Mössbauer characterization of iron oxide nanoclusters grown within aluminosilicate matrices

Mössbauer spectroscopy uses the resonant absorption of nuclear radiation by ⁵⁷Fe to probe the electronic and internal magnetic structure of iron based magnetic materials. The technique has a characteristic measuring time of 10 ns enabling investigation of spin relaxation phenomena in nanoscale particles; and determination of their magnetic properties in the absence of externally applied magnetic fields. We report on Mössbauer studies of γ-Fe₂O₃ nanoparticles synthesized within hexagonally packed mesoporous MCM-41 aluminosilicate matrices with cylindrical pores of 2.5 nm diameter. Data analysis allowed differentiation of particle-matrix interfacial versus particle-core interior iron sites. Interfacial iron atoms experience large electric field gradients resulting in quadrupole splitting values of ΔE_q (surface) = (1.25 ± 0.05) mm/s, while core atoms exhibit smaller values of ΔE_q (core) = (0.73 ± 0.05) mm/s at room temperature. Similarly, differences were observed in the values of the internal hyperfine fields at low temperatures indicating reduced strength in the exchange interactions at the particle surface, with interfacial atoms experiencing internal fields H_hf (surface) = (458 ± 1) kOe reduced relative to the core H_hf (core) = (488 ± 1) kOe at 4.2 K. Particle/matrix interactions at the surface appear to perturb the electronic interactions deeper into the core than the magnetic exchange interactions.