Spin and orbital contributions to magnetically ordered moments in 5d layered perovskite Sr2IrO4.

The ratio of orbital (L) and spin (S) contributions to the magnetically ordered moments of a 5d transition metal oxide, Sr2IrO4 was evaluated by non-resonant magnetic x-ray diffraction. We applied a new experimental setting to minimize the error in which we varied only the linearpolarization of incident x-ray at a fixed scattering angle. Strong polarization dependence of the intensity of magnetic diffraction was observed, from which we conclude that the ordered moments contain substantial contribution from the orbital degree of freedom with the ratio of hLi/hSi ∼5.0, evidencing the pronounce effect of spin-orbit coupling. The obtained ratio is close to but slightly larger than the expected value for the ideal Jeff = 1/2 moment of a spin-orbital Mott insulator, |hJ1/2|Lz|J1/2i|/|hJ1/2|Sz|J1/2i| = 4, which cannot be accounted by the redistribution of orbital components within the t2g bands associated with the elongation of the IrO6 octahedra. Magnetic moments of 3d transition metal complexes predominantly originate from the spins of the d electrons rather than the orbital momenta because of the quenching of the orbital momenta by the crystal fields. The orbital contribution is treated as a perturbation to the spin, the effect of which is renormalized to the g factor of electrons resulting in a magnetic anisotropy. In heavier elements like 5d transition metals, however, the spinorbit coupling (SOC) is pronounced and drastic change of the electronic states is anticipated. An unconventional Mott state was recently found in a layered perovskite with Ir 4+ (5d 5 ), Sr2IrO4, where a half-filled band of Jeff = 1/2 (Jeff = S + Leff = S − L ) is formed by a strong SOC and a moderate Coulomb repulsion opens a charge gap within the Jeff = 1/2 band [1, 2]. Experimental supports for the Jeff = 1/2 character of the ordered moments were given by the measurements of XAS and resonant magnetic x-ray scattering (RXS) [2, 3]. The strong suppression of the magnetic x-ray scattering at the LII edge (2p3/2 → 5d) indicated that the wave function of electrons in charge of magnetism should be reasonably close to that of Jeff = 1/2 state. The Jeff = 1/2 character of the ordered moments was also identified in other complex iridium oxides [4–7]. In reality, even in the prototypical spin-orbital Mott insulator Sr2IrO4, there should be certain deviation from the ideal Jeff = 1/2 state. Factors to destabilize the ionic Jeff = 1/2 band could be the followings. 1. The refinement of the crystal structure of Sr2IrO4