Srinivasan, chairman of my doctoral committee and the head of the Department of Physics, Indian Institute of Technology Guwahati, for valuable discussions. Robi, Head, Department of Mechanical Engineering, Indian Institute of Technology Guwahati, for expanding the optical microscope and other experimental facilities. Chaudhuri, Department of Chemistry, Indian Institute of Technology Guwahati, for valuable discussions and expansion of chemical titration facilities.
Singh, Engineering Section, Indian Institute of Technology Guwahati, for extending his help during installation of electromagnet. SG' is the average particle size obtained from the line broadening of the XRD pattern. SG' is the average particle size calculated from the line broadening of the XRD pattern. 105).
SG' is the average particle size obtained from the line broadening of the XRD pattern. 182).
Introduction
Divalent Doped Materials 3
Materials for 0 < x < 0.50 show paramagnetic to ferromagnetic transitions with Tc ranging from 160 to 272K. At x ≈ 0.50, this compound undergoes a paramagnetic to ferromagnetic transition at about 225K and then a charge-ordered antiferromagnetic phase at TCO ≈ 155K [21]. Such ordering of charges (CO) with hysteresis in electrical resistance has been reported by Zhao et al.
26] performed a detailed study of electrical and magnetic properties on La1-xSrxMnO3 crystals for x = 0 to 0.60. 9] reported an M-I transition above room temperature on a La0.67Ba0.33MnO3 thin film sample and found magnetoresistance values up to 60% at room temperature for a 5T field. Mandal and Ghosh [33] reported an M-I transition at around 270 K on a La0.8Ba0.2MnO3 crystal and found reversible low-temperature semiconductor behavior for x ≤ 0.175.
Other rare earth manganites such as Nd1-xSrxMnO3 have also been found to exhibit M-I and FM transitions at room temperature [39].
Monovalent Doped Materials 4
I have also come across other reports on these materials after the completion of my work [56]. In addition to the divalent/monovalent doping, hole doping can also be achieved in RMnO3 compounds by creating a vacancy in the La or Mn site. There are a few reports on self-doped manganites such as R1-xMnO3 and RMn1-xO3, which are found to exhibit ferromagnetism with CMR properties [57–60].
Electron Doped Materials 5
67] concluded from their neutron diffraction study that there is a coexistence of FM and AFM properties in the temperature range from 150 to 200 K on a Bi0.18Ca0.82MnO3 single crystal. In Sm1-xCaxMnO3 compounds, metal-insulator transitions have been reported at around 110 K for a narrow composition range 0.10 ≤ x ≤ 0.12 and the materials show an FM transition with a weak magnetization near the TMI [70]. They explained that the occurrence of CMR behavior at relatively low magnetic field is unlikely to be due to a double-exchange interaction, but is due to a super-exchange type of ferromagnetic interaction.
Their magnetoresistance values continuously increase with decreasing temperature without showing any peak near the M-I junction. According to them, the appearance of metallic conductivity below the M-I transition is mainly due to the presence of cubic perovskite (ferromagnetic phase) above the percolation threshold. There are a few reports on electron-doped materials in the (La1-xSrx)MnO3 series, which have been studied mostly in the near-x composition range.
Here n = 1 exhibits semiconductor behavior with charge ordering, n = 2 exhibits metal-insulator transitions in the temperature range TMI = 110 to 125K and the TMI value of n = 3 is around 150K.
Other CMR Materials 7
In addition to the above-mentioned perovskites, there is another group of manganite materials with a perovskite structure, these are called layered compounds or Ruddleson-Popper phases with the chemical formula (La, Sr)n+1MnnO3n+1 [84-86]. La' can be replaced by other rare earth elements and 'Sr' can be replaced by other alkaline earth metals. Like manganite perovskites, the above two groups of materials show a large drop in resistivity near FM Tc values.
Crystal Structure 8
In the cubic environment of the octahedron, the hybridization and electrostatic interaction of Mn-3d electrons with O-2p electrons will create a crystal field effect. This field lifts the 5-fold degeneracy of d electrons into free 'Mn' ions and divides the energy levels into two states namely t2g and eg. The t2g state is triple degenerate at lower level and bv is doubled at higher energy level. The t2g triplet consists of dxy, dxz and dyz orbitals, while the bv doublet contains the dx2−y2 and d3z2−r2 orbitals [105].
In practice, a strong Hund's coupling between the t2g and eg states leads to the formation of a high spin state (S = 2) for the four 3d electrons in the Mn3+ ion. The Jahn-Teller distortion then splits the level further and favors the dx2−y2 or the d3z2−r2 orbital. It is known that ions such as Cr2+ (d4), Cu2+ (d9) and Mn3+ (d4) in the center of a regular octahedron split the degenerate orbital levels by distorting the cubic symmetry to a tetragonal one.
In practice, a lowering of the energy of the d3z2−r2 orbital is created by stretching the O6 - octahedron along the 'z' direction, while the perpendicular plane is compressed.
Electrical Resistivity and Magneto-Resistivity 10
- Theoretical Background 14
- Electrical Resistivity in the Metallic Region 16
- Electrical Resistivity in the Semiconducting Region 18
- Effective Medium Approach 21
Magneto-resistance (MR) is a measure of the change in electrical resistance as a function of the magnetic field, H, and is usually calculated as,. The electrical resistance in magnetic materials depends on the direction of the applied magnetic field relative to the orientation of the crystal itself [111], a phenomenon known as anisotropic magnetoresistance. According to Anderson-Hasegawa [16], in the case of a strong coupling limit, i.e. with JH/tij → ∞, the effective jump interaction tij is written as,.
So the absolute size of the effective jump depends on the relative angle θij between adjacent revolutions. According to Zener [14, 15], the magnitude of the exchange energy is given by,. where ν is the electron oscillation frequency. Overall, the double exchange interaction still appears to provide the best explanation of charge transport in the metallic region.
The temperature dependence of the resistivity is designed by taking into account both the metallic and insulating properties.
Magnetic Properties 22
- Theoretical Background. 24
- Charge and Orbital Ordering in Manganite Perovskite 26
The paramagnetic-to-ferromagnetic transition temperature (Tc) in mixed-valence manganites depends on the size of the cations. This effect is supported by the small displacement of the oxygen atoms to accommodate the ordered cations. The driving force is partly due to the direct electrostatic repulsion of the charge clouds, however the associated Jahn–Teller distortions of adjacent octahedra stabilize such an effect.
The order of charges in manganite is governed by the band width e.g., which is directly determined by the weighted average radius of the A-site cations
Eventually, the coulomb interaction wins over the kinetic energy of the electrons to form the long-range CO state.
Miscellaneous Physical Properties 28
- Thermo Electric Power (TEP) 28
- Hall Effect 29
- Optical Properties 30
- Thermal Properties 30
There is a strong magnetic field dependence of 'Sc' near Tc, showing that the carriers are magnetic. Unlike normal metals and semiconductors, the Hall resistivity of magnetic materials contains an additional term which depends on the magnetization. Where 'µ0' is the charge carrier mobility, 'Ba' is the applied magnetic field, 'M' is the magnetization and the normal Hall coefficient 'R0' is 1/ne and the extraordinary Hall coefficient 'RE' depends on asymmetric electron scattering due to of spin-orbit coupling.
Assuming one carrier model, the Hall mobility µH = R0/ρ can be derived from measured values of R0 and the normal resistance ρ = ρxx. The sign of the Hall carriers in the ferromagnetic state of mixed valence manganites should depend critically on the Jahn-Teller splitting of the bv band [184]. In addition, a weaker absorption fraction is observed at around 1.7eV and this is attributed to the promotion of a t2g electron into a vacancy e.g. orbital.
The width of the bv band is estimated to be 0.9eV with 0.7 electron per formula.
Objectives and Motivation of the Present Thesis Work 31
The temperature variation was achieved by using a heating wire wound near the cold tip. La' is smaller than that of 'Ba', the increase in lattice parameters with 'La' can be visualized as the increase in the concentration of Mn3+ ions. Typical plots of M versus T for x = 0.50 sample in the ZFC (circles) and FC (squares) conditions are shown in Fig.
Typical plot of temperature variation of electrical resistance in the absence of field for the hole doped material, x = 0.70 is shown in Fig. XRD patterns of parent compound SrMnO3 (x = 0) and electron doped materials in the composition range x = 0.10 to 0.20 are shown in Fig. The charge sorting was observed in Ba1-xLaxMnO3 series for x ≤ 0.50 i.e. in the electron-doped region.
The resistance data in the metallic region for the Ba1-xLaxMnO3 series can be fitted to the equation ρ = ρ0 + ρ2T2. As discussed in the introduction (section 1.1.1), the substitution of divalent elements in place of 'La' by the general chemical formula La1-xAxMnO3 (where A = Ca, Sr, Ba, Pb etc.) is interesting because these materials have interesting magnetic and electronic properties, including the colossal magneto-resistance effect (CMR). The average valency of the 'Mn' ion in the above compounds, determined from the chemical titration, is shown in Table 4.3.
The temperature change of resistance in zero field and in the presence of magnetic fields of 10 kOe is shown in the figure. It is found to be close to the sharp drop in sensitivity observed at around 100 K. An enlarged view of the splitting of the (110) and (104) peaks is shown in the inset of the corresponding figures.
The solid circles represent peaks due to the presence of minor Mn3O4 impurity in the materials. As discussed in Section 3.1.3, the RSG state is mainly due to the presence of the competing antiferromagnetic interaction. The semiconducting behavior observed for the x ≥ 0.15 samples can be explained on the basis of some of the 'Cu' entering the 'Mn' site.
The resistivity as a function of temperature for the sample x = 0.10 in the presence of a magnetic field of 10 kOe is shown in the figure. The maximum value of negative MR was found to be 20% near Tc. For doping 'Ag' instead of 'La', the ferromagnetic transition temperature Tc is found to increase with.
Ca 0.3 MnO 3 . (54)
