We report on tunnelling magnetoresistance (TMR), current-voltage (IV) characteristics and lowfrequency noise in epitaxially grown Fe(110)/MgO(111)/Fe(110) magnetic tunnel junctions (MTJs) with dimensions from 2x2 to 20x20 µm 2 . The evaluated MgO energy barrier (0.50±0.08 eV) , the barrier width (13.1±0.5 Å) as well as the resistance times area product (7±1 MΩµm 2 )show relatively small variation, confirming a high quality epitaxy and uniformity of all MTJs studied. The noise power, though exhibiting large variation, was observed to be roughly anticorrelated with the TMR. Surprisingly, for the largest junctions we observed a strong enhancement of the normalized low-frequency noise in the antiparallel magnetic configuration.This behaviour could be related to an interplay between the magnetic state and the local barrier defects structure of the epitaxial MTJs.

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The surface electronic band structure of thin, well-ordered epitaxial Fe 3 O 4 ͑111͒ films has been investigated at room temperature by means of angle-resolved photoelectron spectroscopy using synchrotron radiation. In the ⌫ − M direction of the Fe 3 O 4 ͑111͒ surface Brillouin zone (SBZ), two types of dispersion states originating from a periodic multilayered structure of iron and oxygen ions, in which Fe 2+ and Fe 3+ cations are incorporated into the close-packed fcc oxygen sublattice, were identified. Oxygen 2p-derived states at binding energies between 2.5 and 8 eV follow a strong, monotonic dispersion extending from ⌫ to M of the oxygen-sublattice originating SBZ. On the other hand, the iron 3d-derived states near the Fermi energy show a weak dispersion with a period half of the ⌫ − M distance of the latter. The surface electronic band structure of the Fe 3 O 4 ͑111͒ film measured with photoemission is described as composed from matrix-element governed contributions of the oxygen and the iron sublattices which are related to the different symmetries of their SBZs. Oxides with strong electronic correlations exhibit various interesting physical phenomena such as metal-insulator phase transitions, 1,2 superconductivity, 3 and colossal magnetoresistance. [4][5][6] The nature of the electronic states and their influence on the physical properties are of detrimental importance. Magnetite ͑Fe 3 O 4 ͒ is an intensively studied, strongly correlated transition metal oxide, which is ferrimagnetically ordered below a relatively high transition temperature ͑T C = 851 K͒. 7 This material has recently received renewed attention because of its potential application in spintronics related to its high spin polarization due to the half-metallic ferromagnetic electronic structure. 8-10The electronic band structure of Fe 3 O 4 films has been extensively investigated by means of x-ray magnetic circular dichroism, 11,12 ultraviolet photoelectron spectroscopy, 13,14 as well as spin-and angle-resolved photoelectron spectroscopy. [14][15][16] However, the interpretation of the valence-band photoemission spectra of Fe 3 O 4 has been a matter of debate since decades. 13,[15][16][17] Previous photoemission studies were mostly interpreted on the basis of the ligand-field theory, in which the localized 3d cation levels, split further by the oxygen ligand field, were found responsible for the spectral features rather than the recently observed dispersing, band-like 3d states. 13,16 Further theoretical studies at the time found that it was necessary to include configuration interactions within the 3d multiplet as well as charge transfer between the 3d and the ligand orbitals to improve the agreement with experiment and to account for the satellite features observed in photoemission. [17][18][19] The recent investigations, 13,16 however, showed that theoretically calculated band dispersions 8-10 should be taken into account for the interpretation of photoelecton spectra of Fe 3 O 4 .Up to now the surface electronic structure of magnetite w...

Tunnel junctions of Co͑10 nm͒/AlO x ͑nominally 2 nm͒/Co͑20 nm͒ have been prepared by molecular beam epitaxy applying a shadow mask technique in conjunction with an UV light-assisted oxidation process of the AlO x barrier. The quality of the AlO x barrier has been proven by x-ray photoelectron spectroscopy and temperature dependent tunneling magnetoresistance ͑TMR͒ measurements. Optimum-oxidized tunnel junctions show a TMR of 20% at 285 K and up to 36% at 100 K. At 285 K the TMR values as a function of oxidation time are not symmetric about the optimum time. For underoxidized junctions the TMR is reduced more strongly than for overoxidized junctions. The temperature dependence of the junction's resistance is a clear and reliable indicator whether pinholes ͑or imperfections͒ contribute to the conduction across the barrier.Magnetic tunnel junctions ͑MTJs͒ consisting of two ferromagnetic electrodes separated by a thin insulating layer ͑typical AlO x ͒ show large tunnel magnetoresistance ͑TMR͒ at room temperature making them promising candidates for magnetic random access memory ͑MRAM͒ devices.