IEEE TRANSACTIONS ON MAGNETICS, VOL 50, NO 6, JUNE 2014 2502404 Electrical and Magnetotransport Properties of La0.7Ca0.3Mn1−x Co x O3 Tran Dang Thanh1,2, T L Phan1 , Phung Quoc Thanh3, Hoang Nam Nhat4 , Duong Anh Tuan5, and S C Yu1 Department of Physics, Chungbuk National University, Cheongju 361-763, Korea of Materials Science, Vietnam Academy of Science and Technology, Hanoi, Vietnam Hanoi University of Natural Science, Vietnam National University, Hanoi, Vietnam University of Engineering and Technology, Vietnam National University, Hanoi, Vietnam Department of Physics, University of Ulsan, Ulsan 680-749, Korea Institute This paper presents a detailed study on the Co-doping influence on the electrical and magnetotransport properties of La0.7 Ca0.3 Mn1−x Co x O3 (x = 0.09–0.17) prepared by solid-state reaction Magnetic measurements versus temperature revealed a gradual decrease of the magnetization (M) and Curie temperature (T C ) with increasing Co concentration (x) The T C values vary from 194 to 159 K as changing x from 0.09 to 0.17, respectively H/M versus M performances around T C prove the x = 0.09 sample undergoing a first-order magnetic phase transition (FOMT) while the samples with x ≥ 0.11 undergo a second-order magnetic phase transition (SOMT) The other with x = 0.10 is considered as a threshold concentration of the FOMT–SOMT transformation Considering temperature dependences of resistivity, ρ(T), in the presence and absence of the magnetic field, the samples (excepting for x = 0.17) exhibit a metal–insulator transition at T P = 60–160 K, which shifts toward lower temperatures with increasing x In the metallic-ferromagnetic region, the ρ(T) data are well fitted to a power function ρ(T) = ρ0 + ρ2 T + ρ4.5 T 4.5 This indicates electron–electron and electron–magnon scattering processes are dominant at temperatures T < T P In addition, the conduction data at temperatures T > θ D /2 (θ D is the Debye temperature) and T P < T < θ D /2 obey the small-polaron and variable-range hopping models, respectively The values of activation energy E p , and density of states at the Fermi level N(E F ) were accordingly determined Here, N(E F ) increases while E p decreases when an external magnetic field is applied We also have found that N(E F ) increases when materials transfer from the FOMT to the SOMT, and N(E F ) value becomes smallest for the sample having the coexistence of the FOMT and SOMT (i.e., x = 0.10) Index Terms— Magnetic and electrical properties, magnetic phase transition, perovskite manganites I I NTRODUCTION OLOSSAL magnetoresistance (CMR) materials are well known as mixed valence manganites with a general chemical formula of ABO3 (perovskite-type structure); for example, R1−y A’ y BO3 (R = La, Pr, Nd; A’ = Ca, Sr, Ba; and B = Mn) [1], [2] The parent compound LaMnO3 is an antiferromagnetic (AFM) insulator with the Néel temperature TN ≈ 140 K [3] Substituting La by a metallic element A’ makes these materials exhibiting the ferromagnetic (FM) metallic behaviors below the Curie temperature (TC ) The physical properties of manganites are arisen from two classes of competing effects: 1) super-exchange (SE) interactions cause insulating and AFM-ordered ground states and 2) double-exchange (DE) interactions lead to metallic and FM-ordered ground states These interactions can be tuned by a number of external and internal parameters, such as temperature, the magnetic field, concentration of carriers, crystal-structure type, the size of ions, and so forth The ground state thus depends on the relative strength of these two interactions It has been realized that La1−y A’ y MnO3 compounds give the CMR effect around the metal–insulator transition (MIT) temperature (TMI ), which is close to the TC , and attains the maximum as y ≈ 0.3 [1], [2] Recently, many works have focused on La1−y Ca y MnO3 materials because of their special features related to the C Manuscript received November 5, 2013; accepted January 14, 2014 Date of current version June 6, 2014 Corresponding author: S C Yu (e-mail: scyu@chungbuk.ac.kr) Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org Digital Object Identifier 10.1109/TMAG.2014.2301713 magnetic phase transition It has been found a first-order magnetic phase transition (FOMT) in bulk samples with y = 0.3 [4], [5], while a second-order magnetic phase transition (SOMT) is present in the samples with y = 0.2 and 0.4 [6] For other Sr- and Ba-doped lanthanum manganites, the magnetic phase transition is the second-order type [7] Therefore, the effects of La/Ca-site doping by Sr [8], [9] and Ba [10], and of the Mn-site doping by Ga [4] and Co [11], [12] on electrical–magnetic phase transitions have attracted much interest However, these studies only focused on either the magnetic or electrical properties of materials The striking differences in the magnetic and transport behaviors of the perovskite manganites around TC have not been reported yet To get more insight into the relation between the magnetic and transport properties, we have studied the influences of the FOMT to SOMT on the transport properties of La0.7 Ca0.3 Mn1−x Cox O3 compounds II E XPERIMENTAL D ETAILS Polycrystalline samples La0.7 Ca0.3 Mn1−x Cox O3 with x = 0.09, 0.10, 0.11, 0.14, and 0.17 were prepared by solid-state reaction High-purity powders (99.9%) of La2 O3 , CaCO3 , MnCO3 , and Co2 O3 were used as starting materials Stoichiometric ratios of these powders were ground and mixed well, and then calcinated in air at 1000 °C for 24 h After calcinating, the mixtures were pressed into pellets, and annealed at 1300 °C for 48 h in air The crystal structure of the final products was checked by an X-ray diffractometer (Bruker AXS, D8 Discover) as using the Cu–Kα radiation source The magnetic properties versus temperature (in the 0018-9464 © 2014 IEEE Personal use is permitted, but republication/redistribution requires IEEE permission See http://www.ieee.org/publications_standards/publications/rights/index.html for more information 2502404 IEEE TRANSACTIONS ON MAGNETICS, VOL 50, NO 6, JUNE 2014 TABLE I L ATTICE PARAMETERS AND VALUES OF TC , T P , AND THE PARAMETERS O BTAINED FROM F ITTING E XPERIMENTAL D ATA TO (1)–(3) FOR La0.7 Ca0.3 Mn1−x Cox O C OMPOUNDS range of 100–300 K) and the magnetic field (up to 15 kOe) were performed on a vibrating sample magnetometer The electrical transport behaviors were studied by the standard four-probe technique integrated into a physical property measurement system with magnetic fields in the range of 0–4 kOe, and with the temperature range 20–300 K III R ESULTS AND D ISCUSSION X-ray diffraction (XRD) patterns indicated a single phase of La0.7 Ca0.3 Mn1−x Cox O3 in an orthorhombic structure (space group Pbnm) Based on the XRD data, Rietveld refinements have been made for all the samples The obtained lattice parameters shown in Table I reveal that when Co concentration increases, the lattice parameters decrease gradually The variation tendency of the lattice parameters is in good agreement with earlier reports on La0.7 Ca0.3 Mn1−x Cox O3 compounds [12] This indicates an incorporation of Co ions into the Mn site of the manganite host lattice because the radii of Co ions are smaller than those of Mn ions Following the crystal structure analyses of La0.7 Ca0.3 Mn1−x Cox O3 samples, we have investigated the nature of their magnetic phase transition As shown in Fig 1(a), M(T ) curves for an applied field H = 500 Oe indicate the FM–paramagnetic (PM) phase transition This phase transition is effectively tuned to lower temperatures by increasing Co concentration TC values of the samples obtained from the minima of the dM/dT versus T curves are about 194, 188, 182, 168, and 159 K for x = 0.09, 0.10, 0.11, 0.14, and 0.17, respectively, as listed in Table I There is the decrease of magnetization with increasing Co concentration Here, the decrease in both TC value and magnetization in La0.7 Ca0.3 Mn1−x Cox O3 can be explained qualitatively as follows: in La0.7 Ca0.3 MnO3 , FM–DE interactions of Mn3+ –Mn4+ pairs are more dominant than AFM–SE interactions caused by Mn3+ –Mn3+ and Mn4+ –Mn4+ pairs When Co ions (with oxidation sates 3+ and 4+) [11], [12] are doped into the samples, they occupy the Mn site of the host lattice The appearance of Co ions in the manganite lattice reduces Fig (a) Thermomagnetization curves for La0.7 Ca0.3 Mn1−x Cox O3 in the field of 500 Oe, and H /M versus M performances for three representative samples with (b) x = 0.09, (c) x = 0.10, and (d) x = 0.11 FM interactions due to the decrease of the Mn3+ /Mn4+ ratio, and to the enhancement of AFM–SE interactions between Co and Mn ions [11], [12] According to Banerjee’s criteria [13], the sign of slopes of H /M versus M curves indicates the nature of the PM–FM phase transition Negative slopes correspond to an FOMT while positive ones correspond to an SOMT Thereby, to assess the nature of magnetic phase transition of the samples, we measured M(H ) curves at temperatures around the FM–PM transition TC From the M(H ) data, it can be performed the H /M versus M curves [Fig 1(b)–(d)] An existence of negative slopes in the H /M versus M curves for x = 0.09 reflects this sample undergoing the FOMT [Fig 1(b)] Positive slopes observed in Fig 1(d) for the samples x ≥ 0.11 prove that they undergoing the SOMT The other sample with x = 0.10 is considered as a threshold concentration of the FOMT–SOMT transformation [Fig 1(c)] Fig shows representative temperature dependences of resistivity, ρ(T ) curves, at the field H = for La0.7 Ca0.3 Mn1−x Cox O3 For all the samples, resistivity first increases with decreasing temperature, and then exhibits a peak around the MIT corresponding to the peak, T P (excepting for the sample x = 0.17 without the MIT) Focusing on the behavior of the ρ(T ) curves, one can see that dρ/dT is negative at temperatures above T P , which is a characteristic of the semiconducting behavior Below T P , the samples showed the metallic character with dρ/dT > When Co concentration is increased, the MIT shifts toward the low-temperature region For the case H = 0, the values of T P are found to be about 160–60 K, as listed in Table I For each sample, the value of resistivity decreases, and T P shifts toward the high-temperature region when there is the presence of the magnetic field [the low inset shown in Fig 2, ρ(T ) curves of a representative sample of x = 0.09 with H = and kOe] The application of the field causes the local ordering of magnetic moments Due to this ordering, the FM metallic THANH et al.: ELECTRICAL AND MAGNETOTRANSPORT PROPERTIES 2502404 Fig Thermal variation of resistivity, ρ(T ), for La0.7 Ca0.3 Mn1−x Cox O3 compounds for H = Lower inset shows ρ(T ) for a representative sample of x = 0.09 for H = and kOe High inset shows the ρ(T ) data at low-temperature metallic part fitted to (1) (the solid lines) for H = state suppresses the PM insulating state, and hence MIT shifts toward the high-temperature region A similar behavior was also observed in hole-doped LaMnO3 compounds [1], [2], [9] A main mechanism responsible for magnetoresistance is thus due to the magnetic-field influence on magnetic domains Recently, some authors have analyzed the resistivity data of La1−y A’ y MnO3 compounds using power functions [14], [15] Though Snyder et al [14] described the term ρ4.5 T 4.5 as the contribution from the electron–magnon scattering process It might be due to the spin wave scattering in the FM phase [16] The electron–magnon scattering process contributes in the term ρ2.5 T 2.5 [15] This electron–magnon scattering term is important in the low-temperature region To understand the nature of the conduction mechanism at low temperatures (below T P ), experimental ρ(T ) data for the samples with x = 0.09–0.14 were fitted to different power functions shown in [14] and [15] However, we find that, in the metallic regime, the ρ(T ) data in the cases of the presence and absence of the field show the best fit to ρ(T ) = ρ0 + ρ2 T + ρ4.5 T 4.5 (1) where ρ0 is the temperature-independent residual resistivity due to domain and grain boundaries, ρ2 T describes the resistivity associated with electron–electron scattering, and ρ4.5 T 4.5 associated with electron–magnon scattering, see the solid lines in the high inset shown in Fig for a representative case with H = This result suggests that the transport mechanism in the metallic regime is attributed to electron–electron and electron–magnon scatterings The parameters obtained from fitting the ρ(T ) data at the low-temperature metallic part to (1) in the cases of H = and kOe for La0.7 Ca0.3 Mn1−x Cox O3 with x = 0.09–0.14 are shown in Table I Here, the values of ρ0 , ρ2 , and ρ4.5 decrease with the presence of the field, as mentioned above In the insulating regime, at temperatures above T P , the ρ(T ) data can be well described by the small-polaron-hopping (SPH) model [17] It has been found that high-temperature transport properties in the rare-earth manganite system are dominated by the thermally activated hopping of small Fig (a) Ln(ρ/T ) versus T −1 curves and (b) log(1/ρ) versus T −1/4 curves of La0.7 Ca0.3 Mn1−x Cox O3 with x = 0.09–0.14 for H = The inset presents the data with H = kOe Solid lines are the best fit to the SPH model at temperatures above θ D /2 (a) and the best fit to the VRH model in the temperature range between TMI and θ D /2 (b) polarons [14] According to the SPH model, the expression for resistivity is given by [15] ρ(T ) = ρα T exp(E P /k B T ) (2) where ρα and E P are the residual resistivity and activation energy of hopping conduction, respectively The performance of the ln(ρ/T ) versus 1/T curves is shown in Fig 3(a) From the slope of the straight line above θ D /2 [θ D is the Debye temperature, and the values of θ D /2 are estimated from the temperature point where a deviation from the linear behavior at high temperatures is observed, Fig 3(a)], we have calculated the activation energy E P , as shown in Table I In this paper, E P almost decreases with increasing Co concentration, excepting the sample with x = 0.10 It was also found that E P is decreased as H = kOe This can be explained upon the delocalization of eg electrons in the magnetic field According to Mott and Davis [17], the conductivity of semiconducting oxides at temperatures below θ D /2 obeys the variable range hopping (VRH) model of charge carriers, which is expressed by [17] σ = σ0 exp(−T0 /T )1/4 (3) where T0 = 16α [k B N(E F )]−1 , α is the decay constant of an electron wave function and N(E F ) is the density of states at the Fermi level It is well known that above 2502404 TC , eg electrons are localized by the random spin-dependent potential, and conduction is by the VRH Viret et al [18] also applied the VRH model for similar systems over the whole temperature above T P Actually, the VRH model was derived for explaining the conductivity data below θ D /2 Thus, we attempted to apply this model to fit the conductivity data of La0.7 Ca0.3 Mn1−x Cox O3 compounds with x = 0.09–0.14 in the temperature region between θ D /2 and T P for the cases H = and kOe, as shown in Fig 3(b) It is seen that the conductivity data in this temperature range were well fitted to the VRH model From the fitting parameters, we obtained values of T0 for the samples Except x = 0.10 (with a maximum T0 value), the T0 value is found to be decreased with the increasing Co content To estimate the N(E F ) values, we used α = 2.22 nm−1 [19] N(E F ) values obtained for La0.7 Ca0.3 Mn1−x Cox O3 as H = and kOe are shown in Table I One can see that the values N(E F ) ≈ 1020 eV−1 · cm−3 are about one order of magnitude higher than those of usual transition-metal–oxide semiconductors [17], which increases with the presence of the magnetic field A higher value of N(E F ) was also estimated in [18] and [20] As pointed out in [20], a large value of N(E F ) is an indication of adiabatic hopping behavior of carriers in manganites As a function of Co concentration, N(E F ) increases while E P decreases when the materials transfer from the FOMT (for x = 0.09) to the SOMT (for x ≥ 0.11) It is necessary to emphasize that N(E F ) reaches the minimum while E P reaches the maximum when there is the coexistence of the FOMT and SOMT (i.e., x = 0.10) Such the results prove a strong interplay of the structural characterization and transport properties of perovskite manganites IV C ONCLUSION The crystal–structural and electrical–magnetic properties of La0.7 Ca0.3 Mn1−x Cox O3 with x = 0.09–0.17 were investigated in detail The experimental results demonstrated the FM–PM transition and MIT taking place at temperatures TC = 159–194 K and T P = 60–160 K (no MIT for x = 0.17), respectively, which decrease with increasing Co-doping content Excepting for x = 0.17, temperature dependences of resistivity data for all the samples were analyzed and fitted to power functions, SPH and VRH models In the low-temperature region (T < T P ), resistivity obeys a power function ρ(T ) = ρ0 + ρ2 T + ρ4.5 T 4.5 , suggesting that electron–electron and electron–magnon scattering processes are dominant in this temperature region Above θ D /2, ρ(T ) data can be well explained by the adiabatic SPH model, while at temperatures in the range between θ D /2 and TMI , the conductivity data of all the samples are fitted to the VRH model For the samples x = 0.09–0.14, N(E) F is one order of magnitude higher than those of many transition-metal–oxide semiconductors Interestingly, we have found that N(E F ) increases while E P decreases when materials transfer from the FOMT to the SOMT The value of N(E F ) is minimum and E P is maximum for the sample x = 0.10 with the coexistence of the FOMT and SOMT The change in N(E F ) and E P values could be related to crystal structure modifications ACKNOWLEDGMENT This research was supported by the Converging Research Center program through the Ministry of Science, ICT IEEE TRANSACTIONS ON MAGNETICS, VOL 50, NO 6, JUNE 2014 and Future Planning, Korea (2013K000405), and by the National Foundation for Science and Technology Development (NAFOSTED) of Vietnam under Grant 103.02-2012.57 R EFERENCES [1] A P Ramirez, “Colossal magnetoresistance,” J Phys., Condensed Matter, vol 9, no 1, pp 8171–8199, 1997 [2] R V Helmolt, J Wecker, B Holzapfel, L Schultz, and K Samwer, “Giant negative magnetoresistance in perovskitelike La2/3 Ba1/3 MnO x ferromagnetic films,” Phys Rev Lett., vol 71, no 14, pp 2331–2333, 1993 [3] J A Alonso, M J Martinez-Lope, M T Casais, and A Munoz, “Magnetic structures of LaMnO3 + perovskites ( = 0.11, 0.15, 0.26),” Solid State Commun., vol 102, no 1, pp 7–12, 1997 [4] S RoSSler, U K RoSSler, K Nenkov, D Eckert, S M Yusuf, K Dorr, et al., “Rounding of a first-order magnetic phase transition in Ga-doped La0.67 Ca0.33 MnO3 ,” Phys Rev B, vol 70, no 10, pp 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