NANOPARTICLES IN METERORITES: THE CONTRIBUTION OF SURFACES TO THE MAGNETIC CURIE TEMPERATURE AND MAGNETIC RECORDING FIDELITY

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In meteoritic magnetism, the Curie temperature (the mineral dependent ferromagnetic ordering temperature) is routinely measured to identify magnetic mineralogy. The Curie temperature marks the break in a short-range nearest-neighbour ordering energy called the exchange energy, which causes the magnetic moments of atoms to align with each other in ferromagnetic materials. For crystals larger than 10-20 nm in size, the effect of atoms on the surface of a crystal that have fewer nearest neighbour atoms is relatively unimportant. However, for magnetic minerals less than 10 nm, the reduction in the number of nearest neighbour atoms leads to a reduction in the Curie temperature. To determine this effect we numerically solve the Hamiltonian describing the exchange interaction between atomic spins and their interaction with an external field (Figure 1). Variations in the exchange energy should theoretically also affect the recording fidelity of small particles; this is some that we wish to investigate. Starting with an existing three-dimensional numerical algorithm written for metallic iron, the project will develop the code for magnetite and other minerals of geological interest, for realistic three-dimensional geometries. Previous published calculations have only been for one-dimensional systems.