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 CuGe_1-xY_xO₃ (Y=Sn or Al) is studied in magnetic fields up to B∼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∼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 ΔB(T) at T<1 K increases substantially in both the Al- and Sn-doped crystals. The magnitude χ of the magnetic transition along the phase boundary B_DM(T) follows a critical behavior similar to that of the superlattice peak: χ=χ₀[1-(T/T^*)]^β, where T^*≈12 K is the temperature of the triple point and β=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∼B_DM is suggested. The influence of disorder on the properties of the CuGeO₃ crystals is discussed.