Développement de la micromagnétométrie appliquée aux sciences de l'Univers – MagMap

The typical spatial scale for magnetization measurements in paleomagnetism laboratories is 1-10 cm3. This size is indeed perfectly adapted to most "standard" paleomagnetic applications. On the other side, scanning techniques such as Magnetic Force Magnetometry provide very high-resolution magnetic images of magnetic domains (down to a few dozen nm), but do not provide quantitative results and are more adapted to physics or pure rock magnetism studies. Nevertheless, an increasing number of emerging applications of paleomagnetism and rock magnetism require magnetization measurements at an intermediate scale (in the 10-500 µm range) and more importantly require mapping of the magnetization distribution in the sample. The core of this project is to develop a set of new magnetic instruments that can be used for high-resolution (10-500 µm scale) quantitative magnetic mapping of polished geological and extraterrestrial samples. The development of these instruments will fill a gap in the magnetic tools available to the Earth and planetary sciences community and would open a wide range of new applications, in particular in the field of extraterrestrial magnetism. We propose to develop the following complementary systems that we have already validated experimentally: 1) a scanning magnetometer based on a Giant Magneto Resistance (GMR) sensor 2) a magnetic microscope based on the magneto-optical Faraday effect 3) a scanning magnetometer based on microscopic Hall-probe. Concerning, the first instrument, we have already put into operation a fisrt instrument whose promising results encourage further development: improvement of sensitivity and spatial resolution, measurement of magnetic susceptibility. With the second instrument, it will be possible to obtain magnetic maps at variable temperature and under an applied field, allowing mapping of phase transitions, unblocking temperatures and Curie temperatures. The third instrument offers a spatial resolution down to 1 µm. The first instrument measures the in-plane components of the magnetic field above the sample where the two others measure the perpendicular component. A number of specific procedures will be developed to invert the magnetic field data and obtain the distribution of the magnetization in the samples. Beside the interest that the development of these instruments presents in the field of Physics, the main scope will be the applications to Earth and Planetary Sciences. In particular, we will investigate the paleomagnetism and rock magnetism of extraterrestrial material, and the effects of shock on the magnetization and the magnetic properties of rocks (a topic which is also related to the interpretation of planetary magnetic anomalies). In the field of extraterrestrial magnetism, the main focus will be the study of the natural magnetization of meteorites, in particular primitive chondrites that are the most likely to have retained information about the primordial magnetic fields in the early Solar system. Martian meteorites and HED meteorites (that very probably originate from asteroid Vesta) are also of particular interest because they ay hold clues to the possible existence of a dynamo field on their respective parent body. Eventually, equilibrated chondrites also deserve some attention because the very existence of a remanent magnetization in these meteorites is enigmatic. In the field of shock and magnetism, we propose an innovative approach that combines pulsed laser impacts and micromagnetometry. Laser impacts can create shock waves that reach several tens of GPa at the surface of the sample, and that decay rapidly over a few mm. Using micromagnetometry it is possible to explore the magnetic effects over the whole range of pressures from the maximum pressure at the point of impact down to zero away from the point of impact. This aspect will be conducted in parallel with more standard paleomagnetic work on selected terrestrial impact structures. The micromagnetometry techniques described here can also be used to identify the magnetic carriers in any terrestrial rock as part of more "classical" rock magnetic and paleomagnetic studies, through comparison of magnetic maps with elemental maps, or SEM or optical images. Mapping of susceptibility, blocking/unblocking temperatures, and Curie temperatures, will also be part of a new toolbox for our terrestrial magnetic studies. In the long term, the goal is to develop at CEREGE a center for micromagnetometry that will not have its equivalent elsewhere. Such a pole will be publicized and opened to the research scientists in paleomagnetism and rock magnetism, on both the national and the international scale.