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-c hain&ladder material Sr6Ca8Cu24O41 All Of these antiferromagnetic spin systems are characterized by a spin-gap between the singlet groundsta te and the triplet first excited states. In the above-mentioned materi als, we confirmed the absence of magnetic order down to milli-Kelvin r egime, supporting the non-magnetic feature of the ground-state. If a s pin-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 ind uces a bulk magnetic order. We report mu SR investigations of doped ma terials as well, which clarifies the existence/absence of a magnetic o rder upon doping.