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We report muon spin relaxation measurements of the doped spin-Peierls system $({\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Zn}}_{x})\ensuremath{-}({\mathrm{Ge}}_{1\ensuremath{-}y}{\mathrm{Si}}_{y}){\mathrm{O}}_{3}$. Spontaneous muon spin precession in zero applied field was observed, confirming the presence of antiferromagnetic order in this series of compounds. In contrast to usual antiferromagnets, muon spin precession is accompanied by a relaxation signal indicating a large spatial inhomogeneity of the ordered moment size. Assuming an exponential decay of the moment size away from the doping centers, we estimated a decay length of $\ensuremath{\xi}\ensuremath{\sim}10$ lattice units along the chain. We suggest that both Zn and Si doping induces the same maximum moment size around the doping center.

The magnetic susceptibilities of polycrystalline ${\mathrm{Cu}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Zn}}_{\mathit{x}}$${\mathrm{GeO}}_{3}$ were measured to study the effects of nonmagnetic impurities on the spin-Peierls transition. The transition temperature drastically reduces upon Zn doping and the spin-Peierls state collapses around x=0.03. The Zn doping causes another phase transition in the samples with 0.02\ensuremath{\le}x\ensuremath{\le}0.08, which proved to be spin-glass-like. The present Letter provides the first study of the spin-Peierls system with impurities.

Besides being an ancient pigment, BaCuSi2O6 is a quasi-2D magnetic insulator with a gapped spin dimer ground state. The application of strong magnetic fields closes this gap, creating a gas of bosonic spin triplet excitations. The topology of the spin lattice makes BaCuSi2O6 an ideal candidate for studying the Bose-Einstein condensation of triplet excitations as a function of the external magnetic field, which acts as a chemical potential. In agreement with quantum Monte Carlo numerical simulations, we observe a distinct lambda anomaly in the specific heat together with a maximum in the magnetic susceptibility upon cooling down to liquid helium temperatures.