Modeling the Magnetic Properties of Both Insulating and Metallic Double Perovskites

Ferrimagnetism in double
perovskites such as Sr₂FeMoO₆ is thought to be closely linked to
delocalization of the minority spin electrons. 
Therefore, it is surprising that Sr₂CrOsO₆ , a semiconductor with localized
electrons, has the highest magnetic ordering temperature of all double
perovskites (TC 
= 720 K).  This is one of many
observations that illustrate shortcomings in our understanding of magnetism
across the entire double perovskite family. 
To address this need we have recently developed a single unifying
framework that can be applied to all double perovskites, metallic or
insulating.

The decoupling of metallic
conductivity and magnetism in Sr₂CrOsO₆ 
results from the Coulomb U on Os
which leads to Mott insulating behavior. Thus superexchange interactions drive
the high TC. Other intriguing behaviors can
also be explained. For example, why is there a net moment at all given that
both Cr and Os are d3
ions,
and naively one would expect their
moments to cancel? Why does the magnetization show such an unusual temperature
dependence (see figure inset). Both features are due to frustration arising
from competition between antiferromagnetic Cr−Os (J1) and Os−Os (J2) superexchange interactions.  Finally we note that unlike Sr₂CrReO₆, spin-orbit  coupling is completely quenched and has no
qualitative effects on the properties.