A brief theoretical review points out the specific aspects of electronic transport in single-crystal magnetic tunnel junctions employing bcc(100) Fe electrodes and a MgO(100) insulating barrier. The theoretical predictions are compared to the experimental reality in both equilibrium and out-of-equilibrium regimes. For extremely small MgO thickness, we illustrate that the equilibrium tunnel transport in Fe/MgO/Fe systems leads to antiferromagnetic interactions. Artificial antiferromagnetic systems based on coupling by spin polarized tunnelling have been elaborated and studied. In the out-of-equilibrium regime and for large MgO barrier thickness, the tunnel transport validates specific spin filtering effects in terms of symmetry of the electronic Bloch function and symmetry-dependent wavefunction attenuation in the single-crystal barrier. Within this framework, we explain the experimental giant tunnel magnetoresistive effects at room temperature, up to 180%, measured in our simple or double barrier tunnel junction systems. Moreover, we illustrate that the magneto-transport properties of the junctions may be skilfully engineered by adjusting the interfacial chemical and electronic structure.

In this Letter we report patterning of colloidal nanocrystal films that combines direct e-beam (electron beam) writing with cation exchange. The e-beam irradiation causes cross-linking of the ligand molecules present at the nanocrystal surface, and the cross-linked molecules act as a mask for further processing. Consequently, in the following step of cation exchange, which is performed by directly dipping the substrate in a solution containing the new cations, the regions that have not been exposed to the electron beam are chemically transformed, while the exposed ones remain unchanged. This selective protection allows the design of patterns that are formed by chemically different nanocrystals, yet in a homogeneous nanocrystal film. Spatially resolved compositional analysis by energy-dispersive X-ray spectroscopy (EDS) corroborates that the selective exchange occurs only in the nonirradiated regions. We demonstrate the utility of this lithography approach by fabricating conductive wires and luminescent patterns in CdSe/CdS nanocrystal films by converting nonirradiated regions to Cu2-xSe/Cu2-xS. Furthermore, we show that X-ray irradiation too can lead to protection from cation exchange.

Low temperature (10K) high voltage bias dynamic conductivity (up to 2.7V) and shot noise (up to 1V) were studied in epitaxial Fe(100)/Fe-C/MgO(100)/Fe(100) magnetic tunnel junctions, as a function of the magnetic state. The junctions show large tunnel magnetoresistance (185% at 300K and 330% at 4K). Multiple sign inversion of the magnetoresistance is observed for bias polarity when the electrons scan the electronic structure of the bottom Fe-C interface. The shot-noise shows a Poissonian character. This demonstrates a pure spin dependent direct tunneling mechanism and validates the high structural quality of the MgO barrier. PACS numbers:Magnetic tunnel junctions (MTJs) [1,2] are nowadays one of the most active areas of material science and spintronics. Recent theoretical predictions [3,4] and experimental demonstration [5,6,7,8,9] of coherent spin-dependent tunneling in single crystal Fe(100)/MgO(100)/Fe(100) MTJs revolutionized this area providing new ways to create devices with room temperature Tunneling Magneto-Resistance (RT-TMR) exceeding 100%. The large TMR at low bias is mostly due to fully spin polarized ∆ 1 bulk electron states in Fe(001), reflected for antiparrallel ferromagnetic electrodes configuration (AP) or well transmitted for the parallel (P) state [3,4]. However, the tunneling mechanism gets more complex when taking into account the electronic structure of the interfaces[10] and when biasing the junction. Therefore, for finite bias polarities the antiparallel conductance may exceed the parallel one, resulting in TMR suppression [8] or its sign reversal [10]. By engineering the chemical and electronic structure of the Fe/MgO interface, the voltage variation of the TMR in amplitude and sign can be skilfully manipulated. It has been recently demonstrated that the carbon-doping of the bottom Fe/MgO interface leads to strongly asymmetric TMR vs bias, providing a root for creation of high-output voltage device applications [9].Our Letter presents a first study of dynamical conductance and TMR in a large bias window, up to 2.7 V, for Fe(100)/Fe-C/MgO(100)/Fe(100) MTJs. The shot noise analysis in different magnetization configuration is performed at voltages up to 1V. The experiments are done at room temperature (300K) and low temperature (4K-10K). The measured TMR ratio increases from 185% at 300K to 330% at 4K, mostly due to the strong temperature variation of the tunnel conductivity in the antiparallel (AP) state. Moreover, our tunneling spectroscopy experiments show a clear maximum in the AP conductivity for a finite bias and a multiple TMR sign inversion. These experiments demonstrate the role of the minority spin Fe interface resonance state (IRS) to the tunneling. Furthermore, in both parallel (P) and antiparallel magnetization configuration, the shot noise measurements demonstrate an uncorrelated direct tunneling mechanism across the MgO barrier. The shot noise analysis and the large breakdown voltage of the junctions (up to 3V) demonstrates the high quality of our MgO barriers (i.e. absence of...

We report on room temperature 1/f noise in fully epitaxial Fe(45nm)/MgO(2.6nm)/Fe(10nm) magnetic tunnel junctions (MTJs) with and without carbon doping of the Fe/MgO bottom interface.We have found that the normalized noise (Hooge factor) asymmetry between parallel and antiparallel states may strongly depend on the applied bias and its polarity. Both types of MTJs exhibit record low Hooge factors being at least one order of magnitude smaller than previously reported.