Tunnel Magnetoresistance in Self-Assemblies of Exchange-Coupled Core/Shell Nanoparticles

We report the precise control of tunneling magnetoresistance (TMR) in devices of self-assembled core-shell ${\mathrm{Fe}}_3{\mathrm{O}}_4$/${\mathrm{Co}}_{1-x}{\mathrm{Zn}}_x{\mathrm{Fe}}_2{\mathrm{O}}_4$ nanoparticles ($0\le x\le 1$). Adjusting the magnetic anisotropy through the content of ${\mathrm{Co}}^{2+}$ in the shell, provides an accurate tool to control the switching field between the bistable states of the TMR. In this way, different combinations of soft-hard and hard-soft core/shell configurations can be envisaged for optimizing devices with the required magnetotransport response.

Figure 1

Scheme of the process of self-assembly of the particles at the liquid-air interface and the transfer of the assemblies on a substrate. The middle part of the figure shows an optical image (right), along with two SEM images of the glass substrate patterned with $\mathrm{Au}$ electrodes, separated by 7 $\mu \mathrm{m}$, with the self-assembled nanoparticles deposited on top. The bottom part of the figure shows the topography and height profile cross section of the annealed nanoparticles assemblies analyzed by AFM. The height profile across the film edge shows a thickness of approximately 20 nm, consistent with a two-layer assembly.

Figure 2

Top panels: (a),(b) TEM images of the ${\mathrm{Fe}}_3{\mathrm{O}}_4$ core nanoparticles annealed at ${400}^{\circ }$ in vacuum atmosphere, HRTEM where the interplanar distance of (113) spinel phase are signaled, and (c) size histogram fitted with a Gaussian function. Bottom panels: (d) HRTEM image of ${\mathrm{Fe}}_3{\mathrm{O}}_4/{\mathrm{Co}\mathrm{Fe}}_2{\mathrm{O}}_4$ nanoparticles annealed at ${400}^{\circ }$ in vacuum atmosphere, where different crystalline orientations (044) and (222) of spinel structure for the core and shell are indicated. (e) Dark-field TEM image reconstructed with a fraction of the (113) of the spinel diffraction ring and (f) size dispersion for the core/shell nanoparticles with the corresponding Gaussian fitting.

Figure 3

TEM images of the self-assembled ${\mathrm{Fe}}_3{\mathrm{O}}_4/{\mathrm{Co}}_{1-x}{\mathrm{Zn}}_x{\mathrm{Fe}}_2{\mathrm{O}}_4$ core/shell nanoparticles for the different compositions from $x=0$ to $x=1$, along with the corresponding size distribution histograms. The right panel shows representative images where the core/shell structure can be appreciated from dark-field images for $x=0$, $0.25$ and $0.50$ and from high-resolution TEM for $x=0.75$ and $1$. The bright contrast in the dark-field images corresponds to the spinel grains with the particular crystallographic orientation selected by positioning the small objective TEM aperture on the brighter (113) electron diffraction ring of the spinel phase.

Figure 4

(a) Hysteresis loops measured at 100 K and (b) coercive field as a function of the temperature for the annealed ${\mathrm{Fe}}_3{\mathrm{O}}_4/{\mathrm{Co}}_{1-x}{\mathrm{Zn}}_x{\mathrm{Fe}}_2{\mathrm{O}}_4$ core/shell nanoparticle assemblies. The inset shows the $H_C$ for the annealed self-assembled particles as a function of $x$, and also the magnetization measured at 5 T ($M_{5T}$) for the powder sample annealed under the same conditions as the self-assemblies. Both measurements are acquired at 100 K.

Figure 5

(a) Current-voltage characteristics measured at 100 K for all the ${\mathrm{Fe}}_3{\mathrm{O}}_4/{\mathrm{Co}}_{1-x}{\mathrm{Zn}}_x{\mathrm{Fe}}_2{\mathrm{O}}_4$ annealed devices. The inset shows $I$/$V$ versus $V^2$. (b) Logarithm of the electrical resistivity as a function of $1/T$, both in logarithmic scale.

Figure 6

TMR of the devices studied in this work at different temperatures. The magnetoresistance is defined as $\mathrm{MR}=100\times [R(H)-R(0)]/R(0)$, and is measured with $H$ in the plane of the self-assembled films. The red line represents the fitting to Eq. (2) using the experimental $M(H)$ curves shown in Fig. 4, measured for each self-assembly of magnetic nanoparticles.

Figure 7

(a) Switching field ($H_{\mathrm{SF}}$) as a function of temperature for ${\mathrm{Fe}}_3{\mathrm{O}}_4/{\mathrm{Co}}_{1-x}{\mathrm{Zn}}_x{\mathrm{Fe}}_2{\mathrm{O}}_4$ nanoparticles and (b) $C=JP/4k_{B}T$ obtained as the adjusted parameter from Eq. (2) as a function of the inverse of the temperature.