Temperature and field evolution of site-dependent magnetism in $\epsilon$-Fe$_2$O$_3$ nanoparticles

Abstract

8~nm epsilon-Fe2O3 nanoparticles exhibit a spin reorientation transition that begins at 150 K which is a hallmark of this unique iron-oxide polymorph. We find that the change from the high to low temperature magnetic structures has been suppressed by ~50 K. At the spin reorientation temperature, a change of the field-dependent response of the tetrahedral sites in intermediate field strengths (0.25 - 1.5 T) indicates that a collective tetrahedral distortion occurs to which the octahedral sites adjust, altering the magnetic anisotropy. An abrupt step in the hyperfine parameters' temperature dependencies, especially at 125 K for the hyperfine field associated with the Fe4 tetrahedral sites, suggests strongly a change in the superexchange pathways are responsible for the spin reorientation.