Interparticle interactions in composites of nanoparticles of ferrimagnetic $\left(\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3\right)$ and antiferromagnetic $(\mathrm{CoO},\mathrm{NiO})$ materials

The magnetic properties of mixtures of ferrimagnetic $\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ (maghemite) and antiferromagnetic $\mathrm{NiO}$ or $\mathrm{CoO}$ nanoparticles have been studied by use of ${}^{57}\mathrm{Fe}$ Mössbauer spectroscopy, neutron powder diffraction and magnetization measurements. The studies showed that the interaction with antiferromagnetic particles has a significant influence on the magnetic properties of the $\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ nanoparticles. It was found that mixing the $\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ nanoparticles with $\mathrm{NiO}$ nanoparticles resulted in a faster superparamagnetic relaxation and a reduced coercivity compared to a sample consisting solely of $\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ nanoparticles. On the contrary, mixing of $\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ nanoparticles with $\mathrm{CoO}$ nanoparticles resulted in a suppression of the relaxation and an increase in coercivity. These results suggest that the properties of the ferrimagnetic $\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ nanoparticles are influenced by the anisotropy of their neighboring antiferromagnetic particles. The dominating type of magnetic interaction between the particles in the composites seems to be exchange interaction between surface atoms of neighboring particles.

Figure 1

${}^{57}\mathrm{Fe}$-Mössbauer spectra obtained at the indicated temperatures of samples of pure $7\mathrm{nm}$ $\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ particles. (a) Coated with oleic acid and suspended in heptane (b) uncoated, freeze-dried, (c) uncoated, dried at room temperature.

Figure 2

${}^{57}\mathrm{Fe}$-Mössbauer spectra obtained at the indicated temperatures of $7\mathrm{nm}$ $\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ particles (a) mixed with $\mathrm{NiO}$ nanoparticles and dried at room temperature, (b) mixed with $\mathrm{NiO}$ nanoparticles and dried at $200°\mathrm{C}$, and (c) pure $\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ particles dried at $200°\mathrm{C}$.

Figure 3

${}^{57}\mathrm{Fe}$-Mössbauer spectra obtained at the indicated temperatures of $7\mathrm{nm}$ $\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ particles mixed with $\mathrm{CoO}$-ace nanoparticles at room temperature.

Figure 4

Hysteresis loops of $\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$, $\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3+\mathrm{CoO}$-bm and $\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3+\mathrm{NiO}$ at $5\mathrm{K}$. The magnetization is given per mass unit of $\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ in the samples.

Figure 5

The coercivity of pure $\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3$ and $\gamma \text{−}{\mathrm{Fe}}_2{\mathrm{O}}_3+\mathrm{CoO}$-bm nanoparticles plotted as a function of temperature. The solid lines are guides to the eye.