Effect of proton irradiation on the magnetic properties of manganese ferrite.

Cubic-spinel MnFe2O4 magnetic nanoparticles (NPs) were prepared, with an average particle size of about 4 nm determined from a high-resolution transmission electron microscope. When the NPs were proton-irradiated, the lattice constants decreased with increasing proton irradiation. Before the proton irradiation, the NPs exhibited 36.2 +/- 0.1 emu/g magnetization (M(S)) and 11.1 +/- 0.1 Oe coercivity (H(C)). After the irradiation of the samples with 5 and 10 pC/microm2 doses, the M(S) changed to 35.6 and 35.1 +/- 0.1 emu/g, and the H(C) to 11.3 and 12.9 +/- 0.1 Oe, respectively. The room-temperature Mössbauer spectra of the NPs showed superparamagnetic characteristics, with the single-absorption line of two sites and a large relaxation frequency. During the proton irradiation, the relaxation frequency decreased to 156.02 and 134.29 +/- 0.01 Gamma/ħ from the unirradiated sample's 164.02 +/- 0.01 Gamma/ħ. It is suggested that the proton irradiation induced the increase in the anisotropy energy of the MnFe2O4 NPs. Moreover, from the external-field-induced Mössbauer spectra at 4.2 K, an increase in the canted angle of the hyperfine field between sites A (tetrahedral) and B (octahedral) was observed with proton irradiation.