Grain size dependence of low-temperature remanent magnetization in natural and synthetic magnetite: Experimental study

Abstract

Magnetic measurements at cryogenic temperatures (<300 K) proved to be useful in paleomagnetic and rock magnetic research, stimulating continuous interest to low-temperature properties of magnetite and other magnetic minerals. Here I report new experimental results on a grain size dependence of the ratio (R LT ) between a lowtemperature (20 K) saturation isothermal remanent magnetization (SIRM) imparted in magnetite after cooling in a 2.5 T field (field cooling, FC) and in a zero field environment (zero field cooling, ZFC). Synthetic magnetite samples ranged in mean grain size from 0.15 to 100 μm, representing nearly single-domain (SD), pseudosingledomain (PSD), and multidomain (MD) magnetic states. The R LT ratio monotonically increases from 0.58 to 1.12 with the decreasing mean grain size, being close to unity for PSD grains (0.15-5 μm) and smaller than unity for MD magnetite (12-100 μm). The R LT ratio of 1.27 is observed for acicular magnetite characterized by nearly SD behavior. These observations indicate that within the range of ∼0.15 to ∼5 μm, the low-temperature SIRM may be higher than that expected from "normal" magnetic domain wall displacement. Such a behavior can be caused by the presence of a SD-like component in the magnetization of these grains, which origin, however, is uncertain. The natural rocks containing nearly stoichiometric magnetite manifest a dependence of the R LT ratio on magnetic domain state identical to that observed from synthetic magnetites. Therefore, the comparison of FC SIRM and ZFC SIRM at very low temperatures may allow a crude estimate of magnetic domain state in some magnetite-bearing rocks, such as shallow mafic intrusions or some marine sediments.

Vertical Axis Rotation Across the Eastern Mono Basin and West‐Central Walker Lane Revealed by Paleomagnetic Data From the Jack Springs Tuff

“…The FC remanence is slightly higher than the ZFC remanence indicative of single-domain titanomagnetite and the two curves converge at~50 K (JS1, JS8) and~150 K (JS14), respectively ( Figure 9b). The ratio of δFC/δZFC and M1/M5 are all greater than 1.0 and between 1.0 and 1.3, respectively, consistent with a single-domain grain size Smirnov, 2009). The lack of strong field dependence above 100 K for these samples on warming is diagnostic of ferromagnetic ordering behavior of a single domain to pseudosingle-domain magnetic grain size.…”

“…The specimen is then subsequently cooled in a null magnetic field (ZFC) to 10 K, where a LTSIRM is imparted and the remanence is measured again on warming back to room temperature (ZFC remanence). The ratio of the δFC/δZFC (Moskowitz et al, ) and the ratio of M1/M5 (Smirnov, ) provides an estimate of the magnetic domain state. The magnetic domain state of the grain is single domain when the δFC/δZFC ratio is greater than 1 and the M1/M5 ratio is between 1 and ~1.3.…”

Tuning of Luminescent and Magnetic Properties via Metal Doping of Zn-BTC Systems

“…This is followed by cooling the sample in the same 0.001 T field (FC) and again measuring induced sample moment on warming in a 0.001 T field (Figure 9). The ratio of the δFC/δZFC [52] and the ratio of M1/M5 [53] provides an estimate of the magnetic domain state. The magnetic domain state of the grain is single domain when the δFC/δZFC ratio is greater than 1 and the M1/M5 ratio is between 1 to~1.3.…”

Low temperature magnetic properties of the Late Archean Boolgeeda iron formation (Hamersley Group, Western Australia): environmental implications

“…In all cases, except for samples 146.70 and 152.86, which both have a low and noisy signal without clear transition, data show a strong Verwey transition around 110-120 K indicative of magnetite (Figure 6). In all samples the ZFC magnetization is always larger than the FC magnetization, the R LT value define as M fc (20 K)/M zfc (20 K) (Smirnov, 2009) is between 0.62 and 0.73. These are characteristics of large multi domain magnetite (Brachfeld et al, 2002;Smirnov, 2009).…”

“…In all samples the ZFC magnetization is always larger than the FC magnetization, the R LT value define as M fc (20 K)/M zfc (20 K) (Smirnov, 2009) is between 0.62 and 0.73. These are characteristics of large multi domain magnetite (Brachfeld et al, 2002;Smirnov, 2009). The Morin transition, around 262 K (Morrish, 1994), is characteristic of hematite and is only faintly detected in the RT-SIRM cycle in sample 130.45 and Figure 6A).…”