Magnetic studies of nonstoichiometric FezO and evidence for magnetic defect clusters

Abstract

Measurements of the static magnetic susceptibility chi have been carried out on polycrystalline, Fe3O4-free samples of FezO for zI=0.929, zII=0.938 and ZIII=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 TN=199.2, 195.7 and 192.4K for zI, ZII and zIII respectively. It is argued that the variation of TN with z is due to change in J2 (NNN exchange) with lattice constant a which equals 4.303, 4.306 and 4.310 AA for zI, zII and zIII respectively. The observed ratio of chi (0)/ chi (TN) approximately=0.70, suggesting that chi /sub ///(0) approximately=0. The values for JI (NN exchange) and J2 are evaluated from the data using the molecular-field approximation. For FC samples through TN, 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 TN and that chi f varies as (1-z)2 below TN. Neutron diffraction studies in FC samples of FezO are recommended to further understand the defect structure.