Half-metallic Electrodes Research Articles

Spin-Polarized Tunneling Spectroscopy in Tunnel Junctions with Half-Metallic Electrodes

We have studied the magnetoresistance (TMR) of tunnel junctions with electrodes of La(2/3)Sr(1/3)MnO3 and we show how the variation of the conductance and TMR with the bias voltage can be exploited to obtain precise information on the spin and energy dependence of the density of states. Our analysis leads to a quantitative description of the band structure of La(2/3)Sr(1/3)MnO3 including the energy gap delta between the Fermi level and the bottom of the t(2g) minority-spin band, in good agreement with data from spin-polarized inverse photoemission experiments. This shows the potential of magnetic tunnel junctions with half-metallic electrodes for spin-resolved spectroscopic studies.

Performance of Fe3O4/AlOx/CoFe magnetic tunnel junctions based on half-metallic Fe3O4 electrodes

Magnetic tunnel junctions (MTJs) with Fe3O4 electrodes were prepared to observe spin- dependent tunneling effect of half-metallic materials through an insulating AlOx barrier at room temperature (RT). In addition, the plasma treatment with Ar gas was in situ performed to improve surface morphology of an as-deposited Fe3O4 electrode. After Ar plasma treatment, the root mean square of as-deposited Fe3O4 thin film was decreased from 4.5Å to 2.5Å. Magnetoresistance (MR) and electrical breakdown voltage of the MTJ were found to be 11% and 0.8V at RT, respectively. This large MR value is ascribed to the enhanced surface morphology of Fe3O4 electrode after plasma treatment. This MR value at RT gives a potential way of using Fe3O4 thin films in the spintronic devices, together with the enhanced surface treatment technique.

Magnetotransport in Exchange-Coupled Magnetite Junctions

Epitaxial magnetic junctions of theoretically half-metallic electrode material magnetite were fabricated with a cobalt chromite paramagnetic insulating tunnel barrier. This spinel structure barrier was chosen to be isostructural to the magnetite electrodes. Highly crystalline trilayers with low surface roughness were grown. Strong exchange coupling between the electrodes is observed across the barrier. Magnetoresistance measurements are reminiscent of a single ferromagnetic layer and not a junction.

Volt–ampere characteristics of spin half-metallic transistor

A new construction of the spin transistor is proposed and its volt–ampere characteristics (VAC) are theoretically investigated. It is based on the ferromagnetic half-metals and is referred to as a spin half-metallic transistor (SHMT). SHMT is revealed to amplify electric current as a bipolar transistor. At the same time, its properties drastically depend on the reciprocal orientation of magnetizations of its three junctions; so SHMT can be used as a switching device. Also properties of an “F ↑–F ↓ junction” are considered. This device consists of two half-metallic electrodes with antiparallel magnetizations. Its VAC reminds VAC of a diode in some voltage range. Behavior of the F ↑–F ↓ junction under different conditions is considered.

Growth and characterization of single crystalline NiMnSb thin films and epitaxial NiMnSb/MgO/NiMnSb(001) trilayers

NiMnSb(001) high-quality epitaxial thin films have been prepared by molecular-beam epitaxy. The growth and morphology of the films have been extensively studied using electron diffraction and scanning tunneling microscopy. We report the presence of various surface structures on the free NiMnSb(001) surface. The structural properties of the prepared single-crystalline NiMnSb films have been analyzed by transmission electron microscopy and x-ray absorption. The typical $C1b$ structure is observed at a large scale with a very good chemical ordering in the unit cell. The magnetic properties have been investigated by x-ray magnetic circular dichroism and a superconducting quantum interferometer device and are compatible with the predicted half-metallic character. These NiMnSb layers have been integrated in single-crystalline NiMnSb/MgO/NiMnSb heterostructures for preparation of magnetic tunnel junctions with half-metallic electrodes.

Organic-Inorganic Hybrid Spin-Valve: A Novel Approach to Spintronics

The combined use of highly correlated solids opens the possibility to explore new phenomena and to generate new hybrid devices for Spintronics. We have recently demonstrated the spin injection of carriers into an organic semiconductor, sexithienyl (T 6 ). A spin-valve device has been realized by using two colossal magnetoresistance (CMR) half metallic electrodes separated by the organic semiconductor. The total magnetoresistance as large as 30% was detected for channel lengths lower than 140 nm.

Spin-dependent tunnelling in La 0.7Sr 0.3MnO 3/SrTiO 3 superlattices

La 0.7Sr 0.3MnO 3 is predicted to show half-metallic behaviour at low temperature, which gives rise to a metallic character for one spin direction and an insulating character for the other. This 100% polarisation of the conduction band should enhance the spin dependent tunnelling in manganite-based tunnel junctions. La 0.7Sr 0.3MnO 3/SrTiO 3 epitaxial superlattices were grown on LaAlO 3(001) substrates by metal–organic chemical vapour deposition (MOCVD). These multilayers consist of 15 epitaxial bilayers of La 0.7Sr 0.3MnO 3 and SrTiO 3. The junctions were patterned using UV lithography and Ar ion milling to carry out transport measurements in the current perpendicular-to-plane geometry (CPP). A temperature-independent non-linear I– V curve, which is characteristic of a tunnelling conduction mechanism, was observed below 50 K. At higher temperatures, the I– V curves are found to become linear and temperature-dependent. Up to 30 K, a constant tunnel magnetoresistance (TMR) (3%) is measured. The switching field is consistent with the film coercive field (a few 10s of mT). At higher temperatures, the TMR decreases rapidly. This temperature dependence is compared to the expected behaviour of a spin tunnel junction with half-metallic electrodes, with thermal activation or the loss of spin polarisation taken into account.

NiMnSb/MgO/NiMnSb heterostructures grown by MBE

NiMnSb(0 0 1) high quality epitaxial thin films have been prepared by MBE. The growth and morphology of the films have been studied by RHEED and STM, and the magnetic properties have been investigated by XMCD and SQUID. These NiMnSb electrodes have been integrated in fully epitaxial NiMnSb/MgO/NiMnSb heterostructures for preparation of magnetic tunnel junctions (MTJs) with half-metallic electrodes.

Sub-200 Oe Giant Magnetoresistance in Manganite Tunnel Junctions

ABSTRACTMetallic manganite oxides, La1-xDxMnO3 (D=Sr, Ca, etc.), display “colossal” magnetoresistance (CMR) near their magnetic phase transition temperatures (Tc) when subject to a Tesla-scale magnetic field. This phenomenal effect is the result of the strong interplay inherent in this class of materials among electronic structure, magnetic ordering, and lattice dynamics. Though fundamentally interesting, the CMR effect achieved only at large fields poses severe technological challenges to potential applications in magnetoelectronic devices, where low field sensitivity is crucial. Among the objectives of our research effort involving manganite materials is to reduce the field scale of MR by designing and fabricating tunnel junctions and other structures rich in magnetic domain walls. The junction electrodes were made of doped manganite epitaxial films, and the insulating barrier of SrTiO3. The interfacial expitaxy has been imaged by using high-resolution transmission electron microscopy (TEM). We have used self-aligned lithographic process to pattern the junctions to micron scale in size. Large MR values close to 250% at low fields of a few tens of Oe have been observed. The mechanism of the spin-dependent transport is due to the spin-polarized tunneling between the half-metallic electrodes, in which the spins of the conduction electrons are nearly fully polarized. We will present results of field and temperature dependence of MR in these structures and discuss the electronic structure of the manganite inferred from tunneling measurement. Results of large MR at low fields due to the grain-boundary effect will also be presented.