STRUCTURAL, MAGNETIC AND THERMAL PROPERTIES OF DOPED ORTHOFERRITE MATERIALS

Abstract

Among transition metal oxides, Orthoferrites (RFeO3), where R = rare earth element, are widely studied because of their unique magnetic properties. The magnetic ground state of Orthoferrites is primarily determined by the Fe3+ - Fe3+ exchange interaction at ambient temperatures which is antiferromagnetic G - type ordering having easy axis along the x- direction. In most of the Orthoferrites at high temperature, the magnetic ordering is canted antiferromagnetic with a weak ferromagnetic component. This canting is present due to weaker and antisymmetric superexchange interaction known as Dzyaloshinsky – Moriya interaction. A relatively weaker Fe3+ - R3+ exchange interaction is also active in Orthoferrites. Due to antisymmetric and anisotropic-symmetric part of Fe3+ - R3+ exchange interaction, Fe spins undergo spin reorientation with the easy axis changing from x-direction to y-/z-direction. R3+- R3+ exchange interaction in Orthoferrites is the weakest as compared to Fe3+ - Fe3+ and Fe3+ - R3+ interactions. This interaction occurs at very low temperature (below 10 K) and decides the possible magnetic ordering of rare earth ions. In the present work, mainly the Orthoferrite (NdFeO3) has been studied by varying the transition metal site properties in one case and the rare earth site properties in the other case. The main objective of this thesis is to study the structural, magnetic and thermal properties of polycrystalline Orthoferrite. In the first part of the thesis, 50% doping of Mn is done on the Fe site to study the change in structural and magnetic properties of the compound with the doping. In the second part of the thesis, doping was done on the rare earth site, i.e. 50% Dy was doped on the Nd site to study the change in magnetic property and structure of Fe3+ and rare-earth (Dy3+/Nd3+) spins. Spin reorientation of Fe3+ ions in both compounds is studied in detail. Study of magnetocaloric effect is also performed on both the compounds in this thesis. Further, structural and magnetic properties of NdFe0.5Mn0.5O3 thin film on SrTiO3 are studied. The chapter-wise overview of the present thesis work has been discussed below: Chapter 1 gives the general introduction of transition metal oxides and Orthoferrites with a brief description of the origin of magnetism in these compounds. Further, spin reorientation which is observed in all the Orthoferrites with magnetic rare earth ion is discussed in detail. The factors affecting the direction and transition temperature of spin reorientation along with the type of ii magnetic structure is discussed. In this thesis magnetocaloric effect has been studied in Orthoferrite compounds. Thus the magnetocaloric effect is explained in detail. The previous studies on Mn doped Orthoferrites materials have been discussed. A literature survey has been done on the substitution of Mn on Fe site for different rare earth material as well as for doping on rare earth site. Finally, the scope of the thesis is outlined in the last section. Chapter 2 describes the details of experimental techniques, which have been used throughout the work in the present thesis for the synthesis and characterization of perovskite structure based orthoferrites in polycrystalline as well as thin film forms. The polycrystalline compound has been prepared by using conventional solid-state reaction route. The synthesis of thin films has been carried out by pulsed laser deposition technique. The structural properties of the samples were studied using X-Ray diffraction (XRD) and Neutron diffraction. Magnetic properties of all the samples were studied using Superconducting Quantum Interference Device (SQUID). Heat capacity measurements were carried out using specific heat option of Physical Property Measurement System (PPMS). In chapter 3, structural and magnetic properties of NdFe0.5Mn0.5O3 (NFMO) has been investigated in detail. The synthesis of the sample was done by conventional solid-state reaction method. Rietveld refinement of powder X-Ray diffraction data suggest that NFMO possesses an orthorhombic structure with space group Pbnm. Due to the presence of 50% Mn on Fe site, Jahn – Teller distortion is present in the MO6 (M= Fe, Mn) octahedra. The M-T curves of NFMO exhibit paramagnetic to antiferromagnetic transition at 250 K which is intermediate to the TN of NdFeO3 and NdMnO3. Additionally, another magnetic transition is observed at 36 K which denotes the spin reorientation of Fe3+/Mn3+ spins. The spin reorientation is also confirmed by M-H curves measured at different temperatures. According to Mössbauer spectroscopy, Fe3+ spins show paramagnetic behavior at 300 K. At lower temperatures (5 - 100K) Mössbauer spectra is fitted using two sextets which confirms the magnetic ordering of Fe3+ spins. The data analysis of Mössbauer spectra also indicates the spin reorientation of Fe3+ spins in lower temperature range. Chapter 4 deals with the powder neutron diffraction studies of polycrystalline NFMO sample in the temperature range 1.5 – 400 K. Initially, 3- dimensional short-range ordering is discussed that is present over the whole temperature range 400 – 1.5 K and suppressed by the long-range ordering with the decreasing temperature. There is a presence of broad magnetic Bragg peak at iii 300 K. This peak represents the mixture of Γ4 (Gx, Ay, Fz) and Γ1 (Ax, Gy, Cz) magnetic structures, not like other orthoferrites which have Γ4 magnetic structure at 300 K. The possible reason for the presence of Γ1 magnetic structure is the presence of Mn3+ ions which have large single ion anisotropy. Below TN, the contribution from Γ4 magnetic structure starts decreasing gradually with an increase in the contribution of Γ1 structure. At 150 K, contribution of Γ4 vanishes completely, leaving pure Γ1 magnetic structure till 90 K. The magnetic structure undergoes another spin reorientation transition between temperature 75 and 25 K where the magnetic structure exists as a sum of two irreducible representations (Γ1 + Γ2). In this chapter, two-fold spin reorientation is discussed which is seen rarely in orthoferrites. At 1.5 K, antiferromagnetic structure belongs entirely to Γ2 (Gz) with small ferromagnetic canting (fx) due to Nd. To further investigate rare earth ordering at low temperature, specific heat of NFMO have been measured and analyzed where Nd3+ - Fe3+/Mn3+ exchange interactions dominates over Nd3+ - Nd3+ interactions, as evident from the presence of Schottky anomaly. Chapter 5 describes the synthesis and characterization of Nd0.5Dy0.5FeO3 (NDFO). Rietveld refinement of XRD data suggests that NDFO is orthorhombic in structure with Pbnm space-group. In the case of magnetic structure, the magnetic structure of Fe3+ belongs to Γ4 (Gx, Fz) at room temperature. The magnetization data indicate the occurrence of spin reorientation below 60 K. The neutron diffraction studies confirmed the spin reorientation where the magnetic structure of Fe3+ changes gradually from Γ4 to Γ2 between 60 and 20 K while maintaining G type configuration. Between 20 and 10 K, the Fe3+ magnetic structure is represented by Γ2 with Fe3+ magnetic moments arranged in Gz type configuration with a small ferromagnetic moment along a direction. Interestingly, Γ4 magnetic structure of Fe3+ re-emerges below 10 K which also coincides with the development of rare earth (Nd3+/Dy3+) magnetic ordering having Cy configuration with magnetic moment of 1.8 μB. The absence of any signature of second order phase transition in the specific heat confirms the role of R(Nd3+/Dy3+) – Fe3+ exchange interaction in the observed rare – earth ordering unlike DyFeO3 where Dy3+ ordering takes place independently to the magnetic ordering of Fe3+ magnetic structure. In chapter 6, magnetocaloric effect has been studied in detail for Nd0.5Dy0.5FeO3 and NdFe0.5Mn0.5O3. An enhancement in the magnetization led to a large change in magnetic entropy (10.4Jkg-1K-1) at 4K in NDFO. The observed entropy change is remarkable considering the smaller iv value of Nd3+ moment as compared to that of Dy3+. Change in magnetic entropy of NFMO is 2.98 Jkg-1K-1 at 4.75 K which is smaller as compared to NDFO. On comparison with other Nd based transition metal oxide, NFMO has higher value making it suitable for application due to the relatively high abundance of Nd in nature. From application point of view, the presence of such a high value of change in magnetic entropy, as well as relative cooling power of NDFO and NFMO, makes both the compound good alternative for magnetic refrigeration at lower temperature which will also prove to be environment friendly. In chapter 7, the growth and characterization of NdFe0.5Mn0.5O3 thin films have been discussed in detail which is deposited using pulsed laser deposition technique on STO substrate. The thin film deposition was optimized by varying various deposition parameters. X-ray diffraction confirms the out-of-plane growth of the films i.e. they are (00l) oriented. Magnetic property of NFMO thin film on STO substrate has also been studied. In the magnetic studies, magnetic ordering with the low temperature spin reorientation transition has been observed in the thin film. Chapter 8 summarizes the work done in the present thesis and presents a brief conclusion with some future scope of the research findings.