MAGNETIC AND ORBITAL ORDER IN FRUSTRATED SPINEL SYSTEMS

Abstract

Strongly correlated electronic systems exhibit a wide range of very interesting and complex phenomena and therefore have in recent years drawn a lot of attention of the researchers in this field. The salient feature of these materials is the existence of various competing states. Various fascinating properties shown by these materials include colossal magnetoresistance (CMR), high-temperature superconductivity (HTSC), heavy fermionic behavior, metal-insulator transition without disorder, charge-, orbital- and magnetic-orders, and many more. Number of theoretical and experimental approaches have been used to understand the physics of these materials, but till date the basic underlying mechanism of the various exotic properties shown by these materials is still unclear and thus needs to be explored. Among the variety of strongly correlated electronic systems, the geometrically frustrated transition metal spinel systems (AB2X4; with transition metal ion at the B-site) form an interesting category. These have been the subject of intense theoretical and experimental study in recent years because of their potential technological applications. The interplay of spin, orbit and lattice degrees of freedom are thought to be the origin of many interesting phenomena in this type of materials. In the thesis, transition metal spinel systems have been studied through density functional theory (DFT). First-principle calculations have been performed to study various physical properties, such as magnetic order, orbital order, metal-insulator transition, of transition metal spinels MgV2O4, CoV2O4, NiV2O4 and Mn3O4. The spin, charge, crystal structure and orbital are the four main factors which contribute to the complexity present in i ii spinel systems. Two very popular DFT codes have been used to perform these calculations: one is WIEN2k which uses full potential linearized augmented plane wave method and another is VASP which uses projector augmented wave (PAW) method. The first principle calculations have been performed through various approximations such as LSDA, LSDA+U, LSDA+U+SO, GGA, GGA+U and GGA+U+SO. The thesis contains six chapters. Chapter 1 includes a brief description of the strongly correlated materials and spinel systems and also presents a brief introduction to the theoretical background which involves density functional theory along with various approximations employed to do the calculations. In Chapter 2, we have discussed the orbital and magnetic order present in the low temperature phase of MgV2O4. Within LSDA+U approximation, we found that the vanadium chains along [110] and [010] direction have di↵erent partial occupancy of dyz and dxz orbitals. We have also studied the e↵ect of spin-orbit (SO) coupling on the orbital order present in MgV2O4. Our calculations show that the spin-orbit coupling a↵ects the two orbital chains along the c-axis di↵erently. Chapter 3, presents the electronic structure calculations on CoV2O4. We have investigated the nature of itineracy in CoV2O4 under pressure. Our results indicate that the experimentally observed pressure induced metallicity in CoV2O4 could be obtained via two possible routes. One is through the spin-orbit interaction along with Coulomb correlation and other one is based on the presence of two types of “d” electrons in the system: localized and itinerant which can be modelled through an e↵ective Falicov-Kimball model. We find that the second framework presents a better description. In Chapter 4, we have theoretically investigated NiV2O4, a spinel which has eluded the experimentalists so far. There is no experimental structural data available in literature for this compound. We present our results on the possible crystal and magnetic structure of NiV2O4 proposed on the basis of our first principle calculations. We have also explored whether there will be any magnetic or orbital order present in this system. The system has been found to have a vanadium-vanadium distance equal to 2.94 °A which is same as the limit of itineracy predicted by Goodenough for vanadium spinels. It suggests iii that NiV2O4 is close to the limit of instability, probably the reason why experimentalists have not able to materialize it. In Chapter 5, we have studied the complex magnetic structure of Mn3O4 at low temperature. Then we have studied the e↵ect of doping on this system by replacing some magnetic Mn2+ ion by the nonmagnetic Zn2+ ion at A site. The e↵ect of doping on the exchange interaction present between the magnetic ions has been investigated in this chapter. Chapter 6 contains overall summary, conclusions and the future directions.