Variation in particle morphology and Curie temperature of fine ${\mathrm{SrZn}}_2$${\mathrm{Fe}}_{16}$${\mathrm{O}}_{27}$ ceramic powders

Ceramic powders of ${\mathrm{SrZn}}_2$ ${\mathrm{Fe}}_{16}$ ${\mathrm{O}}_{27}$ ferrite exhibit characteristically sharp Curie transition temperatures (${\mathit{T}}_{\mathit{C}}$) in thermomagnetograms, particularly those measured in low magnetic fields of about 2 kOe or lower. The ${\mathit{T}}_{\mathit{C}}$ in this peculiar example is very sensitive to the crystallization temperature ${\mathit{T}}_{\mathit{s}}$. It varied, in a wide 360–450 °C range, by crystallizing the powders at different ${\mathit{T}}_{\mathit{s}}$ between 1100 and 1400 °C, through a solid-state reaction. The powder crystallized at an effectively low ${\mathit{T}}_{\mathit{s}}$∼1100 °C comprises single-domain particles (≤1 μm size) of peculiar uniaxial elongated shapes along the crystallographic c axis, and exhibits a characteristically high ${\mathit{T}}_{\mathit{C}}$ at 440 ° C. The process of the reaction is modified when ${\mathit{T}}_{\mathit{s}}$ is raised up from 1100 to 1200 °C and the particles crystallize as thin platelets (with the c axis lying perpendicular to the platelets), with ${\mathit{T}}_{\mathit{C}}$ lying at much lower temperature ∼361 °C. The platelets experience a sort of superexchange interaction (on which ${\mathit{T}}_{\mathit{C}}$ solely depends) driven by probably a different spin-distribution structure (than those below the surface) being stabilized on the surface.