Ground state of the geometrically frustrated system Y(Sc)Mn2 studied by muon spin relaxation

We performed positive muon spin relaxation measurements of the geometrically frustrated system Y0.97Sc0.03(Mn1-xAlx)2. A spin freezing with a static magnetic field of 0.05 T was found at 2.5 K in the x=0 compound while the x=0.1 compound freezes at 45 K with the internal field of 0.17 T. In the x=0.02 compound, the relaxation rate shows a marked thermal hysteresis; the maximum rates occur at 10 K on cooling and at 20 K on heating, whereas the sudden changes in the initial asymmetries occur at 40 K in both processes. The results for the x=0 compound are interpreted as a freezing of minority paramagnetic Mn moments induced by imperfections such as Sc and/or muon charge while majority Mn moments form singlet tetrahedra.