Magnetic properties of Fe-doped NiO nanoparticles

Abstract

Undoped and Fe-doped NiO nanoparticles were successfully synthesized using a lyophilization method and systematically characterized through magnetization techniques over a wide temperature range, with varying intensity and frequency of the applied magnetic fields. The Ni_1-xFe_xO nanoparticles can be described by a core-shell model, which reveals that Fe doping enhances exchange interactions in correlation with nanoparticle size reduction. The nanoparticles exhibit a superparamagnetic blocking transition, primarily attributed to their cores, at temperatures ranging from above room temperature to low temperatures, depending on the Fe-doping level and sample synthesis temperature. The nanoparticle shells also exhibit a transition at low temperatures, in this case to a cluster-glass-like state, caused by the dipolar magnetic interactions between the net magnetic moments of the clusters. Their freezing temperature shifts to higher temperatures as the Fe-doping level increases. The existence of an exchange bias interaction was observed, thus validating the core-shell model proposed.

Keywords: Cluster-glass; Core-shell model; Fe-doped; Nanoparticles; Nickel oxide; Spin-glass.

Fig. 1

M(T) curves of the undoped NiO sample (ZFC (blue) and FC (red) protocols) obtained for different probe fields: (a) H = 50 Oe; the insert shows an expanded view of the 2–40 K range; (b) H = 500 Oe; and (c) H = 5000 Oe.

Fig. 2

χ(T) curves of the undoped NiO sample obtained for different frequencies f_AC (a); expanded view of the 4–16 K range where the continuous solid lines are spline fits (b).

Fig. 3

ln τ vs. the reciprocal of T_BF for the undoped NiO sample. The solid line corresponds to a fit with the Arrhenius law.

Fig. 4

Dynamic scaling law fit of the AC susceptibility peaks for the NiO sample.

Fig. 5

M(H) curves obtained at RT and 2 K for the NiO sample. The insert is the derivative of the descending magnetization 2 K curve.

Fig. 6

M(T) curves (ZFC (blue) and FC (red) protocols) obtained with H = 50 Oe for the Ni_0.98Fe_0.02O (the insert shows an expanded view of the 80–300 K range) (a) and Ni_0.96Fe_0.04O (the insert shows an expanded view of the 2–25 K range) (b) samples.

Fig. 7

χ(T) curves of the Ni_0.97Fe_0.03O sample obtained for different frequencies, f_AC (a); expanded view of the 7–13 K range where the continuous solid lines are splines fits (b). The red dashed line connects the maxima (T_BF) of the 50 Hz and 10 kHz curves.

Fig. 8

ln τ vs. the reciprocal of T_BF (f), for the Ni_0.97Fe_0.03O sample. The solid line corresponds to a fit using the Arrhenius law.

Fig. 9

Dynamic scaling law fit of the AC susceptibility peaks for the Ni_0.97Fe_0.03O sample.

Fig. 10

M(H) curves obtained at RT and 2 K for the Ni_0.98Fe_0.02O (a) and Ni_0.96Fe_0.04O (b) samples. The inserts are expanded views of the hysteresis central parts.

Fig. 11

M(T) curves - ZFC (blue) and FC (red) protocols obtained with H = 50 Oe for the Ni_0.97Fe_0.03O sample. The inserts show expanded views of the 35–300 K range (right-hand side) and the 2–45 K range (left-hand side).

Fig. 12

M(T) curves - ZFC (blue) and FC (red) protocols obtained with H = 50 Oe for the Ni_0.97Fe_0.03O sample annealed at 500 °C. The insert shows an expanded view of the 150–300 K range.

Fig. 13

M(H) curves obtained at 2 K for the Ni_0.97Fe_0.03O sample annealed at 400 °C, in ZFC (blue curve) and FC (black curve) conditions. The insert is an expanded view of the hysteresis central parts.