Order−Disorder Phenomena in New LaBaMn₂O_6-x CMR Perovskites. Crystal and Magnetic Structure

Abstract

Three new perovskites in the LaBaMn 2 O 6-x family have been synthesized by controlling the oxygen pressure, during both synthesis and postannealing. Structural determination from powder neutron diffraction (PND) data shows that one form of LaBaMn 2 O 6 is cubic (a ) 3.906 Å), with a disordered distribution of La 3+ and Ba 2+ cations, whereas a second form of LaBaMn 2 O 6 is tetragonal (a ) 3.916 Å; c ) 7.805 Å), with an alternate stacking of lanthanum and barium layers along c. The same La/Ba cation order is observed for the ordered, oxygen-deficient perovskite LaBaMn 2 O 5 , which is also tetragonal (a ) 5.650 Å; c ) 7.808 Å) and adopts a YBaCuFeO 5 -related structure. Elucidation of the magnetic structure of LaBaMn 2 O 5 , from low-temperature PND data, leads to a G-type antiferromagnetic model; the superimposed Mn 2+ /Mn 3+ charge order results in ferrimagnetic behavior for this phase and explains its magnetic properties, as obtained from susceptibility measurements. In both forms of LaBaMn 2 O 6 , the PND data show a ferromagnetic contribution. The CMR properties of the "O 6 " forms exhibit a remarkable feature: T C is increased from 270 K for the disordered phase to 335 K for the ordered one, probably owing to the La/Ba ordering.

Effect of oxygen content on magnetization and magnetoresistance properties of CMR manganites

“…3). Surprisingly, the Curie points are much lower than that of La 0.5 Ba 0.5 MnO 3 (T C = 340 K) [14]. The relatively low magnetic moment may be attributed to a canted magnetic state or to a mixture of two magnetic phases.…”

“…However, it is well known that they do not exhibit antiferromagnet-ferromagnet and charge order-disorder transitions. It was found that La 0.5 Ba 0.5 MnO 3 is a ferromagnet with T C ≈ 340 K [14]. The Pr 0.5 Ba 0.5 MnO 3 and Nd 0.5 Ba 0.5 MnO 3 have Curie points of 125 K [15] and 120 K [16], respectively.…”

Colossal Magnetoresistance and Quenched Disorder in Manganese Oxides

“…Millange et al demonstrated that it is possible to synthesize mixed valence compounds A 0.5 A ′ 0.5 MnO 3 in which the A 3+ and A ′2+ cations order periodically in lattice structure. 23 These special compounds, which are called the A-site ordered manganites, exhibit distinct properties from the materials with random configuration of A-site cations. We can compare the 'clean' systems (A-site ordered) and the 'dirty' systems (A-site disordered) both of which have the same chemical formula.…”

Redox characteristics variations in the cation-ordered perovskite oxides BaLnMn2O5+δ(Ln = Y, Gd, Nd, and La) and Ca2Al1−xGaxMnO5+δ(0 ≤ x ≤ 1)

“…For BaYMn 2 O 5+δ , substitutions of the lanthanoid (Ln) series at the yttrium site were extensively investigated, although the interests in previous works were mainly devoted to the electronic properties. [14][15][16][17][18] Recent reports by Swierczek and coworkers evidenced [19][20][21] that…”