Influence of magnetic structure on the magnetic properties of compensated substituted compounds of CoCr2O4.
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Among oxides materials, spinel oxides having structure AB2O4, exhibit a rich spectrum of functional responses due to their interesting magnetic, electric and multiferroic properties. In the family of magnetic spinel oxides, Cobalt chromate (CoCr2O4) is famous as magnetoelectric (ME) multiferroic material which exhibits a rich phase diagrams involving spin and orbital degrees of freedom, geometrical frustration and complex spiral ferrimagnetic (FIM) orderings. CoCr2O4 exhibits a long range collinear FIM ordering at (TC) ∼94 K followed by another magneto-structural transition (TS) ∼24 K below which an incommensurate conical spin-spiral order exists. It is the existence of the conical spin-spiral order in this compound, which gives rise to ME coupling as predicted by the spin-current model. Further, through neutron scattering experiment, another transition is also observed at T ∼15 K, known as lock-in transition (TL) in CoCr2O4. The presence of weak magnetic frustration, which is very much intangible to superexchange interactions present along different paths such as A−O−A, B−O−B and A−O−B give rise to all above enthralling features. This thesis presents detailed study to understand the crystalline and magnetic structure by performing neutron powder diffraction experiments and other physical properties of A and B site substituted spinel compounds. (i) Co(Cr1-xFex)2O4 with x = 0.05, 0.075, (ii) Co(Cr1-xMnx)2O4 with x = 0.27, 0.30, and (iii) Co1-xCuxCr2O4 with x = 0.20. Structural analysis of these polycrystalline compounds shows that substituted compounds of Fe and Mn retains the cubic phase till the lowest measuring temperature (i.e. 3 K) whereas a structural distortion is observed in case of Cu (20%) substituted sample. Magnetization measurements have revealed many interesting magnetic properties including negative magnetization, sign change in exchange bias field and sign change in MCE across Tcomp. Temperature dependent NPD experiments have been performed on the compensated substituted compounds which provide a detailed magnetic structure responsible for observed fascinating magnetic properties of these compounds.
Supervisor: Dilip Pal