Magnetic and Polar Properties of some Geometrically Frustrated Transition Metal Oxides

Abstract

Recently there has been a surge of research interest in the magnetoelectric multiferroics, where magnetic and polar orders co-exist. Intense study of magnetoelectric coupling in several materials has shed light on the different possible scenarios of combining magnetism and ferroelectricity in a single phase material.The discovery of type-II multiferroics, where the electric polarization is induced by a specific type of non- collinear magnetic order- ing, has brought rapid growth in the research area of magnetelectric multiferroic. The magnetic order in geometrically frustrated magnets are often nontrivial and has been a fertile playground for exploring magnetoelectric coupling. In this thesis, we aim to study geometrically frustrated transition metal oxides for designing new magnetoelectric mul- tiferroic materials and to study their physical properties. Chapter 1 gives a brief introduction and evolution of the research area of magnetoelectric multiferroics. The issues associated with the coexistence of magnetism and ferroelectricity are highlighted. Typically, multiferroics are broadly classified in two categories, namely type-I and type-II. In a type-I multiferroic, the magnetism and ferroelectricity occur from different origins and results in a weak coupling between the order parameters. On the other hand, for a type-II multiferroic various type of commensurate/incommensurate spin configurations break spatial inversion symmetry and provide a strong coupling between the order parameters. Possible mechanisms of inducing ferroelectricty and magnetism in both type-I and II multiferroics are described in details. Different theoretical models are qualitatively described and their experimental realization also is presented. Finally, technological applications of these materials are highlighted and the motivation and the scope of the thesis is presented. Chapter 2 describes the following measurement facilities developed during the course of the this work: • Dielectric and pyroelectric measurement setups : Sample holders for low-temperature dielectric and pyroelectric measurements in a closed cycle refrigerator (CCR) have been designed. Both the measurements were automated using LabVIEW program. • Dielectric and pyroelectric measurements under magnetic field: Using a MPMS- XL Manual Insertion Utility Probe, low-temperature dielectric and pyroelectric measurements up to 7 Tesla magnetic field have been performed. • Electric field dependent magnetic measurement: Using a MPMS-XL Manual Insertion Utility Probe, electric field dependent magnetic measurements have been performed successfully on a single crystal specimen of Mn4Ta2O9, and the magnetoelectric coupling has been studied along different orientations of the crystal. A description of experimental techniques/methods used during this work which include synthesis of polycrystalline and single crystal specimen, investigation of the nuclear and magnetic structure from powder X-Ray and Neutron diffraction and bulk physical properties measurement is also presented. Chapter 3 describes the investigation of Dy1−xCaxBaCo4O7 series ( x= 0 to 1) using X-ray diffraction, magnetization and dielectric measurements. Systematic doping causes a lifting of the geometrical frustration through structural transition from a high symmetry trigonal P31c to a low symmetry orthorhombic Pbn21 symmetry at the doping concentration of x = 0.4. For all the orthorhombic specimens, such a structural transition was also observed as a function of temperature. One of the distinguishing features of this structural transition is that the temperature at which this transition occurs in the RBaCo4O7 (R=Rare earth ions, Ca2+) family is observed to scale linearly with the mean ionic radius of the R site ion. However, such a quasilinear relationship is violated for R= Ca2+ comprising of an equal number of Co2+ and Co3+, indicating that charge ordering could possibly influence the structural transition. The ZFC-FC magnetization exhibits a large paramagnetic background arising from Dy3+ ions that apparently masks the magnetic transition appearing from the cobalt sublattice. Using Thermo-remanent Magnetization (TRM) measurements, we have identified the magnetic ordering temperatures of the solid solution Dy1−xCaxBaCo4O7. Finally, we chart out the phase diagram of the Dy1−xCaxBaCo4O7 series. SrBaCo4O7, a new member of Swdenborgite family has been synthesized successfully by the solid state reaction route.We have solved the structure using DICVOL06 and LeBail profile refinement of powder X-ray diffraction data. It stabilizes in the orthorhombic pbam space group. Detailed magnetic and thermodynamic study reveals multiple phase transitions as a function of temperature. Chapter 4 of the thesis presents study of a new near room temperature (≈ 280K) magnetodielectric γ - Fe2WO6. The magnetic, thermodynamic and dielectric properties of the γ - Fe2WO6 system is discussed. Crystallizing in the centrosymmetric Pbcn space group, this particular polymorph exhibits a number of different magnetic transitions, all of which are seen to exhibit a finite magneto-dielectric coupling. At the lowest measured temperatures, the magnetic ground state appears to be glass-like, as evidenced by the waiting time dependence of the magnetic relaxation. Also reflected in the frequency dependent dielectric measurements, these signatures possibly arise as a consequence of the oxygen non-stoichiometry, which promotes an inhomogeneous magnetic and electronic ground state. In Chapter 5, the study of a new type-II multiferroic specimen Fe4Ta2O9 is presented. Combination of magnetic, dielectric, specific heat and pyroelectric measurements reveals a series of magnetic transitions, many of which are associated with the emergence of ferroelectric order. The anomaly observed in low field ZFC magnetic measurement as a function of decreasing temperature are also being identified in dielectric and pyroelectric measurements performed under zero field. The low temperature H-T phase diagram of Fe4Ta2O9 is seen to exhibit a rich variety of coupled magnetic and ferroelectric phases, in similarity with that observed in the distorted Kagome systems. On cooling two distinct multiferroic states of Fe4Ta2O9 labeled as AFM1 FE1, and AFM2 FE2, with onsets at T1= 80K and T2= 60K respectively are seen. On cooling further, and in the presence of magnetic fields in excess of 1 Tesla, we observe a field induced multiferroic state denoted as AFM3 FE3. Within this state, we also identify the presence of a metamagnetic (spin-flop) transition which appears to have no discernible influence on the polar state (SF-AFM3 FE3). We further observe the onset of possibly a different kind of antiferromagnetic order at temperatures of the order of 5K, (AFM4 FE3) as is evidenced by magnetic and dielectric measurements. We suggest that these properties arise due to an effective reduction in the dimensionality of the magnetic lattice, with the magnetically active Fe2+ ions preferentially occupying a quasi 2D buckled honeycomb structure. Chapter 6 presents the crystal growth of magnetoelectric Mn4Ta2O9 and temperature dependent magnetization and electric field dependent magnetic measurements along the different crystallographic axes of this hitherto unexplored single crystal. The magnetic structure solved from high-resolution T.O.F powder neutron diffraction data reveals that the magnetic structure comprises of antiferromagnetically coupled ferromagnetic chains of Mn2+ spins aligned along the trigonal c axis, in sharp contradiction to that reported in other isostructural members of this family. DC magnetic measurements performed with magnetic fields parallel and perpendicular to the crystallographic c axis of the crystal reveals anisotropic behavior with the presence of Dzyloshinskii-Moriya driven weak FM phase, making the magnetic phase diagram of Mn4Ta2O9 far richer than the other members of this family. More importantly, magnetic measurements performed under a period electric field indicate that the magnetoelectric response is also anisotropic, with this coupling along the trigonal c axis and that perpendicular to it having different signs. Chapter 7 summarizes the conclusions of the complete work presented in this thesis. Also, possible methods and avenues for future work are listed Appendix-I of the thesis presents a detailed description of solving the magnetic structure of Mn4Ta2O9 from Neutron powder diffraction pattern using the Fullprof Suite. A step by step guide of using the K_Search Program for identifying k vector from the Neutron diffraction data is described. Use of the BasIreps program for representational analysis in solving magnetic structure is also discussed. Finally, construction of *.pcr (input control file for running the FULLPROF program ) for incorporating the calculation of magnetic contribution and testing the different representations to find the correct magnetic model is highlighted.