G. Fillion

Magnetic transition at 30–34 Kelvin in pyrrhotite: insight into a widespread occurrence of this mineral in rocks

Abstract A characteristic magnetic transition at 30–34 K is shown to provide a powerful tool for the identification of pyrrhotite with concentration down to 10 ppm through the same low-temperature techniques as applied to magnetite and hematite, extended down to liquid helium temperature. A review of rock magnetic and petrological data on pyrrhotite suggests that this mineral should be considered as a major carrier of paleomatnetic signals. Unblocking temperature up to 350°C and extreme resistance against AF may be encountered in fine grained pyrrhotite.

Pyrrhotite and the remanent magnetization of SNC meteorites: a changing perspective on Martian magnetism

Abstract A combined magnetic and mineralogical study of 10 Shergotty–Nakhla–Chassigny type (SNC) meteorites, including six recent desert finds, provides strong evidence that, like in ALH84001, the natural remanence (NRM) of basaltic shergottites (apart from Los Angeles, LA) is carried by pyrrhotite. Low Curie point titanomagnetite is the major magnetic carrier only in nakhlites, Chassigny and LA. LA appears to be by far the most magnetic SNC. The dominant role of pyrrhotite has major implications on the interpretation of Martian magnetic anomalies and SNC paleomagnetism: in particular pyrrhotite undergoes magnetic and/or phase transition at room temperature in a pressure range of 1.6–3.6 GPa, much lower than the peak shock pressure experienced by shergottites. Any impact-related pressure above this value should thus totally reset remanent magnetism. It is also suggested that the very low thermal stability of remanence in some SNCs may be due to hexagonal pyrrhotite in metastable ferrimagnetic state or shock-induced substructures in monoclinic pyrrhotite.

Rock magnetism of remagnetized Paleozoic carbonates: Low-temperature behavior and susceptibility characteristics

We have conducted a new set of rock magnetic experiments on samples of remagnetized Paleozoic carbonates of eastern North America. These experiments were designed to investigate the origin of the unusual hysteresis behavior of these rocks, by evaluating (1) the importance of ferrimagnetic pyrrhotite as a remanence carrier, and (2) the sources of low-field susceptibility. Low-temperature measurements of saturation isothermal remanent magnetization (SIRM) indicate that the pyrrhotite magnetic transition at 32 K is absent in the Trenton and Onondaga limestones of New York. This transition is observed but poorly expressed in the Knox Group of east Tennessee: room-temperature SIRMs, cooled to 10 K in zero field, lose a small fraction of their intensity between 30 and 35 K. Samples from all three formations show a broad peak in IRM intensity at about 200 K, which is typical of pyrrhotite. In the Trenton and Onondaga samples, the ferrimagnetic component of low-field susceptibility is significantly larger than the ratio Mrs/Hcr, and is thus probably due dominantly to magnetite; in some of the Knox samples the reverse is true, suggesting an important pyrrhotite contribution. For all of the samples, the ferrimagnetic susceptibility, normalized by the saturation magnetization, is anomalously high, about a factor of 5 or 10 higher than the typical value for magnetite. We believe that this indicates a very substantial contribution from superparamagnetic particles. Strong frequency dependence of susceptibility and very high ratios of anhysteretic to saturation remanence confirm the importance of ultrafine particles, spanning the superparamagnetic—single-domain boundary. All three of these chemically remagnetized carbonates units exhibit the following properties, which have not previously been found together for any rock or synthetic analog, and which therefore appear to constitute a diagnostic set of rockmagnetic criteria for recognizing chemically-remagnetized rocks: Mrs/Ms ≈ 0.89 (Hcr/Hc)−0.6; kƒ/Ms ≈ 50 μm/A; ARM/SIRM ≈ 20%; ka/(Mrs/Hcr) ≈ 50.

4f magnetism in CeNi, PrNi and NdNi single crystals☆

Abstract The magnetic properties of the orthorhombic CeNi, PrNi and NdNi compounds (CrB-type structure) have been studied on single crystals. PrNi and NdNi are ferromagnetically ordered below 20 and 28 K, respectively, whereas CeNi is an enhanced paramagnet in which the susceptibility passes through a maximum at about 140 K. Large anisotropic effects are observed in the three compounds and can be well accounted for by crystal electric field considerations. CeNi behaves as an anisotropic magnetic Fermi liquid in which the spin fluctuations are mainly longitudinal. These properties can be understood in the light of the band structure of this type of alloy in which a large hybridization occurs between the 3d Ni electrons and the 5d rare earth electron, as well as the 4f electrons in the case of CeNi.

Impact demagnetization by phase transition on Mars

Recent solar system exploration has revealed the presence of a complex pattern of strong crustal magnetization on Mars [Acuna et al., 1999; Figure l]. The north-south dichotomy of the magnetization might be interpreted in terms of shutdown of the Martian dynamo before the formation or reworking of the younger and/or thinner northern crust, as well as the Tharsis dome. On the other hand, there is no apparent difference in the older, southern “Noachian” heavily-cratered terrane between the magnetized and non-magnetized crust. One could invoke the proximity of the Hellas and Argyre giant impact craters (Figure 1) if impacts occurred after dynamo shutdown, and if a mechanism is available to demagnetize the crust at distances of several crater radii. This mechanism cannot be thermal, as the thermal effect is very limited at such distance; thus, the mechanism must be pressure-related.

The Pyrrhotite 32 K Magnetic Transition

Pyrrhotite [1] is a common ferrimagnetic mineral in terrestrial rocks and has been identified recently as the major remanence carrier in Martian rocks, as in SNC (Shergotty-Nakhla-Chassigny) type meteorites. This compound undergoes a low temperature magnetic transition around 32 K with a change in the natural magnetic remanence [2]. This transition is going to be of growing importance in paleomagnetism and rock magnetism. To determine the structure change at 32K neutron diffraction was performed on two crystal samples at 50K and 20K (D10-ILL). Based on the magnetic torque measurements, performed on the same samples, a model of low-temperature triclinic structure was build. This model, explains previous Mössbauer experiments [4], the torque measurements, the magneto-crystalline anisotropy decreasing.

Septet spin state in the lithium phthalocyanine π-radical compound. Role of dioxygen

Abstract The lithium phthalocyanine π-radical compound is a molecular semiconductor which exhibits exceptional magnetic properties. These latter are interpreted as resulting from two distinct spin species. One species is delocalized and its behaviour may be interpreted in the frame of itinerant magnetism. It is responsible of an extremely narrow ESR line. The other species is due to localization of the PcLi spins interacting with molecular oxygen. At low temperature this magnetic coupling is shown to stabilize a septet spin state which involves indirect ferromagnetic coupling between neighbouring π-spins. The experimental results obtained up to now on this unexpected spin state are presented and discussed.

Magnetic flux penetration process in two‐dimensional superconductor covered with ferromagnetic particle array

The magnetostatic effect of a ferromagnetic particle array on an underlying two‐dimensional superconducting Nb thin film was investigated in terms of flux penetration processes by means of high sensitive magnetometry. For bare Nb film, a large drop in magnetization due to a flux rearrangement appeared in the low field region below 2.5 mT. However, once magnetic particles were fabricated on the surface of the Nb film, the drop was not observed. In this configuration, the ferromagnetic particles seem to behave as flux guides for the external magnetic field, whereby the flux distribution inside the superconductor is well regulated. Furthermore, a periodical oscillation was observed in the hysteresis loops. This behavior is associated with a flux quantization in the regions where the superconductivity is suppressed by the stray field of the ferromagnetic particles. The local flux density distribution around the ferromagnetic particles was evaluated by using the London equation, and the magnetization curve of ...

Néel temperatures of synthetic substituted goethites and their rapid determination using low‐field susceptibility curves

The Neel temperatures (T N ) of synthetic goethites (α-FeOOH), substituted with diamagnetic elements (Al, Ga) have been determined from the temperature dependence of their low-field susceptibility using a Kappabrige AC bridge. These temperatures proved to be identical to those derived from more conventional measurements of high-field or remanent magnetization versus temperature, and in good agreement with those inferred from Mossbauer models. The low-field susceptibility based-technique appears to offer a precise and rapid method of determining T N While Neel temperature coincides with maximum unblocking temperature in Gagoethites, supporting the assumption that antiferromagnetism and parasitic ferromagnetism have a similar origin in goethite, the lack of clear correlation between saturation remanent magnetization and increasing diamagnetic substitution suggests that the weak ferromagnetism of goethite is due to substitution independent defects induced during crystal growth.

Magnetic flux penetration process in superconducting Nb film covered with lithographic array of ferromagnetic particles

Abstract The effect of a micron size Sm 33 Co 67 or Fe-Ni ferromagnetic particles array fabricated on a 2D superconducting niobium film was investigated by means of a magnetoresistivity measurement and rotating SQUID magnetometry. The magnetic particles seem to act as flux accumulators or flux guides through which flux lines penetrate into the superconducting Nb film.