LaSrMn0.5Ni0.5O4+δ: spin and oxidation state in the K2NiF4 structure

Abstract

Magnetic and structural data on the K₂NiF₄ structure oxide LaSrMn_0.5Ni_0.5O_4+δare reported. Powder X-ray and electron diffraction show that the samples adopt the tetragonalI4/mmm space group without cation ordering. The absence of cation ordering together with structural considerations and magnetic data indicate that both cations are in the +III oxidation state, with high spin Ni^III in contrast to both the three-dimensional perovskite LaMn_0.5Ni_0.5O₃ and the K₂NiF₄ phase LaSrCr_0.5Ni_0.5O₄ . The spin and oxidation state distributions account for the suppression of the antiferromagnetic long-range order of LaSrMnO₄ and the observation of hysteresis characteristic of spin- or cluster glass behaviour below 90 K. The irreversible magnetism, but not that in the high-temperature Curie–Weiss region, is slightly altered by annealing single phase samples. The higher divergence temperature in the annealed sample is consistent with greater homogeneity of the cation distribution, as found for canonical RKKY spin glasses.