Effects of oxygen vacancies on magnetic properties of Ca-substituted yttrium iron garnet (abstract)

Abstract

Ca-substituted YIG samples, Y3−xCaxFe5O12−0.5x−z for 0.14≤x≤0.23 were annealed for 6–8 h at various temperatures between 600 °C and 1000 °C in N2 atmosphere. The magnetization barely shows any change by annealing under 900 °C but by annealing between 900 °C and 1000 °C a significant change over the temperature range from 4.2 K to the Curie temperature is observed. The lattice constant is also observed to change. The magnetization and the lattice constant of the samples did not change within experimental error by annealing in O2 atmosphere at 1000 °C. For the sample with x=0.23, annealing at 1000 °C in N2 resulted in a decrease of 4πM at 4.2 K by 560 G and a decrease of the lattice constant by 0.031 Å. The Curie temperature also changes from 531 K in the as-grown sample to 508 K for this annealing condition. The change in 4πM for various annealing conditions is linear with the change in the lattice constant. For the samples tested, the oxygen vacancies, z, in the as-grown state shows z≳0 and that z did not change by annealing in O2 atmosphere. A microscopic molecular field model based on the Heisenberg Hamiltonian in which a singly charged oxygen vacancy, VȮ, mediates a ferromagnetic superexchange between the iron ions at adjacent sites is proposed. The model explains the temperature dependence of 4πM, the magnitude of superexchange constant mediated by VȮ and the variation of the Curie temperature with annealing conditions. This model also explains the positive slope of 4πM at low temperature for Ca-substituted YIG.