Mn ferrite (MnFe(2)O(4)) nanoparticles, having diameters from 4 to 50 nm, were synthesized using a modified co-precipitation technique in which mixed metal chloride solutions were added to different concentrations of boiling NaOH solutions to control particle growth rate. Thermomagnetization measurements indicated an increase in Néel temperature corresponding to increased particle growth rate and particle size. The Néel temperature is also found to increase inversely proportionally to the cation inversion parameter, delta, appearing in the formula (Mn(1-delta)Fe(delta))(tet)[Mn(delta)Fe(2-delta)](oct)O(4). These results contradict previously published reports of trends between Néel temperature and particle size, and demonstrate the dominance of cation inversion in determining the strength of superexchange interactions and subsequently Néel temperature in ferrite systems. The particle surface chemistry, structure, and magnetic spin configuration play secondary roles.

Rietveld structure refinement, cation distribution and magnetic properties of Al3+ substituted NiFe2O4 nanoparticles J. Appl. Phys. 109, 053909 (2011); 10.1063/1.3559266 Control of the cation occupancies of MnZn ferrite synthesized via reverse micelles Structure and magnetic properties of rf thermally plasma synthesized Mn and Mn-Zn ferrite nanoparticlesMnFe 2 O 4 nanoparticles with diameters ranging from about 4 to 50 nm were synthesized using a modified coprecipitation method. X-ray diffractograms revealed a pure phase spinel ferrite structure for all samples. Transmission electron microscopy showed that the particles consist of a mixture of both spherical ͑smaller͒ and cubic ͑larger͒ particles dictated by the reaction kinetics. The Néel temperatures ͑T N ͒ of MnFe 2 O 4 for various particle sizes were determined by using high temperature magnetometry. The ϳ4 nm MnFe 2 O 4 particles showed a T N of about 320°C whereas the ϳ50 nm particles had a T N of about 400°C. The high Néel temperature, compared with the bulk MnFe 2 O 4 T N of 300°C, is due to a change in cation distribution between the tetrahedral and octahedral sites of the spinel lattice. Results of extended x-ray absorption fine structure measurements indicate a systematic change in the cation distribution dependent on processing conditions.

Articles you may be interested inHigh temperature phase transformation studies in magnetite nanoparticles doped with Co2+ ion J. Appl. Phys. 112, 054320 (2012); 10.1063/1.4748318 Magnetoelectric behavior of carbonyl iron mixed Mn oxide-coated ferrite nanoparticles Control of the cation occupancies of MnZn ferrite synthesized via reverse micelles Structure and magnetic properties of rf thermally plasma synthesized Mn and Mn-Zn ferrite nanoparticles J. Appl. Phys. 93, 7495 (2003); 10.1063/1.1557953Mn-Zn ferrite with higher magnetization for temperature sensitive magnetic fluid

In this work, Fe3O4 nanoparticles with two distinct diameters of 10 and 2nm are synthesized with different chemical methods in order to investigate the critical size effects on the magnetic resonance of Fe3O4 nanoparticles. Our results clearly demonstrate a difference in the dynamic behaviors of resonance field (Hr) and resonance linewidth between these two nanoparticles. Furthermore, the nonlinear temperature dependency of Hr in 2-nm Fe3O4 suggests that the surface anisotropy becomes dominating on the magnetodynamics of nanoparticles when the diameter of the nanoparticle is smaller than the critical size.