A model for doping-dependent magnetic frustration and ordering in a triangular lattice
1 226, Bagalur, Bangalore North Taluk, Karnataka - 562149, India
2 RIKEN SPring-8 Centre - Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan
Received: 15 June 2013
Accepted: 18 October 2013
The family of cobaltates AxCoO2 (A = Na, Li, K), with a crystal structure consisting of a layered triangular lattice of Co ions, exhibits a fascinating range of coupled spin, charge and ion ordering phenomena as a function of electron doping content x. Based on these experimental observations, we investigate an extended Ising model for suppression of magnetic frustration caused by electron doping in a quasi–2-dimensional nearest-neighbour antiferromagnetic triangular lattice. As determined by geometry and bond-counting, it is found that magnetic frustration can be quantified by a frustration index fub and its magnitude is a non-monotonic function of x. A mean-field calculation is carried out to determine the temperature-dependent magnetization, spin-entropy and heat capacity for the obtained structures with the lowest energy (≡ lowest fub) for each x. The calculations reveal that the magnetic-ordering temperature TC exhibits an anti-correlation with fub as a function of x. In addition, as seen in thermopower experiments, an anomalous spin-entropy is obtained for the high-doping cases when . The results indicate the importance of doping-dependent magnetic frustration in the layered cobaltates.
PACS: 75.10.Hk – Classical spin models / 75.10.Jm – Quantized spin models, including quantum spin frustration / 65.40.-b – Thermal properties of crystalline solids
© EPLA, 2013