IEEE TRANSACTIONS ON MAGNETICS, VOL 50, NO 6, JUNE 2014 2503104 Synthesis and Magnetic Properties of Perovskite La1−xSrx MnO3 Nanoparticles Nguyen Hoang Nam1,2 , Duong Thi Mai Huong1 , and Nguyen Hoang Luong1,2 Center for Materials Science, Faculty of Physics, Vietnam National University of Science, Hanoi, Vietnam Nano and Energy Center, Vietnam National University of Science, Hanoi, Vietnam Perovskite nanoparticles La1−x Srx MnO3 were successfully synthesized by modified sol-gel method Their structure and magnetic properties were systematically investigated in dependence on doped Sr ratio x The structure was investigated by X-ray diffraction and showed a slight change, but magnetic properties varied strongly with changing x Magnetic properties of samples studied by physical property measurement system show spin-glass behavior with Curie temperature and blocking temperatures above room temperature The blocking temperature as well as Curie temperature varies by changing x At room temperature, the samples show superparamagnetic properties with high saturation magnetization M S up to 87 emu/g, which strongly depends on the doped Sr ratio x Index Terms— Magnetic properties, nanoparticle, perovskite, sol-gel I I NTRODUCTION T HE Lanthanum Strontium manganite is a perovskitebased crystal-structured ceramic material with the formula of La1−x Srx MnO3 , where x describes the doping ratio It has attracted much attention due to its good magnetic, electrical, and catalytic properties and is becoming an attractive potential material in several biomedical applications, particularly with nanosize In industry, this material is commonly used as a cathode material in commercially produced solid oxide fuel cells [1] In [2] and [3], this is one of the perovskite manganites that have the colossal magnetoresistance effect as well as an observed half-metal at x = 0.3 suggesting its possible use in spintronics Magnetic properties of this bulk material are quite well known, however, in the nanoscale, which can be applicable in biomedicine; the material has been reported in the literature in limited numbers [4]–[11] Pradhan et al [4], Thorat et al [5], [7], Zhang et al [6], and Daengsakul et al [8] produced its nanoparticles for biomedical applications such as hyperthermia applications because it has Curie temperature at above room temperature and is relatively nontoxic These nanoparticles are of a superparamagnetic nature, predicting the suitability for biomedical application Krivoruchko et al [9] and Rostamnejadi et al [10] also showed the superparamagnetism of these nanoparticles at x = 0.3 Recently, several synthetic procedures were developed to synthesize a fine and homogeneous La1−x Srx MnO3 such as the citrate-gel [11], sol-gel [12], molten salt [13], autocombustion [14], and hydrothermal [15] methods to produce the biomedical applicable material with proper magnetic properties However, the magnetic properties of those nanoparticles strongly varied with the changing in synthetic conditions and the synthetic processes Almost all reports show that a sample with x = 0.3 has the saturation magnetization of ∼45–50 emu/g [9], [10], [13] The blocking temperature as Manuscript received November 10, 2013; revised February 7, 2014 and February 16, 2014; accepted February 20, 2014 Date of current version June 6, 2014 Corresponding author: N H Nam (e-mail: namnh@hus.edu.vn) 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.2014.2307834 well as Curie temperature vary with controlling particles size and strongly depend on the sample preparation procedures In this paper, we present the study on magnetic properties of the nanosize La1−x Srx MnO3 at x = 0, 0.3, 0.4, and 0.5 prepared by a modified sol-gel method hopefully a method to produce biomedical applicable La1−x Srx MnO3 material Our results show higher saturation magnetization up to 87 emu/g compared with previous reports In our study, the blocking as well as the Curie temperature not have maximum at x = 0.3 but increase with increasing x II E XPERIMENT Magnetic nanoparticles of Lax Sr1−x MnO3 perovskite were prepared by a modified sol-gel method Stoichiometric amounts of the nitrate precursor reagents La(NO3 )3 , Mn(NO3)2 , and Sr(NO3 )2 were dissolved in distilled water This solution was mixed with citric acid, forming a stable solution The stable solution was then heated on a thermal plate under constant stirring at 80 °C for h to eliminate the excess water and to obtain a viscous gel The obtained gel was dried at 120 °C and then calcinated at 300 °C for 0.5 h The second calcinated process was carried out at 1000 °C for h after an hour of milling to obtain final powder products The morphology and crystal structure of the powders were checked by scanning electron microscope (SEM) and X-ray diffraction (XRD) pattern using Cu Kα radiation source in the 2θ scan range from 20° to 70° The average particle sizes of the samples were estimated from the X-ray peak width at half maximum by using the Scherrer formula The magnetization was recorded by vibrating sample magnetometer mode in the physical property measurement system, Evercool II, quantum design III R ESULTS AND D ISCUSSION Fig shows XRD patterns of La1−x Srx MnO3 nanoparticles with x = 0, 0.3, 0.4, and 0.5 In the XRD pattern of sample with x = 0, one can see the diffraction peaks corresponding to (102), (110), (104), (202), (006), (204), (212), (116), (214), (220), and (208) reflection planes, of which (110) peak has 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 2503104 IEEE TRANSACTIONS ON MAGNETICS, VOL 50, NO 6, JUNE 2014 Fig XRD patterns of La1−x Srx MnO3 nanoparticles with x = 0, 0.3, 0.4, and 0.5 Fig SEM images of La1−x Srx MnO3 with x = 0, 0.3, 0.4, and 0.5 annealed at 1000 °C for h, shown in order from left to right and top to bottom the strongest intensity The XRD pattern indicated that the sample crystallized in a typical ABO3 perovskite rhombohedral structure with R-3c symmetry The lattice constants are calculated to a = 5.51 Å, b = 5.52 Å, and c = 13.78 Å When x increases to 0.3, 0.4, and 0.5, XRD patterns show that samples have pseudocubic structure with Pm3m symmetry, where the reflection peaks of (104), (006), (116), (301), and (220) could not be clearly seen There are no peaks of other phases indicating that the samples are of single phase The reflection peaks become stronger and sharper with increasing x indicating the continuing of the crystallization These results are slightly different from the results of [8] and [12], where the sample with x = 0.3 has a rhombohedral structure while samples with x = 0.4 and 0.5 have a cubic structure In [8], samples with x ≤ 0.3 exhibit structure change from a cubic to a rhombohedral structure when annealing temperature increases from 700 °C to 900 °C If Sr2+ was substituted into La3+ sites of LaMnO3 , Mn4+ ions will be formed but the content of them also will be fixed by creation of ion vacancies or non-stoichiometry (La1−x Srx MnO3+δ ), depending on preparation procedure [16], [17] However, the ability to form over the stoichiometry of the compound in air decreases with increasing the Sr content and almost disappears at high Sr concentration [17] Hence, there is no structure transition at high annealing temperature of 900 °C Our samples were annealed at 1000 °C and still have pseudocubic structure, but our samples may have larger nominal doping concentration than in afore-mentioned studies According to XRD patterns, the slight decrease of lattice constants with increasing x may be due to both the substitution of Mn4+ to Mn3+ , which have larger radii, and/or the reduction of the bending electrostatic forces from the creation of vacancies at oxygen sites The average size of crystallites of La1−x Srx MnO3 were calculated from XRD using the Scherrer formula D = 0.9λ/βcosθ , where D (Å) is the particle size, λ(Å) is the X-ray wavelength of Cu Kα , β(rad) is the full-width at half-maximum of the diffraction line, and θ (rad) is the Bragg angle The calculation showed that La1−x Srx MnO3 crystallites have average sizes of about 17, 10, 12, and 13 nm for samples with x = 0, 0.3, 0.4, and 0.5, respectively Our results show high crystallization of the sample and the crystallite sizes are smaller than those observed from samples produced by other methods [4]–[15] Thus, on comparing our observation with previous results, the structure of final products strongly depends on the preparation conditions Fig shows SEM images of La1−x Srx MnO3 with x = (top-left), x = 0.3 (top-right), x = 0.4 (bottom-left), and x = 0.5 (bottom-right) It shows that the particles are quasisphere with much larger particle sizes than the crystallites’ sizes calculated from XRD results A sample with x = has particles size of 1–2 μm while a sample with x = 0.3 has smaller size particles of ∼500 nm alongside a large particle, being aggregated due to high temperature annealing A sample with x = 0.4 has a narrow distribution of particles size, comparable to smallest particles of a sample with x = 0.3 and the aggregation is much less than that of a sample with x = 0.3 A sample with x = 0.5 has average particles size a little smaller than that of a sample with x = 0.4 and also does not have much aggregation The average size of particles decreases while the average size of the crystallites calculated from XRD results increases with increasing the doping level These tendencies are different from [8] and [12], where both sizes decrease with increasing doping level The particles’ sizes are comparable to those obtained in previous reports, for example, reported in [6] In samples with x = 0.4 and 0.5, particles are quasi-sphere and quite homogenous They can be proofed for the biomedical application if one more process of milling is applied after the calcinations, making particles more dispersive and smaller in size Fig shows the room-temperature magnetization curves of La1−x Srx MnO3 nanoparticles with various x A sample with x = shows paramagnetic behavior with almost linear dependence of magnetization on magnetic field When Sr was doped into samples, the samples show soft magnetic properties with small hysteresis in magnetization curves as shown, for example, in the inset of the sample with x = 0.3 The NAM et al.: SYNTHESIS AND MAGNETIC PROPERTIES OF PEROVSKITE La1−x S rx MnO3 NANOPARTICLES 2503104 Fig Room-temperature magnetization curves of La1−x Srx MnO3 nanoparticles with x = 0, 0.3, 0.4, and 0.5 Inset: hysteresis in magnetization curve of sample with x = 0.3 high saturation magnetization M S was observed for the doped samples and increases with increasing the doping ratio x M S is 29, 52, and 87 emu/g for the samples with x = 0.3, 0.4, and 0.5, respectively The saturation magnetization of 87 emu/g is higher than those observed in [6], [8], [10], and [11] For instance, Daengsakul et al [8] reported the highest saturation magnetization of ∼45 emu/g observed in samples prepared by the citrate gel method In addition, almost all samples prepared by other methods showed the highest magnetization at x = 0.3 while in our results, the saturation magnetization increases with increasing x This is in agreement with the XRD results where the sample with x = 0.5 has the best quality of the crystallization, where the content of Mn4+ is most appropriate for the double exchange interactions These results indicated that the Sr was successfully doped into the lattice of LaMnO3 In our experiments, Sr ion may have better conditions to be doped into perovskite lattice when compared to other methods The observed high saturation magnetization may be due to the superparamagnetic state in the sample, where the magnetization of individual nanoparticles can be considered as a super magnetic moment The small hysteresis of the magnetization curve at room temperature, as shown in Fig 3, also indicates the existence of remanent magnetization in individual particles These properties of samples could be applied and used in the hyperthermia applications Fig shows the zero field cooled (ZFC) and FC temperature dependence of magnetization of La1−x Srx MnO3 nanoparticles with x = 0.3, 0.4, 0.5 under the magnetic field of 100 Oe At x = 0.3 and 0.4, samples show clear spin-glass behavior with blocking temperature of ∼312 and 320 K and Curie temperature of ∼360 and 362 K, respectively These blocking temperatures, which are higher than room temperature, enhance the applicability of samples in industry A sample with x = 0.5 does not show clear blocking temperature but still shows spin-glass-like behavior with different FC and ZFC curves FC curve has an abnormality near 300 K and goes under the ZFC curve at ∼300–350 K This sample Fig FC and ZFC temperature dependence of magnetization of La1−x Srx MnO3 nanoparticles with x = 0.3, 0.4, and 0.5 under the magnetic field of 100 Oe has the highest Curie temperature of 366 K The abnormal behavior of the sample with x = 0.5 can be understood by the inhomogeneous doping of Sr into the sample In samples prepared by the citrate gel method, Zhang et al [6] reported a lower blocking temperature as well as Curie temperatures of 287 and 350 K, respectively Meanwhile, Lipham et al [11] showed a little higher Curie temperature of 370 K Lipham et al [11] also showed the almost linear dependence of the saturation magnetization and the Curie temperature on the particles size When comparing reported results, we note that the magnetic properties of samples depend on the preparation procedure, which can control particles size, crystallites size, and so on As discussed above, using this modified sol-gel method, the Sr ion can be more easily doped into perovskite lattice than by using other methods However, at a high level of doping, the inhomogeneity in the sample may result in the abnormal magnetic properties of the sample Furthermore, the sample with x = 0.4 shows the highest splitting of FC and ZFC curves It can also be understood by the inhomogeneity of doping in this sample, which gives high anisotropy of sample particles 2503104 IEEE TRANSACTIONS ON MAGNETICS, VOL 50, NO 6, JUNE 2014 IV C ONCLUSION La1−x Srx MnO3 nanoparticles were successfully prepared by the modified sol-gel method from nitrate precursors The structure analysis shows that the structure of the samples is a typical perovskite structure and the Sr was successfully doped into the sample The crystallite sizes of samples are calculated to ∼10–17 nm The structure of the sample exhibits small changes with changing doping ratio x, but magnetic properties of the sample vary strongly with doping ratio of Sr Undoped samples have paramagnetic properties at room temperature while doped samples show superparamagnetic properties with highest saturation magnetization of 87 emu/g, which is applicable in biomedicine The temperature dependence of the magnetization of doped samples expressed the spin-glass behavior of the sample with blocking temperature higher than room temperature ACKNOWLEDGMENT This work was supported by the National Foundation of Science and Technology Development NAFOSTED under Grant 103.02-2010.08 R EFERENCES [1] T J Armstrong and A V Virkar, “Performance of solid oxide fuel cells with LSGM-LSM composite cathodes,” J Electrochem Soc., vol 149, no 12, pp A1565–A1571, 2002 [2] A P Ramirez, “Colossal magnetoresistance,” J Phys., Condens Matter, vol 9, no 39, pp 8171–8199, 1997 [3] J H Park, E Vescovo, H J Kim, C Kwon, R Ramesh, and T Venkatesan, “Direct evidence for a half-metallic ferromagnet,” Nature, vol 392, pp 794–796, Apr 1998 [4] A K Pradhan et al., “Synthesis and magnetic characterizations of manganite-based composite nanoparticles for biomedical 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TRANSACTIONS ON MAGNETICS, VOL 50, NO 6, JUNE 2014 Fig XRD patterns of La1 x Srx MnO3 nanoparticles with x = 0, 0.3, 0.4, and 0.5 Fig SEM images of La1 x Srx MnO3 with x = 0, 0.3, 0.4, and 0.5 annealed... 0.4, 0.5 under the magnetic field of 100 Oe At x = 0.3 and 0.4, samples show clear spin-glass behavior with blocking temperature of ∼312 and 320 K and Curie temperature of ∼360 and 362 K, respectively