Tuning structural and magnetic properties of Fe oxide nanoparticles by specific hydrogenation treatments

Abstract

Structural and magnetic properties of Fe oxide nanoparticles prepared by laser pyrolysis and annealed in high pressure hydrogen atmosphere were investigated. The annealing treatments were performed at 200 °C (sample A200C) and 300 °C (sample A300C). The as prepared sample, A, consists of nanoparticles with ~ 4 nm mean particle size and contains C (~ 11 at.%), Fe and O. The Fe/O ratio is between γ-Fe2O3 and Fe3O4 stoichiometric ratios. A change in the oxidation state, crystallinity and particle size is evidenced for the nanoparticles in sample A200C. The Fe oxide nanoparticles are completely reduced in sample A300C to α-Fe single phase. The blocking temperature increases from 106 K in A to 110 K in A200C and above room temperature in A300C, where strong inter-particle interactions are evidenced. Magnetic parameters, of interest for applications, have been considerably varied by the specific hydrogenation treatments, in direct connection to the induced specific changes of particle size, crystallinity and phase composition. For the A and A200C samples, a field cooling dependent unidirectional anisotropy was observed especially at low temperatures, supporting the presence of nanoparticles with core–shell-like structures. Surprisingly high MS values, almost 50% higher than for bulk metallic Fe, were evidenced in sample A300C.

The crystal growth and properties of novel magnetic double molybdate RbFe5(MoO4)7with mixed Fe3+/Fe2+states and 1D negative thermal expansion

“…Fe-Containing materials can exhibit simultaneous ferrimagnetism and ferroelectricity with magnetic and electrical Curie transitions. [4][5][6][7] Thus, many investigations of crystal and electronic structures have already been carried out with complex Fe-containing compositions, and the materials are of great interest to scientists. [8][9][10][11][12][13][14] On the other hand, it is well known that high-quality molybdate crystals can be prepared at comparatively low temperatures, [15][16][17][18][19][20] and this feature is extremely significant for industrial production.…”

Enhanced Magnetization in a Frustrated Spin System by Ni+ Ion Beam Irradiation

“…Because of the random orientation of the easy axis, the net magnetization approaches zero at very low temperatures. 15,19,20 Then, as the particles are warmed up to 300 K in the presence of the field (500 Oe), they can switch their magnetization direction from the easy axis to the applied field due to thermal energy, leading to an increase in the overall magnetization, as shown in the ZFC curves of Figs. 4(a) and (b).…”

“…Given that the particles are in the regime of being blocked or superparamagnetic, the blocking temperature T B can be expressed as, 14,15,17…”

“…Typically, in superparamagnetic nanoparticles, the magnetic anisotropy on the surface may increase, but in the bulk they may have the ferromagnetic single domain. [15][16][17] A superparamagnetic behavior shows an increasing magnetization with increasing applied magnetic field with no remanent magnetization and coercive field in the nanoparticles. However, in our hysteresis curves, the M s was observed at ∼10 kOe, which is unlikely to the superparamagnetic behavior.…”

“…Typically, the ZFC curve in non-interacting single domain particles shows a decrease in the magnetization above the blocking temperature T B , where an assembly of identical single-domain particles goes from blocked to superparamagnetic-type behavior. 15,17,21,22 It is thus in a superparamagnetic regime at room temperature.…”