F. Montaigne

High tunnel magnetoresistance in epitaxial Fe/MgO/Fe tunnel junctions

We report on spin-polarized tunneling in fully epitaxial Fe/MgO/Fe/Co tunnel junctions. By increasing the thickness of the insulating layer (tMgO), we have strongly enhanced the tunnel magnetoresistance. Values up to ∼100% at 80 K (∼67% at room temperature) have been observed with tMgO=2.5 nm. This tunnel magnetoresistance ratio, which is much larger than the one predicted by the Julliere’s model, can be understood in the framework of ab initio calculations.

Enhanced tunnel magnetoresistance at high bias voltage in double-barrier planar junctions

Single Co/Al2O3/NiFe and double Co/Al2O3/Co/Al2O3/NiFe planar tunnel junctions were grown by sputtering and subsequently patterned in a four-step process using optical lithography. The Al2O3 barriers are formed by radio frequency plasma oxidation of 1.5 nm aluminum layers. The double junctions exhibit three clear resistance levels depending on the relative configuration of the magnetizations. Both single and double junctions exhibit maximum magnetoresistance (MR) ratios above 10% at room temperature and 20% at 30 K and a decrease of MR with increasing bias voltage. With regard to its low bias value, the MR is reduced by a factor of 2 at 0.26 V for the single junctions and at values above 0.8 V for the double junctions. The decay of the MR of double junctions with bias voltage is significantly slower than expected from two independent junctions in series.

Spin tunnelling phenomena in single-crystal magnetic tunnel junction systems

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.

Wide range and tunable linear magnetic tunnel junction sensor using two exchange pinned electrodes

A magnetic tunnel junction sensor with both the detection and the reference layers pinned by IrMn is fabricated. Using the differences in the blocking temperatures of the IrMn films with different thicknesses, crossed anisotropies can be induced between the detection and the reference electrodes. The pinning of the sensing electrode ensures a linear and reversible output. It also allows tuning both the sensitivity and the linear range of the sensor. The sensitivity varies linearly with the ferromagnetic thickness of the detection electrode. It is demonstrated that an increased thickness leads to a rise of sensitivity and a reduction of the operating range.

Epitaxial MgO layer for low-resistance and coupling-free magnetic tunnel junctions

Epitaxially grown magnetic tunnel junctions MgO(100)/Fe/MgO/Fe/Co/Pd have been elaborated by molecular beam epitaxy, with insulating layer thickness down to 0.8 nm. The continuity of this layer was checked at different spatial scales by means of morphological (high resolution transmission electronic microscopy), electric (local impedance), and magnetic (magnetoresistance and hysteresis loop) measurements. These junctions show a low resistance (4 kΩ μm2), tunnel magnetoresistance up to 17%, and a very small interlayer magnetic coupling.

Development of magnetoresistive sensors based on planar Hall effect for applications to microcompass

We present here a new magnetoresistive sensor based on planar Hall effect for detection of low magnetic fields 10 nT in the 1-1000 . Hz frequency range. These sensors are suitable for low-cost fabrication. The growth of a Permalloy FeNi active layer by conventional sputtering on misoriented silicon substrates leads to a well controlled in-plane uniaxial magnetic anisotropy. Moreover, a magnetisation switching system allows to remove any offset of the measure. The association of two orthogonal sensors will give a micro-compass with an angular resolution, below 0.58, limited by the precision of assembling, with a device size of the order of 1 mm 2 . q 2000 Elsevier

Fe/MgO interface engineering for high-output-voltage device applications

The magnetotransport characteristics of Fe∕MgO∕Fe epitaxial tunnel junctions are reported. For clean Fe∕MgO interfaces, a tunnel magnetoresistance of 150% is measured. However, the magnetoresistance decreases rapidly with the applied voltage. Consequently, the main parameter to optimize for device application, namely the output voltage, remains relatively low. This limitation has been solved by interface engineering through the insertion of carbon impurities at the Fe∕MgO interface. Although the tunnel magnetoresistance amplitude is slightly reduced, its variation versus the applied voltage becomes strongly asymmetric with large magnetoresistance maintained up to 1.5V. This determines a large increase of the tunnel junction output voltage.

Static and dynamic aspects of spin tunnelling in crystalline magnetic tunnel junctions

Single-crystal magnetic tunnel junctions employing bcc (100) Fe electrodes and MgO(100) insulating barrier are elaborated by molecular beam epitaxy. The magneto-transport properties are investigated in two extreme regimes. First, for extremely small MgO thickness, we show that the equilibrium tunnel transport in Fe/MgO/Fe systems leads to antiferromagnetic interactions, mediated by the tunnelling of the minority spin interfacial resonance state. Second, 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 present giant tunnel magnetoresistive effects at room temperature (125–160%). Moreover, we illustrate that the interfacial chemical and electronic structure plays a crucial role in the spin filtering. We point out imperfect filtering effects and a strong implication of the minority surface state of Fe on the low voltage variation of tunnel magnetoresistance. The insertion of carbon impurities at the Fe/MgO interface changes radically the voltage response of the tunnel magnetoresistance and activates a resonant tunnelling mechanism via the interfacial resonance state.

On the quality of molecular-beam epitaxy grown Fe∕MgO and Co∕MgO(001) interfaces

We investigated in detail the structural, chemical, and magnetic properties of Fe at the interface with MgO. The samples were grown by molecular-beam epitaxy and analyzed by using reflection high-energy electron diffraction, scanning tunneling microscopy, Auger electron spectroscopy, x-ray photoemission spectroscopy (XPS), x-ray-absorption spectroscopy (XAS), and x-ray magnetic circular dichroism (XMCD) measurements. We discuss the presence or absence of C and O contaminants at the surface of the metallic film. We actually show that when growing the first Fe layer on the initial MgO substrate, a carbon contamination at the surface of the annealed Fe layer is observed leading to a C(2×2) surface structure. We propose a method to eliminate this carbon contamination and to get interfaces free of carbon. On the other hand, we observed that the reactivity of the Fe surface to oxygen contamination is greatly influenced by the presence of carbon or absence on the surface. The influence of oxygen adsorption at th...

Review of recent results on spin polarized tunneling and magnetic switching by spin injection

We review recent results obtained at Orsay on two topics in the field of spin electronics: (i) Spin polarized tunneling in magnetic tunnel junctions combining electrodes of ferromagnetic transition metal and half-metallic oxide: we will describe the influence of the nature of the barrier on the sign of the spin polarization of electrons tunneling from the transition metal and we also discuss the temperature dependence of the TMR obtained with half metallic oxides. (ii) Magnetization reversal by spin injection: we will present and interpret experimental results obtained with pillar-shaped Co/Cu/Co trilayers.