Spin-1/2 quantum antiferromagnet on a three-dimensional honeycomb lattice formed by a new organic biradical F4BIPBNN

We have succeeded in synthesizing a new organic biradical F4BIPBNN [= 2,2′-(3,3′,5,5′-tetrafluorobiphenyl-4,4′-diyl)bis(4,4,5,5-tetramethylimidazolin-1-oxyl 3-oxide)] which forms an S = 1=2 Heisenberg three-dimensional honeycomb antiferromagnet. Each site of a honeycomb layer alternately couples with upper or lower layers, which results in the unique three-dimensional honeycomb network with four nearest neighbors. At zero magnetic field, antiferromagnetic long-range order has been observed below TN = 2.7 K. Magnetic susceptibility in both paramagnetic and antiferromagnetic states and the magnetization curves are well reproduced by quantum Monte Carlo calculations with three antiferromagnetic interactions in the range of 4.3 to 6.6 K. From the concave shape of the magnetization curve, the shrinkage of spin due to spin fluctuations is evaluated to approximately 30% with respect to its classical value. The phase diagram of magnetic field versus temperature was determined by heat capacity and magnetization. In the field region below 3 T, a slight increase of TN was observed, which reflects the effect of spin fluctuations.
Dr. A. Arauzo from General Services for Research of University of Zaragoza is acknowledged. JC and FP acknowledge grant number MAT2015-68200-C2-2-P. This work was partly supported by JSPS KAKENHI Grant Number JP15H03695 for YH. This work was performed in part under the Inter-University Cooperative Research Program of the Institute for Materials Research, Tohoku University, and in Institute for Molecular Science, supported by Nanotechnology Platform Program (Molecule and Material Synthesis) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.