Nuclear forward scattering application to the spiral magnetic structure study in ε−Fe2O3

Abstract

The $\ensuremath{\epsilon}\ensuremath{-}{\text{Fe}}_{2}{\text{O}}_{3}$ magnetic structure has been analyzed using the synchrotron radiation source. Time spectra of nuclear forward scattering for isolated nanoparticles with an average size of 8 nm immobilized in a xerogel matrix have been recorded in the temperature range of $4--300\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ in applied magnetic fields of $0--4\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ in the longitudinal direction at the European Synchrotron Radiation Facility (ESRF, Grenoble, France). It has been found that the external magnetic field does not qualitatively change the ${H}_{\mathrm{hf}}(\mathrm{T})$ behavior, but makes a strong opposite impact on the hyperfine fields in the nonequivalent iron sites, leading to the divergence of ${H}_{\mathrm{hf}}$ polar angle dependences below 80 K. A complete diagram of the $\ensuremath{\epsilon}\ensuremath{-}{\text{Fe}}_{2}{\text{O}}_{3}$ magnetic structure in the temperature range of $4--300\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ is proposed. At 300 K, the $\ensuremath{\epsilon}\ensuremath{-}{\text{Fe}}_{2}{\text{O}}_{3}$ compound is confirmed to be a collinear ferrimagnet. The experimental results show that the magnetic transition at $150--80\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ leads to the formation of a noncollinear magnetic structure. Furthermore, in the range of the 80--4 K, the ground state of a magnetic spiral is established. The experimental results are supplemented by the analysis of the exchange interactions and temperature dependence of the magnetization in a magnetic field of 7 T.