Magnetic properties of Mn-oxide nanoparticles dispersed in an amorphous SiO2 matrix

Abstract

Samples of Mn-oxide nanoparticles dispersed in an amorphous SiO2 matrix with manganese concentration 0.7 and 3at% have been synthesized by a sol–gel method. Transmission electron microscopy analysis has shown that the samples contain agglomerates of amorphous silica particles 10–20nm in size. In silica matrix two types of Mn-rich particles are dispersed, smaller nanoparticles with dimensions between 3 and 10nm, and larger crystalline areas consisting of aggregates of the smaller nanoparticles. High-temperature magnetic susceptibility study reveals that dominant magnetic phase at higher temperatures is λ-MnO2. At temperatures below TC=43K strong ferrimagnetism originating from the minor Mn3O4 phase masks the relatively weak magnetism of λ-MnO2 with antiferromagnetic interactions. Magnetic field dependence of the maximum in the zero-field-cooled magnetization for both the samples in the vicinity of 40K, and a frequency shift of the real component of the ac magnetic susceptibility in the sample with 3at% Mn suggest that the magnetic moments of the smaller Mn3O4 nanoparticles with dimensions below 10nm are exposed to thermally activated blocking process just below the Curie temperature TC. Appearance of a maximum in the zero-field-cooled magnetization for both the samples below 10K indicates possible spin glass freezing of the magnetic moments at low temperatures which might occur in the geometrically frustrated Mn sublattice of the λ-MnO2 crystal structure.