Exchange bias in $\text{Fe}/{\text{Fe}}_3{\text{O}}_4$ core-shell magnetic nanoparticles mediated by frozen interfacial spins

The magnetization curves of monodisperse $\text{Fe}/{\text{Fe}}_3{\text{O}}_4$ core-shell and ${\text{Fe}}_3{\text{O}}_4$ hollow-shell nanoparticles reveal an unusual exchange-bias effect. Hysteresis measurements of core-shell particles at 5 K after field cooling exhibit a large loop shift associated with unidirectional anisotropy whereas ${\text{Fe}}_3{\text{O}}_4$ hollow-shell nanoparticles support much smaller shifts. Both core-shell and hollow-shell particles exhibit sharp demagnetization jumps at low fields associated with a sudden switching of shell moments. Temperature-dependent magnetization of core-shell particles at high fields shows a deviation between field-cooled and zero-field-cooled curves below 30 K, suggesting the presence of frozen spins at the interface. These frozen interfacial spins play an important role in mediating the exchange coupling between the ferromagnetic core and ferrimagnetic shell.

Figure 1

(Color online) (a) TEM image of $\text{Fe}/{\text{Fe}}_3{\text{O}}_4$ core-shell nanoparticles $\left(13.8\pm 1.0\text{nm},N=100\right)$. Inset: HR-TEM image of the core-shell structure with average shell thickness of 2.5 nm. Dashed lines demarcate nanocrystalline domains in the ${\text{Fe}}_3{\text{O}}_4$ shell. (b) ${\text{Fe}}_3{\text{O}}_4$ hollow-shell nanoparticles $\left(16.0\pm 1.2\text{nm},N=230\right)$ with a shell thickness of 4.5 nm. Inset: HR-TEM image of the hollow-shell nanoparticles.

Figure 2

(Color online) $M\text{−}H$ hysteresis loops for (a) $\text{Fe}/{\text{Fe}}_3{\text{O}}_4$ core-shell and (b) ${\text{Fe}}_3{\text{O}}_4$ hollow-shell nanoparticles, under FC (filled circles) and ZFC (open triangles) conditions. Insets: $dM/dH$ plots for core-shell and hollow-shell nanoparticles during the negative sweep direction (from 10 to $-10\text{kOe}$).

Figure 3

(Color online) Magnetization vs temperature curves of $\text{Fe}/{\text{Fe}}_3{\text{O}}_4$ core-shell nanoparticles under ZFC (filled squares) and FC (open circles) using cooling and measurement fields of (a) 3 Oe and (b) 10 kOe. Inset in panel (b) shows the data plotted vs $T^{1.9}$.

Figure 4

(Color online) Model of spin orientations in core-shell nanoparticle during hysteresis sweep from 10 to $-10\text{kOe}$ under FC. Ferromagnetic and local ferrimagnetic moments are represented by heavy arrows; frozen interfacial spins are represented by small black arrows.

Figure 5

(Color online) For core-shell particles: (a) exchange-bias field $H_E$, coercivity $H_C$, and low-field jump $\Delta M$ as a function of applied cooling field; measurements were performed at 5 K. (b) $H_E$, $H_C$ and $\Delta M$ measured after the sample was cooled down to different temperatures under a 10 kOe field (Ref. 36).