Orbital ordering and quasi-two-dimensional magnetism in AMnF4 (A = K, Rb): A first-principles study

Strongly correlated systems with the interplay of electronic, charge, spin, and orbital degrees of freedom have, in recent times, received a surge of interest because of their rich physics, novel physical properties, and potential applications. In the present work, we study the structural, magnetic, and electronic properties and orbital ordering in layered perovskite-type AMnF(4) (A=K, Rb) from first principles. A detailed analysis of the electronic properties in both compounds reveals an interesting nodal-line-like dispersion at similar to 0.4 eV below the Fermi level at the hinges of the Brillouin zone. Magnetic properties reflect the quasi-two-dimensional magnetism in these compounds, with very weak exchange interaction between the layers. Our results report the robust in-plane ferromagnetic spin order in AMnF(4) (A=K, Rb) with the critical temperatures estimated to be around 30-60 K. We also find an anti-ferro-orbital ordering within the ab plane and a ferro-orbital ordering out of plane favoring the C-type orbital order in these compounds.