Low-Temperature Magnetic Properties of LuBaCuFeO5+δ and TmBaCuFeO5+δ

The crystal and magnetic structures of the perovskites LuBaCuFeO5+δ and TmBaCuFeO5+δ have been studied in the temperature range 2–170 K by neutron powder diffraction. The best fit using the Rietveld method is obtained for the acentric tetragonal space group P4 mm, with lattice constants a=3.774(1) Å, c=7.474(2) Å and a=3.7812(2) Å, c=7.4854(4) Å, for LuBaCuFeO5+δ and TmBaCuFeO5+δ, respectively (T=2 K). The lattice parameters of the unit cell of the magnetic structure for these samples are aM=2a and cM=2c, with a magnetic propagation vector k=(12, 12, 12). A commensurate magnetic superstructure is detected by the presence of satellites surrounding the (12, 12, 12) magnetic peak at d∼5.1 Å, determined by cMcomm.=8c. This superstructure could evolve at T>170 K to a distribution of commensurate and even incommensurate superstructures. A transition to a simple antiferromagnetic structure occurs at T2=303(3) K for LuBaCuFeO5+δ. Evidence of a strong spin freezing effect at T1=14.0(5) K is provided through DC-susceptibility measurements.