ELECTRONIC STRUCTURE OF NOVEL MAGNETIC SYSTEMS

Abstract

The present thesis is devoted to the study of low dimensional quantum spin systems and low dimensional systems that exhibit multiferroic behavior. Using ab initio density functional theory we have studied the electronic and magnetic properties of a spin gap compound Sr2Cu(BO3)2. We have calculated the hopping and exchange interactions between various Cu ions and derived the low energy spin model for the system. The spin model turns out to be a system of decoupled spin ladders with strong rung coupling. The validity of the model is checked by calculating the magnetic susceptibility as a function of temperature and magnetization as a function of temperature as well as magnetic field using Quantum Monte Carlo technique and comparison of the calculated results with the available experimental data. Our results suggest that the above model is appropriate to describe the low energy physics of Sr2Cu(BO3)2. We have studied the electronic structure and magnetic properties of frustrated quantum spin systems which include diamond chain antiferromagnets Ba3Cu3X4O12 (X = Sc, In) and also the proposed staircase Kagome lattice system PbCu3TeO7. With the aid of first principles calculations we have identified the dominant exchange paths of these systems which are not obvious from structural considerations. Our estimation for the Curie-Wiess temperature from the computed exchange interactions compares well with experiments. The calculated exchange couplings lead to long range magnetic order, in contrast to the expectation from the structural considerations. Using first principles density functional calculations, we have studied the electronic structure of the low dimensional multiferroic compound FeTe2O5Br to investigate the origin of magnetoelectric (ME) effect and the role of Te ions in this system. We find that without magnetism even in the presence of Te 5s lone pairs, the system remains centrosymmetric due to the antipolar orientation of the lone pairs. Our study shows that the exchange striction within the Fe tetramers as well as between them is responsible for the magnetoelectric (ME) effect in FeTe2O5Br. We also find that the Te4+ ions play an important role in the inter tetramer exchange striction as well as contribute to the electric polarization in FeTe2O5Br, once the polarization is triggered by the magnetic ordering. Finally we have studied the magnetic and ferroelectric properties of two dimensional triangular lattice antiferromagnetic AgFeO2 and compared with the isostructural system CuFeO2. Our calculations reveal spin orbit coupling has a profound effect on the magnetic and ferroelectric properties of AgFeO2. Calculations of ferroelectric polarization suggest that the spontaneous polarization arises from noncollinear spin arrangement via spin orbit coupling. Our calculations also indicate that in addition to electronic contribution, the lattice mediated contribution to the polarization are also important for AgFeO2.The research was conducted under the supervision of Prof. Indra Dasgupta of the Solid State Physics division under SPS [ School of Physical Sciences]The research was carried out under sponsorship Council of Scientific and Industrial Research (CSIR) (Grant No.09/080(0615)/2008-EMR -1) for research fellowship, financial support from MONAMI and infrastructural support from IACS