Effect of contacts between $\gamma$-Fe$_2$O$_3$ nanoparticles on their phase-transition temperature

The effect of mechanical contacts between $\gamma$-Fe$_2$O$_3$ particles on the temperature of the $\gamma$–$\alpha$ structural transition in them is established by magnetic studies and differential thermal analysis (DTA). The sample in which $\gamma$-Fe$_2$O$_3$ particles had no mechanical contacts with one another remained in the ferromagnetic state up to $T_{\mathrm{C}}$ = 630$^\circ$C and had two exothermal DTA peaks. The first peak almost coincided with the Curie temperature, while the second peak attributed to the $\gamma\to\alpha$ structural transition corresponded to 760$^\circ$C. The magnetic transition for particles with a larger number of contacts was shadowed by the $\gamma\to\alpha$ structural transition with a temperature lowered to 550$^\circ$C.