**3. Magnetic studies**

The ESR spectra obtained from room temperature of all LSrT-t samples are shown together in **Figure 4**. In the first observation, the absorption signal changes radically with crystallite sizes. Two limiting cases are distinguished. For the sizes smaller than

**Figure 4.** *ESR spectra measured at room temperature for La0.8Sr0.2MnO3 samples with different crystallite sizes [20].*

**25**

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

27 nm, the spectra are formed by a single narrow line observed at the higher magnetic field (greater than 1500 G), which can be straightforwardly associated with the electron magnetic resonance absorption (EMR). The EMR is specifically associated with resonant absorption and hence resonant transitions between energy levels by Zeeman effect. This absorption is symmetric with a Lorentzian line shape. Resonance field position stays roughly constant around 3500 G, with g-factor ~ 2 for samples not exceeding 24.5 nm. These spectra are comparable to those of perovskite manganite in a parametric state observed above the Curie temperature. For sizes larger than 27 nm, signal is formed by two absorptions. A strong absorption at high field corresponds to EMR absorption. An additional absorption in the low-field range less than 1000 G is associated with the low-field microwave absorption. As the crystallite size increases, the EMR absorption mode changes toward a broad asymmetric line of Dyson-type

EMR is a resonant absorption that satisfies Larmor's condition defined by the expression ω = γ H, where ω is the resonance frequency, γ is the gyromagnetic fac-

In the ferromagnetic samples with spontaneous magnetization resulting from parallel alignment of spins by magnetic interaction effect, the internal fields Hint is added to the applied field giving rise to a total field H = H0 + Hint [42]. The resonance condition is reached at low values of external field to satisfy the Larmor

Strontium doping transforms La1−xSrxMnO3 manganites into a ferromagnetic state at room temperature, due to the reinforcement of the double exchange (DE) interactions between Mn3 + and Mn4 + ions, which is a ferromagnetic interaction. The ferromagnetic transition depends on the strontium content [43, 44]. The Curie temperature (TC) increases monotonically with x, and TC is increased from 220 to

Zoom of the spectral region from 3100 to 3700 G **Figure 5(a)**) shows that the samples with crystallite sizes below 28 nm have a resonance field (Hres) between 3200 and 3500 G (g-factor between 2.04 and 2.21). These values are lower than the typical value of paramagnetic manganites which is equal to Hres = 3528 G (g = 1.98) [46]. In addition to that, g-factor is bigger than the value of the free electron (ge = 2.0023) [47]. All the samples are attracted by a magnet which confirms the ferromagnetic state. Thus, the presence of the internal field leads to the displacement of the resonance field toward the weak field compared to that of the paramagnetic manganites. On the other hand, the decrease of Hres with decreasing crystallite size is an indication of a reduction in the internal field. Ferromagnetic manganite nanoparticles of few tens of nanometers are formed by a core-shell structure [34]. The core is a ferromagnetic volume enveloped by a magnetically dead layer that contains

most of the oxygen defects and the faults in crystallographic structure.

When the particle size is reduced to the nanoscale, two sources contribute to decreasing ferromagnetic volume of manganite. First, the percentage of dead layer increases with increased surface/volume ratio. In addition to that, the shell thickness increases and the volume of the magnetic core decreases [34, 48]. Magnetization of the shell is considered as null, and the contributory portion of

On the other hand, the state of agglomeration in the polycrystalline nanoparticles has an effect on the macroscopic magnetic properties [19]. Considering that the crystallites are in intimate contact, the increase in the shell thickness decreases the magnetic exchange energy between the two cores of neighboring particles, which promotes and improves their separation. In addition, the decrease in the size of the magnetic volume provides another source of new properties. In the nanometric state under the effect of the size reduction, the ferromagnetic material goes from a

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

and resonance field shifts toward lower values [19, 41].

tor, and H is the total field on the spins.

325 K for values of x between 0 and 0.2 [18, 45].

each crystallite to the magnetization is the core.

relation [21, 23].

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

27 nm, the spectra are formed by a single narrow line observed at the higher magnetic field (greater than 1500 G), which can be straightforwardly associated with the electron magnetic resonance absorption (EMR). The EMR is specifically associated with resonant absorption and hence resonant transitions between energy levels by Zeeman effect. This absorption is symmetric with a Lorentzian line shape. Resonance field position stays roughly constant around 3500 G, with g-factor ~ 2 for samples not exceeding 24.5 nm. These spectra are comparable to those of perovskite manganite in a parametric state observed above the Curie temperature. For sizes larger than 27 nm, signal is formed by two absorptions. A strong absorption at high field corresponds to EMR absorption. An additional absorption in the low-field range less than 1000 G is associated with the low-field microwave absorption. As the crystallite size increases, the EMR absorption mode changes toward a broad asymmetric line of Dyson-type and resonance field shifts toward lower values [19, 41].

EMR is a resonant absorption that satisfies Larmor's condition defined by the expression ω = γ H, where ω is the resonance frequency, γ is the gyromagnetic factor, and H is the total field on the spins.

In the ferromagnetic samples with spontaneous magnetization resulting from parallel alignment of spins by magnetic interaction effect, the internal fields Hint is added to the applied field giving rise to a total field H = H0 + Hint [42]. The resonance condition is reached at low values of external field to satisfy the Larmor relation [21, 23].

Strontium doping transforms La1−xSrxMnO3 manganites into a ferromagnetic state at room temperature, due to the reinforcement of the double exchange (DE) interactions between Mn3 + and Mn4 + ions, which is a ferromagnetic interaction. The ferromagnetic transition depends on the strontium content [43, 44]. The Curie temperature (TC) increases monotonically with x, and TC is increased from 220 to 325 K for values of x between 0 and 0.2 [18, 45].

Zoom of the spectral region from 3100 to 3700 G **Figure 5(a)**) shows that the samples with crystallite sizes below 28 nm have a resonance field (Hres) between 3200 and 3500 G (g-factor between 2.04 and 2.21). These values are lower than the typical value of paramagnetic manganites which is equal to Hres = 3528 G (g = 1.98) [46]. In addition to that, g-factor is bigger than the value of the free electron (ge = 2.0023) [47].

All the samples are attracted by a magnet which confirms the ferromagnetic state. Thus, the presence of the internal field leads to the displacement of the resonance field toward the weak field compared to that of the paramagnetic manganites. On the other hand, the decrease of Hres with decreasing crystallite size is an indication of a reduction in the internal field. Ferromagnetic manganite nanoparticles of few tens of nanometers are formed by a core-shell structure [34]. The core is a ferromagnetic volume enveloped by a magnetically dead layer that contains most of the oxygen defects and the faults in crystallographic structure.

When the particle size is reduced to the nanoscale, two sources contribute to decreasing ferromagnetic volume of manganite. First, the percentage of dead layer increases with increased surface/volume ratio. In addition to that, the shell thickness increases and the volume of the magnetic core decreases [34, 48]. Magnetization of the shell is considered as null, and the contributory portion of each crystallite to the magnetization is the core.

On the other hand, the state of agglomeration in the polycrystalline nanoparticles has an effect on the macroscopic magnetic properties [19]. Considering that the crystallites are in intimate contact, the increase in the shell thickness decreases the magnetic exchange energy between the two cores of neighboring particles, which promotes and improves their separation. In addition, the decrease in the size of the magnetic volume provides another source of new properties. In the nanometric state under the effect of the size reduction, the ferromagnetic material goes from a

*Smart Nanosystems for Biomedicine, Optoelectronics and Catalysis*

powder of sizes between 9 and 57 nm [20].

**3. Magnetic studies**

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

The ESR spectra obtained from room temperature of all LSrT-t samples are shown together in **Figure 4**. In the first observation, the absorption signal changes radically with crystallite sizes. Two limiting cases are distinguished. For the sizes smaller than

**24**

**Figure 4.**

*ESR spectra measured at room temperature for La0.8Sr0.2MnO3 samples with different crystallite sizes [20].*

**Figure 5.**

*Variation depending on the crystallite size of: (a) resonance fields in the spectral zone of 3100–3700 G, (b) low field microwave absorption and (c) linewidth ΔHpp [20].*

multi-domain state to a magnetic single-domain state and then to the superparamagnetic state. The changes observed in the EMR and LFMA signal appear to be indicative of these magnetic state transitions.
