Magnetism in Mn-doped ZnO nanoparticles prepared by a co-precipitation method

Abstract

We report the synthesis of nominal 2 and 5 at.% Mn-doped ZnO nanocrystalline particles by a co-precipitation method. Rietveld refinement of x-ray diffraction data revealed that Mn-doped ZnO crystallizes in the monophasic wurtzite structure and the unit cell volume increases with increasing Mn concentration. DC magnetization measurements showed ferromagnetic ordering above room temperature with Hc∼150 Oe for nominal 2 at.% Mn-doped ZnO nanoparticles annealed at 675 K. A distinct ferromagnetic resonance (FMR) signal was observed in the EPR spectra of the 2 at.% Mn-doped ZnO nanoparticles annealed at 675 K. EPR measurements were used to estimate the number of spins participating in ferromagnetic ordering. Of the total Mn present in the 2 at.% Mn ZnO lattice, 25% of the Mn2+ ions were responsible for ferromagnetic ordering, whereas nearly 5% of the Mn2+ ions remained uncoupled (isolated spins). A well resolved EPR spectrum of 5% Mn-doped ZnO samples annealed at 875–1275 K (g = 2.007, A = 80 G, D = 210 G and E = 15 G) confirmed that Mn was substitutionally incorporated into the ZnO lattice as Mn2+. On increasing the temperature of annealing beyond 1075 K an impurity phase emerges in both the 2 and 5 at.% Mn-doped ZnO samples, which has been identified as a variant of (Zn1−XMn(II)X)Mn(III)2O4 with Tc∼15 K. Our results indicate that the observed room temperature ferromagnetism in Mn-doped ZnO can be attributed to the substitutional incorporation of Mn at Zn-sites rather than due to the formation of any metastable secondary phases.

Ferromagnetism in Fe-doped ZnO nanocrystals: Experiment and theory

“…For Mn-doped ZnO nanoparticles, as seen in Ref. 44, the FMR signal was observed at room temperature for nominal 2 at. % Mn 2+ in a ZnO matrix.…”

“…With the gradual increase of the applied field value, the meeting point of the ZFC and FC curves shifts toward lower temperature and at a sufficiently high field, there will be absolutely no irreversibility. Figure 4͑a͒ indicates that the system will be weakly ferromagnetic even at room temperature, unlike the other two recent studies, 43,44 where the superparamagnetism is the pre- ͑c͒ Highresolution TEM micrograph for the same showing the singlecrystalline nature of each nanoparticle. ͑d͒ Nanometric particle size distribution of the same sample fitted with a log-normal distribution function, from which the estimated average particle size is ϳ7 nm.…”

“…Earlier studies on Co-doped ZnO nanoparticles prepared by the vaporization condensation method 43 and on Mn-doped ZnO nanoparticles prepared by the coprecipitation technique 44 have observed a ferromagnetic behavior at low temperature and superparamagnetic behavior at high temperature. For some of the samples, depending on the preparation procedure, Curie-Weiss behavior is observed, confirming an antiferromagnetic ground state.…”

“…In addition to this, the technique is used to extract information about the oxidation state of the dopant cation involved in the spin coupling. 44,[57][58][59] Figures 8͑a͒ and 8͑b͒ show the EPR spectra of Zn 0.9 Fe 0.1 O recorded at room temperature and at 100 K. Both of the EPR spectra of Zn 0.9 Fe 0.1 O can be considered as a superposition of two overlapping signals, an intense and broad Gaussian signal ͑⌬H pp = 1675 G͒ with effective g factor g eff = 2.126 and another weak and narrow Lorentzian signal with g eff = 2.005. For the sake of convenience, the broad and narrow signals are designated as signal A ͑g eff = 2.126͒ and signal B ͑g eff = 2.005͒, respectively.…”

“…Earlier studies in the literature on the Co-͑Ref. 43͒ and Mndoped ZnO nanoparticles 44 were mostly dominated by superparamagnetism, a widely studied finite-size effect.…”