• Sukumaran Vasudevan

      Articles written in Pramana – Journal of Physics

    • Magnetism, exchange and crystal field parameters in the orbitally unquenched Ising antiferromagnet FePS3

      Nirmala Chandrasekharan Sukumaran Vasudevan

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      FePS3 is a layered antiferromagnet (TN=123 K) with a marked Ising anisotropy in magnetic properties. The anisotropy arises from the combined effect of the trigonal distortion from octahedral symmetry and spin-orbit coupling on the orbitally degenerate5T2g ground state of the Fe2+ ion. The anisotropic paramagnetic susceptibilities are interpreted in terms of the zero field Hamiltonian, ℋ=Σi [δ(Liz2−2)+|λ|Li.Si]−ΣijJijSi.Sj. The crystal field trigonal distortion parameter Δ, the spin-orbit coupling λ and the isotropic Heisenberg exchange,Jij, were evaluated from an analysis of the high temperature paramagnetic susceptibility data using the Correlated Effective Field (CEF) theory for many-body magnetism developed by Lines. Good agreement with experiment were obtained for Δ/k=215.5 K; λ/k=166.5 K;Jnnk=27.7 K; andJnnnk=−2.3 K. Using these values of the crystal field and exchange parameters the CEF predicts aTN=122 K for FePS3, which is remarkably close to the observed value of theTN. The accuracy of the CEF approximation was also ascertained by comparing the calculated susceptibilities in the CEF with the experimental susceptibility for the isotropic Heisenberg layered antiferromagnet MnPS3, for which the high temperature series expansion susceptibility is available.

    • Spin state and exchange in the quasi-one-dimensional antiferromagnet KFeS2

      Satish Kumar Tiwary Sukumaran Vasudevan

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      We report the optical spectra and single crystal magnetic susceptibility of the one-dimensional antiferromagnet KFeS2. Measurements have been carried out to ascertain the spin state of Fe3+ and the nature of the magnetic interactions in this compound. The optical spectra and magnetic susceptibility could be consistently interpreted using aS=1/2 spin ground state for the Fe3+ ion. The features in the optical spectra have been assigned to transitions within thed-electron manifold of the Fe3+ ion, and analysed in the strong field limit of the ligand field theory. The high temperature isotropic magnetic susceptibility is typical of a low-dimensional system and exhibits a broad maximum at ∼565K. The susceptibility shows a well defined transition to a three dimensionally ordered antiferromagnetic state atTN=250 K. The intra and interchain exchange constants,J andJ′, have been evaluated from the experimental susceptibilities using the relationship between these quantities, andχmax,Tmax, andTN for a spin 1/2 one-dimensional chain. The values areJ=−440.71 K, andJ′=53.94 K. Using these values ofJ andJ′, the susceptibility of a spin 1/2 Heisenberg chain was calculated. A non-interacting spin wave model was used belowTN. The susceptibility in the paramagnetic region was calculated from the theoretical curves for an infiniteS=1/2 chain. The calculated susceptibility compares well with the experimental data of KFeS2. Further support for a one-dimensional spin 1/2 model comes from the fact that the calculated perpendicular susceptibility at 0K (2.75×10−4 emu/mol) evaluated considering the zero point reduction in magnetization from spin wave theory is close to the projected value (2.7×10−4 emu/mol) obtained from the experimental data.

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