D. Lacour

Thermal effects on the magnetic-field dependence of spin-transfer-induced magnetization reversal

The effects of thermal fluctuations on the magnetic-field dependence of spin-transfer-induced magnetization reversal was studied using a pinned synthetic antiferromagnet as a reference layer. These effects were analyzed by numerical solution of the thermal-field plus spin-transfer-augmented LLG equations for a single domain free layer.

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.

Field sensing using the magnetoresistance of IrMn exchange-biased tunnel junctions

An original concept of high sensitivity magnetic field sensor using the spin-dependent tunneling effect has been investigated. The required crossed-biased configuration is obtained by combining both shape energy originating from vicinal step bunched Si substrates and unidirectional exchange anisotropy supplied by an Ir20Mn80 film in the “top-biased” geometry. We demonstrate a linear and reversible signal at room temperature and above. The smooth loss of sensitivity at higher temperature is shown to be correlated to the thermal dependence of the exchange bias property when IrMn is deposited above the insulating Al2O3 barrier.An original concept of high sensitivity magnetic field sensor using the spin-dependent tunneling effect has been investigated. The required crossed-biased configuration is obtained by combining both shape energy originating from vicinal step bunched Si substrates and unidirectional exchange anisotropy supplied by an Ir20Mn80 film in the “top-biased” geometry. We demonstrate a linear and reversible signal at room temperature and above. The smooth loss of sensitivity at higher temperature is shown to be correlated to the thermal dependence of the exchange bias property when IrMn is deposited above the insulating Al2O3 barrier.

Experimental evidence of multiple stable locations for a domain wall trapped by a submicron notch

The presence of a domain wall trapped by a submicron notch is probed in two ways: through electronic transport measurements and by magnetic force microscopy (MFM). MFM data, exhibiting complex magnetic features, are consistent with numerical simulations predicting the existence of multiple magnetic configurations stabilized at the notch. By changing the MFM scanning history we demonstrate experimentally the existence of multiple stable locations for a domain wall trapped by a submicron notch.

Large inverse magnetoresistance in fully epitaxial Fe∕Fe3O4∕MgO∕Co magnetic tunnel junctions

Fully epitaxial Fe(001)∕Fe3O4(001)∕MgO(001)∕Co micron-sized magnetic tunnel junctions have been elaborated on MgO(001) substrates. X-ray reflectivity and high-resolution transmission electron microscopy revealed a good quality and epitaxial growth of the stack with abrupt interfaces. The magnetotransport measurements exhibit a large negative tunneling magnetoresistance (TMR) value for magnetic tunnel junctions including an Fe3O4 layer and a MgO tunnel barrier (−8.5% at 300K and −22% at 80K). Moreover, the sign of the TMR changes with the applied bias. We discuss here the structural quality of the samples and the transport measurement results.

360° domain wall generation in the soft layer of magnetic tunnel junctions

High spatial resolution x-ray photoemission electron microscopy technique has been used to study the influence of the dipolar coupling taking place between the NiFe and the Co ferromagnetic electrodes of micron sized, elliptical shaped magnetic tunnel junctions. The chemical selectivity of this technique allows us to observe independently the magnetic domain structure in each ferromagnetic electrode. The combination of this powerful imaging technique with micromagnetic simulations allows us to evidence that a 360° domain wall can be stabilized in the NiFe soft layer. In this letter, we discuss the origin and the formation conditions of those 360° domain walls evidenced experimentally and numerically.

Magnetic properties of postoxidized Pt∕Co∕Al layers with perpendicular anisotropy

The magnetic properties of ultrathin Co layers sandwiched between Pt and Al layers are studied as a function of the Al layer oxidation time. The association of three batches of complementary experiments (extraordinary Hall effect, x-ray photoelectron spectroscopy, and tunneling magnetoresistance) allows the authors to finely characterize their samples both magnetically and chemically. The authors show that their oxidation process reduces the coercive field of ultrathin Co layers with perpendicular anisotropy (case of short oxidation time) and can even induce transition from a ferromagnetic to a superparamagnetic state (lengthy oxidation time).

Domain structures during magnetization reversal in exchange-biased layers

The magnetization reversal of an exchange-biased Co/NiO layer is studied with the help of magneto-optical microscopy, as a function of the angle between the applied magnetic field and the biasing direction. Based on domain patterns, a model of the magnetization reversal in these layers is presented. The drastic changes in the domain patterns indicate different domain nucleation conditions for different directions of the effective field.

Telegraph noise due to domain wall motion driven by spin current in perpendicular magnetized nanopillars

Telegraph noise induced by stochastic domain wall nucleation, pinning, and annihilation in the presence of spin transfer torque is observed in nanopillar spin valves with perpendicular anisotropy. By measuring the dwell times corresponding to domain wall pinning and to domain wall nucleation, the telegraph noise is found to be consistent with thermal activation over an effective potential barrier.

Fragmentation of magnetism in artificial kagome dipolar spin ice

Geometrical frustration in magnetic materials often gives rise to exotic, low-temperature states of matter, such as the ones observed in spin ices. Here we report the imaging of the magnetic states of a thermally active artificial magnetic ice that reveal the fingerprints of a spin fragmentation process. This fragmentation corresponds to a splitting of the magnetic degree of freedom into two channels and is evidenced in both real and reciprocal space. Furthermore, the internal organization of both channels is interpreted within the framework of a hybrid spin–charge model that directly emerges from the parent spin model of the kagome dipolar spin ice. Our experimental and theoretical results provide insights into the physics of frustrated magnets and deepen our understanding of emergent fields through the use of tailor-made magnetism.