Exchange bias and surface effects in bimagnetic $\mathrm{CoO}\text{−}\mathrm{core}/{\mathrm{Co}}_{0.5}{\mathrm{Ni}}_{0.5}{\mathrm{Fe}}_2{\mathrm{O}}_4$-shell nanoparticles

Bimagnetic nanoparticles have been proposed for the design of new materials with controlled properties, which requires a comprehensive investigation of their magnetic behavior due to multiple effects arising from their complex structure. In this work we fabricated bimagnetic core/shell nanoparticles formed by an $\sim 3$-nm antiferromagnetic (AFM) CoO core encapsulated within an $\sim 1.5$-nm ferrimagnetic (FiM) ${\mathrm{Co}}_{0.5}{\mathrm{Ni}}_{0.5}{\mathrm{Fe}}_2{\mathrm{O}}_4$ shell, aiming at studying the enhancement of the magnetic anisotropy and the surface effects of a ferrimagnetic oxide shell. The magnetic properties of as-synthesized and annealed samples were analyzed by ac and dc magnetization measurements. The results indicate that the magnetic response of the as-synthesized particles is governed by the superparamagnetic behavior of the interacting nanoaggregates of spins that constitute the disordered ferrimagnetic shell, whose total moments block at $\langle T_B\rangle =49$ K and collectively freeze in a superspin-glass-type state at $\langle T_g\rangle =3$ K. On the other hand, annealed nanoparticles are superparamagnetic at room temperature and behave as an exchange-coupled system below the blocking temperature $\langle T_B\rangle =70$ K, with enhanced coercivity $H_C\left(10\text{K}\right)\sim 14.6$ kOe and exchange bias field $H_{EB}\left(10\text{K}\right)\sim 2.3$ kOe, compared with the as-synthesized system where $H_C\left(10\text{K}\right)\sim 5.5$ kOe and $H_{EB}\left(10\text{K}\right)\sim 0.8$ kOe. Our results, interpreted using different models for thermally activated and surface relaxation processes, can help clarify the complex magnetic behavior of many core/shell and hollow nanoparticle systems.

Figure 1

X-ray diffraction patterns of as-synthesized and annealed $\text{CoO}/{\mathrm{Co}}_{0.5}{\mathrm{Ni}}_{0.5}{\mathrm{Fe}}_2{\mathrm{O}}_4$ nanoparticles. The positions of bulk CoO (dashed vertical lines) and ${\mathrm{CoFe}}_2{\mathrm{O}}_4$ (solid vertical lines) reflections are shown for comparison. The inset shows the thermogravimetric curves for both samples and oleic acid measured under air atmosphere.

Figure 2

(a) Bright-field image, (b) size-distribution histogram with the corresponding lognormal fit and (c-d) high-resolution TEM images of as-synthesized $\text{CoO}/{\mathrm{Co}}_{0.5}{\mathrm{Ni}}_{0.5}{\mathrm{Fe}}_2{\mathrm{O}}_4$ nanoparticles, where shell nanograins are highlighted.

Figure 3

(a) Bright-field and (b) dark-field TEM images, (c) size-distribution histogram and the corresponding lognormal fit, and (d) high-resolution TEM image of annealed $\text{CoO}/{\mathrm{Co}}_{0.5}{\mathrm{Ni}}_{0.5}{\mathrm{Fe}}_2{\mathrm{O}}_4$ nanoparticles.

Figure 4

(a) ZFC and FC dc magnetization curves measured at $H=50$ Oe [the inset shows the energy-barrier distribution $f\left(T_B\right)$ and the corresponding lognormal fit used to estimate $\langle T_B\rangle ]$ and (b) the real $\left({\chi }^{\prime }\right)$ component of the ac susceptibility measured with $H_{ac}=10$ Oe at selected frequencies (the inset shows a detail of the low-temperature peak) for as-synthesized $\text{CoO}/{\mathrm{Co}}_{0.5}{\mathrm{Ni}}_{0.5}{\mathrm{Fe}}_2{\mathrm{O}}_4$ nanoparticles.

Figure 5

(a) ZFC and FC dc magnetization curves measured at $H=50$ Oe [the inset shows the energy-barrier distribution $f\left(T_B\right)$ and the corresponding lognormal fit used to estimate $\langle T_B\rangle ]$ and (b) the real $\left({\chi }^{\prime }\right)$ component of the ac susceptibility measured with $H_{ac}=10$ Oe at selected frequencies for annealed $\text{CoO}/{\mathrm{Co}}_{0.5}{\mathrm{Ni}}_{0.5}{\mathrm{Fe}}_2{\mathrm{O}}_4$ nanoparticles.

Figure 6

(a) Frequency dependence of the high-temperature maximum for as-synthesized and annealed nanoparticles. The solid lines correspond to the fit of the experimental data with the Vogel-Fulcher law, Eq. (1). (b) Frequency dependence of the low-temperature maximum for as-synthesized nanoparticles and the corresponding fit with the power law, Eq. (2).

Figure 7

ZFC hysteresis cycles for several temperatures and FC hysteresis curves at 10 K for as-synthesized and annealed $\text{CoO}/{\mathrm{Ni}}_{0.5}{\mathrm{Co}}_{0.5}{\mathrm{Fe}}_2{\mathrm{O}}_4$ nanoparticles.

Figure 8

Temperature dependence of the coercive field $H_C$, the exchange-bias field $H_{EB}$, and the saturation magnetization $M_S$ for as-synthesized and annealed $\text{CoO}/{\mathrm{Ni}}_{0.5}{\mathrm{Co}}_{0.5}{\mathrm{Fe}}_2{\mathrm{O}}_4$ nanoparticles.