Manipulating the Magnetic Structure of Co Core/CoO Shell Nanoparticles: Implications for Controlling the Exchange Bias

We present an experimental study of the effects of oxidation on the magnetic and crystal structures of exchange biased $ϵ\mathrm{\text{−}}\mathrm{Co}/\mathrm{CoO}$ core-shell nanoparticles. Transmission electron microscopy measurements reveal that oxidation creates a Co-CoO interface which is highly directional and epitaxial in quality. Neutron diffraction measurements find that below a Néel temperature $T_N$ of $\sim 235\mathrm{K}$ the magnetization of the CoO shell is modulated by two wave vectors, ${\mathbf{q}}_{\mathbf{1}}=(\frac{1}{2}\frac{1}{2}\frac{1}{2})\frac{2\pi }{a}$ and ${\mathbf{q}}_{\mathbf{2}}=(100)\frac{2\pi }{a}$. Oxidation affects the ${\mathbf{q}}_{\mathbf{1}}$ component of the magnetization very little, but hugely enhances the ${\mathbf{q}}_{\mathbf{2}}$ component, resulting in the magnetic decompensation of the core-shell interface. We propose that the large exchange bias effect results from the highly ordered interface between the Co core and CoO shell, and from enhanced core-shell coupling by the uncompensated interface moment.

Figure 1

(a), (b) Z-contrast TEM images of moderately oxidized nanoparticles. (c) TEM image showing crystallinity of Co core and CoO shell. The Co-CoO contact plane is Co (100). (d) TEM image of core-shell interface. (e) Size distributions for core and particle diameters from TEM. (f) The temperature dependence of $H_{\mathrm{EB}}$ for light (○, $H_{\mathrm{EB}}$ is multiplied by 100), medium (grey circles) and heavily oxidized (●) nanoparticles.

Figure 2

(a) Neutron diffraction for moderately oxidized particles, as well as the 300 K x-ray diffraction pattern. (b) 12 K neutron powder patterns for lightly (○), medium (gray circles), and heavily (●) oxidized particles, normalized to intensities of the corresponding (200) nuclear peaks. Solid lines are fits to Gaussian line shapes. (c) Comparison of the 300 K x-ray diffraction pattern (gray points) and the refined structure (black line). Arrow indicates (100) peak, omitted from refined structure.

Figure 3

Temperature dependencies of the moment factors $p_{1,\perp }$ and $p_{2,\perp }$. Light oxidation (●), medium oxidation (○), and heavy oxidation (◆). Solid lines are guides for the eye.

Figure 4

The ($\frac{1}{2}$, $\frac{1}{2}$, $\frac{1}{2}$) (a) and (100) (b) magnetic modulations. Only Co atoms are shown. (c) Zero temperature values of $p_{1,\perp }$ (○) and $p_{2,\perp }$ (●). Bulk CoO has $x=0$ [24, 30].