Nicolas Vernier

Magnetic properties and domain structure of (Ga,Mn)As films with perpendicular anisotropy

The ferromagnetism of a thin GaMnAs layer with a perpendicular easy anisotropy axis is investigated by means of several techniques, that yield a consistent set of data on the magnetic properties and the domain structure of this diluted ferromagnetic semiconductor. The magnetic layer was grown under tensile strain on a relaxed GaInAs buffer layer using a procedure that limits the density of threading dislocations. Magnetometry, magneto-transport and polar magneto-optical Kerr effect (PMOKE) measurements reveal the high quality of this layer, in particular through its high Curie temperature (130 K) and well-defined magnetic anisotropy. We show that magnetization reversal is initiated from a limited number of nucleation centers and develops by easy domain wall propagation. Furthermore, MOKE microscopy allowed us to characterize in detail the magnetic domain structure. In particular we show that domain shape and wall motion are very sensitive to some defects, which prevents a periodic arrangement of the domains. We ascribed these defects to threading dislocations emerging in the magnetic layer, inherent to the growth mode on a relaxed buffer.

Magnetic patterning of (Ga,Mn)As by hydrogen passivation

We present a method to create magnetic patterns in thin layers of (Ga,Mn)As. It relies on local hydrogen passivation to significantly lower the hole density, and thereby locally suppress the carrier-mediated ferromagnetic phase. The sample surface is thus maintained continuous, and the minimal structure size is of about 200nm. In micron-sized ferromagnetic dots fabricated by hydrogen passivation on perpendicularly magnetized layers, the switching fields can be maintained closer to the continuous film coercivity, compared to dots made by usual dry etch techniques.

Modified current-induced domain-wall motion in GaMnAs nanowires

We report on current induced domain wall propagation in a patterned GaMnAs microwire with perpendicular magnetization. An unexpected slowing down of the propagation velocity has been found when the moving domain wall extends over only half of the width of the wire. This slowing down is related to the elongation of a longitudinal wall along the axis of the wire. By using an energy balance argument, the expected theoretical dependence of the velocity change has been calculated and compared with the experimental results. According to this, the energy associated to the longitudinal domain wall should change when a current passes through the wire. These results provide possible evidence of transverse spin diffusion along a longitudinal domain wall.

Extrinsic pinning of magnetic domain walls in CoFeB-MgO nanowires with perpendicular anisotropy

In this work, we have studied the mechanism of domain wall motion in 0.2-1.5 μm wide nanowires based on Ta/CoFeB/MgO films with perpendicular magnetic anisotropy. We show that domain wall propagation can be completely stopped due to the presence of strong pinning sites along the nanowires. From the analysis of the distribution of the strongest depinning fields as a function of the wire width, we evidence the presence of extrinsic pinning sites in nanowires, probably induced by edge damages, that dominate over the intrinsic pinning of the magnetic films even for these large wire widths.

Magnetoresistive sensors based on the elasticity of domain walls

Magnetic sensors based on magnetoresistance effects have promising application prospects due to their excellent sensitivity and their advantages in terms of integration. However, the competition between higher sensitivity and a larger measuring range remains a problem. Here, we propose a novel mechanism for designing magnetoresistive sensors: probing the perpendicular field by detecting the expansion of the elastic magnetic domain wall in the free layer of a spin valve or a magnetic tunnel junction. The performances of devices based on this mechanism, such as the sensitivity and the measuring range, can be tuned by manipulating the geometry of the device. This can be achieved without changing the intrinsic properties of the material, thus promising a higher integration level and a better performance. The mechanism is theoretically explained based on the experimental results. Two examples are proposed and their functionality and performances are verified via a micromagnetic simulation.

Current-induced fingering instability in magnetic domain walls

The shape instability of magnetic domain walls under current is investigated in a ferromagnetic (Ga, Mn)(As, P) film with perpendicular anisotropy. Domain wall motion is driven by the spin transfer torque mechanism. A current density gradient is found either to stabilize domains with walls perpendicular to current lines or to produce fingerlike patterns, depending on the domain wall motion direction. The instability mechanism is shown to result from the nonadiabatic contribution of the spin transfer torque mechanism.