Spin reorientation in $\alpha$-Fe${}_2$O${}_3$ nanoparticles induced by interparticle exchange interactions in $\alpha$-Fe${}_2$O${}_3$/NiO nanocomposites

We report that the spin structure of $\alpha$-Fe${}_2$O${}_3$ nanoparticles rotates coherently out of the basal (001) plane at low temperatures when interacting with thin plate-shaped NiO nanoparticles. The observed spin reorientation (up to ∼70${}^{\circ }$) in $\alpha$-Fe${}_2$O${}_3$ nanoparticles has, in appearance, similarities to the Morin transition in bulk $\alpha$-Fe${}_2$O${}_3$, but its origin is different—it is caused by exchange coupling between aggregated nanoparticles of $\alpha$-Fe${}_2$O${}_3$ and NiO with different directions of easy axes of magnetization.

Figure 1

Rietveld refined x-ray diffraction data of (a) 8-nm $\alpha$-Fe${}_2$O${}_3$ particles, (b) NiO nanoparticles, and (c) $\alpha$-Fe${}_2$O${}_3$/NiO nanoparticle composite. The refinement of $\alpha$-Fe${}_2$O${}_3$ is shown in blue/medium gray, that of NiO in red/dark gray, and the total refinement (c) in black.

Figure 2

Mössbauer spectra of 8-nm $\alpha$-Fe${}_2$O${}_3$ nanoparticles obtained at the indicated temperatures; (a) ferrofluid sample, (b) powder sample, and (c) mixed with NiO nanoparticles. The magenta/light gray numbers in panel c show how the lines are numbered. The solid (magenta/light gray and blue/dark gray) lines in panel b, at 80 K, and in panel c, at 20 K and 80 K, represent fits to the data by two sextets (sextets 1 and 2); the green/gray solid line is the sum of the fit components. The data in panel a at 25 K are fitted with one sextet and a doublet (orange/light gray), due to particles with fast superparamagnetic relaxation. The data in panel b obtained at 20 K are fitted with one sextet. The fit results for all the sextet components in panels a–c are summarized in Table .

Figure 3

The quadrupole shift of the two sextet components (sextet 1 (filled circles) and sextet 2 (open squares)) in the Mössbauer spectra of the $\alpha$-Fe${}_2$O${}_3$/NiO nanoparticles as a function of temperature.

Figure 4

Neutron powder diffraction data obtained at 20, 80, and 300 K of (a) 8-nm $\alpha$-Fe${}_2$O${}_3$ particles and (b) $\alpha$-Fe${}_2$O${}_3$/NiO nanoparticles. The solid lines are fits to data with Lorentzian lines and a background with constant slope.

Figure 5

The integrated intensities of the magnetic reflections [(003), filled circles, and (101), open circles] of $\alpha$-Fe${}_2$O${}_3$ nanoparticles mixed with NiO particles as a function of temperature. The integrated intensities are given relative to that of the structural (104) reflection. The cyan/medium gray and red/dark gray arrows to the right indicate the mean integrated intensities (scaled) of the (003) and (101) reflections, respectively, found for pure $\alpha$-Fe${}_2$O${}_3$ nanoparticles at temperatures between 20 and 300 K.

Figure 6

Schematic illustration of two interacting nanoparticles with easy axes ${\vec{e}}_p$ and ${\vec{e}}_q$ and sublattice magnetization directions ${\vec{M}}_p$ and ${\vec{M}}_q$. $\alpha$ is the angle between the two easy axes, and ${\theta }_p$ and ${\theta }_q$denote the angles between the easy axes and the sublattice magnetization of the two particles.

Figure 7

The dependence of ${\theta }_r$ as a function of $\alpha$, as given by Eq. (10), for $KV/E_{\mathrm{int}}$ = 0.02, 0.2, 1, and 5.

Figure 8

Schematic illustration of epitaxial attachment of $\alpha$-Fe${}_2$O${}_3$ and NiO nanoparticles.

Figure 9

TEM images of $\alpha$-Fe${}_2$O${}_3$/NiO nanoparticle composite. (a) Elemental map of aggregate of $\alpha$-Fe${}_2$O${}_3$ and NiO particles. Blue/medium gray and yellow/light gray represent enrichment in iron ($\alpha$-Fe${}_2$O${}_3$) and nickel (NiO), respectively. (b, c) TEM images of assemblies of more than 14 particles and of four particles, respectively. $\alpha$-Fe${}_2$O${}_3$ nanoparticles are indicated by black outlines and NiO nanoparticles by white outlines. The lower panels in panel b show an enlargement of the area with red/dark gray outline and the fast Fourier transform (FFT) of this. The particle with a blue/medium gray outline in panel b is not identified (lattice plane spacings correspond to both $\alpha$-Fe${}_2$O${}_3$ and NiO).