Antiferromagnetic Mott insulating state in single crystals of the honeycomb lattice materialNa2IrO3

Abstract

We have synthesized single crystals of Na2IrO3 and studied their structure, transport, magnetic, and thermal properties using powder x-ray diffraction (PXRD), electrical resistivity, isothermal magnetization M versus magnetic field H, magnetic susceptibility χ versus temperature T , and heat capacity C versus T measurements. Na2IrO3 crystallizes in the monoclinic C2/c (No. 15) type structure which is made up of Na and NaIr2O6 layers alternately stacked along the c axis. The χ(T ) data show Curie-Weiss behavior at high T > 200 K with an effective moment µ eff = 1.82(1)µB indicating an effective spin S eff = 1/2 on the Ir 4+ moments. A large Weiss temperature θ = −116(3) K indicates substantial antiferromagnetic interactions between these S eff = 1/2, Ir 4+ moments. Sharp anomalies in χ(T ) and C(T ) data indicate that Na2IrO3 undergoes a transition into a long-range antiferromagnetically ordered state below TN = 15 K. The magnetic entropy at TN is only about 20% of what is expected for S eff = 1/2 moment ordering. The reduced entropy and the small ratio TN /θ ≈ 0.13 suggest geometrical magnetic frustration and/or low-dimensional magnetic interactions in Na2IrO3. In plane resistivity measurements show insulating behavior. This together with the local moment magnetism indicates that bulk Na2IrO3 is a Mott insulator.

Crystal Structure, Transport, and Magnetic Properties of an Ir6+ Compound Ba8Al2IrO14

“…The main point of this analysis is that the fitted slope of the data corresponds to the ratio (g ⊥ /g ∥ ) 2 . Ba 8 Al 2 IrO 14 has the anisotropy of (g ⊥ /g ∥ ) 2 = 0.50 (5).…”

“…8−12 In this work we show the ambient-pressure synthesis and physical properties of a new all-Ir 6+ iridate Ba 8 Al 2 IrO 14 . The preparation of all-Ir 6+ iridate without using traditional highpressure techniques has to our knowledge previously only been reported in Nd 2 = 26:4:1 was heated for 2 h at 1250°C and rapidly cooled to room temperature. After the reaction, small black single crystals were found to have a needlelike shape.…”

New material for probing spin-orbit coupling in iridates

“…Here we present a new iridium oxide that due to the characteristics of its structure and its doping flexibility provides straightforward support for the importance of SOC in iridium oxides through measurements of its magnetic susceptibility as its iridium is continuously tuned between 5d 5 Ir 4+ and 5d 4 Ir 5+ . This material has the general formula Sr x La 11-x Ir 4 O 24 and features an array of isolated IrO 6 octahedra.…”

“…Its consequences appear to be manifested in materials as diverse as oxides, superconductors, and topological insulators (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12).…”

“…Our ability to change the value of x between 1 and 5 in Sr x La 11-x Ir 4 O 24 allows for the continuous tuning of the iridium oxidation state from Ir 4+ to Ir 5+ that is critical for proving the point: 5d 5 Ir 4+ in an ideal octahedron will display the behavior of a single unpaired electron in its magnetic susceptibility, regardless of whether SOC is present or not. Conversely, 5d 4 Ir 5+ will either have no unpaired electrons when strong SOC is present, or two unpaired electrons when SOC is absent. This substantial difference allows us to make an assessment of the importance of SOC in iridium oxide octahedra simply through measurements of the magnetic susceptibility, supported by an estimate of the influence of crystal field effects through electronic structure calculations.…”