Magnetic phase diagram of the spin-Peierls compound CuGeO3 doped with Al and Sn

The magnetic phase diagram of the doped inorganic spin-Peierls compound CuGe1-xYxO3 (Y = Sn or Al) is studied in magnetic fields up to B similar to 50 T at temperatures 0.32-20 K. The comparative analysis of the magnetization data and the resonant magnetoabsorption spectra for the frequency range 20-120 GHz shows that the spin-Peierls phase is not destroyed up to x(Sn) = 0.01 and x(Al) = 0.02. The strong effects of doping on the magnetic phase transition between the spin-Peierls (D) and the incommensurate (M) phases are observed. The temperature dependence of the transition field B-DM(T) demonstrates a nonmonotonous behavior with a maximum at T similar to 9 K. Doping by Sn and Al reduces the amplitude of the B-DM(T) maximum by a factor of 2 with respect to the undoped crystal. The magnetic hysteresis width Delta B(T) at T<1 K increases substantially in both the Al- and Sn-doped crystals. The magnitude chi of the magnetic transition along the phase boundary B-DM(T) follows a critical behavior similar to that of the superlattice peak: chi = chi(0)[1-(T/T*)](beta), where T* approximate to 12 K is the temperature of the triple point and beta = 0.75 +/- 0.04. A model of the magnetic hysteresis in the spin-Peierls compounds based on a universal scaling of the magnetic phase diagram and g-factor renormalization in the transition region B similar to B-DM, is suggested. The influence of disorder on the properties of the CuGeO3 crystals is discussed.