GP24A-01
Transition Warming and Cooling Remanences in Magnetite, Hematite and Pyrrhotite
One way of gaining insight into the size and morphology of assemblages of magnetite particles is to compare either remanence or susceptibility at low temperatures after zero-field cooling (ZFC) and after field cooling (FC) through the Verwey transition around Tv = 120 K. At 10 K a sample is demagnetized following ZFC, while in the FC initial state before warming the sample has a transition cooling remanence (TCRM), acquired largely but not entirely in crossing Tv. There is also a reciprocal remanence acquired as a result of heating a demagnetized sample from low temperature across Tv. This transition warming remanence (TWRM) is often called an inverse TRM. Hematite and pyrrhotite also acquire TCRM and TWRM in crossing a low-T magnetic phase transition in the presence of a field. In TCRM experiments, initially demagnetized samples were cooled in a 2 mT field from 300 K to 10 K and measured every 1 K to 5 K. At 10 K, the field was zeroed, and the remanence was then monitored during zero-field warming back to 300 K. The properties of TCRMs were generally similar to those of TWRMs produced by heating a ZFC sample in a 2 mT field from 10 K, but the increase in induced magnetization for the magnetites and pyrrhotites in cooling through their phase transitions was relatively small because of the very high coercivity of the low-temperature phase of both minerals. In the case of hematite, where the low-temperature phase is almost non-ferromagnetic, the induced magnetization actually decreased across the transition. On the other hand, 70-90 percent of the induced magnetization was retained as remanence when the field was zeroed at 10 K, and up to 20 percent of this remanence survived zero-field warming through the transition. This room- temperature memory was much larger than the remanence below the Morin transition. A remarkable property is the observed mirror-image symmetry between in-field warming curves tracking the acquisition of TWRM and zero- field warming curves of TCRM between 10 K and 300 K. The symmetry, with increases in the field-on induced + remanent magnetization curves exactly mirroring decreases in the field-off remanent magnetization curves, is almost perfect from 10-110 K.
GP24A-02
Is the Morin Transition Size Dependent?
Morin transition temperatures Tm were measured for submicron (SD) synthetic hematites with grain sizes between 120 and 520 nm and on 0.5 and 6 mm (MD) natural single crystals. The Morin transition temperature is quite variable in both SD and MD hematites. Tm ranged from 250 to 260 K for natural crystals and even more widely, from 241 to 254 K, for the more heterogeneous synthetic hematites. The scatter must be due to different methods of preparation, crystal morphology, strain and crystal imperfections that are common for both synthetic and natural crystals. The present Tm data for SD and MD hematites are compatible with published data by other workers in the 100 nm - 10 mm range. In this interval, Tm decreases slightly with decreasing particle size but is almost size independent. However, in nanoparticles with grain sizes between 30 and 90 nm, the spin-flop transition is strongly dependent on particle size: Tm decreases sharply with decreasing size and the transition disappears below 20 nm. This phenomenon is due to surface effects that lead to spin directions deviating from the easy axis. The present SD and MD hematites exhibit a thermal hysteresis in the Morin transition: the values of Tm in cooling and in heating are different. For the same cooling/warming rate, the width of the Morin transition in the submicron crystals is broader than for the natural single crystals. This broadening in SD hematites is directly attributable to the wide distribution of particle sizes. In natural single crystals the entire width of the transition could be due to crystal imperfections and the internal stresses that result from these defects.
GP24A-03
Ferrian Ilmenites: Investigating the Magnetic Phase Diagram
The main objective of this study is to investigate the magnetic phase changes within the hematite-ilmenite solid solution, yFeTiO3·(1-y)·Fe2O3. Two sets of synthetic ferrian ilmenites of y-values equal to 0.7, 0.8, 0.9, and 1.0 were available for this study. As currently drawn, the magnetic phase diagram, proposed by Ishikawa et al. [1985, J. Phys. Soc. Jpn. v.54, 312-325], predicts for increasing y values (0.5<y<1.0) magnetic odering from paramagnetism to (1) ferrimagnetism, or (2) to superparamagnetism then ferrimagnetism, or (3) to superparamagnetism then antiferromagnetism. Moreover, for y values ranging between 0.65 and 1.0 a transition into a spin glass state is expected at 100K or below. Ilmenite, y=1.0, is antiferromagnetic. Various low tempreature experiments including temperature dependance of remanence and induced magnetizations and AC susceptibility were conducted in order to characterize the magnetic behaviour and changes of magnetic states. In general, the data confirms the predicted phase changes for the different compositions investigated. The y=1.0 sample, pure ilmenite, is antiferromagnetic below 57K, the measured Néel temperature. The y=0.9 sample magnetically orders at about 100K in a superparamagnetic state. Hysteresis loops remain effectively closed down to 60K below which an antiferromagnetic order prior to reaching the spin glass state is ambiguous. The y=0.8 sample magnetically orders at about 270K in an initially superparamagnetic states before entering a ferrimagnetic state below about 250K. Lastly, as previously demonstrated in Lagroix et al. [2004, JGR-B, v.109, doi:10.1029/2004JB003076], the y=0.7 samples order ferrimagnetically at 380K. However, like the y=0.7 samples which also demonstrated an antiferromagnetic state at temperature above the Curie temperature, hysteresis loops for y=0.9 and y=0.8 only achieve perfect linearity at 190K and 340K respectively. All samples (except y=1.0) show a frequency dependent amplitude non-dependent quadrature susceptbility peak at 30K which is most likely associated with the spin glass like state. Time-dependant DC and AC induced magnetization experiments provide convincing evidence and help define the range of temperatures over which the behaviour persists.
GP24A-04
Effects of Temperature on Ilmenite-Hematite: Microstructure and Magnetic Properties in the Ecstall Pluton, British Columbia
The ~91 Ma Ecstall pluton exhibits paleomagnetic directions and NRM intensities that vary spatially in a systematic way (Butler et al. 2002, Brownlee et al. 2006). This spatial variation may be related to reheating from the adjacent ~58 Ma Quottoon pluton of the Coast Mountains batholith (CMB) (Hollister et al. 2004, Brownlee et al. 2006). In order to determine the mineralogic origins of the magnetic variations we studied single grains of ilmenite-hematite from samples along a transect on the South side of the Skeena River using hysteresis, FORC, and remanence v. temperature experiments in concert with petrographic, SEM, and TEM observations. Light microscope and SEM observations indicate 3 basic types of grains; (1) hematite host with ilmenite lamellae (found >13 km from the CMB); (2) hematite with large, irregular ilmenite host regions (found ≤ 13 km from the CMB); and (3) hematite host with blitz texture formed by many large rutile needles in 6 orientations (found <8 km from the CMB). Single crystals distant from the CMB have coercivities from 250-350 mT, and FORC coercivity distributions that are curved and have a negative Ha offset, which we attribute to exchange coupling between hematite lamellae. Single crystals closer to the CMB appear to have two magnetic phases. Hysteresis loops are wasp-waisted, and FORC coercivities have two distinct peak distributions; one at 300-350 mT with a negative Ha offset, and another at 10-15 mT with no apparent offset. Remanence v. temperature, and TEM analyses indicate magnetite as the second magnetic phase. Samples far from the CMB show no evidence of this second magnetic phase indicating that magnetite growth may be related to reheating. Significant grain-to-grain variability in the ratio of magnetite to hematite within a sample makes FORC analysis useful in distinguishing the hysteresis properties of the two magnetitc phases. Magnetite coercivity distributions change only in peak intensity, while for hematite there are trends in the shape of the coercivity distribution, maximum coercivity, and Ha offset that may be correlated with distance from the CMB.
GP24A-05
Fundamental Magnetic Properties from Pure Synthetic Greigite
Greigite (Fe3S4), an authigenic mineral that forms in sulfate-reducing environments, has been widely identified in marine and lake sediments. It is often the main magnetic carrier in some settings, and can therefore be significant in paleomagnetic and environmental magnetic studies. However, unlike its iron oxide counterpart, magnetite (Fe3O4), the fundamental magnetic characteristics of greigite are still poorly understood, which is partially due to the metastability of greigite and the difficulty in obtaining high quality greigite samples. We have successfully synthesized pure greigite samples with good crystallinity using a new hydrothermal method. Our detailed low- and high-temperature magnetic measurements document the previously poorly known magnetic properties of greigite, including the first accurate measurement of its saturation magnetization. We have for the first time unambiguously determined the magnetic structure of greigite by combined neutron powder diffraction and neutron polarization analysis. Low temperature (LT) neutron diffraction spectra reveal the temperature dependence of sublattice magnetizations. The pure synthetic greigite samples are large enough to show pseudo-single-domain (PSD) and multi-domain (MD) behavior. LT cycling (LTC) of saturation isothermal remanent magnetization (SIRM) measurements indicate a continuous demagnetization of remanence during cooling. Preservation of the main features of first-order reversal curve distributions at LT, coupled with LT SIRM warming curves, rule out the presence of substantial superparamagnetic behavior in the studied samples. No LT magnetic transition has been detected; however, a local coercivity minimum is observed at around 130 K. These fundamental studies provide new constraints on the magnetic behaviour of greigite.
GP24A-06
Low Temperature Magnetic Properties of Daubreelite, Troilite and Enstatite Chondrites
Various FeNi phases are dominant magnetic phases in most chondritic meteorites. In addition, iron-bearing sulphides are present in meteorites and in cometary dust. The low-temperature magnetic properties of the daubreelite (FeCr2S4) and troilite (FeS) were investi-gated. Daubreelite is ferrimagnetic below its Curie temperature Tc = 150 - 170 K, depending on its exact chemical composition. On cooling through the Tc its saturation magnetization sharply increases up to a maximum of ~32 Am2/kg at 80 K, while the magnetic susceptibility ranges between 0.5-3.5 10 4 m3/kg reaching maximum immediately below Tc. Further cooling reveals a magnetic transition around 60 K (characterized by spin-glass-like features and cubic-to-triclinic symmetry reduction) manifesting itself in a local maximum of induced magnetization and susceptibility. Remanence acquired at 5 K shows a major drop at the transition. Troilite is antiferromagnetic at room temperature. At Tm = 62 +- 4 K we have found a magnetic transition which is likely due to spin canting. Below the transition both saturation magnetization and coercivity increase dramatically, the latter approaching 0.5 T at 5 K. Magnetic saturation is not reached in 5 T field at this temperature. However, susceptibility remains low (~4 10-7 m3/kg) with a local maximum at Tm (~1.7 10-6 m3/kg). Compared to FeNi, saturation magnetization of daubreelite at 10 K is lower by a factor of 7 and that of troilite is lower by a factor of about 200. Daubreelite can therefore contribute to or even control magnetic properties of bodies in the cold regions of Solar System. The low temperature magnetic signature of daubreelite is recognizable in magnetic measurements made on various enstatite chondrites and can thus be used to identify its presence.
GP24A-07
In-situ TEM observation of the interaction between magnetic domain walls and twin domain walls below the Verwey transition in magnetite
The Verwey transition has an enormous impact on the magnetic properties of magnetite at low tempeatures - the magnetocrystalline anisotropy increases by an order of magnitude and the magnetic easy axis switches from the <111> directions of the cubic phase to the [001] direction of the monoclinic phase. On cooling through the transition, the [001] easy axis of the monoclinic phase may be chosen to lie along any one of three <100> directions of the parent cubic phase, resulting in the development of transformation twinning. Numerous studies have proposed that a strong interaction exists between the ferroelastic twin walls and the ferrimagnetic domain walls in magnetite. Nevertheless, the nature of this interaction remains highly controversial. Key questions include: i) are the ferroelastic twin walls strongly pinned, or can they be moved by application of a stress and/or magnetic field? ii) are magnetic domain walls strongly pinned by the twin walls or can they be moved indepedently? and iii) how does the twin microstructure that develops on cooling through through the transition depend on the magnetic microstructure that exists above the transition, and vice versa? To address these questions we have performed an in-situ study of the cubic to monoclinic phase transition in synthetic multi-domain magnetite using low-temperature transmision electron microscopy. The Fresnel mode of Lorentz microscopy was used to make simultaneous observations of the nucleation and translation of transformation twins and magnetic domain walls as the sample was repeatedly cycled through the phase transition. The phase transition is first-order in character and proceeds by the rapid movement of an abrupt phase interface separating the cubic and monoclinic phases. For temperatures just below the transition point, heating of the sample by the electron beam is sufficient to cause rapid movement of the phase interface and internal rearrangement of the transformation twins within the monoclinic phase. There appears to be little "twin memory", i.e. a different set of transformation twins is often observed each time the sample is cooled through the transition. The distribution of magnetic domains above and below the transition was generally very different. The cubic phase is characterised by a low density of magnetic domain walls, whereas the monoclinic phase contains a higher density of closely-spaced lamellar domains. In contrast to previous studies, magnetic closure domains within the monoclinic phase were also observed to be relatively common. Regions showing a clear interaction between magnetic domain walls and twin domain walls were observed. Typical features include the pinning of magnetic domain walls at the tips of needle twin domains and the shearing of needle twins by an intersecting magnetic domain wall.
GP24A-08
Early Developments and Applications of SQUID Rock Magnetometers.
The SQUID rock magnetometer at the University of Pittsburgh was designed by Bill Goree and was operational by 1969. This was only some 7 years after Josephson had published his work predicting the preservation of long range order between two superconductors across a thin insulating barrier: the Josephson junction, whose incredible sensitivity to magnetic flux made the rock magnetometer possible. The intial rock magnetometer used RF SQUIDs. It was designed with an open room temperature access, so that long cores could be passed through the instrument to be measured. Other experiments such as thermal demagnetization, stress effects, or viscosity could also be carried out with continuous observations. Unfortunately, at this time the RF SQUIDs were somewhat unreliable. The intial design had to be abandoned because replacements of SQUIDs required complete disassembly of the instrument, which was costly and time consuming. A standard vertical dewar design was then adopted with the SQUIDs easily accessable in the liquid helium. With the development of more reliable SQUIDs a return to the intial design was undertaken. This intrument with its DC SQUIDs is now used in laboratories all around the world.