Spin-orbit coupling control of anisotropy, ground state and frustration in 5d(2) Sr(2)MgOsO(6)

The influence of spin-orbit coupling (SOC) on the physical properties of the 5d(2) system Sr(2)MgOsO(6) 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 Sr(2)MgOsO(6) 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 5d(2) system of 0.60(2) μ(B )–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 Sr(2)ScOsO(6), it is demonstrated that SOC-induced anisotropy has the ability to relieve frustration in 5d(2) systems relative to their 5d(3) counterparts, providing an explanation of the high T(N) found in Sr(2)MgOsO(6).