**1. Introduction**

Magnetic nanoparticles (MNPs) display physical and chemical properties different those found in their corresponding bulk materials. These properties make them attractive in widespread applications such as energy, electronics, sensor designs of all kinds, catalysts, magnetic refrigeration, optics, and in various biomedical applications [1–6].

In medicine, MNPs has attracted attention because they are detectable, remotely manipulable, stimulable by a magnetic field, and can combine both diagnosis and therapy in one dose. These multifunctional nanomaterials can be used as contrast agents for medical imaging and nano-vectors to transport therapeutic agents to their target, in local delivery of drugs, or to destroy the cancer cells by local hyperthermia [7–10]. These magnetic platforms should possess very small size and must combine high magnetic susceptibility and loss of magnetization after removal of the magnetic field [11, 12]. The optimization of the

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

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

#### **Figure 1.**

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

**21**

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

magnetization which will presage a precession movement [20].

as the temperature is lower than Curie temperature [25, 26].

tion is higher than that of most materials for such applications [28].

11 nm [28], and between 18 and 24 nm [33, 34, 36].

with their corresponding crystallite size.

**2. Experimental methods**

**2.1 Autocombustion synthesis**

important than Zeeman energy. Thus, in the presence of an applied field, it is the total

ESR spectra is formed by two absorption, one to the higher fields known as the electronic magnetic resonance (EMR) and an absorption around zero magnetic field appointed low-field microwave absorption (LFMA) [21]. EMR spectrum is a resonant absorption characterized by means of two parameters, the resonant magnetic field (Hres) and the linewidth (∆Hpp). These parameters give information on magnetic nature of the materials [22]. LFMA signal is a nonresonant absorption considered as a sensitive detector of magnetic ordering [23, 24]. This signal has been used to detect the magnetic states in materials and provide highly sensitive detection of magnetic order [23]. More importantly, this signal is not present in the paramagnetic state and emerges

Magnetic homesteads of La1−xSrxMnO3 perovskites are influenced by intrinsic properties (composition and structure) and extrinsic properties (particle and crystallite sizes depending on the synthesis procedure). Their Curie temperature strongly depends on the Sr-content, and it varies between 320 and 370 K for 0.2 ≤ x ≤ 0.3, which makes them attractive for self-controlled magnetic hyperthermia applications. Thus, by applying a high-frequency magnetic field, the magnetic nanoparticles reach their own heating temperature, which does not exceed TC [6, 27]. Their magnetiza-

Magnetic phase transitions as a function of temperature and crystallite size have been extensively studied in the past [29–31]. But there are controversial studies in critical sizes of the ferromagnetic- single magnetic domain-superparamagnetic transitions. The critical size is given as a function of the particle size determined by transmission electron microscopy (TEM) or by the crystallite size from DRX data calculated by the Scherrer method. For manganite particles, single magnetic domain is observed in the literature less than diameters which range between 50 and 80 nm [32–34]. Based on crystallites, the change from a multi-domain state to a single-domain state is given in the literature for sizes varying between 26 and 36 nm [34, 35], and crystallite size of the superparamagnetic transition is estimated at

The observed disparity between crystallite size and particle size is attributed to the polycrystalline nature of the particles. In this work, we will refer to the samples

For this reason, in order to determine critical sizes, we discuss crystallite size dependence of the transition from ferromagnetism to superparamagnetism at room temperature of La0.8Sr0.2MnO3 nanoparticles by using electron spin resonance technique. These transitions are quantified by means of Hres, ∆Hpp, and LFMA. To our knowledge, studies on nanoparticles of manganite with LFMA signal are scarce. Therefore, in this work, LFMA signal is used to give knowledge on magnetic state. For this purpose, to obtain the La0.8Sr0.2MnO3 (LSrT-t) nanoparticles with different crystallite sizes, the autocombustion method was adopted with a two-step synthesis process. This method permits to prepare powders in a wide range of crystallite sizes

depending on the annealing temperature (T) and heat treatment time (t).

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

*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*

important than Zeeman energy. Thus, in the presence of an applied field, it is the total magnetization which will presage a precession movement [20].

ESR spectra is formed by two absorption, one to the higher fields known as the electronic magnetic resonance (EMR) and an absorption around zero magnetic field appointed low-field microwave absorption (LFMA) [21]. EMR spectrum is a resonant absorption characterized by means of two parameters, the resonant magnetic field (Hres) and the linewidth (∆Hpp). These parameters give information on magnetic nature of the materials [22]. LFMA signal is a nonresonant absorption considered as a sensitive detector of magnetic ordering [23, 24]. This signal has been used to detect the magnetic states in materials and provide highly sensitive detection of magnetic order [23]. More importantly, this signal is not present in the paramagnetic state and emerges as the temperature is lower than Curie temperature [25, 26].

Magnetic homesteads of La1−xSrxMnO3 perovskites are influenced by intrinsic properties (composition and structure) and extrinsic properties (particle and crystallite sizes depending on the synthesis procedure). Their Curie temperature strongly depends on the Sr-content, and it varies between 320 and 370 K for 0.2 ≤ x ≤ 0.3, which makes them attractive for self-controlled magnetic hyperthermia applications. Thus, by applying a high-frequency magnetic field, the magnetic nanoparticles reach their own heating temperature, which does not exceed TC [6, 27]. Their magnetization is higher than that of most materials for such applications [28].

Magnetic phase transitions as a function of temperature and crystallite size have been extensively studied in the past [29–31]. But there are controversial studies in critical sizes of the ferromagnetic- single magnetic domain-superparamagnetic transitions. The critical size is given as a function of the particle size determined by transmission electron microscopy (TEM) or by the crystallite size from DRX data calculated by the Scherrer method. For manganite particles, single magnetic domain is observed in the literature less than diameters which range between 50 and 80 nm [32–34]. Based on crystallites, the change from a multi-domain state to a single-domain state is given in the literature for sizes varying between 26 and 36 nm [34, 35], and crystallite size of the superparamagnetic transition is estimated at 11 nm [28], and between 18 and 24 nm [33, 34, 36].

The observed disparity between crystallite size and particle size is attributed to the polycrystalline nature of the particles. In this work, we will refer to the samples with their corresponding crystallite size.

For this reason, in order to determine critical sizes, we discuss crystallite size dependence of the transition from ferromagnetism to superparamagnetism at room temperature of La0.8Sr0.2MnO3 nanoparticles by using electron spin resonance technique. These transitions are quantified by means of Hres, ∆Hpp, and LFMA. To our knowledge, studies on nanoparticles of manganite with LFMA signal are scarce. Therefore, in this work, LFMA signal is used to give knowledge on magnetic state. For this purpose, to obtain the La0.8Sr0.2MnO3 (LSrT-t) nanoparticles with different crystallite sizes, the autocombustion method was adopted with a two-step synthesis process. This method permits to prepare powders in a wide range of crystallite sizes depending on the annealing temperature (T) and heat treatment time (t).
