Magnetic properties of La0.7Ca0.3MnO3 nanoparticles prepared by reactive milling

Journal of Alloys and Compounds 479 (2009) 828–831

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Magnetic properties of La0.7 Ca0.3 MnO3 nanoparticles prepared by reactive milling Do Hung Manh a,∗ , Nguyen Chi Thuan a , Pham Thanh Phong b , Le Van Hong a , Nguyen Xuan Phuc a a b

Institute of Materials Science, Vietnamese Academy of Science and Technology, 18 Hoang Quoc Viet Road, Cau Giay Distr., Hanoi, Viet Nam Nha Trang Pedagogic College, Khanh Hoa Province, Viet Nam

a r t i c l e

i n f o

Article history: Received 8 December 2008 Received in revised form 12 January 2009 Accepted 18 January 2009 Available online 29 January 2009 Keywords: Manganite La0.7 Ca0.3 MnO3 Nanoparticles Magnetic property

a b s t r a c t La0.7 Ca0.3 MnO3 (LCMO) nanoparticles were synthesized by reactive milling in ambient conditions. Magnetic properties of LCMO single-phase nanocrystalline particles were studied. LCMO nanoparticles exhibit superparamagnetism with blocking temperature that decreases in the logarithmic function as increasing applied magnetic field. Besides, the blocking temperature decreases as increasing milling time from 8 h to 16 h. The temperature dependence of the saturation magnetization shows a strong collective excitation due to the spin wave that depends on temperature in form T˛ with ˛ = 1.7, which slightly deviates from the Bloch law. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Nanoscale magnetism provides wealthy scientific knowledge and potentials for applications, which include magnetic recording media, ferrofluid, catalysis, magnetic refrigeration, bioprocessing, drug delivery system, etc. [1]. Magneto- and electronic-transport properties of manganites at the nanometer scale comprise an issue of great interest nowadays [2–3]. Some salient features observed as we reduce the particle size of manganite systems are (a) a decrease and broadening of the ferromagnetic transition temperature TC , (b) a decrease in the magnetization in comparison with single-crystal and bulk polycrystalline samples, showing superparamagnetic behavior at very low particle sizes. It is generally believed that a high value of the surface to volume ratio with large fraction of atoms residing at the grain boundaries is what differentiates them from the bulk materials in their properties. The net magnetic behavior is dominated by surface magnetic properties [4]. Formation of grain boundaries causes broken bonds at the surface, which induces a decrease in the magnetization value. The recognition and elucidation of the grain size effect are crucial if manganites are expected to be used in nano-electronic devices. In our previous papers [5,6], we presented experiment results and theoretical analysis on La0.7 Sr0.3 MnO3 nanoparticles system fabricated in SPEX D8000 mill. Magnetic measurements showed the existence of strong magnetic interaction between nanoparti-

∗ Corresponding author. E-mail address: [email protected] (D.H. Manh). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.01.049

cles and decrease in saturation magnetization (MS ) with increasing milling time. Mean field approximation formalism and core–shell structure of nanoparticles were used to describe the magnetic interaction and obtain the average of particle diameter ranging from 12 nm to 6 nm when MS decreases of 48.5–19 emu/g. In this paper we present a systematic investigation on crystalline structure, morphology and magnetic characteristics for La0.7 Ca0.3 MnO3 nanoparticles. 2. Experimental Nanocrystalline samples of La0.7 Ca0.3 MnO3 were synthesized by reactive milling method. We used La2 O3 , CaCO3 , Mn3 O4 powders of 99.5% purity or more. La2 O3 powder was annealed at 950 ◦ C for 4 h in order to transform any lanthanum hydroxide to lanthanum oxide, CaCO3 was annealed at 1100 ◦ C for 4 h to decompose the CO2 before weighting. The powder milling process was performed with commercial high energy SPEX 8000D shaker mill. The powders were put in vial with two 1/2 in. and four 1/4 in. balls. The vial and the balls are made with tempered steel. Milling was conducted in the ambient atmosphere for a period of milling time tmil varied from 1 h to 20 h and interrupted at different times (4 h, 8 h, 12 h, 16 h and 20 h) for sampling. The crystal structure was determined by X-ray diffraction using a SIEMENS D5000 with Cu K␣ radiation. A scanning rate of 0.02◦ s−1 was adopted and the data were taken at room temperature in the 2 range of 20–70◦ . The pure polycrystalline LCMO sample was used as a standard sample to correct the instrumental line broadening. After analyzing the measurement data by a special program packet, the profiles of diffracting peaks were fitted to the ps-vogt1 function [7]. Then the obtained parameters of these peaks’ profiles were used to determine grain sizes and size distribution by the WIN-CRYSIZE program packet based on the Warren–Averbach analysis. The size and the shape of particles were directly observed by a field emission scanning electron microscopy (FESEM) (Hitachi S-4800). To study magnetic property, the samples with the milling time longer than 8 h are chosen to ensure that there does not exist any unexpected phases out of LCMO at least under X-ray diffraction analysis. Magnetic measurements were carried out as functions of temperature and

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Fig. 1. X-ray diffraction patterns of La0.7 Ca0.3 MnO3 samples with various milling time.

magnetic field using the PPMS at the temperature range 5–250 K in fields up to 5 T. For zero-field-cooled (ZFC) magnetization, the sample was first cooled from room temperature down to 10 K in the field of 0 Oe. After applying the magnetic field at 5 K, the magnetization was measured in the warning up cycle with field on. Whereas, for field-cooled (FC) magnetization measurements, the sample was cooled in the same field (measuring field in the ZFC case) down to 5 K and FC magnetization was measured in the warning up cycle under the same field. The applied field dependence of magnetization, at various temperatures, was carried out after cooling the sample from room temperature down to the measurement temperature in the absence of the magnetic field.

3. Results and discussions 3.1. Structural characterization Fig. 1 presents the XRD patterns showing the evolution of the crystalline structure of the initial powder found in the vial at different milling time. After 4 h of milling, the typical patterns of the starting oxides La2 O3 , Mn3 O4 and CaO are not seen. The perovskite lines are broad indicating the formation of a nanophase but the base line shown no indication for the presence of an amorphous phase. After 8 h of milling in particular conditions of this experiment, the powder in the vial is essentially converted into perovskite. When milling further the peaks are broaden indicating that grain size of nanoparticles decreases with increasing milling time. Similar observation for ball-milled La0.8 Sr0.2 MnO3 nanoparticles is revealed by Roy et al. [2]. It is worth noting from the authors that accompanying with the reduction of the size due to further milling the nanoparticles possess surface layer with increasing in degree of defects resulting a decrease the magnetization in comparison with bulk samples. Reactive milling is demonstrated to be a simple technique which allows preparation of perovskite samples. The grain size distribution curves for the studied samples are presented in Fig. 2. We can see that size distributions for small grains (<8 nm) have asymmetrical shape, while for bigger grains they become more symmetrical shape. For the sample of 12 h milling, the size distribution has two maximums at 2.3 nm and 10.5 nm. In the case of reactive milling samples, it is observed that the lattice structure is more likely cubic symmetry than orthorhombic (for standard ceramic sample). Fig. 3 shows the morphology and size of samples after milling 4, 8, 12 and 16 h with the mean diameter is greater than 15 nm. After 4 and 8 h of milling, the particles are clearly spherical in shape and appear to be composed of extremely fine material (Fig. 3a and b). As one can see in Fig. 3c and d, the 15 nm spherical particles tend to bind with others to form larger clusters with the increase of milling time. The cause of grain clustering is likely related to magnetic dipolar interaction between neighboring nanograins. In comparison with the average crystal size deduced from XRD data, the particle size provided by FESEM images used to be bigger.

Fig. 2. Relative frequency distribution of LCMO nanocrystalline samples.

3.2. Magnetic property We have investigated field-cooled and zero-field-cooled temperature dependence of dc magnetization at an applied field of 10 Oe for LCMO nanoparticles. A typical ZFC/FC curve of LCMO particles after milling of 8 h is shown in Fig. 4a. The FC/ZFC curve shows the presence of a peak in ZFC curve and FC magnetization has an irreversible behavior below the peak temperature. This indicates that below the peak temperature the magnetic system goes onto a disordered blocked state. Blocking temperature TB had been determined from the peak of the ZFC curve. For the other nanoparticles, there is a huge reduction of the magnetization (as shown on the inset of Fig. 4a). Such a drastic decrease of the magnetization could be explained by considering a nonmagnetic layer on the surface, which thickness increases as the milling time goes up. The zero-field-cooled temperature variation of the magnetization of the LCMO nanoparticles with various milling time is shown in Fig. 4b. The blocking temperature TB of LCMO nanoparticles decreases from 88 K after 8 h to 65 K after 16 h of milling time. The inset in Fig. 4b shows a monotonic decrease of blocking temperature in range of milling time from 8 h to 16 h. From TEM result and magnetization data reported previously [5] we noted a systematic decrease of particle size with increasing milling time. Therefore, it is clear that the TB increases with decreasing particle size. This result is in accordance with the results of Roy et al. [2]. In order to understand the behavior of nanoparticles in the presence of field, applied magnetic field dependent experiments were performed for LCMO after 8 h of milling. Since the blocking temperature equals the energy at which the moment requires to get aligned along the field, so when an external field is applied, the energy of the barrier decreases to KV–H, where K is anisotropy constant. Therefore, the alignment of the moments along the field requires less energy. Consequently the blocking temperature also decreases. A logarithmic dependence of TB with applied field is reported by some authors [2,8]. The TB in our case have also been fitted well by TB = 132.93 − 11 × ln (H) Fig. 5a shows the variation of blocking temperature TB vs. the applied magnetic field. Inset shows the change in the magnetization curve sharp as the applied magnetic field increases. Apart from the shift of the peak towards lower temperature at higher field, the curves are broaden at lower applied fields. The broadening of the curves at low fields reflects the distribution of the particles sizes. At low enough fields, the energy gained due to the

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Fig. 3. FESEM images of LCMO after milling for (a) 4 h, (b) 8 h, (c) 12 h and (d) 16 h, respectively.

Fig. 4. (a) Typical ZFC/FC curves of LCMO particles after 8 h of milling time. Inset shows the M(ZFC) vs. T of a bulk sample (B) and LCMO nanoparticles (N) after 8 h of milling at the applied field of 100 Oe and (b) zero-field magnetization vs. temperature curve of LCMO samples with various milling time at an applied field of 10 Oe. Inset shows the change of blocking temperature as the milling time changes.

Fig. 5. (a) The dependence of blocking temperature on the applied magnetic field for LCMO nanoparticles after 8 h of milling. Inset shows the change in the magnetization curve sharp as a function of the applied magnetic field and (b) saturation magnetization as a function of temperature for LCMO nanoparticles after 8 h of milling. Dashed curve is power law fitted with the form MS = M0 (1 − H)T˛ to the data points.

field is small and the unblocking takes place mainly due to increasing temperature. Consequently the rate of unblocking is slow. As the temperature is high enough so that all the particles are unblocked and we can observe the superparamagnetic behavior. The rate is increased as the contribution of field to the unblocking process increases. We have also performed isothermal magnetization measurements as a function of magnetic field at different temperatures. Fig. 5b shows the behavior of the saturation magnetization MS vs. temperature for LCMO nanoparticles after 8 h of milling. The MS value is obtained by extrapolating the applied magnetic field H → ∞ and finding the magnetization from M(H) curve. In the case of Heisenberg spin clusters, the temperature dependence of magnetization including finite size effects is given by a power law of the

form: MS = M0 (1 − BT ˛ ) A fit to the magnetization data is shown in Fig. 5b (dashed curve) with M0 = 36.87 emu/g, B = 1.1 × 10−4 K−1.7 , and ˛ = 1.7. The magnetization does not follow Bloch’s T3/2 law but rather it follows T1.7 law. The slightly greater value of ˛ compared to the bulk value (3/2) is due to effect of small particle size [9]. 4. Conclusions La0.7 Ca0.3 MnO3 nanoparticles were successfully synthesized by the reactive milling method. The average crystalline size of about

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15 nm was deduced from XRD data for La0.7 Ca0.3 MnO3 nanoparticles and also reconfirmed by FESEM images. The reactive milling process creates unusual defects at surface of LCMO nanoparticles, results in a huge decrease of magnetic moments in comparison with the bulk material. The temperature dependence of magnetization in the case of LCMO nanoparticles follows a T˛ law where ˛ = 1.7, which slightly deviates from the Bloch law. La0.7 Ca0.3 MnO3 nanoparticles exhibit superparamagnetic behavior with a blocking temperature that decreases as increasing milling time from 8 h to 16 h. Besides, the blocking temperature decreases in form of logarithmic function as increasing applied magnetic field. Acknowledgements This work was done under the support of the Fundamental Research Program of Vietnam and Project of Vietnamese Academy

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