Spin-Dependent STM Tunnelling Study of the Patterned Magnetite (111) Surface
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Spin-Dependent STM Tunnelling Study of the Patterned Magnetite (111) Surface N.Berdunov, S.Murphy, G.Mariotto, I.V.Shvets SFI Nanoscience Laboratory, Physics Department Trinity College, Dublin 2, Ireland ABSTRACT Under oxidizing preparation conditions the magnetite (111) surface reconstructs to a highly ordered superlattice. This surface reconstruction represents an oxygen-termination of the magnetite bulk. We employ spin-polarized (SP) STM to study the spin-dependent tunnelling between a magnetite (111) sample and an antiferromagnetic tip through a vacuum barrier. Atomic scale STM images show significant magnetic contrast corresponding to variations in the local surface states induced by oxygen vacancies. The local variations of the tunnelling magnetoresistance around these vacancies correspond to 150%. By employing SP-STM measurements and First principles calculations we could conclude that an oxygen top-layer considerably changes the SP properties of the magnetite surface. We explain the appearance of the superstructure in terms of electron-lattice instability due to the surface strain. INTRODUCTION Magnetite, Fe3O4, predicted to be a half-metallic ferromagnet, attracts a lot of interest from the spin electronics community. It is expected that a magnetic tunnel junction (MTJ) with a Fe3O4 electrode could exhibit a high Tunnelling Magnetoresistance (TMR) effect. In practice, at room temperature such MTJs do not demonstrate a sizable magnetoresistance and at low temperature the MR values reported are still much lower than expected1. However, the photoelectron spectroscopy measurements on the magnetite surface do show much greater values of the Spin Polarization (SP)2. It was suggested that this reduction of SP is due to the disorder at the electrode/barrier drastically changing the spin-polarized properties of the interface3. In complex structures, like magnetite, the surface can possess a number of non-identical terminations, which have a variety of spin-electronic properties. For example, it was shown that an oxygen layer deposited on a magnetic electrode could change MTJ properties drastically, even reversing the sign of spin-polarization4. Therefore, an understanding of the relationship between spin-electronic properties and the structure of the surfaces/interfaces at the nanometer and atomic scale is of much interest. In the present work we apply Spin-Polarized Scanning Tunnelling Microscopy (SP-STM) measurements to analyze the electronic structure of the oxygen-terminated magnetite (111) surface. We aim to understand the impact of point defects on the surface electronic structure and on the spin dependent tunnelling between a magnetite sample and an antiferromagnetic MnNi tip through a vacuum barrier. The potential of SP-STM as a useful tool to study the surface magnetic properties has been demonstrated in recent experimental and theoretical works5. Spindependent tunnelling can be achieved by selecting the tunnelling conditions for the majority and minority spin states. SP-STM techniques can be broadl
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