Study of the magnetic properties of variable spin network formed by organic radical crystals

Programa de Doctorado en Física por la Universidad de Zaragoza.Organic radicals consisting only of light elements have negligibly small spin-orbit coupling and therefore behave as ideal Heisenberg spin systems bearing an S = 1/2 for each unpaired electron. Organic radicals have attracted current interests because of the emergence of novel quantum magnetic phenomena due to the highly isotropic nature of its electronic spin. There are some persistent organic radical species reported which are stabilized by -conjugation. The -conjugated planar molecules easily stack themselves with their planes parallel each other and give one-dimensional structures. Low-dimensional spin systems with small number of nearest neighbors are known to show large quantum fluctuations. As the simplest quantum spin system, the one-dimensional antiferromagnet has been extensively studied both theoretically and experimentally. Recently quantum phenomena in more complicated spin systems have shown a renewed interest. In this study, new organic radicals were designed and synthesized to realize one-, two- and three-dimensional spin network. The author focuses on nitroxide-based materials, which have partial electric charges on the radical unit. Due to the electrostatic interaction, the intermolecular contact between the radical units is realized which results in the formation of the antiferromagnetic interaction. In order to realize high-dimensional spin network, the polyradical approach was applied. The introduction of two or three radical units within a molecule increases the number of intermolecular network path. The author also demonstrated that the large dihedral angle in a -conjugated molecule realized the intermolecular network in multiple directions. The magnetic properties at low temperature and in magnetic field were studied with the aid of molecular orbital and numerical calculations.Peer reviewe

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.Peer reviewe