Search for magnetic order in undoped and doped spin-gap systems by mu SR

We introduce our mu SR investigations of spin-gap systems, such as, (1) a 2-leg spin-ladder material SrCu2O3, (2) a Haldane material (S = 1 spin-chain) Y2BaNiO5, (3) a spin-Peierls material CuGeO3, (4) a spin-chain&ladder material Sr6Ca8Cu24O41 All Of these antiferromagnetic spin systems are characterized by a spin-gap between the singlet groundstate and the triplet first excited states. In the above-mentioned materials, we confirmed the absence of magnetic order down to milli-Kelvin regime, supporting the non-magnetic feature of the ground-state. If a spin-gap system is doped with charges and/or vacancies at the spin site, unpaired spins are induced out of the singlet ground-state. In some materials, doping completely destroys the singlet ground-state and induces a bulk magnetic order. We report mu SR investigations of doped materials as well, which clarifies the existence/absence of a magnetic order upon doping.