Large magnetoresistance effects in Fe₃O₄

We investigated the magnetoresistance (MR) of a single crystal of magnetite, Fe₃O₄. In an effort to distinguish between different contributions to the MR the samples were prepared in two different initial magnetic states, i.e. by either zero-field or by field cooling from room temperature. The different magnetic structures in this sample have a dramatic effect on the magnetoresistance: for initially zero-field-cooled conditions a negative MR of about  −20% is observed just below the Verwey transition at K. For decreasing temperature the MR increases, changes sign at  ∼78 K and reaches a record positive value of  ∼45% at around 50 K. This behavior is completely absent in the field-cooled sample. Magnetization measurements corroborate an alignment of the easy magnetization direction in applied magnetic fields below as a cause of the strong effects observed in both, magnetization and MR. Our results point to a complex interplay of structural and magnetocrystalline effects taking place upon cooling Fe₃O₄ through . (paper)