arXiv subjects
Publications and source records attributed to A. J. A. de Oliveira.
This article was withdrawn by the authors due to misinterpretation of experimental data.
We report on resonant tunneling magnetoresistance via localized states through a ZnSe semiconducting barrier which can reverse the sign of the effective spin polarization of tunneling electrons. Experiments performed on Fe/ZnSe/Fe planar junctions have shown that positive, negative or even its sign-reversible magnetoresistance can be obtained, depending on the bias voltage, the energy of localized states in the ZnSe barrier and spatial symmetry. The averaging of conduction over all localized states in a junction under resonant condition is strongly detrimental to the magnetoresistance.
We have investigated the magnetic and chemical properties of very thin Cr films, CrAs, and arsenized Cr grown by molecular beam epitaxy on Ga As (001), using x-ray photoemission spectroscopy and SQUID magnetometry. Distintic preparation procedures have been used with the purpose to undestand the origin of the ferromagneti signal observed for this system. It results that Ga segregation and chemical reactivity between Ga and Cr have negligible contribution in the formation of different thi films. A clear ferromagnetic response even at room temperature suggests the existence of a very thin interfacial layer formed that can eventually be burid during the growth process.
We have put into evidence the existence of an antiferromagnetic coupling between iron epilayers separated by a ZnSe crystalline semiconductor. The effect has been observed for ZnSe spacers thinner than 4 nm at room-temperature. The coupling constant increases linearly with temperature with a constant slope of ~5.5x 10-9 J/m2K. The mechanisms that may explain such exchange interaction are discussed in the manuscript. It results that thermally-induced effective exchange coupling mediated by spin-dependent on and off resonant tunnelling of electrons via localized mid-gap defect states in the ZnSe spacer layer appears to be the most plausible mechanism to induce the antiferromagnetic coupling.