Structural and Magnetic Properties of La 0 . 9 Sr 0 . 1 MnO 3 Micro and Nanometer-Sized Manganite Samples

The structural and magnetic properties of micrometer and nanometer-sized samples of La0.9Sr0.1MnO3 manganite have been studied, using powder X-ray diffraction (XRD), transmission electron microscope, field emission scanning electron microscope and magnetic measurements. The XRD refinement result indicates that both samples have rhombohedral structure. The micrometer-sized sample shows long-range ferromagnetic ordering with transition temperature, Tc ~ 225 K. In a contrary, the temperature dependence of Ac susceptibility of nanometer-sized sample shows broad peak which is frequency dependent. The result of Ac susceptibility data for nanometer-sized sample revealed super-spin glass behavior for this sample at low temperatures. Also, the Dc magnetization measurements confirmed the superparamgnetic behavior of nanometer-sized sample at room temperature.


Introduction
Manganites La 1-x A x MnO 3 (A = Sr, Ca, Ba or vacancies) has created an enormous considerable interest due to the discovery of the so-called colossal magnetoresistance (CMR) and its applications as magnetic field sensors (Jin et al.,1994;Asamitsu et al., 1995;Zener, 1951).The properties of these compounds have been interpreted by double-exchange model (Zener, 1951) and Jahn-Teller distortion (Millis et al., 1995).By varying the composition x, and consequently tuning the Mn 3+ / Mn 4+ ratio, the compound La 1-x A x MnO 3 indicates various electrical, magnetic and structural behaviors.These behaviors were related to strong coupling among spin, charge, orbital degree of freedom and lattice vibrations.
It is well known that the magnetic properties of these materials strongly depend on the particle size (Sánchez et al., 1996;Zhang et al., 1997;Wang et al., 1999;Rostamnejadi et al., 2009;Daengsakul et al., 2012).Manganite nanoparticles display features like lower values of magnetization (Kameli et al., 2006), higher values of low field magnetoresistance (Kuberkar et al., 2012), exhibit superparamagnetic (SPM) phenomena (Rostamnejadi et al., 2012;Dormann et al., 1999) and etc. SPM nanoparticles with single domain microstructure have a high potential as carriers for biomedical applications (Pardhan et al., 2008;Thorat et al., 2013).Additionally the nanosize particles of manganites can have zero coercivity at room temperature, which are applicable for hyperthermia investigation.
In this work we have investigated the structural and magnetic behaviors of micrometer and nanometer-sized La 0 .9 Sr 0 . 1 MnO 3 (LSMO) manganite samples which are prepared by Microwave-assisted Sol-Gel grown.To the best of our knowledge this is a first report about the microwave synthesizing method for LSMO manganite samples.

Experimental
The LSMO nanometer and micrometer-sized manganite samples were synthesized by the microwave-assisted Sol-Gel method using a kitchen-type microwave furnace equipped with a 2.45 GHz generator with the output power of 1000 W. The appropriate amounts of La(NO 3 ) 3 •6H 2 O, Mn(NO 3 ) 2 •4H 2 O, and Sr(NO 3 ) were dissolved in water and mixed with ethylene glycol and citric acid, forming a stable solution.The solution was then kept in a microwave furnace with 60% of output power.The obtained powder is calcined at 450 °C for 6 h.Sample S1 and sample S2 were obtained by annealing the powder at 575 °C and 1200 °C for 6 h, respectively, to obtain powders with nanometer and micrometer-sized particles.
The synthesized samples were characterized by X-ray diffraction (XRD) using Cu Kα radiation source.The morphology of the samples was determined by transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM).The dynamic magnetic properties have been characterized by using Ac susceptometer (Lake Shore Ac susceptometer model 7000).The Dc magnetization of the samples were measured at room temperature on a vibrating sample magnetometer (VSM, model Meghnatis DaghighKavir).

Structural Characterization
The XRD patterns of the samples (S1 (a) and S2 (b)) are shown in Figure 1.The XRD data is analyzed with Rietveld refinement using FULLPROF program (Carvajal et al., 1993).It is found that the samples are single phase with the rhombohedral structure (R-3c space group) and there is a good agreement between the experimental and calculated patterns.The estimated average lattice parameters are a = b = 5.511 Å and c = 13.352Å which are found close agreement with reported values (Shinde et al., 2011).

Magnetic Characterization
Figure 3 shows temperature dependence of Ac susceptibility measurements (real part) for S1 and S2 samples in an Ac field of 1 mT and frequency of 333 Hz.The micrometer-sized sample shows long-range ferromagnetic (FM) ordering with transition temperature, Tc ~ 225 K.In a contrary, the temperature dependence of Ac susceptibility of nanometer-sized sample shows broad peak which is frequency dependent (see Figure 4).This peak is normally related to the existence of SPM/spin glass behavior in magnetic systems (Dormann et al., 1997;Suzuki et al., 2009;Aslibeiki et al., 2012).The peak temperature corresponds to the average blocking temperature (T B ) of the nanoparticles.Also, the magnitude of susceptibility of nanometer-sized sample is less than the micrometer-sized sample due to the spin disorders at surface of nanoparticles and presence of SPM behavior in this sample (Millis, 1998).To study the dynamic behavior of samples, frequency dependence of Ac susceptibility was measured on sample S1. Figure 4 shows the Ac susceptibility of sample S1 in an Ac field of 1 mT and frequency range of 33-1000 Hz.It is evident that the blocking temperature shifts to higher temperature as the frequency increases.In the non-interacting system, the frequency dependence of T B obeys the Néel-Brown law (Tadic et al., 2012): Where τ is related to measuring frequency (τ = 1/f) and τ 0 is the magnetic moment flip time, k B is the Boltzmann's constant and E B is the energy barrier of the nanoparticles.When the energy of the barrier is less than thermal energy, the particles show SPM relaxation.Below T B the magnetization of each nanoparticle is blocked along their easy anisotropy axes.Figure 5 shows the plot of ln (f) versus 1/T B .The value of τ 0 ~1.26×10 -90 was estimated from data, were too small in comparison to the values of 10 -9 -10 -13 for SPM systems.As expected, this result simply indicates that there exists strong interaction between nanoparticles and the modified model is required to analysis the data.The Vogel-Fulcher law with an additional parameter T 0 could be an appropriate model to describe the behavior of nanoparticles with medium interaction (Dormann et al., 1999), Figure 6 shows the results of the fit with the Vogel-Fulcher law.It is well known that the Vogel-Fulcher law is valid only if T 0 << T B (Dormann et al., 1999).The estimated value of T 0 for S1 sample is about 199 K which is comparable with the average value of T B , ~ 202 K. Therefore, the Vogel-Fulcher law cannot explain the behavior of sample.
Figure 6.The best fits of ac magnetic susceptibility data for sample S1, using Vogel-Fulcher model In the case of strong interaction between nanoparticles, magnetic nanoparticles indicate super-spin glass behavior (Aslibeiki et al., 2010).Critical slowing down model as a useful model has widely used to study the possibility of presence of a spin-glass (SG) behavior.This model is given as (Mydosh et al., 1996): Where T f is peak temperature in χ '' component of Ac susceptibility, T 0 corresponds to DC value of T f for f→0.υ is the critical exponent of correlation length and z is a dynamic critical exponent (Mydosh et al., 1996;Thatar et al., 2009).From the best fits of this model (Figure 7), τ 0 was estimated as 4.17×10 -11 and zυ value as 4.8, which were in the predicted range for SG systems.This kind of SG behavior has been reported for interacting nanoparticles of γ-Fe 2 O 3 (Fiorani et al., 2002) and Mn 0.5 Zn 0.5 Fe 2 O 4 ferrite nanoparticles (Parekh et al., 2010).
Since each particle contains 10 3 -10 4 atoms (spin), this behavior for interacting nanoparticles is called super-spin-glass behavior.It should be noted that the generated heat by nanoparticles in hyperthermia based therapy method is dependent on interactions between nanoparticles (Rahimi et al., 2013).
Figure 7.The best fits of ac magnetic susceptibility data for sample S1, using Critical slowing down model The room temperature hysteresis loop of sample S1 is shown in Figure 8.Since the size of this sample is within the single domain limits the coercivity of this sample is zero and it shows SPM behavior at room temperature.
Figure 8.The room temperature Magnetization vs. magnetic field for sample S1

Conclusions
The La 0 .9 Sr 0 . 1 MnO 3 nanometer and micrometer-sized manganite samples could be synthesized successfully by the Microwave-assisted Sol-Gel auto-combustion method.The micrometer-sized sample shows long-range ferromagnetic ordering.The zero coercivity and non-saturated magnetization show SPM behavior for the nano-sized sample at room temperature.It was found that the nanometer-sized sample has super-spin-glass

Figure 1 .
Figure 1.The observed and calculated (Rietveld analysis) XRD patterns of samples (a) S1 and (b) S2 at room temperature

Figure 3 .
Figure 3. Temperature dependence of Ac magnetic susceptibility (real part) for (a) S1 and (b) S2 samples

Figure 4 .Figure 5 .
Figure 4. Frequency dependence of real (a) and imaginary (b) parts of Ac susceptibility for sample S1, in frequencies of 33-1000 Hz and ac field of 1 mT