Magnetic correlation in the ordered phase of CeOs₄Sb ₁₂

We performed neutron diffraction and magnetization measurements to reveal the nature of phase transition of a Kondo semiconductor CeOs₄Sb ₁₂. Weak antiferromagnetic (AFM) reflections characterized by a wave vector q = (1 0 0) were observed, and the ordered moment value is around 0.07μ_B/Ce at zero magnetic field. The AFM structure is rapidly suppressed by magnetic field of 1 T, in contrast to the transition temperature suggested to be enhanced by magnetic field. Only ferromagnetic correlation is observed above 1 T. Such drastic variation of the magnetic correlation is closely connected into the transport properties; the AFM ordered phase below 1 T and the induced ferromagnetic state above 1 T correspond to the semiconducting and metallic states, respectively.