Phase evolution and magnetic properties of a high-energy ball-milled hematite–alumina system

Abstract

The system (␣-Fe 2 O 3 ) x (␣-Al 2 O 3 ) 1Ϫx was subjected to 24 h of high-energy ball-milling varying its nominal concentration, x. The milled samples were structurally and magnetically characterized at room temperature by x-ray diffraction, Mössbauer spectroscopy, and magnetic measurements. Mössbauer studies were also performed in the temperature range 250-6 K. As a result of the earlier analyses, it was observed that the milling products were extremely dependent on the hematite starting concentration. In samples with low ␣-Fe 2 O 3 initial concentration ͑i.e., xр0.12), the paramagnetic solid solution ␣-(Fe ⌬Y Al 1Ϫ⌬Y ) 2 O 3 , the ␣-Fe and the FeAl 2 O 4 phases were identified, along with alumina, which was always residual. The presence of spinel and metallic iron was attributed to the stainless-steel vial and balls abrasion. For xϾ0.12, the iron component was no longer present but another magnetic component, corresponding to an aluminum-substituted hematite phase, ␣-(Fe 1Ϫ⌬W Al ⌬W ) 2 O 3 , could be seen to increase with increasing x. This solid solution was shown to be transitional, at room temperature, between two ordering states, weak ferromagnetic and superparamagnetic, the latter resulting from the milling induced particle size reduction. Low temperature Mössbauer spectra revealed the magnetic ordering of both solid solutions, ␣-(Fe ⌬Y Al 1Ϫ⌬Y ) 2 O 3 and ␣-(Fe 1Ϫ⌬W Al ⌬W ) 2 O 3 , and indicated the suppression of the Morin transition for the iron-rich solid solution. The magnetization versus magnetic field curves obtained for samples with xу0.12 revealed, besides a general superparamagnetic character, some hysteretic behavior due to the magnetic phases eventually existing.

High-energy ball-milled (α-Fe2O3)(α-Al2O3) system: A study on the milling time effects

“…For t = 3 and 6 h, the spectra were fitted with a hyperfine field distribution, a discrete sextet and two doublets. The former is related to an ␣-(Fe 1 − w Al w ) 2 O 3 type of solid solution, as previously verified for 24 h milled samples of diverse concentrations [15], and the second one with magnetic ␣-Fe, hence confirming the above pointed out contamination. The Fe 2+ paramagnetic component is attributed to hercynite (FeAl 2 O 4 ), a spinel synthesizable from iron and alumina and, eventually, hematite, through a number of processes including ball-milling [17,18].…”

“…In some of our previous contributions, we have systematically studied the starting composition, milling velocity and milling medium dependencies on isochronally 24 h high-energy ball-milled ␣-Fe 2 O 3 /␣-Al 2 O 3 system [15,16]. The results revealed the formation of an iron-rich (␣-(Fe 1 − w Al w ) 2 O 3 -magnetically ordered) and an ironpoor (␣-(Fe y Al 1 − y ) 2 O 3 -magnetically ordered at lowtemperatures) solid solutions for most of the initial nominal concentrations and milling conditions.…”

Mössbauer Spectroscopy X-Ray Diffraction and SEM Studies on Multiferroic Bi5Ti3FeO15Ceramics

“…The size of the quadrupole splitting is determined by valence electrons and ligands and therefore provides information on the oxidation state, coordination number, and site distortion. Values of IS = 0.25 mm/s and QS = 0.45 mm/s are typical of Fe 3+ ions in an octahedral environment [15][16][17]. Therefore, we can conclude that in Bi 5 Ti 3 FeO 15 phase there are two iron sites, both occupied by Fe 3+ and surrounded by a distorted octahedron of oxygen atoms with different degrees of distortion.…”

Structural Studies with the Use of XRD and Mössbauer Spectroscopy οf Bi5Ti3FeO15Ceramic Powders Obtained by Mechanical Synthesis

“…Zeeman sextet (Z) can be attributed to desired Bi 5 Ti 3 FeO 15 phase. Two ferric doublets can be associated with different phases: nanostructured hematite [18], (Bi,Fe) 2 O 3 (which presents a sextet spectrum at room temperature for single crystalline samples [19]). The kinetics of the formation of the desired Bi 5 Ti 3 FeO 15 phase during high--energy ball milling can be derived from the analysis of the Mössbauer spectra.…”