Magnetic Properties of Fe<i>x</i>O as Related to the Defect Structure

The magnetic properties of FexO with long-range order of defects was studied. The crystal structure appears to consist of periodically spaced complexes of vacant octahedral and occupied tetrahedral sites, with most of the octahedral ions in an ideal NaCl-type environment. The Néel temperature, determined from measurements of susceptibility and modulus, increases very slightly as x is decreased. Octahedral-tetrahedral ion exchange interactions are proposed to account for the increase in TN. The molar susceptibility obeys a Curie-Weiss law, χ = 3.56/(T+136). With θ = 136°K and TN about 200°K, θ/TN is less than one. According to a treatment by Van Vleck one expects θ/TN greater than one for negative first-near-neighbor and second-near-neighbor interactions. A positive first-near-neighbor interaction may possibly arise through double exchange between Fe2+ and Fe3+.

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