HR: 0830h
AN: GP31B-0750    [PDF]
TI: First-order reversal curve (FORC) diagrams of elongated single-domain grains at high and low temperatures
AU: * Carvallo, C
EM: carvallo@physics.utoronto.ca
AF: Physics Department University of Toronto at Mississauga, 3359 Mississauga Road, Mississauga, ON L5L1C6 Canada
AU: Ozdemir, O
EM: ozdemir@physics.utoronto.ca
AF: Physics Department University of Toronto at Mississauga, 3359 Mississauga Road, Mississauga, ON L5L1C6 Canada
AU: Dunlop, D J
EM: dunlop@physics.utoronto.ca
AF: Physics Department University of Toronto at Mississauga, 3359 Mississauga Road, Mississauga, ON L5L1C6 Canada
AB: First-order reversal curve (FORC) diagrams were measured on elongated single-domain maghemite particles at temperatures ranging from 20 K to 853 K (580$^{\rm o}$C). Below room temperature, the FORC distribution is almost temperature invariant because both FORC variables, coercivity $H_c$ and interaction field $H_i$, are proportional to spontaneous magnetization $M_s$, which changes only 15% from 20-300 K. Above room temperature, the FORC distribution contracts along both axes above 430$^{\rm 0}$C and the bulk coercive force $$ decreases rapidly. These features are caused by the increasing thermal activation of single-domain moments pinned by shape anisotropy. A profile f($H_c$) through the FORC peak along the $H_c$ axis can be integrated to give a synthetic alternating field demagnetization curve or isothermal remanence curve. The synthesized curves match measured curves quite well at different temperatures, and the FORC median $H_c$ varies approximately in proportion to $(T)$. A profile g($H_u$) through the FORC peak parallel to the $H_u$ axis contracts with heating approximately as $M_s(T)$, and the half-width of g($H_u$) is similar to independent measures of local interaction fields $H_i$. These properties support the interpretations of FORC variables $H_c$ and $H_u$ as being equivalent to microcoercivity $H_c$, and interaction field $H_i$, respectively.
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
DE: 1594 Instruments and techniques
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting