Stable Magnetic Remanence in Antiferromagnetic Goethite

A paleomagnetic study of some felsite and basic intrusions in Virginia revealed the presence of presumably stable magnetic directions of normal and reverse polarities in close agreement with the axis of the present geomagnetic field. Results of ore microscopy, Curie temperatures, and X ray diffraction analysis suggest primary titanomagnetites to be the major magnetic mineral present. The paleomagnetic results suggest a Tertiary age of all intrusions in accordance with a radiometrically determined Middle Tertiary age of the felsites.

supporting

confidence: 76%

Rock magnetism and paleomagnetism of some intrusions from Virginia

Løvlie¹,

Opdyke²

1974

“…The basic magnetic properties of both natural and synthetic goethite have been extensively investigated [ Strangway et al , 1967; 1968; Banerjee , 1970; Dekkers , 1988, 1989a, 1989b; Rochette and Fillion , 1989; Pollard et al , 1991; Özdemir and Dunlop , 1996; Mathé et al , 1999; Guyodo et al , 2003; Liu et al , 2004; Rochette et al , 2005]. The spin coupling of goethite lies along the crystallographic b axis (in the Pnma space group, Figure 1a), and defects or isomorphous cation substitutions (e.g., aluminum, Al) result in a net magnetic moment along the b axis.…”

Section: Introductionmentioning

confidence: 99%

Characteristic low‐temperature magnetic properties of aluminous goethite [α‐(Fe, Al)OOH] explained

Liu

Torrent

et al. 2006

[1] Goethite (a-FeOOH) is an antiferromagnetic iron oxyhydroxide that forms as a weathering product of iron-bearing minerals. We systematically investigated the low-and room temperature properties of well-defined aluminous goethites [a-(Fe, Al)OOH] with varying grain size and Al content. A marked decrease in the Néel temperature with increasing Al content for goethite lowers the blocking temperature distribution, which produces an increase in the remanent magnetization on cooling. The zero-field-cooled (ZFC) and field-cooled (FC) curves are irreversible. This is due to an additional partial thermal remanent magnetization (pTRM), which is acquired at low temperatures during the FC process because the initial 300 K remanence is far from being saturated due to the extremely high saturation field of goethite. This pTRM can be thermally demagnetized when reheating a sample back to 300 K. Finally, a sharp decrease in the bulk coercivity at 11-13 mol % Al is mostly caused by a broad coercivity distribution due to nonuniformity of Al substitution, which becomes more significant with increasing Al content. The positive correlation between pTRM acquisition during the FC process and the bulk coercivity strongly indicates that the ZFC/FC behavior is controlled by the bulk coercivity, which is, in turn, determined by Al substitution. This explanation of the characteristic low-temperature magnetic properties of Al goethite provides important constraints for identifying natural Al goethite bearing samples and quantitatively estimating the contributions of this mineral to the bulk magnetic properties of such samples.

“…[15] T B has been previously proposed to represent T N [Strangeway et al, 1967;Banerjee, 1970;Mathé et al, 1999]. However, we found that they are significantly different.…”

Section: Discussionmentioning

confidence: 74%

Mechanism of the parasitic remanence of aluminous goethite [α‐(Fe, Al)OOH]

Liu¹

2004

[1] The Néel temperature (T N ) of cation-substituted goethites is controlled by the degree of isomorphous substitution and crystal defects. In particular, the exact relationship between the T N of the antiferromagnetism (AFM) and the unblocking temperature (T B ) of the parasitic remanence is undetermined. We found that T B is systematically higher than T N for a set of well-characterized aluminous goethite samples. In addition, the difference between T B and T N increases from $8-9 K for a pure goethite (that contains vacancies) to >20 K for Al-substituted goethites. This indicates that T B and T N change independently with the diamagnetic substitutions, suggesting a fundamental difference of magnetization process between parasitic remanence and AFM. A zonal distribution of the diamagnetic substitutions has been incorporated to account for the observed magnetic properties of the AFM and the parasitic remanence. Nonetheless, both parasitic remanence and AFM could exist along the goethite c axis.