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

[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 Mossbauer 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 Tcrit is not completely understood.