Ferromagnetism in 2D organic iron hemoglobin crystals based on nitrogenated conjugated micropore materials

Abstract

In this work we study a low-cost two-dimensional ferromagnetic semiconductor with possible applications in biomedicine, solar cells, spintronics, and energy and hydrogen storage. From first principle calculations we describe the unique electronic, transport, optical, and magnetic properties of a π-conjugated micropore polymer (CMP) with three iron atoms placed in the middle of an isolated pore locally resembling heme complexes. This material exhibits strong Fe-localized d_z² bands. The bandgap is direct and equal to 0.28 eV. The valence band is doubly degenerate at the Γ-point and for larger k-wavevectors the HOMO band becomes flat with low contribution to charge mobility. The absorption coefficient is roughly isotropic. The conductivity is also isotropic with the nonzero contribution in the energy range 0.3–8 eV. The xy-component of the imaginary part of the dielectric tensor determines the magneto-optical Faraday and Kerr rotation. Nonvanishing rotation is observed in the interval of 0.5–5.0 eV. This material is found to be a ferromagnet of an Ising type with long-range exchange interactions with a very high magnetic moment per unit cell, m = 6 μ_B. The exchange integral is calculated by two independent methods: (a) from the energy difference between ferromagnetic and antiferromagnetic states and (b) from a magnon dispersion curve. In the former case J_nn = 27 μeV. In the latter case the magnon dispersion is fitted by the Ising model with the nearest and next-nearest neighbor spin interactions. From these estimations we find that J_nn = 19.5 μeV and J_nnn = −3 μeV. Despite the different nature of the calculations, the exchange integrals are only within 28% difference.