Low‐temperature magnetic behavior of multidomain titanomagnetites: TM0, TM16, and TM35

Abstract

[1] An unpredicted effect of the Verwey transition in magnetite is that a field-cooled (FC) remanent magnetization can be less intense than a zero-field cooled (ZFC) isothermal remanence. The effect, only documented in a handful of multidomain (MD) samples, is thought to be unique to MD material. Data for new MD samples all show an elevation of ZFC over FC remanences. Current theory suggests that the FC easy axis bias alone produces the effect. We measured hysteresis loops after three cooling pretreatments; the results are inconsistent with the aforementioned theory. They are, however, consistent with a previous hypothesis which cites the absence of transformational twins in FC samples as an important factor. Our initial low-temperature domain observations in FC and ZFC magnetite further support this theory. We also present data for MD titanomagnetites (x = 0.16, 0.35). These samples also show elevated ZFC remanences below a critical temperature (T crit ). The titanomagnetites' frequency dependence of susceptibility around T crit , the suppression of the amplitude dependence of susceptibility below T crit , and Mössbauer data suggest that the change in magnetic anisotropy at T crit is related to a suppression of B site electron hopping at low temperature, at least on the timescale of the magnetic measurements. Given our remanence data, field cooling must affect the orientation of the new low-temperature magnetic easy axis. We appeal to the same process as we did for magnetite to explain the elevation of ZFC moments, noting that the exact nature of the transition across T crit is not completely understood.Citation: Carter-Stiglitz, B., B. Moskowitz, P. Solheid, T. S. Berquó, M. Jackson, and A. Kosterov (2006), Low-temperature magnetic behavior of multidomain titanomagnetites: TM0, TM16, and TM35,

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

“…Because in MD grains saturation remanence is related to coercivity through the demagnetizing factor (SIRM ∝ H c /N ; e.g., Dunlop andÖzdemir, 1997), ZFC SIRM will be larger than FC SIRM, resulting in R LT < 1. Recently, Carter-Stiglitz et al (2006) convincingly argued that, in addition to Kosterov's "geometrical" mechanism, higher coercivity of MD grains after zero field cooling can be caused by the presence of "hard" magnetic 90…”

“…For single-domain (SD) magnetite, the FC SIRM is always greater than the ZFC SIRM (e.g., Moskowitz et al, 1993;Carter-Stigliz et al, 2002). In contrast, for large multidomain (MD) magnetite grains, the FC SIRM has been found to be significantly lower than the ZFC SIRM (e.g., Kosterov, 2001;Brachfeld et al, 2002;Carter-Stiglitz et al, 2006). Such a difference in the R LT ratio reflects different mechanisms governing the SIRM acquisition in single-domain and multidomain magnetite below T V (Kosterov, 2003).…”

“…The character of this control, however, depends on magnetic domain state (and, hence, on the grain size and shape) of magnetite (e.g., Kosterov, 2001;Smirnov and Tarduno, 2002;Carter-Stiglitz et al, 2006). In particular, magnetic domain state affects the ratio (R LT ) between an SIRM imparted at a very low temperature (10-20 K) after cooling in a strong magnetic field (>1 T) (field cooling, FC) and an SIRM imparted after cooling in a zero field environment (zero field cooling, ZFC).…”

Rock-magnetic properties of single zircon crystals sampled from the Tanzawa tonalitic pluton, central Japan

“…One sample (TNZ03-08-41) showed clear transition temperature~35 K. If this transition indicates the pyrrhotite, it has remanent magnetization at room temperature and may affect paleointensity experiment. The intensity of FC remanence was larger than that of ZFC remanence, which is a characteristic behavior of SD and fine-grained PSD magnetite (Moscowitz et al 1993;Kosterov 2003;Carter-Stiglitz et al 2006), although the zircon crystals may contain magnetic minerals other than pure magnetite.…”

Progressive and Punctuated Magnetic Mineral Diagenesis: The Rock Magnetic Record of Multiple Fluid Inputs and Progressive Pyritization in a Volcano‐Bounded Basin, IODP Site U1437, Izu Rear Arc

“…The FC and ZFC paths for this sample track closely together, except below about 50 K, where the FC curve is higher, and both FC and ZFC warming curves decrease rapidly between 20 and 50 K and continue to decrease above T V . When MD magnetite is the dominant remanence carrier, the ZFC curve is usually higher than the FC curve, at least up to T V (Carter‐Stiglitz et al, ), and most of the remanence is lost in a steep decrease around the Verwey transition. Brachfeld et al () attributed similar FC/ZFC curve behavior to that shown in Figure to the presence of mixed “PSD” (vortex state) and SP grains.…”