Drastic anion substitution effect in deformed pyrochlore lattice Co2(OH)3Cl1-xBrx

As a part of an extensive study of the new geometric frustration series M-2(OH)(3)X (M: 3d magnetic ion; X: halogen ion) on deformed pyrochlore lattice we have investigated in detail the magnetism of Co-2(OH)(3)Cl1-xBrx, in which the magnetic state was found to be very sensitive to the anion substitution. Co-2(OH)(3)Br was found to undergo anti ferromagnetic transitions at T-N1 = 6.2 K and T-N2 = 4.8 K, respectively, which are contrasting as compared to the partial ferromagnetic state in Co-2(OH)(3)Cl. Muon spin precession was observed with additional fast relaxation below T-N1, implying coexisting spin-fluctuation. Meanwhile, the partially substituted Co-2(OH)(3)Cl1-xBrx transforms from ferromagnetic to anti ferromagnetic by increasing the x ratio. The present work presents a unique system to investigate the critical role of the tetrahedral, distortion in geometric frustration. (C) 2008 Elsevier B.V. All rights reserved.