If a rock is to retain a geologically meaningful magnetic record of its history, it is essential that it contains magnetic minerals which are capable of carrying stable magnetic remanence. Of the natural occurring magnetic minerals, magnetite is the most important because of its abundance and strong magnetic signature. The stability, i.e., the resistance to demagnetisation or reorientation, of magnetic remanence is related to grain size; in smaller grains the magnetic moments align to have single domain (SD) structures, in larger grains complex magnetic patterns are formed (multidomain (MD)). “Classical” domain theory predicts that SD remanence is stable, whilst MD remanence is not. However experimental evidence has shown that both SD and MD grains can have stable remanences. In this thesis the origin of stable MD remanence is examined. There are two opposing theories; one suggests that the stability is due to independent SD-like structures, the other postulates that the stability is due to metastable MD structure. A series of experiments were designed to examine the stability using a selection of characterised synthetic and natural samples. Low-stress hydrothermal recrystallised samples where grown for this study. For the first time, the stability of thermoremanence induced in hydrothermal crystals to cooling was examined. The results agree with previous observations for crushed and natural magnetites, and support kinematic models. The behaviour of SIRM and thermoremanences in MD magnetite to low-temperature cooling to below the crystallographic Verwey transition at 120-124 K (T_v) and the cubic magnetocrystalline anisotropy isotropic point (T_k) at 130 K was investigated. On cooling through T_v, SIRM was observed to decrease and demagnetise, however thermoremanence was found to display a large increase in the magnetisation at T_v, which was partially re- versible on warming. The size of the anomaly is shown to be dependent on the temperature at which the thermoremanence is acquired, internal stress and grain size. The anomaly is attributed to the large increase in the magnetocrystalline anisotropy which occurs on cooling through T_v . It is postulated that low-temperature cycling demagnetisation is due to kinematic processes which occur on cooling between room temperature and T_k. Characterisation of low-temperature treated remanence and partially alternating field demagnetised remanence, suggest that the stable remanence is multidomain. Low-temperature cooling of remanence in single sub-micron crystals was simulated using micromagnetic models. The models predict the observed anomaly for thermoremanence on cooling through T_v, and also the relative behaviour of SIRM and thermoremanence. The single domain threshold was calculated for the low-temperature phase of magnetite, and was found to be 0.14 microns, compared to 0.07 microns at room temperature.

We have performed detailed measurements of magnetic and dielectric properties of high quality magnetite (Fe3O4) single crystals in weak magnetic and electric fields. These measurements can reveal details of phase transitions and other features that are not yet fully understood. We focused not only at the Verwey transition - a well known phase transition taking place at about 120 K in stoichiometric samples - but we also explored and described new relaxation effects in magnetite at low temperatures. The low-temperature properties were also found to be exceptionally sensitive to sample quality, stoichiometry and homogeneity. The results presented in this thesis were acquired on sensitive non-commercial SQUID magnetometer complemented by four-probe dielectric spectroscopy and dc conductivity measurements.

Ce travail de thèse traite de la dynamique ultrarapide de spins et de charges dans des oxydes de fer. Dans un premier temps, on montre à l'aide d'un montage pompe-sonde résolu en temps et exploitant l'effet Faraday magnéto-optique, que le temps de désaimantation dans une assemblée de nanoparticules de maghémite est plus rapide que le temps de désaimantation dans une assemblée de nanoparticules de magnétite. Une superposition des temps de thermalisation des électrons et de désaimantation est observée dans la maghémite. Cette accélération du temps de désaimantation est interprétée comme étant la conséquence d'un renforcement des interactions antiferromagnétiques dans la maghémite. La seconde partie prouve qu'il est possible de caractériser la transition de Verwey dans un film de magnétite grâce à des signaux de dynamique de charges et de spins. La dynamique ultrarapide d'aimantation se caractérise par un mouvement de précession dépendant de la température. D'importantes modifications des oscillations sont visibles de part et d'autre de la température de Verwey, reflétant un changement d'anisotropie caractéristique de cette transition. / This work deals with spins and charges ultrafast dynamics in iron oxide. Thanks to a time-resolved magneto-optical Faraday effect measurements, we show the demagnetization time in an assembly of maghemite nanoparticles is faster than the demagnetization time in an assembly of magnetite nanoparticles. A superposition of thermalization times of electron and demagnetization times is observed in maghemite. This acceleration of the demagnetization time is interpreted as the effect of an enhancement of antiferromagnetic interactions in maghemite. The second part demonstrates the possibility to characterize the Verwey transition in a thin film of magnetite thanks to charges and spins dynamics signals. The ultrafast magnetization dynamic shows a temperature-dependent precession motion. Huge modifications of oscillations are visible on both side of Verwey temperature, reflecting an anisotropy change typical of this transition.