Exchange Interactions in Double Perovskite and Related Transition Metal Oxides*

The double perovskite structure type is extremely flexible and accommodates a large fraction of the elements in the periodic table. Of particular interest are double perovskites containing magnetic 4d or 5d transition metal elements. Such materials exhibit novel phenomena including high ferrimagnetic transition temperatures in conjunction with half metallic or insulating behavior. One of the key remaining challenges is to accurately determine effective spin Hamiltonians to achieve a deeper understanding of this class of materials. This talk will focus on our efforts to determine the effective spin Hamiltonians with inelastic neutron scatting in materials including Sr₂ScOsO₆, Sr₂FeOsO₆, Sr₂MgOsO₆, Ca₃LiOsO₆, Ca₃LiRuO₆, and Ba₂LaRuO₆. Interestingly, all of the Osmates in the above list exhibit significant gaps in the magnetic excitation spectrum, despite being nominally in the 5d³ electronic configuration. This provides an indication of strong spin-orbit effects that would not normally be expected for a 5d³ half-filled t_2g multiplet. We will also discuss what can be learned about the nature of the exchange interactions from inelastic neutron scatting on polycrystalline samples and how such experimental studies can be used to constrain models for effective spin Hamiltonians in double perovskites and related transition metal oxides.