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Currently there is significant interest in the magnetic properties of 5d-transition metal based oxide systems due to the prominence of spin-orbit coupling in them which appears to drive them towards a Mott insulating state in spite of large covalency and a small on-site Coulomb repulsion energy U. Novel magnetism might be expected in low-dimensional and/or geometrically frustrated versions of such systems. We present our recent bulk and NMR results on three such Ir-based systems. These are: (i) an edge shared triangular system Ba3YIr2O9, (ii) high-pressure cubic phase of Ba3YIr2O9, and (iii) a possible spin-liquid system on a triangular lattice Ba3IrTi2O9.
Ba3YIr2O9 crystallizes in a hexagonal (P63mmc) structure where Ir-Ir structural dimers form an edge shared triangular network. In this structure, Ir occupies a unique crystallographic site and has a fractional (+4.5) oxidation state in a purely ionic picture thereby suggesting metallicity. However, it orders antiferromagnetically at ~4K confirmed by our susceptibility, heat capacity and 89Y NMR measurements. This appears to be a case of spin-orbit driven Mott insulator [1]. We reacted the as prepared Ba3YIr2O9 under high-pressure (8GPa 10000C for 30min) to see the changes in its magnetic properties. After the above treatment, the crystal symmetry changed from hexagonal to cubic (similar to Ref [2]) while the magnetic ordering was completely suppressed. Further, heat capacity measurement evidences the presence of a linear term in C/T vs. T2 in the range 2-9K with γ=8mJ/mol Ir-K2. Susceptibility data fits with a Curie-Weiss law over a large temperature range with θ=0K and Curie constant is reduced to almost 1% of the spin half value. 89Y NMR shift had no T-dependence in the range 4-120K which might be expected when magnetic moments are absent. On the other hand, the 89Y NMR spin-lattice relaxation rate varies linearly with temperature in the range 4-25K (above which it is T-independent) suggesting Korringa-like behaviour. We will discuss the possibility of a transition to spin-liquid behaviour below about 25 K in the cubic phase of Ba3YIr2O9. Ba3IrTi2O9 crystallizes in a hexagonal (P63mc) structure where Ir4+ spins form a 2D corner shared triangular network. However, in our sample there appears to be a large atomic site disorder between Ir4+ and Ti4+ cations. In-spite of this, the system shows no magnetic ordering or spin-glass behaviour down to 2K but has a large θ~-110K (AF) as inferred from our susceptibility data. The Curie term is strongly reduced (almost 1/3 of the spin ½ value). Based on our heat capacity measurements, we infer the presence of a magnetic contribution to the heat capacity which follows a power law at low-temperature. These measurements suggest that Ba3IrTi2O9 is a spin-liquid system. References: [1] B. J. Kim et al., Phys. Rev. Lett. 101, 076402 (2008) [2] J. G. Cheng et al., Phys. Rev. Lett. 107, 197204 (2011) |
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