Orbital ordering in the two - dimensional ferromagnetic semi - conductor

PACS. 71.20.-b – Electron density of states and band structure of crystalline solids. PACS. 71.20.Be – Transition metals and alloys. Abstract. – We present the results of electronic structure calculations for the two-dimensional ferromagnet Rb2CrCl4. They are obtained by the augmented spherical wave method as based on density functional theory and the local density approximation. In agreement with experimental data Rb2CrCl4 is found to be semiconducting and displays long-range ferromagnetic order of the localized Cr 3d moments. The magnetic properties are almost independent of the structural modifications arising from the Jahn-Teller instability, which leads from the parent body-centered tetragonal K2NiF4 structure to a side-centered orthorhombic lattice. In contrast, our calculations give evidence for a strong response of the optical band gap to the corresponding structural changes. Introduction. – Since the discovery of CrBr 3 as a ferromagnetic semiconductor by Tsub-okawa [1] this narrow class of materials has attracted increasing interest. To some part this is for fundamental reasons, since these compounds serve as ideal candidates for the Heisen-berg model. However, the prospect of using ferromagnetic semiconductors in spin electronics has motivated an even greater amount of research activities in the last years. Only recently, La 2 MnNiO 6 has been shown to exhibit long-range ferromagnetic order below T C ≈ 280 K with a magnetization of 5 µ B per formula unit [2] and an optical band gap of about 0.6 eV as determined by electronic structure calculations [3]. Rubidium tetrachlorochromide, Rb 2 CrCl 4 , also belongs to this class of non-metallic fer-romagnets albeit with a much lower ordering temperature of T C = 52.4 K [4, 5]. According to neutron scattering measurements of the spin-wave dispersion this compound behaves as a two-dimensional, S = 2, easy-plane ferromagnet with an in-plane exchange constant of J = 7.7 K [6]. Since the interlayer coupling was estimated to be about four orders of magnitude smaller than the intralayer interaction, three-dimensional long-range order was also attributed to the weak uniaxial anisotropy. The latter, in addition, leads to a small energy gap of about 0.1 meV in the spin wave dispersion at the center of the Brillouin zone, which precludes true XY-type behaviour [5, 7]. Furthermore, the anisotropy causes a canting of the ordered spins of 5.2 • away from the 110 direction [7]. From susceptibility measurements and neutron scattering investigations of the static critical properties a crossover from an ideal