Recent experiments reported giant magnetoresistance at room temperature in LaOMnAs. Here a density functional theory calculation is performed to investigate magnetic properties of LaOMnAs. The ground state is found to be the G-type antiferromagnetic order within the $ab$ plane but coupled ferromagnetically between planes, in agreement with recent neutron investigations. The electronic band structures suggest an insulating state which is driven by the particular G-type magnetic order, while a metallic state accompanies the ferromagnetic order. This relation between magnetism and conductance may be helpful to qualitatively understand the giant magnetoresistance effects.
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