Magnetization Process of the S = 1/2 Two-Leg Organic Spin-Ladder Compound BIP-BNO

We have measured the magnetization of the organic compound BIP-BNO [3,5′-bis(N-tert-butylaminoxyl)-3′,5-dibromobiphenyl] up to 76 T, at which the magnetization is saturated. The S = 1/2 antiferromagnetic Heisenberg (AFH) two-leg spin-ladder model accounts for the magnetization curve, which is well reproduced by quantum Monte Carlo calculations. The exchange constants on the rung and leg of the ladder are evaluated as Jrung/kB = 65.7 K and Jleg/kB = 14.1 K, respectively, which is in the strong coupling region of Jrung/Jleg > 1. The results provide strong evidence that BIP-BNO is an S = 1/2 AFH two-leg spin ladder.