科研之友
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摘要
We have synthesized single crystals of Na2IrO3 and studied their structure, transport, magnetic, and thermal properties using powder x-ray diffraction, electrical resistivity, isothermal magnetization M versus magnetic field H, magnetic susceptibility chi 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 chi(T) data show Curie-Weiss behavior at high T > 200 K with an effective moment mu(eff)=1.82(1)mu(B) indicating an effective spin S-eff= 1/2 on the Ir4+ moments. A large Weiss temperature theta = -116(3) K indicates substantial antiferromagnetic interactions between these S-eff= 1/2, Ir4+ moments. Anomalies in chi(T) and C(T) data indicate that Na2IrO3 undergoes a transition into a long-range antiferromagnetically ordered state below T-N= 15 K. The magnetic entropy at T-N is only about 20% of what is expected for S-eff= 1/2 moment ordering. The reduced entropy and the large ratio theta/T-N approximate to 8 suggest geometrical magnetic frustration and/or low-dimensional magnetic interactions in Na2IrO3. In plane resistivity measurements show insulating behavior. This behavior together with the local-moment magnetism indicates that bulk Na2IrO3 is a Mott insulator.
- 出版日期2010-8-12
