Investigation of the role of 3d transition metal atoms (M=Ti-Ni) in a Y3Al5O12 matrix by first principles electronic structure calculations

We report here the ground state electronic properties of the Y3Al2M3O12 (M=Ti-Ni), where M atoms are substituted in the tetrahedral sites of YAG garnet, Y3Cr2Al3O12, where the Cr atoms are substituted in the octahedral sites and Y3Fe5O12 (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 Y3Cr2Al3O12 and Y3Al2Fe3O12, which are shown to be a semi-metal and a semiconductor, respectively. The local magnetic moment calculated for the M atoms in Y3Al2M3O12 are consistent with Hund rules. The magnetic moment of Cr atom with octahedral position agrees well with that in LaCrO3, in which the Cr atom has the similar octahedral coordination. The calculated cohesive energy (Ecoh) per molecule decreases continuously from Ti-Ni in the Y3Al2M3O12 system, this trend is similar to that of the LaMO3, with M=Sc-Ni.