Ab initio determination of magnetic ground state of pyrochlore Y$_2$Mn$_2$O$_7$

Mohammad Amirabbasi; Mojtaba Alaei

doi:10.1103/PhysRevB.102.125105

paper

Ab initio determination of magnetic ground state of pyrochlore Y $_{2}$ Mn $_{2}$ O $_{7}$

arXiv:2006.15002 · doi:10.1103/PhysRevB.102.125105

Abstract

There are two discrepant experimental results on the magnetic ground state of Y $_{2}$ Mn $_{2}$ O $_{7}$ , one study proposes a spin glass state, while another introduces the material as a ferromagnet. In this study, we attempt to resolve this issue by employing density functional theory and Monte Carlo simulations. We derive different spin models by varying the Hubbard $U$ parameter in ab initio GGA+ $U$ calculations. For the most range of Hubbard $U$ , We obtain that the leading terms in the spin Hamiltonian are bi-quadratic and the nearest neighbor Heisenberg exchange interactions. By comparing Monte Carlo simulations of these models with the experiments, we find a ferromagnetic ground state for Y $_{2}$ Mn $_{2}$ O $_{7}$ as the most compatible with experiments. We also consider Y $_{2}$ Mo $_{2}$ O $_{7}$ as a prototype of the defect-free pyrochlore system with spin-glass behavior and compare it with Y $_{2}$ Mn $_{2}$ O $_{7}$ . The orbital degrees of freedom are considered as a leading factor in converting a defect-free pyrochlore such as Y $_{2}$ Mn $_{2}$ O $_{7}$ to a spin glass system. By changing the $d$ orbital occupations of Mo atoms, our GGA+ $U$ calculations for Y $_{2}$ Mo $_{2}$ O $_{7}$ indicate many nearly degenerate states with different $d$ orbital orientations which reveals $d$ orbital degrees of freedom in this material. While for Y $_{2}$ Mn $_{2}$ O $_{7}$ , we find a single ground state with a fixed orbital orientation. Consequently, all of our ab initio approaches confirm Y $_{2}$ Mn $_{2}$ O $_{7}$ as a ferromagnetic system.

Ab initio determination of magnetic ground state of pyrochlore Y2​Mn2​O7​

Abstract

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Ab initio determination of magnetic ground state of pyrochlore Y $_{2}$ Mn $_{2}$ O $_{7}$