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 particlesby a co-precipitation method. Rietveld refinement of x-ray diffraction data revealedthat Mn-doped ZnO crystallizes in the monophasic wurtzite structure and theunit cell volume increases with increasing Mn concentration. DC magnetizationmeasurements showed ferromagnetic ordering above room temperature withHc∼150 Oe for nominal 2 at.% Mn-doped ZnO nanoparticles annealed at 675 K. A distinctferromagnetic resonance (FMR) signal was observed in the EPR spectra of the 2at.% Mn-doped ZnO nanoparticles annealed at 675 K. EPR measurementswere used to estimate the number of spins participating in ferromagneticordering. Of the total Mn present in the 2 at.% Mn ZnO lattice, 25% of theMn2+ ions were responsible for ferromagnetic ordering, whereas nearly 5% of theMn2+ ions remained uncoupled (isolated spins). A well resolved EPRspectrum of 5% Mn-doped ZnO samples annealed at 875–1275 K(g = 2.007,A = 80 G,D = 210 Gand E = 15 G) confirmed that Mn was substitutionally incorporated into the ZnO lattice asMn2+. On increasing the temperature of annealing beyond 1075 K an impurity phase emerges inboth 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 inMn-doped ZnO can be attributed to the substitutional incorporation of Mn atZn-sites rather than due to the formation of any metastable secondary phases.