Magnetic studies of nonstoichiometric FezO and evidence for magnetic defect clusters

Measurements of the static magnetic susceptibility chi have been carried out on polycrystalline, Fe 3 O 4 -free samples of Fe z O for z I =0.929, z II =0.938 and Z III =0.950, in the temperature range of 4.2-320K. The data were taken for both FC (field-cooled in 8 kOe) and ZFC (zero-field cooled) conditions. For ZFC samples, the chi measured with increasing temperatures from 4.2K in 200 Oe showed antiferromagnetic ordering with T N =199.2, 195.7 and 192.4K for z I , Z II and z III respectively. It is argued that the variation of T N with z is due to change in J 2 (NNN exchange) with lattice constant a which equals 4.303, 4.306 and 4.310 AA for z I , z II and z III respectively. The observed ratio of chi (0)/ chi (T N ) approximately=0.70, suggesting that chi /sub ///(0) approximately=0. The values for J I (NN exchange) and J 2 are evaluated from the data using the molecular-field approximation. For FC samples through T N , a field-enhanced excess susceptibility chi f is observed, whose presence is interpreted to be due to the polarisation of the defect clusters present as a result of non-stoichiometry. Some support for the model of the defect clusters proposed by Battle and Cheetham (1979) is obtained. The temperature dependence of chi f shows that the defects order at T N and that chi f varies as (1-z) 2 below T N . Neutron diffraction studies in FC samples of Fe z O are recommended to further understand the defect structure.