Interactions in Various Magnetic Compounds

Abstract

Susceptibility measurements on single crystals of KFe11O17 show an anomalous antiferromagnetic behavior, which we propose to call antiferrimagnetism. The behavior follows from Weiss field theory on a simplified (i.e., two-sublattice) model. The magnetic structure of K2NiF4 has been determined by neutron diffraction by Legrand and Plumier at C.E.N., Mol (Belgium). Measurements on their single crystal show that χ∥ and χ⊥ become nearly equal at ≈100°K, i.e., far below the temperature where χ is maximum. This is believed to be due to a gradual breaking up of the long-range order in the basal layers. Results obtained on powders of La0.5Sr1.5MnO4 and La1.5Sr0.5CoO4 are also given. Pure polycrystalline samples of EuS, EuSe, and EuTe with rocksalt structure show θ values of +16, +6, and −7°K; EuTe has a Néel temperature at +11°K. The ferromagnetism of EuO, EuS, and EuSe is due to a predominant direct positive exchange interaction Eu-Eu, the antiferromagnetism of EuTe to a predominant next-nearest neighbor superexchange interaction Eu-X-Eu. Neither interaction is believed to change sign in the series. Measurements of M vs T at various field strengths are in very good agreement with Weiss field theory. Oxidic spinels with diamagnetic ions in A sites have so far shown negative θ values. The substances Ge[Fe2]O4, Ge[Co2]O4, and Ge[Ni2]O4 have θ values of −15, +90, and 0°K, respectively. The materials Ge[Ni2−2δLiδFeδIII]O4 have positive θ values, increasing linearly with δ to 180°K for δ=0.5. Here the Ni2+–Fe3+ interaction must be positive and this interaction is thought to be a direct one, as is discussed.