The effect of Coulomb correlation and magnetic ordering on the electronic structure of two hexagonal phases of ferroelectromagnetic YMnO3

The electronic structure of YMnO3 in its high- and low-temperature hexagona l phases has been investigated within the local spin-density approximation (LSDA) and by the LSDA + U method which takes into account the local Coulom b interaction between d electrons of transition-metal ions. In contrast to the case for orthorhombic manganites, the d(4)-configuration degeneracy is already lifted in the high-temperature symmetric hexagonal phase, indicatin g that Mn3+ is not a Jahn-Teller ion; hence, we argue that the lowering of the symmetry is not connected with Jahn-Teller instability in hexagonal YMn O3. Each of these two hexagonal phases is found to be semiconducting, with a band gap of about 1.5 eV. It is shown that magnetism and correlation effe cts are important in band-gap formation for both crystal structures. Using the Green function method, we estimated die Ncel temperature from the calcu lated effective exchange interaction parameters, and found it to be in good agreement with experiment.