Investigation of the role of 3d transition metal atoms (M=Ti–Ni) in a [formula omitted] matrix by first principles electronic structure calculations

Abstract

We report here the ground state electronic properties of the Y 3 Al 2 M 3 O 12 (M=Ti–Ni), where M atoms are substituted in the tetrahedral sites of YAG garnet, Y 3 Cr 2 Al 3 O 12 , where the Cr atoms are substituted in the octahedral sites and Y 3 Fe 5 O 12 (YIG garnet) systems using the tight-binding linearised muffin-tin orbital (TB-LMTO) method. We have made the calculations for the paramagnetic and ferromagnetic phases with the experimental lattice constant of YAG. For YIG, we have done the same and also with the experimental lattice constant of YIG. Total energy calculations clearly favour, for all the systems under investigation, the ferromagnetic ground state phase (high spin configuration), except the M=Ti case whose paramagnetic phase has lower energy. Interestingly for YIG, a ferrimagnetic phase is observed as the Fe atoms in the octahedral and tetrahedral positions have opposite magnetic ordering. This is in very good agreement with the experimental observations, even though our calculated local magnetic moment for the octahedral Fe sites and therefore the total magnetic moment per molecule deviates largely. Our calculations show all these systems to be metallic, except Y 3 Cr 2 Al 3 O 12 and Y 3 Al 2 Fe 3 O 12 , which are shown to be a semi-metal and a semiconductor, respectively. The local magnetic moment calculated for the M atoms in Y 3 Al 2 M 3 O 12 are consistent with Hund rules. The magnetic moment of Cr atom with octahedral position agrees well with that in LaCrO 3 , in which the Cr atom has the similar octahedral coordination. The calculated cohesive energy ( E coh ) per molecule decreases continuously from Ti–Ni in the Y 3 Al 2 M 3 O 12 system, this trend is similar to that of the LaMO 3 , with M=Sc–Ni.