interaction is dominant in this temperature range And, the obtained value of g is appropriate to the one-phonon process [12,33], i.e the variation of the ESR linewidth above Tmin obeys to a linear function As can be seen in Fig 6, a function DHTị ẳ A ỵ BT fits well to the ESR linewidth data above Tmin, where A is a constant and B is a parameter being related to exchange, dipolar, lattice distortions, and/or Jahn–Teller fluctuations [33] The B value is determined to be 7.02 and 6.64 Oe/K for the x ¼ and 0.02 samples, respectively According to earlier studies [12,32–36], we realize that La1Àx A0x MnO3 manganites were often found to have the low B value ($3 Oe/K) [34–36] compared to praseodymium manganites Pr1Àx A0x MnO3 ($5–7 Oe/K) [33] This is perhaps due to influences of crystal fields, which are caused by La3+ and Pr3+ ions, on Mn ions In our case, the estimated value of B is $7.0 Oe/K and this is acceptable because of the presence of both La3+ and Pr3+ ions in the samples The temperature dependence of the ESR intensity I(T) at T4T , determined by taking double integration of the experimental curve, for the samples is shown in Fig With increasing temperature, I(T) exponentially decreased According to the one-phonon process, I(T)pwDC(T), where wDC is the DC susceptibility, [33,36,37] and the relation between DHðTÞ and r(T) [6,32], I(T) can be expressed by the function ITị ẳ I exp ðÀE a =kB TÞ, where Ea is the thermal activation energy for the dissociation of the FM spin clusters [26,32,35] This expression describes well the experimental I(T) data as shown in Fig The Ea 321 x=0 10 θ 330 360 390 420 450 480 T (K) x=0 x = 0.02 Fitting curve 360 380 400 420 T (K) 440 460 480 Fig The temperature dependence of the ESR intensity for (La0.5Pr0.5)0.7Pb0.3Mn1ÀxCuxO3 (x ¼ 0, 0.02) samples; the symbols shows the experimental data and the solid lines are tting curves with the function ITị ẳ I exp ðÀE a =kB TÞ The inset shows the 1=IðTÞ vs T curve, according to the Curie–Weiss law value is determined to be $0.16 and 0.14 eV for the x ẳ and 0.02 samples, respectively The 1=ITị vs T plot and the extrapolation of the linear-high-temperature part of this curve to zero value, according to the Curie-Weiss law, allowed us to determine the Curie–Weiss temperatures of the samples, Y, which are summarized in Table [4,34–36] A reasonable agreement of this law was found at high temperatures (see the inset of Fig 7) implying an existence of FM clusters as exposed earlier by the magnetization data and the activation energy Ea Finally, to interpret the magneto-transport mechanism of the samples, the electronic configuration of ions present in the compounds has been taken into account In the case of the x ¼ sample, both Mn3+ and Mn4+ ions (Mn3+ is 3d4, t32g e1g , with S ¼ 2, whereas Mn4+ is 3d3, t32g e0g , with S ¼ 3=2) coexist and follow the strong Hund’s rule coupling The spins of these ions orient parallel to each other thus leading to the fact that the interaction governing them is FM [17] Meanwhile, the exchange couplings of Mn4+–Mn4+ and/or Mn3+–Mn3+ ions are AFM superexchange interactions [38,39] When substituting a small amount of Cu for Mn, the Mn4+ concentration in the sample increases, and the AFM interaction is intensified due to the additional appearance of the super-exchange couplings of Cu2+–O–Mn3+,4+ and Cu2+–O–Cu2+ As a result, the FM interaction is declined That is why both the TC and the activation energy value Ea decreased in the Cudoped sample It should be furthermore noted that the x ¼ 0:02 sample contains Cu2+ ions with 3d9 configuration, t82g e3g and S ¼ 1=2, one free-electron spin on the eg level which couples weakly with the t2g-core [38–40] Therefore, the interaction between Cu2+ and Mn3+/4+ ions is actually AFM ARTICLE IN PRESS 322 T.L Phan et al / Physica B 371 (2006) 317–322 Conclusions The electrical and magnetic properties of (La0.5Pr0.5)0.7 Pb0.3Mn1ÀxCuxO3 (x ¼ 0, 0.02) perovskites have been thoroughly studied It was found that with Cu addition the Curie temperature decreased from 326 K for x ¼ composition to 300 K for x ¼ 0:02 composition The resistivity of the Cu-doped sample was higher than that of the Cu-free sample It is interesting to note that the x ¼ sample underwent a metallic–insulator transition at $130 K, whereas the x ¼ 0:02 sample exhibited an insulating behavior in the entire temperature investigated The results obtained from ESR measurements show that asymmetrical ESR signals at low temperatures became Lorentzian at high temperatures above Tmin Temperature dependence of the linewidth, DHðTÞ, at T4T tted well to the one-phonon model, DHTị ẳ AT þ B The activation energy Ea are estimated to be 0.16 and 0.14 eV for x ¼ and 0.02 compositions, respectively In terms of our experimental results, it is reasonable to conclude that the Cu addition led to a suppression of the ferromagnetism and conductivity of the parent compound (x ¼ 0) Acknowledgements This work in Vietnam was supported by the National Fundamental Research Program (Project 421004), and in Korea was supported by the Korea Research Foundation Grant (KRF-2003-005-C00018) References [1] G.H Jonker, Physica XXII (1956) 707; 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