Role of orbital degrees of freedom in investigating the magnetic properties of geometrically frustrated vanadium spinels

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• 作者：Sohan Lal ; ^{goluthakur2007@gmail.com" class="auth_mail" title="E-mail the corresponding author} ; Sudhir K. Pandey ^{sudhir@iitmandi.ac.in" class="auth_mail" title="E-mail the corresponding author}
• 关键词：Frustration indices ; Magnetic moments ; Band gap ; Exchange coupling constant ; Curie-Weiss temperature ; Magnetic transition temperature
• 刊名：Computational Materials Science
• 出版年：2017
• 出版时间：January 2017
• 年：2017
• 卷：126
• 期：Complete
• 页码：373-381
• 全文大小：984 K

文摘

The inconsistency about the degree of geometrical frustration has been a long issue in AV₂O₄ (A ≡$\equiv$ Zn, Cd and Mg) compounds, which arises from the two experimental results: (i) frustration indices and (ii) magnetic moments. In the present study, we try to understand such inconsistency by using ab initio electronic structure calculations. The orbital degrees of freedom are found to play an important role in understanding the geometrically frustrated magnetic behavior of these compounds. The magnitude of the maximum calculated values of orbital magnetic moment per formula unit for ZnV₂O₄, MgV₂O₄ and CdV₂O₄ compounds are found to be ∼$\sim$1.54 8ed4b" title="Click to view the MathML source">μ_B${\mu }_B$, ∼$\sim$0.92 8ed4b" title="Click to view the MathML source">μ_B${\mu }_B$ and ∼$\sim$1.74 8ed4b" title="Click to view the MathML source">μ_B${\mu }_B$, respectively. The inclusion of the orbital and spin angular momenta for calculating the frustration indices improves the understanding about the degree of geometrical frustration in these compounds. The calculated values of the frustration indices (9e3ea0e1b8aac14d37c38fb9b3c506dc" title="Click to view the MathML source">f_J$f_J$) are largest for MgV₂O₄ and smallest for CdV₂O₄ for 3.3 99869c68c9b0a2aee6" title="Click to view the MathML source">⩽$⩽$ 9d3126f4565c4941afb361cc5b" title="Click to view the MathML source">U⩽$U⩽$ 5.3 eV. In this range of U  , the calculated values of Δ$\mathrm{\Delta }$M₂ = M_total-M_exp$M_{\text{total}}-M_{\text{exp}}$ (where, 99c40fbdf7bff81" title="Click to view the MathML source">M_total=M_spin-|M_orbital|$M_{\text{total}}=M_{\text{spin}}-|M_{\text{orbital}}|$) are also found to be largest for MgV₂O₄ and smallest for CdV₂O₄. Hence, the consistency about the degree of geometrical frustration, which arises from the 9e3ea0e1b8aac14d37c38fb9b3c506dc" title="Click to view the MathML source">f_J$f_J$ as well as from the Δ$\mathrm{\Delta }$M₂ is achieved and improves the understanding about the degree of geometrical frustration in these compounds. Calculated values of band gap in this range of U   are found to be closer to that of experimentally observed values for all three compounds. The absolute values of the nearest neighbor exchange coupling constant (990a7bc64b95e3109dc47ae6302d7" title="Click to view the MathML source">J_nn$J_{\mathit{nn}}$) between V spins are found to be largest for MgV₂O₄ and smallest for CdV₂O₄, which indicate that the calculated absolute values of the Curie-Weiss temperature (Θ_CW)_J${({\Theta }_{\mathit{CW}})}_J$ are highest for MgV₂O₄ and smallest for CdV₂O₄ for 3.3 99869c68c9b0a2aee6" title="Click to view the MathML source">⩽$⩽$ 9d3126f4565c4941afb361cc5b" title="Click to view the MathML source">U⩽$U⩽$ 5.3 eV. In this range of U  , the magnetic transition temperaturee6870df4617eadb0390b904ebf" title="Click to view the MathML source">(T_N)_J${(T_N)}_J$ is found to be ∼$\sim$150 K, ∼$\sim$60 K and ∼$\sim$22 K for MgV₂O₄, ZnV₂O₄ and CdV₂O₄, respectively, which shows that the order of e6870df4617eadb0390b904ebf" title="Click to view the MathML source">(T_N)_J${(T_N)}_J$ is similar to that of 93000a919022479fe8e8aad78f0c988" title="Click to view the MathML source">(T_N)_exp${(T_N)}_{\text{exp}}$ for these compounds. Hence, all the magnetic properties studied in the present work are well explained in these spinels for 3.3 99869c68c9b0a2aee6" title="Click to view the MathML source">⩽$⩽$ 9d3126f4565c4941afb361cc5b" title="Click to view the MathML source">U⩽$U⩽$ 5.3 eV. This work is expected to provide a valuable input in understanding the geometrically frustrated magnetic behavior for those systems for which the orbital part of the angular momenta are not quenched.