Thermomagnetic, dielectric and EPR studies on four new multiferroics

Multiferroics are materials which exhibit at least two or all three of the ferroic (ferroelectric, ferromagnetic and ferroelastic) orders co-existing in them, and are of high current interest in both the fundamental research and in the discovery of new multifunctional materials. This dissertation presents thermomagnetic, dielectric and electron paramagnetic resonance (EPR) studies on some new multiferroics. The primary focus of this work is on these four compounds: [(CH3)2NH2]Mn(HCOO)3, K3Fe5F15, Pb(Fe1/2Nb1/2)O 3, and MnV2O4. Chapter 3 discusses our discovery of multiferroic behavior of the metal-organic framework (MOF) compound dimethylammonium manganese formate, [(CH3)2NH2]Mn(HCOO) 3, with the aid of single crystal heat capacity and EPR measurements on the compound. Simultaneous occurrence of ferroelectric and antiferromagnetic long-range orders in this compound established its multiferroic nature. Chapter 4 describes the multiferroic nature of the 'tetragonal tungsten bronze'-type ferroelectric potassium iron fluoride, K3Fe5 F15, studied by magnetic susceptibility, magnetization, dielectric and EPR characterization of single crystal and polycrystalline samples. A weak ferromagnetic (FM) ordering due to spin canting caused by the antisymmetric exchange interaction between the Fe ions, and its influence on the dielectric constant evident by the observation of an anomaly around the FM ordering temperature, together indicate an intrinsic magnetoelectric coupling present in this compound. Chapter 5 describes multiferroic behavior of the complex oxide perovskite lead iron niobate, Pb(Fe1/2Nb1/2)O3 abbreviated as PFN. Systematic (9.4, 34, 381, 683 GHz) EPR and magnetic susceptibility measurements on polycrystalline PFN samples show an antiferromagnetic (AFM) ordering, the presence of a small finite internal field well above the AFM ordering temperature, and the first observation of an anomaly in the susceptibility near the ferroelectric Curie point, together implying the presence of magnetoelectric effect. Chapter 6 presents ac and dc magnetic susceptibility experiments on the magnetoelastic spinel compound manganese vanadate, MnV2O4. The compound is shown to exhibit a re-entrant spin glass behavior. The results and perspectives presented here should contribute to the advancement and fundamental understanding of multiferroic materials.