Magnetic exchange interactions and antiferromagnetism of ATcO3 (A = Ca, Sr, Ba) studied from first principles

The magnetic properties of ATcO(3) (A = Ca, Sr, Ba) perovskites were studied from first principles using multiple-scattering theory. For each system, the most preferable magnetic order is antiferromagnetic where the magnetic moments of the nearest Tc atoms are aligned antiparallel to each other. In the case of CaTcO3 and SrTcO3, their recently observed antiferromagnetism is supported by our calculation of the Tc-Tc exchange coupling parameters. The Neel temperatures T-N estimated for CaTcO3 and SrTcO3 are in a good agreement with experiment. For BaTcO3, which is not synthesized so far, we predict a fundamental band gap of 0.3 eV and T-N below 900 K. In general, T-N increases with volume expansion, whereas the presence of vacancies on both the oxygen and cation sites of ATcO(3) should significantly suppress the critical temperature as well as the magnetic moment of Tc. Electronic correlations play a minor role in the formation of the magnetic state, although they may improve the calculated value of T-N.