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

8-nm $\epsilon -{\mathrm{Fe}}_2{\mathrm{O}}_3$ 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 $\sim 50$ 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 ${\mathrm{Fe}}_4$ tetrahedral sites, suggests strongly that a change in the superexchange pathways is responsible for the spin reorientation.

Figure 1

XRD pattern (red $\circ$) at 20 K (top) and room temperature (bottom) with refinements (solid black line), as described in the text. Residuals and Bragg markers are in blue.

Figure 2

(a) A representative TEM image of the $\epsilon -{\mathrm{Fe}}_2{\mathrm{O}}_3$ nanoparticles where the majority of the silica coating was stripped (postsynthesis, as described in the text) to permit imaging. Representative nanoparticles in the remaining ${\mathrm{SiO}}_2$ shells are identified with red arrows. The inset shows a single nanoparticle with the lattice planes. (b) Distributions of the long and short axis of $\sim 200$ nanoparticles (solid lines are a guide to the eye).

Figure 3

10-K transmission Mössbauer spectrum with the fit (solid green line) and spectral components presented above the data. ${\mathrm{Fe}}_{\mathrm{RO}}$ (blue) and ${\mathrm{Fe}}_{\mathrm{DO}(1\&2)}$ (orange) denote the regular octahedral (${\mathrm{Fe}}_3$) and distorted octahedral (${\mathrm{Fe}}_1$ and ${\mathrm{Fe}}_2$) sites, and ${\mathrm{Fe}}_{\mathrm{T}}$ (red) is the tetrahedral ${\mathrm{Fe}}_4$ site component.

Figure 4

Temperature and frequency (10 Hz $\circ$, 100 Hz $\square$, 500 Hz $\diamond$, and 1000 Hz $▵$) dependence of the in-phase ${\chi }^{\prime }(T,\nu )$ and out-of-phase $[{\chi }^{\prime \prime }\left(T\right)$ at 10 Hz, inset] AC susceptibility of the $\epsilon -{\mathrm{Fe}}_2{\mathrm{O}}_3$ nanoparticles.

Figure 5

Temperature dependence of the zero-field-cooled (ZFC) and field-cooled (FC) DC susceptibility ${\chi }_{\mathrm{DC}}$ in 100 Oe (bottom) and 1 kOe (top) fields. The inset shows the difference between 1 kOe field-cooled and zero-field-cooled susceptibilities, $\mathrm{\Delta }{\chi }_{\mathrm{DC}}$ at temperatures between 200 and 400 K.

Figure 6

Respective field-dependent magnetizations of the $\epsilon -{\mathrm{Fe}}_2{\mathrm{O}}_3$ nanoparticles at (a) 2 K, (b) 100 K, (c) 200 K, and (d) 350 K.

Figure 7

Temperature dependence of the saturation magnetization $M_s\left(T\right)$ for the $\epsilon -{\mathrm{Fe}}_2{\mathrm{O}}_3$ nanoparticles. The solid line is a Bloch $T^{3/2}$-law fit as described in text. The inset shows the temperature dependence of the remanent magnetization ($M_r$) normalized to $M_s$ at that temperature.

Figure 8

Temperature dependence of the $\epsilon -{\mathrm{Fe}}_2{\mathrm{O}}_3$ nanoparticle coercivity ($H_c$, top) and the exchange bias field ($H_{ex}$, bottom).

Figure 9

Selected Mössbauer spectra at (a) 25 K, (b) 125 K, and (c) 300 K, with the green solid line the full component fit and the blue, red, and orange solid lines present the site-assigned spectral decompositions, as discussed in the text.

Figure 10

Temperature dependence of the Mössbauer spectra total relative absorption $f$ (Debye-Waller factor) for the $\epsilon -{\mathrm{Fe}}_2{\mathrm{O}}_3$ nanoparticles [35].

Figure 11

Temperature evolution of the hyperfine parameters determined from fits to the Mössbauer spectra. (a) Isomer shifts $\delta$, (b) hyperfine fields $B_{\mathrm{hf}}$, (c) quadrupolar shifts $\mathrm{\Delta }$, and linewidths $\mathrm{\Gamma }$ of the distorted octahedral ($D_{\text{Oh}}$), regular octahedral ($R_{\text{Oh}}$) and tetrahedral (Td) Fe sites.

Figure 12

Representative XAS (top) and artifact-free XMCD (bottom) spectra at 10 K in 5 T. The red line is the integrated XMCD spectrum, and the energies chosen for $p$ and $q$ are marked, as described in the text. Inset shows the simulated XMCD spectrum (blue line) of the combined $\sim (3{\mathrm{Fe}}_{\mathrm{Oh}}^{3+}+{\mathrm{Fe}}_{\mathrm{Td}}$) components superposed over the data, described in the text.

Figure 13

Temperature dependence of the orbit-to-spin magnetic moment ratios for the $\epsilon -{\mathrm{Fe}}_2{\mathrm{O}}_3$ nanoparticles. The line is a guide to the eye.

Figure 14

Relative AF TEY XMCD absorption amplitude vs temperature of the Fe sites from $L_3$ edge analysis. Inset shows the field dependence of the sites from the $L_3$ edge spectra at 10 K (open symbols) and 150 K (closed symbols).