**2. Experimental methods**

#### **2.1 Autocombustion synthesis**

After several years of intense research efforts, it has emerged that a large number of synthesis approaches to a wide variety of nanoparticles are available. In this work, La0.8Sr0.2MnO3 (LSr) nanocrystalline were synthesized by the autocombustion process

*Smart Nanosystems for Biomedicine, Optoelectronics and Catalysis*

magnetic nanoparticles in diverse fields.

nanoparticles' size, size distribution, agglomeration, coating, and shapes along understanding the changes in magnetic properties prompted the application of

The magnetic properties arise from the magnetic moment associated with electron spin. In ferromagnetic materials, groups of atoms band together into areas called domains, in which all the electrons have the same magnetic orientation. Fundamental changes occur in ferromagnetic materials when their physical size is reduced. The magnetic structure of the ferromagnetic material consists of several magnetic domains and thus retains an important magnetic moment in zero fields [13]. When the particle size becomes smaller, there is a limit when it becomes energetically unfavorable with the formation of several domains and the particle becomes magnetic single domain. A single-domain particle presents all the spins aligned in the same direction. The critical size value depends on the material. A single-domain particle presents all the spins aligned in the same direction. The total magnetic moment of the nanoparticles can be regarded as one giant magnetic moment, composed of all the individual magnetic moments of the atoms. These nanoparticles show a certain preference for the direction, along which their magnetization aligns to (directions of easy magnetization). As particle size continues to decrease below a critical size, the ferromagnetic material is transformed into a superparamagnetic one. In this case, magnetization can randomly flip direction under the influence of temperature, causing the residual magnetization to be null (**Figure 1**) [14]. Superparamagnetic nanoparticles are preferred in biomedical applications because they have zero

magnetization at room temperature and do not agglomerate [15, 16].

Magnetic measurements such as vibrating sample magnetometry and associated methods for determining magnetization according to the applied field and Mössbauer spectroscopy have been extensively used to observe the superparamagnetic behavior [17–19]. In this study, electron spin resonance (ESR) spectroscopy was used to study magnetic properties of La0.8Sr0.2MnO3 nanopowders. The basic concepts of ESR are based on the Zeeman effect which leads to the separation of the energy levels of the electrons under the effect of an external magnetic field. The magnetic moments of the electrons perform a precessional motion around the direction of the applied magnetic field with the angular velocity of Larmor. Classically, the resonance event occurs when a transverse alternating field is applied at the Larmor frequency. For a ferromagnetic solid, a strong coupling exists between the electrons. In general, this energy is more

*Transition from multidomain to single-domain to superparamagnetic state whith increasing the particle* 

**20**

**Figure 1.**

*diameter.*

with two-step synthesis process. It is an interesting synthetic route for the preparation of a compound with differing crystallite sizes. The reaction is very simple and involves just lanthanum nitrate, strontium nitrate, and manganese acetate. Manganese acetate was dissolved in the minimum of distilled water under agitation. On the other hand, stoichiometric amounts of lanthanum nitrate and strontium nitrate are dissolved in distilled water under stirring. The solution was prepared by mixing aqueous solutions of (La(NO3)3 + Sr(NO3)2) and manganese acetate in 1:1 molar ratio. To evaporate the water, the solution was stirred in a beaker placed on a hot plate at 80°C until a viscous product was obtained. The solutions are mixed and were kept stirred in a beaker on a hot plate at 80°C to evaporate the water until you get a product with viscous appearance. After that, the gel obtained was inserted into a preheated oven at 350°C for 2 h. After a few seconds, a violent flame was produced by releasing large amounts of gas, with formation of a spongy powder of dark brown color. Finally, the powder resulting from autocombustion was calcined at 700°C under different heat treatment times and at 800, 900, and 1000°C for 15 h. The samples are designated by LSrT-t (T, calcination temperature; t, heat treatment times).

Phase analysis of all products was performed by using powder X-ray diffraction (XRD) using an X'Pert Pro PANAnalytical diffractometer with CuKα radiation (λ = 1.5418 Å) at room temperature. The crystalline phases were determined by comparison of the registered patterns with the International Center for Diffraction Data (ICDD) powder diffraction files (PDF).

The average crystallite sizes of the samples were estimated from the (0 2 4) reflections at 2θ = 46°, by means of the Debye–Scherrer equation: D = Kλ/β cosθ, where K is a constant equal to 0.89, λ is the wavelength of the X-ray used, and β is the full width at half maxima (FWHM) of the X-ray reflection at 2θ [37]. The reflection at 2θ = 46° was selected to calculate the crystallite sizes because it does not overlap with other profiles [38, 39].

Magnetization at various fields was measured at temperature 300 K using a vibrating sample magnetometer (VSM) in fields up to 5 T.

We have recorded the ESR spectra of La0.8Sr0.2MnO3 at room temperature, with a Bruker ER-200D spectrometer, operating in the X-Band (9.30 GHz). These measurements are performed on 20 mg of loosely packed fine powder. Data were acquired with 2 mW of power, with a spectral width of 0–8000 Gauss. The ESR signal measured presents the first derivative of the microwave absorption over the magnetic field. The different magnetic state transitions are quantified by means of Landé factor (g), linewidth (∆Hpp), resonant field Hres as function of crystallite sizes, and low-field microwave absorption. The maxima and minima in the derivative signal define the peak-to-peak distance (ΔHpp); the Landé g-factor (g) is calculated based on the equation g = hν/μBHres where h is Planck's constant, ν is microwave frequency, μB is Bohr magneton, and Hres defines the resonance field determined by the zero-crossing point (dp/dH = 0) [40].

#### **2.2 Structural properties**

**Figure 2(a)** shows the XRD patterns of the precursor and the powders calcined at 700°C (LSr700- t). It seems that all patterns share the same characteristic peaks, indicating that the manganite with perovskite structure has been formed for all samples. Their widths confirm that the La0.8Sr0.2MnO3 powders obtained were in the form of nanocrystals. For precursor powder (sample without any extra heat treatment LSrp), the characteristic peaks of perovskite phase appear with small extra peaks at θ = 25.6, 30 and 39° related to La2O3 phase (JCPDS 83-1355); hence we did not include it in our studies. The high heat released during autocombustion leads to forming the perovskite phase with small amounts of impurities. Calcination

**23**

**Figure 3.**

**Figure 2.**

*crystallite size with heat treatment time [20].*

*Synthesis and ESR Study of Transition from Ferromagnetism to Superparamagnetism…*

The sizes rapidly increase with treatment durations less than 5 h, to reach an

*(a) X-ray diffraction patterns of the samples prepared at 700°C for different processing times. (b) Variation of* 

*X-ray diffraction patterns of samples calcined for 15 h at 700, 800, 900 and 1000°C.*

at 700°C during 0.5 h is sufficient to obtain the pure phase. This shows that precursor powder obtained from the autocombustion is very reactive. All peak positions were indexed with perovskite-type crystalline structure of La0.8Sr0.2MnO3 (JCPDS 53–0058) with a rhombohedral space group R-3c. By increasing the heat treatment times, the width of the diffraction peaks decreased, suggesting an increase in the crystallite size. The evolution curve of the average sizes given by the Scherrer formula as a function of the heat treatment times is of sigmoid shape (**Figure 2(b)**).

*DOI: http://dx.doi.org/10.5772/intechopen.89951*

*Synthesis and ESR Study of Transition from Ferromagnetism to Superparamagnetism… DOI: http://dx.doi.org/10.5772/intechopen.89951*

at 700°C during 0.5 h is sufficient to obtain the pure phase. This shows that precursor powder obtained from the autocombustion is very reactive. All peak positions were indexed with perovskite-type crystalline structure of La0.8Sr0.2MnO3 (JCPDS 53–0058) with a rhombohedral space group R-3c. By increasing the heat treatment times, the width of the diffraction peaks decreased, suggesting an increase in the crystallite size. The evolution curve of the average sizes given by the Scherrer formula as a function of the heat treatment times is of sigmoid shape (**Figure 2(b)**). The sizes rapidly increase with treatment durations less than 5 h, to reach an

#### **Figure 2.**

*Smart Nanosystems for Biomedicine, Optoelectronics and Catalysis*

temperature; t, heat treatment times).

profiles [38, 39].

**2.2 Structural properties**

Data (ICDD) powder diffraction files (PDF).

vibrating sample magnetometer (VSM) in fields up to 5 T.

determined by the zero-crossing point (dp/dH = 0) [40].

with two-step synthesis process. It is an interesting synthetic route for the preparation of a compound with differing crystallite sizes. The reaction is very simple and involves just lanthanum nitrate, strontium nitrate, and manganese acetate. Manganese acetate was dissolved in the minimum of distilled water under agitation. On the other hand, stoichiometric amounts of lanthanum nitrate and strontium nitrate are dissolved in distilled water under stirring. The solution was prepared by mixing aqueous solutions of (La(NO3)3 + Sr(NO3)2) and manganese acetate in 1:1 molar ratio. To evaporate the water, the solution was stirred in a beaker placed on a hot plate at 80°C until a viscous product was obtained. The solutions are mixed and were kept stirred in a beaker on a hot plate at 80°C to evaporate the water until you get a product with viscous appearance. After that, the gel obtained was inserted into a preheated oven at 350°C for 2 h. After a few seconds, a violent flame was produced by releasing large amounts of gas, with formation of a spongy powder of dark brown color. Finally, the powder resulting from autocombustion was calcined at 700°C under different heat treatment times and at 800, 900, and 1000°C for 15 h. The samples are designated by LSrT-t (T, calcination

Phase analysis of all products was performed by using powder X-ray diffraction (XRD) using an X'Pert Pro PANAnalytical diffractometer with CuKα radiation (λ = 1.5418 Å) at room temperature. The crystalline phases were determined by comparison of the registered patterns with the International Center for Diffraction

The average crystallite sizes of the samples were estimated from the (0 2 4) reflections at 2θ = 46°, by means of the Debye–Scherrer equation: D = Kλ/β cosθ, where K is a constant equal to 0.89, λ is the wavelength of the X-ray used, and β is the full width at half maxima (FWHM) of the X-ray reflection at 2θ [37]. The reflection at 2θ = 46° was selected to calculate the crystallite sizes because it does not overlap with other

Magnetization at various fields was measured at temperature 300 K using a

We have recorded the ESR spectra of La0.8Sr0.2MnO3 at room temperature, with a Bruker ER-200D spectrometer, operating in the X-Band (9.30 GHz). These measurements are performed on 20 mg of loosely packed fine powder. Data were acquired with 2 mW of power, with a spectral width of 0–8000 Gauss. The ESR signal measured presents the first derivative of the microwave absorption over the magnetic field. The different magnetic state transitions are quantified by means of Landé factor (g), linewidth (∆Hpp), resonant field Hres as function of crystallite sizes, and low-field microwave absorption. The maxima and minima in the derivative signal define the peak-to-peak distance (ΔHpp); the Landé g-factor (g) is calculated based on the equation g = hν/μBHres where h is Planck's constant, ν is microwave frequency, μB is Bohr magneton, and Hres defines the resonance field

**Figure 2(a)** shows the XRD patterns of the precursor and the powders calcined at 700°C (LSr700- t). It seems that all patterns share the same characteristic peaks, indicating that the manganite with perovskite structure has been formed for all samples. Their widths confirm that the La0.8Sr0.2MnO3 powders obtained were in the form of nanocrystals. For precursor powder (sample without any extra heat treatment LSrp), the characteristic peaks of perovskite phase appear with small extra peaks at θ = 25.6, 30 and 39° related to La2O3 phase (JCPDS 83-1355); hence we did not include it in our studies. The high heat released during autocombustion leads to forming the perovskite phase with small amounts of impurities. Calcination

**22**

*(a) X-ray diffraction patterns of the samples prepared at 700°C for different processing times. (b) Variation of crystallite size with heat treatment time [20].*

**Figure 3.** *X-ray diffraction patterns of samples calcined for 15 h at 700, 800, 900 and 1000°C.*

asymptote at 28 nm for times greater than 15 h. By increasing the calcination temperature to 800, 900, and 1000°C for 15 h, the widths of the peaks gradually decreased (**Figure 3**). Their crystallite sizes are equal to 32, 55, and 57 nm, respectively.

The autocombustion method offers the advantage of being an exothermic process, self-propagated, and initiated at low temperature. The exothermic reaction between acetate and nitrate ions leads to the formation of the perovskite phase. The nucleation by rearrangement of short-range networks of neighboring atoms is favored by heat treatment. Modified heat treatment conditions such as the temperature and the duration of the heat treatment allowed to prepare a nanocrystalline powder of sizes between 9 and 57 nm [20].
