Field-induced staggered magnetic moment in the quasi-two-dimensional organic Mott insulator κ-(BEDT-TTF)2Cu[N(CN)2]Cl

We investigated the magnetism under a magnetic field in the quasi-two-dimensional organic Mott insulator kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Cl through magnetization and (13)C-NMR measurements. We found that in the nominally paramagnetic phase (i.e., above Neel temperature) the field-induced local moments have a staggered component perpendicular to the applied field. As a result, the antiferromagnetic transition well defined at a zero field becomes crossover under a finite field. This unconventional behavior is qualitatively reproduced by the molecular-field calculation for Hamiltonian including the exchange, Dzyaloshinsky-Moriya (DM), and Zeeman interactions. This calculation also explains other unconventional magnetic features in kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Cl reported in the literature. The present results highlight the importance of the DM interaction in field-induced magnetism in a nominally paramagnetic phase, especially in low-dimensional spin systems.