Anatomy of a pressure-induced, ferromagnetic-to-paramagnetic transition in pyrrhotite: Implications for the formation pressure of diamonds

Meteorites and diamonds encounter high pressures during their formation or subsequent evolution. These materials commonly contain magnetic inclusions of pyrrhotite. Because magnetic properties are sensitive to strain, pyrrhotite can potentially record the shock or formation pressures of its host. Moreover, pyrrhotite undergoes a pressure-induced phase transition between 1.6 and 6.2 GPa, but the magnetic signature of this transition is poorly known. Here we report room temperature magnetic measurements on multidomain and single-domain pyrrhotite under nonhydrostatic pressure. Magnetic remanence in single-domain pyrrhotite is largely insensitive to pressure until 2 GPa, whereas the remanence of multidomain pyrrhotite increases 50\% over that of initial conditions by 2 GPa, and then decreases until only 33\% of the original remanence remains by 4.5 GPa. In contrast, magnetic coercivity increases with increasing pressure to 4.5 GPa. Below ∼1.5 GPa, multidomain pyrrhotite obeys Néel theory with a positive correlation between coercivity and remanence; above ∼1.5 GPa, it behaves single domain-like yet distinctly different from uncompressed single-domain pyrrhotite. The ratio of magnetic coercivity and remanence follows a logarithmic law with respect to pressure, which can potentially be used as a geobarometer. Owing to the greater thermal expansion of pyrrhotite with respect to diamond, pyrrhotite inclusions in diamonds experience a confining pressure at Earth’s surface. Applying our experimentally derived magnetic geobarometer to pyrrhotite-bearing diamonds from Botswana and the Central African Republic suggests the pressures of the pyrrhotite inclusions in the diamonds range from 1.3 to 2.1 GPa. These overpressures constrain the mantle source pressures from 5.4 to 9.5 GPa, depending on which bulk modulus and thermal expansion coefficients of the two phases are used

Effet Mössbauer en ligne et effet de recul dans les alliages de substitution Fe1-xAlx(b.c.c.) et Fe-Ni (c.f.c.)

On-line Môssbauer experiments show that, in substitutional alloys such as Fe-Al and Fe-Ni, the vacancy rich zone surrounding the primary knock-on collapses at the end of the recoil event. It is preferentially filled by iron atoms.Les expériences d'effet Môssbauer en ligne sur les alliages de substitution Fe-A1 et Fe-Ni montrent que la fin de l'événement de recul est marquée par l'effrondrement de la zone riche en lacune qui entoure l'atome primaire et son remplissage sélectif par des atomes de fer

Perfect magnetic compensation of gravity along a vertical axis

International audienceMagnetic compensation of gravity allows for ground-based experiments to be carried out under weightless conditions at reasonable cost and without the time limitation of systems such as zero-g airplanes or drop towers. Most of the time classical superconducting solenoids are operated to perform such experiments, which lead to a poor quality of the gravity compensation due to the non-ideal shape of the current and field distribution. In order to improve the quality of simulated microgravity, scientists need to build novel ground-based systems fully dedicated to magnetic levitation. The magnetic design of these levitation apparatuses would be based on theoretical studies of magnetic forces and associated field distributions. The work presented in this paper demonstrates the possibility of producing a magnetic force on paramagnetic and diamagnetic materials that is constant, thus providing a uniform gravity compensation, along a segment in a 3D geometry. These results come from both the decomposition in spherical harmonics of the magnetic field and specific conditions applied on the magnetic force. The magnetic field configuration leads to conical-shape isohomogeneity of the resulting acceleration. As an additional remark to previous works, the impossibility to get a magnetic force varying as 1/r(2) is briefly described

Diamagnetic levitation based digital microfluidics

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The Magnetic Metrology Laboratory for Low Field in Grenoble

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155Gd MOSSBAUER EXPERIMENTS ON (Gd0.8RE0.2)l+ε Fe4B4

Mössbauer experiments using the 86.5 keV resonance of 155Gd have been performed on Gd1+εFe4B4 with and without substitution of 20 % of Gd by Nd or Sm. All these alloys are characterized by a very large quadrupolar interaction. At T = 4.2 K, i.e. below the magnetic temperature order, the Mössbauer spectra are quite different for the three alloys, owing to the different direction of the Gd moments with respect to the c-axis. The preferred moment direction for the Gd atoms is imposed by the R.E. atoms in substitution for Gd, and determined by a magnetic anisotropy resulting chiefly from the second order crystal electric field interactions

Magnetophoretic trapping of microparticles

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Magnetic levitation of microdroplets in air

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An Optimization Method for Magnetic Field Generator

International audienceWe propose an optimization method to design an elongated three-axes magnetic field generator with given criteria specified over a large volume. The approach is based on the field expansion in Spherical Harmonics and Tchebychev polynomials, for noncircular symmetrical coils arrangement. We developed a specific tool, to get a “flat” or a given "equal-ripple" solution, over a chosen length. The parameters to be defined are: dimensions, coil positions and Amp-turns, associated with the different axes. Once these parameters have been computed, a program predicts the field for the whole structure. A major interest of using such a method lies on the fact that, once the true optimal solution is found, any deviation from theoretical results (for instance building inaccuracy) can be compensated by adjustment of any other design parameters (i.e. current), restoring the initial homogeneity. This method has been successfully applied to the simulator of the Magnetic Metrology Laboratory for Low Field (Laboratoire de Métrologie Magnétique en Champ Faible in French). The experimental results correspond to the theoretical computation