Title: Thermoremanent
magnetization of multidomain hematite
Source: JOURNAL
OF GEOPHYSICAL RESEARCH-SOLID EARTH 110 (B9): Art. No. B09104 SEP 24
2005
Document Type: Article
Language: English
Abstract: We
have studied thermoremanent magnetization (TRM) produced by fields of
10-140 mu T in the (0001) basal plane of a 10 x 6 x 2 mm natural single
crystal of hematite, both before and after zero-field cycling through
the Morin transition at T-M = 260 K. Stepwise thermal demagnetization
of TRM indicated high-unblocking temperatures between 680 degrees C and
the Curie-Neel temperature T-N = 690 degrees C. In contrast, TRM was
easily demagnetized by alternating fields, TRM intensity decreasing
exponentially with increasing field in typical multidomain fashion. The
observed 100-mu T M-TRM is 1.1 kA/m. This strong TRM, almost equal to
the saturation remanence, results from hematite's weak internal
demagnetizing field. Domain walls move almost unhindered to their
limiting positions, and TRM intensity approaches saturation. On cooling
through T-M, spins rotate to the antiferromagnetic c axis, and
hematite's weak ferromagnetism is largely lost. However, on reheating
in zero field through T-M, as the spins rotate back into the basal
plane, a "memory" remanence is regenerated in the original TRM
direction. This TRM memory was about 25% of M-TRM for our crystal and
was even more resistant to thermal demagnetization than the original
TRM. The 25% memory of TRM is similar to that of 0.12- to 0.42-mm
single-domain hematites. High-unblocking-temperature TRM and TRM memory
must be due to magnetoelastic pinning of spins in the basal plane by
lattice defects, because both TRM and memory decrease with
high-temperature treatment, which anneals out defects. The memory
phenomenon seems to be in essence an amplification of residual
magnetism that survives below the Morin transition. Remanence produced
in a demagnetized sample below T-M and room temperature remanence that
has been cooled through T-M increase in identical ways on warming
through the transition. We propose that small regions of canted spins,
pinned by crystal defects, remain below T-M when the bulk of spins have
aligned with the antiferromagnetic c axis. These nuclei serve to
regenerate room temperature domain structure and remanence in warming
through T-M.
KeyWords Plus: SINGLE
CRYSTALS; PHASE TRANSITIONS; MORIN TRANSITION; NATURAL HEMATITE; DOMAIN
STRUCTURE; ALPHA-FE2O3; HAEMATITE; TEMPERATURE; RESONANCE; PATTERNS
Addresses: Ozdemir
O (reprint author), Univ Toronto, Dept Chem & Phys Sci,
Mississauga, ON L5L 1C6 Canada
Univ Toronto, Dept Chem & Phys Sci, Mississauga, ON L5L 1C6 Canada
Publisher: AMER
GEOPHYSICAL UNION, 2000 FLORIDA AVE NW, WASHINGTON, DC 20009 USA
Subject Category: GEOCHEMISTRY
& GEOPHYSICS
IDS Number: 968SJ
ISSN: 0148-0227