Magnetism out of antisite disorder in the J=0 compound Ba2YIrO6

Abstract

We systematically investigate the magnetic properties and local structure of Ba2YIrO6 to demonstrate that Y and Ir lattice defects in the form of antiphase boundary or clusters of antisite disorder affect the magnetism observed in this d 4 compound. We compare the magnetic properties and atomic imaging of (1) a slow cooled crystal, (2) a crystal quenched from 900• C after growth, and (3) a crystal grown using a faster cooling rate than the slow cooled one. Atomic imaging by scanning transmission electron microscopy (STEM) shows that quenching from 900• C introduces antiphase boundary to the crystals, and a faster cooling rate during crystal growth leads to clusters of Y and Ir antisite disorder. STEM study suggests the antiphase boundary region is Ir-rich with a composition of Ba2(Y1−xIrx)IrO6. The magnetic measurements show that Ba2YIrO6 crystals with clusters of antisite defects have a larger effective moment and a larger saturation moment than the slow-cooled crystals. Quenched crystals with Ir-rich antiphase boundary shows a slightly suppressed saturation moment than the slow cooled crystals, and this seems to suggest that antiphase boundary is detrimental to the moment formation. Our DFT calculations suggest magnetic condensation is unlikely as the energy to be gained from superexchange is small compared to the spin-orbit gap. However, once Y is replaced by Ir in the antisite disordered region, the picture of local non-magnetic singlets breaks down and magnetism can be induced. This is because of (a) enhanced interactions due to increased overlap of orbitals between sites, and, (b) increased number of orbitals mediating the interactions. Our work highlights the importance of lattice defects in understanding the experimentally observed magnetism in Ba2YIrO6 and other J = 0 systems.

On Jeff=0 Ground State Iridates(V): Tracking Residual Paramagnetism in New Bi2NaIrO6

“…Theoretical explanations for the breakdown of the J =0 magnetic state suffer from similar discrepancies; mechanisms discussed are for example, band structure effects, effects of local symmetry, or excitonic singlet‐triplet excitations . Finally, low levels of spurious unpaired spins and particular lattice perturbations were regarded relevant . For such extrinsic contributions caused by the presence of local electronic defects, for example, in the form of Ir 4+ or Ir 6+ , experimental evidence has been provided, however, on samples prepared by solid‐state synthesis, where it is difficult to avoid minute amounts of by‐phases…”

Octupolar order in d -orbital Mott insulators

“…Recent experimental and theoretical studies on the DP Ba 2 CeIrO 6 , which hosts such a j eff = 1/2 Mott insulator on the FCC lattice, have found evidence of magnetic ordering with a strong frustration parameter, suggesting proximity to a quantum spin liquid state [20,21]. Stepping down to a d 4 configuration, strong SOC favors a total J eff = 0 singlet ground state, with a gap to all excitations [22], which appears to be realized in Ba 2 YIrO 6 [23][24][25][26]. However, if intersite exchange competes with SOC, it can lead to magnetic ordering from exciton condensation [22,27]; clear experimental candidates for such an exciton condensate are yet to be found.…”

“…PACS numbers: 75. 25.aj, 75.40.Gb, 75.70.Tj Multipolar symmetry-breaking orders have been extensively discussed in f -orbital based lanthanide and actinide compounds, which host ions where spin-orbit coupling (SOC) is a dominant energy scale [1]. For instance, the "hidden order" state of URu 2 Si 2 has been proposed to host hexadecapolar symmetry breaking [2].…”

Influence of the multiplet structure on the photoemission spectra of spin-orbit driven Mott insulators: Application to Sr2IrO4

“…In that case, the multiplet structure of d 4 configuration is well described by the LS coupling scheme. This is indeed the assumption made in many of the earlier works, [14][15][16][17] for instance in 18 while deriving the t-J -like model of Sr 2 IrO 4 to calculate the PES spectra. The PES spectra, thus obtained, reproduces the low-energy features of the experimental spectra remarkably well, which is both interesting and intriguing.…”