5.1 Introduction A lot of information about the magnetic properties of a material can be obtained by studying its hysteresis loop. Hysteresis is a well known phenomenon in ferromagnetic materials. A wide range of applications, from electric motors to transformers, permanent magnets, various types of electronic devices, magnetic recording devices, magnetic refrigeration etc. and many more rely heavily on various aspects of hysteresis. The lanthanum manganites for a particular application are dependent on its magnetic properties. Magnetic properties are strongly dependent on chemical composition, sintering temperature, grain size and crystal structure and porosity of the material [1]. Hence, in this chapter we have reports on the magnetic properties for different composition of LSMO. 5.2 Theoretical background In a physical phenomenon an electric current loop generates a region of magnetic field represented by a magnetic flux line. Figure 5.1 shows a simple illustration of this phenomenon. The magnetic field vector at any given point near to the electric current loop is given by H, vector quantity. There are also other type of materials that are inherently magnetic, that is, they can generate magnetic field without a macroscopic electric current [2]. Figure 5.2 shows a magnetic bar as an example which exhibits a particular dipole North South that give the orientation. The best utility of magnetism is the force of attraction that can provide many of our present technology devices based on magnetism of magnetic materials; these include electrical power generators and transformers, electric motors, radio, television, telephone and magnetic refrigerator. There are five basic types of magnetism have been observed and classified on the basis of magnetic behavior of the material with respect to applied magnetic field. These types are Diamagnetism, Paramagnetism, Ferromagnetism, Anti-ferromagnetism and Ferrimagnetisms. If the particle Department of Physics, Shivaji University, Kolhapur. 113
size of some magnetic materials is in nanometer range then materials shows superparamganetism. Figure 5.1 Magnetic field generated around an electrical current loop. Figure 5.2 Magnetic material can generate a magnetic field without an electric current.
5.2.1 Nanomaterials Nanomaterials are defined as having at least one dimension less than 100 nm and typically are engineered to have unique properties which make them desirable for commercial applications. The two main reasons why materials at the nano scale can have different properties are increased relative surface area and quantum effects. Nanomaterials have a much greater surface area to volume ratio than their conventional forms, which can lead to greater chemical reactivity and affect their strength. Also at the nano scale, quantum effects can become much more important in determining the materials properties and characteristics, leading to novel optical, electrical and magnetic behaviours. Nanomaterials can be nanoscale in one dimension, two dimensions and three dimensions. They can exist in single, fused, aggregated or agglomerated forms with spherical, tubular, and irregular shapes. The common types of nanomaterials include nanotubes, dendrimers, quantum dots and fullerenes. Products containing engineered nanomaterials are already in commercial use, with some have been available for several years or decades. The range of commercial products available today is very broad, including stain-resistant and wrinkle-free textiles, cosmetics, sunscreens, electronics, paints and varnishes. Nanotechnology is a multidisciplinary grouping of physical, chemical, biological, engineering, and electronic processes, materials, applications, and concepts in which the defining characteristic is size. In this policy, nanotechnology is a generic term that encompasses the manipulation of matter at atomic or near atomic scales to produce new materials, structures, or devices. The size of the nanoparticle grains strongly affects the property changes in the bulk material. For instance, the overlapping of different grain sizes affects the physical strength of the material [3]. The nanotechnology is as well as evolutionary as revolutionary in nature. Evolutionary are the many applications where the same material is Department of Physics, Shivaji University, Kolhapur. 115
incrementally improved by using nanotechnology. Revolutionary it can be called where new properties originate from nanotechnology like for example in quantum dots. Those new properties can be divided as Properties based on the fact that the surface is large compared to the weight/volume. In addition to size, low energy dissipation and high processing speeds are important. New properties not found in bulk or micro sized particles. 5.2.2 Classification of nanomaterials Nanomaterials consisting of nanometer sized crystallites or grains and interfaces may be classified according to their chemical composition and shape. According to the shape of the crystallites or grains we can broadly classify nanomaterials into four categories and shown in Figure 5.3. 1. Clusters or powders (MD=0) 2. Multilayers (MD=1) 3. Ultrafine grained overyaers or buried layers (MD=2) 4. Nanomaterials composed of equiaxed nanomter-sized grains (MD=3) Figure.5.3 Classification of nanomaterials according to dimensions. Department of Physics, Shivaji University, Kolhapur. 116
5.2.3 Properties of nanomaterials As we know the properties of materials are drastically changes with size. In case of magnetic nanomaterials particles changes then their mechanical, optical and magnetic properties changes accordingly. 5.2.3.1 Mechanical properties Due to the nanometer size, many of the mechanical properties of the nanomaterials are modified to be different from the bulk materials including the hardness, elastic modulus, fracture toughness, scratch resistance and fatigue strength etc. An enhancement of mechanical properties of nanomaterials can result due to this modification, which are generally resultant from structural perfection of the materials [4, 5]. The small size either renders them free of internal structural imperfections such as dislocations, micro twins, and impurity precipitates or the few defects or impurities present cannot multiply sufficiently to cause mechanical failure. The imperfections within the nano dimension are highly energetic and will migrate to the surface to relax themselves under annealing, purifying the material and leaving perfect material structures inside the nanomaterials. Moreover, the external surfaces of nanomaterials also have less or free of defects compared to bulk materials, serving to enhance the mechanical properties of nanomaterials [19]. The enhanced mechanical properties of the nanomaterials could have many potential applications both in nano scale such as mechanical nano resonators, mass sensors, microscope probe tips and nano tweezers for nano scale object manipulation and in macro scale applications structural reinforcement of polymer materials, light weight high strength materials, flexible conductive coatings, wear resistance coatings, tougher and harder cutting tools etc. 5.2.3.2 Optical properties The optical properties are very much sensitive to size of the particles. The applications based on optical properties of nanomaterials include optical Department of Physics, Shivaji University, Kolhapur. 117
detector, laser, sensor, imaging, phosphor, display, solar cell, photocatalysis, photo electrochemistry and biomedicine. Many of the underlying principles are similar in these different technological applications that span a variety of traditional disciplines including chemistry, physics, biology, medicine, materials science and engineering, electrical and computer science and engineering. The optical properties of nanomaterials depend on parameters such as feature size, shape, surface characteristics, and other variables including doping and interaction with the surrounding environment or other nanostructures. The simplest example is the well-known blue-shift of absorption and photoluminescence spectra of semiconductor nanoparticles with decreasing particle size, particularly when the size is small enough. For semiconductors, size is a critical parameter affecting optical properties. By simply controlling the physical dimensions, one can generate gold nanostructures with absorption covering the entire visible and near IR regions of the optical spectrum. 5.2.3.3 Magnetic properties For a single isolated particle, coercivity or remanence as function of the particle size, are sketched in the Figure 5.4. The large magnetic particles are subdivided by Bloch walls into magnetic domains. Therefore, remanence and coercivity are independent of the particle size. Decreasing particle size leads to a size range, where the particles consist of only a single magnetic domain. Therefore, coercivity and remanence are increasing drastically. This range of particle sizes is used for magnetic data storage. Further decrease of the particle size leads to a decrease of remanence and coercitivity to zero. However, this step depends on the measurement time. The shorter the time constant of the measuring method τm the more is this step shifted to smaller particle sizes. Magnetic properties of a single isolated particle are strongly influenced by the particle size. This phenomenon, called Department of Physics, Shivaji University, Kolhapur. 118
"Superparamganetism" is observed, when the thermal energy of the particle kt is larger than the energy of magnetic anisotropy KV. Figure 5.4 Coercivity as a function of the particle size. 5.3 Coercivity The effect of reducing the particle size of materials is of great importance from both fundamental considerations and application point of view. A brief discussion of magnetic behavior of low dimensional systems is focused based on literature. Magnetic nanoparticles exhibit specific properties such as coercivity and superparamganetism, generally attributed to reduced size. Coercivity: The coercivity is also called the coercive field or coercive force. It is defined for a ferromagnetic material as the intensity of the applied magnetic field required to reduce the magnetization of that material to zero after the magnetization of the sample has been driven to saturation. The coercivity of fine particles has a striking dependence on their size. Figure 5.5 shows dependence of coercivity on size schematically and how the Department of Physics, Shivaji University, Kolhapur. 119
size range is divided according to the variation of coercivity with particle radius r. The following regions can be distinguished: (i) Multi-domain (MD): It is observed for r >rc and in this region, the coercivity decreases as the particle size increases and the coercivity (Hc) is found to vary with size as ~ 1/ r n. (ii) Single-domain (SD): For r0< r <rc, the particles become single domain and in this size range, the coercivity reaches a maximum. (iii) Superparamagnetic (SP): Below a critical size r0, the coercivity is zero because of thermal effect, which is strong enough to spontaneously demagnetize the assembly of magnetic particles. Figure 5.5 Schematic of the size dependence of coercivity Superparamganetism: Superparamganetism is a form of magnetism, which appears in small ferromagnetic or ferrimagnetic nanoparticles (1-10nm). In sufficiently small nanoparticles, magnetization can randomly flip direction under the influence of temperature. The typical time between two flips is called the Néel
relaxation time. In the absence of external magnetic field, when the time used to measure the magnetization of the nanoparticles is much longer than the Néel relaxation time, their magnetization appears to be in average zero and they are said to be in the superparamagnetic state. In this state, an external magnetic field is able to magnetize the nanoparticles, similarly to a paramagnet. However, their magnetic susceptibility is much larger than the one of paramagnets. The effective magnetic moment of a ferromagnetic particle is determined by its size. A ferromagnetic sample with a volume greater than a critical value V divides into multiple magnetic domains, each magnetized along the local easy axis but in one of two opposite directions. The multiple domain structure is, however, no longer favorable below the critical volume, and the particle becomes a single domain with ferromagnetic alignment of all its moments along the easy axis in the same direction. Thermal fluctuations of the moment exist on a microscopic scale, but to reverse the direction of the single domains magnetization requires an energy ΔE to overcome the crystal-field anisotropy. If single domain particles become small enough, KV would become so small that thermal fluctuations could overcome the anisotropy forces and spontaneously reverse the magnetization of a particle from one easy direction to the other, even in the absence of an applied field. Each particle has a magnetic moment μ = MsV and, if a field is applied, the field will tend to align the moments of the particles and the thermal energy will tend to disalign them. This is called superparamganetism. The probability of such a reversal by thermal activation is proportional to exp (-ΔE/kT). This differs from conventional paramagnetism because the effective moment of the particle is the sum of its ionic particles, which can be several thousand spins in a ferromagnetic particle small enough to show superparamganetism [6]. Very fine ferromagnetic particles have very short relaxation times even at room temperature and behave superparamagnetically; and their behavior is paramagnetic but magnetization values are typical of ferromagnetic Department of Physics, Shivaji University, Kolhapur. 121
substances. The individual particles have normal ferromagnetic moments but very short relaxation times so that they can rapidly follow directional changes of an applied field and on removal of the field, do not hold any remanent moment. Superparamganetism is characterized by two experimental features 1. There is no hysteresis; (i.e., both the retentivity and the coercivity are zero) in the field dependence of magnetization. 2. Magnetization curves measured at different temperatures superimpose when magnetization (M) is plotted as a function of field (H) temperature (T). Superparamganetism can be destroyed by cooling. This follows because the characteristic fluctuation time for a particle's moment varies exponentially with temperature, so the magnetization appears to switch sharply to a stable state as the temperature is reduced. The temperature at which this occurs is called the blocking temperature (TB) and it depends linearly on the sample's volume and on the magnitude of the crystal field anisotropy. In the case of superparamagnetic materials, the magnetization shows temperature and path dependence which is shown schematically in Figure 5.6. Figure 5.6 Schematic diagram of ZFC and FC magnetization curves as a function of temperature taken in an applied field H. Department of Physics, Shivaji University, Kolhapur. 122
The two curves zero field cooled (ZFC) and field cooled (FC) show different behavior at low temperatures. As the temperature increases the magnetic moment in the FC curve decreases. However, as the temperature begins to rise from 5K, the moment in the ZFC curve begins to increase. At a certain temperature, the ZFC curve reaches a peak and this temperature is called the blocking temperature (TB). The divergence of ZFC and FC curve and the blocking temperature depend on the particle size and its distribution. The blocking temperature of a substance should decrease with increasing applied field and eventually disappear when the field reaches a critical value. The higher field is expected to lower the barriers between the two easy axis orientations. For a particle of constant size below the blocking temperature TB, the magnetization will be stable and shows hysteresis. It refers to particles which have relaxation time for demagnetization longer than 100 sec. For uniaxial particles using the same criterion for stability gives, TB =..(5.1) where K = Anisotropy constant V = Volume of the particle k = Boltzmann s constant (1.38 10-23 JK -1 ) If one considers Ni as a classical example with an anisotropy constant K = 4.5 10 3 Jm -3 then for a size 20 nm, the particle will show a blocking temperature (TB) at ~ 55 K by using Eq. 5.1. Below TB, the magnetization will have relatively stable and shows ferromagnetic behavior. While above TB, the thermal energy will be sufficient to suppress the ferromagnetic behavior and thus the particles become superparamagnetic. 5.2.5 Applications of nanomaterials Since the properties of nanomaterials all together different from bulk, there is wide scope of applications of nanomaterials. By using nanomaterials, Department of Physics, Shivaji University, Kolhapur. 123
everyday consumer products may be made lighter, stronger, cleaner, less expensive, more efficient, more precise, or more aesthetic. Products containing nanomaterials may improve our quality of life through more efficient target driven pharmaceuticals, better medical diagnosis tools, faster computers, cleaner energy production, etc. Several of the consumer end products available today that utilize nanomaterials have been developed from existing products, for example by the incorporation of nanomaterials into solid, viscous or liquid matrices. About one third of the products are sunscreen lotions or cosmetics such as skin-care and colorant products. For sunscreens, titanium dioxide and zinc oxide nanoparticles are used as they absorb and reflect ultraviolet rays but are still transparent to visible light and, thus, the resulting sunscreen becomes both more appealing to the consumer and is claimed to be more effective. Also liposomes, i.e. tiny vesicles made out of the same material as cell membranes, are known to be used in the cosmetics industry. In fact, there are very many cosmetic products that have nanomaterial content and the frequent use of nanoparticles in cosmetics has indeed raised a number of concerns about consumer safety, since they are applied directly on the human body. The area where nanotechnology has a considerable impact, include these are Medical and pharmaceutical sector Bo-nanotechnology, bio-sensors Energy sector, including fuel cells, batteries and photovoltaics Environment sector including water remediation Construction sector, including reinforcement of materials Electronics and optoelectronics, photonics 5.3 Experimental The La1-xSrxMnO3 (x=0.1, 0.2, 0.3 and 0.35) samples were synthesized by solution combustion method with polyvinyl alcohol as a fuel. The LSMO samples were structurally and morphologically characterized by x-ray diffraction and Scanning electron microscope. The microstructure was Department of Physics, Shivaji University, Kolhapur. 124
determined by Transmission electron microscopy. The magnetization vs temperature of all the LSMO sample heat treated at 600 o C, 900 o C and 1200 o C were measured by using Vibrating Sample Magnetometer (Lakeshore-7307 Model) at room temperature. The powder samples filled into a sample holder (plastic tube) having 2mm diameter. The weight of plastic tube before filling LSMO powder and after filling powder were taken and weight of LSMO powder calculated by subtracting these weights. The sample holder is hanged at the central part of two magnetic poles. At room temperature, magnetization (emu) in the LSMO samples were measured with an applied magnetic field -9 koe to +9 koe. The temperature dependence of magnetization such as the fieldcooled (FC) and zero-field cooled magnetization for all the LSMO sample measured in temperature range 100-350K, at the constant field of 500 Oe was carried with a SQUID magnetometer (Quantum Design MPMS). 5.3 Results and discussion The nanocrystalline nature was confirmed by x-ray diffraction study and Transmission electron microscopy. The detail descriptions are described elsewhere in section 3.4.2 and 3.4.5. 5.4.1 Study of superparamganetism of La1-xSrxMnO3(x=0.1-0.35) at room temperature The field dependence magnetization (emu/gm) of sample La0.9Sr0.1MnO3, La0.8Sr0.2MnO3, La0.7Sr0.3MnO3 and La0.65Sr0.35MnO3 heat treated at 600 o C, 900 o C and 1200 o C are as shown in Figure 5.7 and Figure 5.8. The S- shaped nature of loop shows the LSMO particles shows superparamagnetic behavior at room temperature. The magnetic hysteresis loss is very low or almost zero which is beneficial for room temperature magnetic refrigeration. From Figure 5.7 (a) it is observed that as annealing temperature increases, increase in saturation magnetization. This is attributed to increase in crystallite size with annealing temperature from 600 o C to 1200 o C. Similarly for Department of Physics, Shivaji University, Kolhapur. 125
other composition of LSMO heat treated at 600 o C, 900 o C and 1200 o C shows same behavior from hysteresis loop as shown in Figure 5.7 (b) and Figure 5.8 (a, b). Saturation Magnetization (emu/gm) 60 (a) M3 40 M2 20 M1 0-8000 -6000-4000 -2000 0 2000 4000 6000 8000-20 -40-60 M agnetic field (O e) Saturation Magnetization (emu/gm) 60 (b) M6 40 M5 20 M4 0-9000 -6000-3000 0 3000 6000 9000-20 -40-60 Magnetic Field (Oe) Figure 5.7 Room temperature hysteresis loops (a) sample M1, M2 and M3 (b) sample M4, M5 and M6. Department of Physics, Shivaji University, Kolhapur. 126
Saturation Magnetization (emu/gm) (c) 60 M9 40 M8 20 M7 0-9000 -6000-3000 0 3000 6000 9000-20 -40-60 Magnetic Field (Oe) Saturation Magnetization (emu/gm) 60 (d) M12 40 M11 20 M10 0-9000 -6000-3000 0 3000 6000 9000-20 -40-60 Magnetic Field (Oe) Figure 5.8 Room temperature hysteresis loops (a) sample M7, M8 and M9 (b) sample M10, M11 and M12. Department of Physics, Shivaji University, Kolhapur. 127
The room temperature field dependent saturation magnetization for La1- xsrxmno3 with x=0.1, 0.2, 0.3 and 0.35 samples heat treated at 1200 o C shown in Figure 5.9. There is not enough change in saturation magnetization with variation of strontium doping for x=0.1 to 0.35. The LSMO saturates above the 2000 Oe and remains parallel to applied field axis. The saturation magnetization and coercivity for the samples M1, M2 and M3 are shown in Figure 5.10 and for M3, M6, M9 and M12 are shown in Figure 5.11. The hysteresis parameter such as saturation magnetization (Ms), remanant magnetization (Mr), coercive field (Hc), remanent magnetization and Bohr magneton (nb) are listed in Table 5.1 for samples M1-M3 and for M3, M6, M9 and M12 samples heat treated at 1200 o C in Table 5.2. From Figure 5.9 it is observed that the saturation magnetization increases with annealing temperature. Saturation Magnetization (emu/gm) 60 40 20 0-9000 -6000-3000 0 3000 6000 9000-20 -40-60 Magnetic Field (Oe) Figure 5.9 Room temperature hysteresis loop for sample M3, M6, M9 and M12 heat treated at 1200 o C. Department of Physics, Shivaji University, Kolhapur. 128
The coercive field increases first and then decreases with annealing temperature. The substitution of Strontium in La site from x=0.1-0.35 increase the saturation magnetization linearly from 52.10 to 57.27 emu/gm while the coercivity decreases. Hence, we can say annealing plays an important role in magnetic properties of LSMO. Saturization magnetization (emu/gm) 60 50 40 30 20 10 28 26 24 22 20 18 16 14 12 10 Coercivity (Oe) 600 700 800 900 1000 1100 1200 Temperature o C Figure 5.10 Variation of saturation magnetization and coercive field with annealing temperature of LSMO. Saturization magnetization (emu/gm) 58 57 56 55 54 53 52 0.1 0.2 0.3 11 10 9 8 7 6 Coercivity (Oe) Concentration of S r Figure 5.11 Variation of saturation magnetization and coercive field for La1-xSrxMnO3 with x=0.1, 0.2, 0.3 and 0.35. Department of Physics, Shivaji University, Kolhapur. 129
The magnetic moment per formula unit in Bohr magneton (nb) was calculated by using the following relation [7], (5.2) where, M is the molecular weight of particular composition and Ms is saturation magnetization (emu/gm). Table 5.1: Room temperature saturation magnetization (MS), remnant magnetization (Mr), Bohr magneton (nb) and coercivity (Hc) for LSMO samples heat treated at 600 o C (M1), 900 o C (M2) and 1200 o C (M3) Sample Saturation Remanant Coercivity Bohr magnetization (emu/gm) magnetization (emu/gm) (Oe) magneton (emu/gm) M1 10.52 0.03 18.62 0.44 M2 35.65 1.39 26.94 1.51 M3 52.02 0.32 10.87 2.21 Table 5.2: Room temperature saturation magnetization (MS), remnant magnetization (Mr), Bohr magneton (nb) and coercivity (Hc) for LSMO samples heat treated at 1200 o C. Sample Saturation magnetization (emu/gm) Remanant magnetization (emu/gm) Coercivity (Oe) Bohr magneton (emu/gm) M3 52.02 0.32 10.87 2.21 M6 54.20 0.55 10.84 2.24 M9 56.32 0.39 7.78 2.28 M12 57.27 0.37 7.30 2.30 Department of Physics, Shivaji University, Kolhapur. 130
The net magnetic moment (nb) increases with increase in annealing temperature for sample M1, M2 and M3 and shown in Table 5.2. However, doping of Sr increases, the magnetic moment increases. Thus we can control the saturation magnetization and coercivity by controlling the annealing temperature and doping level of Strontium. 5.5.2 Effect of Sr doping on Curie temperature of La1-xSrxMnO3 (x=0.1-0.35) The temperature dependence of magnetization such as the field-cooled (FC) and zero-field cooled magnetization for the La0.65Sr0.35MnO3 sample heat treated at 600 o C, measured at the constant field of 500 Oe was carried with a SQUID magnetometer (Quantum Design MPMS) and shown in Figure 5.12. It is observed that the smooth variation of magnetic moment with temperature from 200K-375K. Figure 5.12 Temperature dependence of the magnetization in the FC and ZFC process upon an application of 500 Oe for La0.65Sr0.35MnO3 heat treated at 600 o C. Department of Physics, Shivaji University, Kolhapur. 131
-0.30-0.25 Tc=295K dm/dt (emu/gm.k) -0.20-0.15-0.10-0.05 0.00 0.05 160 240 320 400 480 Temperature (K) Figure 5.13 dm/dt Vs T plot for sample M1 Magnetization (emu/gm) 40 30 20 10 ZFC 100 Oe FC 100 Oe ZFC 500 Oe FC 500 Oe ZFC 1 koe FC 1 koe ZFC 10 koe FC 10 koe 0 100 150 200 250 300 350 Temperature (K) Figure 5.14 Field cooled temperature dependence of the magnetization of La0.9Sr0.1MnO3 in magnetic field 100Oe, 500Oe, 1kOe and 10kOe. Department of Physics, Shivaji University, Kolhapur. 132
The FC and ZFC superimpose above 260K which confirms the La0.65Sr0.35MnO3 shows superparamganetism at room temperature. The Curie temperature (Tc), defined by the maximum in the absolute value of dm/dt. Figure 5.13 shows dm/dt-t curve for the determination of Curie temperature (Tc) and has been found to be 295 K. The M-T plot for the La0.9Sr0.1MnO3 (M1) heat treated at 600 o C is shown in Figure 5.14. It contains the M-T (Field cooled and Zero Field cooled) plot at field 100Oe, 500Oe, 1kOe and 10kOe. The effect of applied magnetic field on magnetization and transition temperature very much. The magnetization and transition or Curie temperature increases with increase in applied magnetic field. Below the Curie temperature one may distinguish the prominent split between the field cooled (FC) and Zero Field cooled (ZFC) curve 100Oe and 500Oe. Above the Curie temperature at magnetic field 1 koe and 10kOe the field-cooled (FC) and zero-field cooled (ZFC) curves coincides. This type of irreversibility behavior suggests that the magnetic anisotropy is not large in the sample. The calculated value of Tc of the sample M1 is 248K. 18 ZFC 500 Oe FC 500 Oe Magnetization (emu/gm) 15 12 9 6 3 0 100 150 200 250 300 350 Temperature (K) Figure 5.15 Temperature dependence of the magnetization in the FC and ZFC process upon an application of 500Oe for La0.8Sr0.2MnO3 (M4). Department of Physics, Shivaji University, Kolhapur. 133
Magnetization (emu/gm) 30 20 10 ZFC 500 Oe FC 500 Oe 0 100 150 200 250 300 350 Temperature (K) Figure 5.16 Temperature dependence of the magnetization in the FC and ZFC process upon an application of 500 Oe for La0.7Sr0.3MnO3 (M7). The MT plot for samples La0.8Sr0.2MnO3 (M4), La0.7Sr0.3MnO3 (M7)is shown in Figure 5.15 and Figure 5.16 respectively and Tc observed are 254K and 257K. The variations in compositional homogeneity or oxygen stoichiometry could cause variations of the Curie temperature on a narrow scale and thereby a spreading of the overall Curie temperature. The Curie temperature may be affected by the oxygen deficiency and the partially disordered structure of grain boundaries in the sample. The broadening of the magnetic transition for La0.9Sr0.1MnO3 (M1), La0.8Sr0.2MnO3 (M4), La0.7Sr0.3MnO3 (M7) and La0.65Sr0.35MnO3 (M10) could be attributed to the decreasing grain size associated with the increased Sr content. A smaller grain size gives a larger proportion of surface-near spins, which may be weaker ferromagnetically coupled than spins in the bulk of the grains. This could give a distribution of Curie temperatures and thus a broadened magnetic transition. A grain size dependent broadening of the Curie temperature has previously been observed in granular Sr-doped lanthanum manganites [8, 9]. In perovskite manganites, the mechanisms Department of Physics, Shivaji University, Kolhapur. 134
which govern the magnetic property of the material are the antiferromagnetic super-exchange interaction in Mn 3+ O 2 Mn 3+ /Mn 4+ O 2 Mn 4+ bonds and the ferromagnetic double-exchange interaction in Mn 3+ O 2 Mn 4+ bonds. The relative strength of these two interactions can be strongly influenced by replacing La-ion by divalent/monovalent ion of different size and oxidation states which results in change of Mn 3+ /Mn 4+ ratio as well as Mn O bond length and Mn O Mn bond angle. In case of polycrystalline La1 xsrxmno3 samples, substitution of smaller La 3+ ion (ionic radii 0.121nm ) by larger Sr 2+ ion (ionic radii 0.131nm replace ) increases the average ionic radius of La-site, thereby introduces crystallographic distortion and an increase in Mn O Mn bond angle [10]. This results in increased double-exchange interaction and a corresponding increase in TC of the system [11]. 5.6 Conclusions The magnetic properties of LSMO are found to be depends on Sr doping and heat treatment. It is observed that saturation magnetization varies linearly with increase in annealing temperature and Sr content. The Tc observed for all LSMO samples investigated is in the range of 230-300K. The variation in Curie temperature observed may be due to the oxygen deficiency and the partially disordered structure of grain boundaries in the sample. It is revealed, Tc of lanthanum manganites can be vary with substitution of Strontium and hence LSMO can be tuned such that it may be used as refrigerant material for room temperature magnetic refrigeration. Department of Physics, Shivaji University, Kolhapur. 135
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