  The influence of spin-orbit coupling (SOC) on the physical properties of the
5d2 system Sr2MgOsO6 is probed via a combination of magnetometry, specific heat
measurements, elastic and inelastic neutron scattering, and density functional
theory calculations. Although a significant degree of frustration is expected,
we find that Sr2MgOsO6 orders in a type I antiferromagnetic structure at the
remarkably high temperature of 108 K. The measurements presented allow for the
first accurate quantification of the size of the magnetic moment in a 5d2
system of 0.60(2) muB - a significantly reduced moment from the expected value
for such a system. Furthermore, significant anisotropy is identified via a spin
excitation gap, and we confirm by first principles calculations that SOC not
only provides the magnetocrystalline anisotropy, but also plays a crucial role
in determining both the ground state magnetic order and the size of the local
moment in this compound. Through comparison to Sr2ScOsO6, it is demonstrated
that SOC-induced anisotropy has the ability to relieve frustration in 5d2
systems relative to their 5d3 counterparts, providing an explanation of the
high TN found in Sr2MgOsO6.
