Ferro and antiferromagnetism of ultrafine‐grained hematite

Abstract

Aluminum-substituted hematite (here referred to as Al-Hm) is an important magnetic mineral for paleo and environmental magnetism. However, the magnetic properties of nanosized Al-Hm are poorly known. In this study, a series of Al-Hm samples (fourteen samples) and their nonsubstituted counterparts (referred to as pure-Hm) (seven samples) were synthesized with particle sizes ranging from 14 to 124 nm. With decreasing particle size, coercive force (B c ), magnetic remanence (M r ) acquired in a 5 T field at room temperature, and the maximum blocking temperature (T b ) of Al-Hm descend gradually; T b drops abruptly from 800 to 200 K at 17.2 6 3.7 nm. This size defines the superparamagnetic (SP)/single-domain (SD) threshold of Al-Hm which is significantly lower than that of pure-Hm (27.5 6 1.5 nm). The antiferromagnetic highfield susceptibility (v anti ) and the saturation magnetization (M s ) for the weak ferromagnetic component in hematite are both correlated negatively with temperature between 20 and 300 K, and grain size in the vicinity of the SP threshold. For very small grains (below 16-17 nm), T b increases with increasing particle size. However, beyond this size, it descends with increasing particle size up to 19 nm before increasing with grain size again. These trends are attributed to the competition of surface anisotropy and bulk anisotropy (elastic anisotropy). This new study substantially improves our understanding of the complex magnetic properties of fine-grained Al-Hm and pure-Hm in natural sample, which is significant for the paleoenvironmental and climatic studies of natural samples, e.g., red beds/soils.

Estimating the concentration of aluminum‐substituted hematite and goethite using diffuse reflectance spectrometry and rock magnetism: Feasibility and limitations

“…Defects resulting from Al incorporation inhibit magnetic domain rotation and flipping, which causes an increase in magnetic hardness. The coercivity of hematite also increases with grain size in the SD size range [ Jiang et al ., , ; Özdemir and Dunlop , ]. Özdemir and Dunlop [] demonstrated that the traditional HIRM (1 T, −300 mT) parameter is only sensitive to hematite particles with sizes from ~0.3 to 3 µm, which is consistent with results for HFh8 (Figure a).…”

“…The ubiquity of Al in weathering environments results in most Fe oxides in soils being Al substituted [ Cornell and Schwertmann , ; Fontes and Weed , ; Friedl and Schwertmann , ; Wiriyakitnateekul et al ., ]. According to previous studies and results presented here, Al substitution affects both DRS and magnetic properties [ Dekkers , ; Dekkers and Linssen , ; Jiang et al ., , ; Liu et al ., ; Malengreau et al ., ; Torrent and Barrón , ], such as HIRM.…”

“…In order to determine the blocking temperature of goethite ( T b ) in natural samples, which is the temperature at which the goethite changes from the stable single domain (SD) to the superparamagnetic (SP) state, temperature‐dependent HIRM curves were measured. Hematite in natural samples is almost saturated at 2.5 T, so HIRM (5 T, −2.5 T) was measured from 150 to 400 K using a Quantum Design Magnetic Property Measurements System at IGGCAS [ Jiang et al ., , ] to eliminate the influence of both hematite and magnetite. First, samples were cooled from room temperature to 150 K in zero field, and then an IRM was imparted in a 5 T field.…”

Iron oxide minerals in dust-source sediments from the Bodélé Depression, Chad: Implications for radiative properties and Fe bioavailability of dust plumes from the Sahara

“…It is not unusual for the Morin transition to be suppressed in natural hematite due to poor crystallinity, cation-substitution, or nanoparticle effects (Vandenberghe et al, 2001;Maher et al, 2004;Özdemir et al, 2008;Jiang et al, 2014).…”

Influence of hydrothermal synthesis conditions on size, morphology and colloidal properties of Hematite nanoparticles

“…The magnetic properties of Hematite nanocrystals are a topic of interest as well [16][17][18] because crystalline α-Fe 2 O 3 has the characteristics of an antiferromagnet below the Morin temperature T M . When the temperature is above T M , it exhibits weakly parasitic ferromagnetic properties [19].…”