Co-doped MnFe2O4 nanoparticles: magnetic anisotropy and interparticle interactions

Abstract

The effect of cobalt doping on the magnetic properties of Mn_{1- x} Co _x Fe₂O₄ nanoparticles was investigated. All samples consist of ensembles of nanoparticles with a spherical shape and average diameter of about 10 nm, showing small structural changes due to the substitution. Besides having the same morpho-structural properties, the effect of the chemical composition, i.e., the amount of Co doping, produces marked differences on the magnetic properties, especially on the magnetic anisotropy, with evident large changes in the coercive field. Moreover, Co substitution has a profound effect on the interparticle interactions, too. A dipolar-based interaction regime is detected for all samples; in addition, the intensity of the interactions shows a possible relation with the single particle anisotropy. Finally, the sample with the strongest interaction regime shows a superspin glass state confirmed by memory effect dynamics.

Keywords: cobalt doping; collective dynamics; ferrite nanoparticles; interparticle interactions; magnetic properties.

Figure 1

Room temperature X-ray diffraction patterns of all samples. All diffraction peaks are compatible with the spinel structure.

Figure 2

As an example of the size, shape and crystalline quality of the samples, TEM images of C0 (a) and C100 (b) samples are reported. The insets show the respective particle size distribution fitted with a Gaussian distribution (solid line).

Figure 3

(a) Comparison of ZFC curve during cooling (blue circles) and subsequent warming up (red circles). The difference, ΔM, is reported in (b) as a function of temperature.

Figure 4

Mössbauer spectra recorded at 300 K for all samples. Only sample C0 is fully superparamagnetic at room temperature, while the only blocked sample is C100.

Figure 5

(a) Magnetization vs field curves and (b) switching field distribution measured at 5 K for sample C0 (black squares), C25 (red circles), C50 (blue triangles), C75 (dark cyan reversed triangles), and C100 (magenta diamonds).

Figure 6

(a) ΔM plot curves measured at 5 K for sample C0 (black squares), C25 (red circles), C50 (blue triangles), C75 (dark cyan reversed triangles), and C100 (magenta diamonds). (b) The absolute intensity of interactions obtained from the ΔM plot is shown with respect to Co content (red triangles). The trend is the same exhibited by the dipolar coupling energy scaled by the effective anisotropy of each sample (black circles).