Research Article | Open Access
Mahin Eshraghi, Parviz Kameli, "Structural and Magnetic Properties of Microwave Assisted Sol-Gel Synthesized Manganite Nanoparticles", International Scholarly Research Notices, vol. 2014, Article ID 867139, 6 pages, 2014. https://doi.org/10.1155/2014/867139
Structural and Magnetic Properties of Microwave Assisted Sol-Gel Synthesized Manganite Nanoparticles
The structural and magnetic properties of nanoparticles of (LSMO) were studied using powder X-ray diffraction (XRD), transmission electron microscope (TEM), and magnetic measurements. The XRD refinement result indicates that samples crystallize in the rhombohedral structure with R-3C space group. The dc magnetization measurements revealed that samples exhibit no hysteretic behavior at room temperature, symptomatic of the superparamagnetic (SPM) behavior. The results of ac magnetic susceptibility measurements show that the susceptibility data are not in accordance with the Néel-Brown model for SPM relaxation but fit well with conventional critical slowing down model which indicates that the dipole-dipole interactions are strong enough to cause superspin-glass-like phase in LSMO samples.
Magnetic nanoparticles are currently the subject of intense research because of their potential applications in high density magnetic storage and biomedical applications [1–3]. The perovskite manganite with the formula (, Ca, Ba, or vacancies) has attracted considerable attention due to the discovery of the phenomenon of colossal magnetoresistance (CMR) and its potential application [4–7]. The properties of these materials are explained by double exchange theory of Zener  and electron lattice interaction . By varying the composition and consequently tuning the ratio, the compound reveals various electronic, magnetic, and structural phase transitions at different temperatures. These phase transitions have been attributed to strong coupling among spin, charge, orbital degree of freedom, and lattice vibrations.
It is well known that the magnetic and transport properties of these materials strongly depend on the particle size, due to the influence of structural and magnetic disorders at the grain boundaries [9–13]. The size effects and the large surface area of magnetic nanoparticles intensely change some of the magnetic properties compared to bulk counter parts. Manganite nanoparticles display features like lower values of magnetization , higher values of low field magnetoresistance , exhibiting superparamagnetic (SPM) phenomena , and so forth. SPM nanoparticles with single domain microstructure have a high potential as carriers for biomedical applications. According to the Stoner-Wohlfarth theory, the magnetocrystalline anisotropy energy, , of a single-domain particle can be approximated by , where is the magnetocrystalline anisotropy constant, is the volume of the nanoparticle, and is the angle between the magnetization direction and the easy axis of the nanoparticle. serves as an energy barrier for blocking the flips of magnetic moments. When becomes comparable with thermal activation energy, with as the Boltzmann constant, thermal activation can overcome the anisotropy energy barrier. This state occurs above the blocking temperatures, , called SPM state. At blocking temperature, thermal energy is equal to the anisotropy energy. The absence of hysteresis, almost incalculable coercivity and remanence, and the nonachievement of saturation even at high magnetic field are the typical characteristics of SPM behavior .
In this paper we have studied the effect of annealing temperature on the structural and magnetic properties of the microwave-assisted sol-gel grown (LSMO) manganite samples.
The LSMO manganite samples were prepared by the microwave-assisted combustion method using a domestic LG microwave oven operating at frequency of 2.45 GHz with the output power of 1000 W. Stoichiometric amounts of La(NO3)36H2O(Merck, 99%) n(NO3)H2O(Merck, 99%) and Sr(NO3) (Merck, 95%) were dissolved in water and mixed with ethylene glycol and citric acid, forming a stable solution. The mixture was then kept in a microwave oven with 60% of output power. This mixture undergoes dehydration followed by decomposition and spontaneous combustion with the evolution of voluminous gases. The obtained final product is in the form of a foamy powder and is calcined at 450°C (S1) for 6 h. Different packages of powder were sintered at 550°C (S2), 575°C (S3), and 600°C (S4) for 6 h to obtain powders with different particle sizes.
Phase formation and crystal structure of the powders were checked by XRD pattern using Cu K radiation source in the scan range from 20° to 80°. The average particle sizes of the samples were estimated from TEM micrographs. The ac magnetic susceptibility has been measured versus temperature at different frequencies in the selected range of 33–1000 Hz and using a Lake Shore ac susceptometer model 7000. The dc magnetization was carried out as a function of magnetic field at 295 K using a Meghnatis Daghigh Kavir vibrating sample magnetometer (VSM).
3. Results and Discussion
Figure 1 shows the XRD patterns at room temperature for samples. Figure 1(a) shows that S1 sample is amorphous and its crystal structure is not formed. By sintering the powder at higher temperatures, 550°C to 600°C, the perovskite crystal structure is formed. The Rietveld analyses of the XRD pattern of the samples have been carried out using FULLPROF program . Figure 1(b) shows the XRD pattern with Rietveld analysis of the S3 sample. The estimated average lattice parameters are and which are found in close agreement with reported values . Typically, the TEM micrograph of S3 sample is shown in Figure 2. TEM micrograph shows that the mean particle size of the S3 sample is about 50 nm. Also, the selected area diffraction patterns of sample S3 confirms the crystalline nature of this sample.
To study the magnetic response of samples to an external field, we measured field dependent magnetization using VSM at room temperature. Figure 3 shows the room temperature hysteresis curves of the samples sintered at different temperatures. It can be seen that by increasing the sintering temperature the magnetization increases slightly. An increase in magnetization with the sintering temperature can be attributed mainly due to the enhancement of grain size. The increase of magnetization by increasing the grain size could be ascribed to the lower surface area of larger grains. This is because of the magnetically disordered state in the surface. In the case of nanometric-size grains, defects are expected to occur to a higher extent due to the vacancies, microstrain, and dislocations. Since the sizes of samples are within the superparamagnetic and single domain limits, these samples are SPM in nature with zero coercivity and nonsaturated magnetization.
To further study the SPM behavior of samples, we have performed ac susceptibility measurements on sample S3. Figure 4 shows the real, , and imaginary, , parts of ac magnetic susceptibility as function of temperature in an ac field of 10 Oe and frequencies of 33, 111, 333, 666, and 1000 Hz. It is evident that by decreasing the temperature, the and show a peak related to the blocking/freezing temperature of the SPM/spin glass systems [20–22]. Above , the nanoparticles considered being in the SPM regime and below which the nanoparticles are in the so-called blocked/freezed regime. is frequency dependent and shifted to higher temperature with increasing frequency. The frequency dependence of the ac magnetic susceptibility is a characteristic of SPM/spin glass systems. There are several models to interpret the ac susceptibility behavior of the magnetic nanoparticles. In the noninteracting SPM system, the frequency dependence of blocking temperature follows the Néel-Brown model : where is related to measuring frequency () and is related to the jump attempt frequency of the magnetic moment of nanoparticle between the opposite directions of the magnetization easy axis. For SPM systems, is in the range of . Blocking temperature is the one that the thermal energy overcomes to anisotropy energy. When the energy of the potential barrier is comparable to thermal energy, the magnetization direction of the nanoparticles starts to fluctuate and goes through a rapid SPM relaxation. Above the magnetization direction of nanoparticles can follow the direction of the applied field. Below the blocking temperature the thermal energy is less than the anisotropy energy; hence the direction of magnetization of each nanoparticle which may lie in the direction of easy axis is blocked. Since the nanoparticles and consequently their easy axes are randomly oriented, by decreasing the temperature, the total magnetic susceptibility is reduced. The linear plot of versus the reciprocal of blocking temperature is shown in Figure 5. The and can be obtained from the slope and intercept of the plot. By fitting the experimental data we have found an unphysical small in comparison to the values of s for SPM systems. As expected, this result simply indicates that there exists strong interaction between nanoparticles of S3 sample. The interactions between nanoparticles affected the blocking/freezing temperature via modifying the energy barrier. On the other hand, the Vogel-Fulcher law describes the behavior of magnetically interacting (medium interaction) nanoparticles as follows : In this model, is an effective temperature, representing the existence of the interaction between nanoparticles and is a time-scale constant typically on the order of s. Fitting result with the Vogel-Fulcher law is given in Figure 6. According to Dormann et al. studies , the Vogel-Fulcher law is valid only if . In our work is 199 K, while the average value of is 202 K. This point revealed that Vogel-Fulcher law cannot explain the behavior of the sample.
Two criteria have often been used to compare the frequency sensitivity of peak temperature in different samples and to distinguish between canonical spin glasses, other disordered magnetic compounds, where the freezing is progressive, and fine particles. They are given as  where is the difference between measured at the frequency interval. ( Hz ( Hz) and is the mean value of blocking/freezing temperature in the range of experimental frequencies and is the characteristic temperature of the Vogel-Fulcher law, (2). The value of , which is independent of any model, represents the relative shift of blocking temperature per decade of frequency. The value of can be useful to compare the variation between various systems. The experimental values of and depend on the interaction strength between magnetic nanoparticles. Dormann et al. distinguish three different types of dynamical behavior based on the values of and : for noninteracting particles and , for weak interaction regime (inhomogeneous freezing) and , and in the medium to strong interaction regime (homogeneous freezing) and . Both and decrease with increasing the interactions between nanoparticles. The calculated values of and for our sample are 0.01 and 0.015, respectively. By comparing the and values, with values just given above, one can claim that there is a strong interaction between LSMO nanoparticles.
In the case of strong interaction between nanoparticles, magnetic nanoparticles indicate superspin glass behavior . The possibility of presence of a spinglass behavior was examined by the critical slowing down model  as follows: where is the freezing temperature and is the glass-transition temperature. The exponent depends on the law governing the phase transition, is related to the relaxation time of the individual particle magnetic moment, is the critical exponent of correlation length, , and relates and as . The parameter as a dynamic critical exponent shows interactions strength and varies between 4 and 12 and is in the range of s for frustrated three-dimensional systems such as spin glasses, reentrant spin glasses, and superspin glasses (SSG) [25, 26]. Figure 7 shows the best fits of ac magnetic susceptibility data using the critical slowing down model. The obtained values of and are and 4.8, respectively. Based on the calculated values of and , the behavior of our sample is consistent with those expected for spin glass systems and could suggest the existence of a phase transition to a SSG state below the peak temperature. This type of SSG-like behavior has been reported elsewhere for different samples. For example, Fiorani et al. reported the values of and for interacting nanoparticles of -Fe2O3 . Also the SSG-like behavior has been reported in oleic acid-coated ferrite nanoparticles by Parekh and Upadhyay .
The (LSMO) manganite nanoparticles could be synthesized successfully in nanocrystalline form by the microwave-assisted sol-gel auto-combustion method. The estimated negligible coercivity and remanent magnetization indicate superparamagnetic behavior for these nanoparticle samples. Spin dynamics analysis, using ac magnetic susceptibility measurements, showed that these nanoparticles have superspin glass behavior with strongly interacting superspins.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors would like to thank the Payame Noor University for supporting this work.
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Copyright © 2014 Mahin Eshraghi and Parviz Kameli. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.