Magnetic properties of rare-earth-doped La_0.7Sr_0.3MnO₃

Rare-earth-doped ferromagnetic manganites La_0.63RE_0.07Sr_0.30MnO₃ (RE  =  Gd, Tb, Dy, and Ho) are synthesized in the form of sintered ceramics and nanocrystalline phases with the mean size of crystallites  ≈30 nm. The electronic states of the dopants are investigated by SQUID magnetometry and theoretically interpreted based on the calculations of the crystal field splitting of rare-earth energy levels. The samples show the orthorhombic perovskite structure of Ibmm symmetry, with a complete FM order of Mn spins in bulk and reduced order in nanoparticles. Non-zero moments are also detected at the perovskite A sites, which can be attributed to magnetic polarization of the rare-earth dopants. The measurements in external field up to 70 kOe show a standard Curie-type contribution of the spin-only moments of Gd³⁺ ions, whereas Kramers ions Dy³⁺ and non-Kramers ions Ho³⁺ contribute by Ising moments due to their doublet ground states. The behaviour of non-Kramers ions Tb³⁺ is anomalous, pointing to singlet ground state with giant Van Vleck paramagnetism. The Tb³⁺ doping leads also to a notably increased coercivity compared to other La_0.63RE_0.07Sr_0.30MnO₃ systems.