Ursula Ebels

Spin transfer induced coherent microwave emission with large power from nanoscale MgO tunnel junctions

In low resistance-area product MgO magnetic tunnel junction nanopillars, we observe high integrated power (up to 43nW), narrow linewidth (down to 10MHz), spin transfer induced microwave emission at frequencies up to 14GHz due to precession of the free layer magnetization at room temperature. Although all devices were fabricated on the same wafer, they present bimodal transport and precessional characteristics. The devices in which the narrowest linewidths were observed exhibited low resistance and tunneling magnetoresistance (30%), while maintaining large integrated power.

Amplitude and phase noise of magnetic tunnel junction oscillators

The microwave emission linewidth of spin transfer torque nano-oscillators is closely related to their phase and amplitude noise that can be extracted from the magnetoresistive voltage signal V(t) using single shot time domain techniques. Here we report on phase and amplitude noise studies for MgO based magnetic tunnel junction oscillators. The analysis of the power spectral densities allows one to separate the linear and nonlinear contributions to the phase noise, the nonlinear contribution being due to the coupling between phase and amplitude. The coupling strength as well as the amplitude relaxation rate can be directly extracted.

In-plane reversal mechanisms in circular Co dots

The reversal process of polycristalline circular Co dots is investigated for dot sizes for which the vortex state corresponds to the ground state configuration. Using magneto-optic Kerr magnetometry and magnetic force microscopy imaging, it is found that during the reversal of the magnetization, the system does not necessarily pass through its ground state configuration. For large dot diameters and thickness, the reversal occurs via nucleation, propagation, and expulsion of a single vortex, whereas for a dot thickness below 20 nm the reversal process may occur by a coherent rotation of the quasi-single-domain configuration. Furthermore, a double-vortex state formation has been evidenced for 1000-nm-large dots.

Micromagnetic simulations of magnetisation in circular cobalt dots

Abstract Using tri-dimensional micromagnetic simulations, the phase diagram including different magnetic ground states is computed for circular Co dots as a function of the dot dimensions. The presence of a large perpendicular magnetocrystalline anisotropy induces a shift of the boundary between the ground states and increases the number of possible stable states.

Spin-dependent phenomena and their implementation in spintronic devices

The general purpose of spinelectronis is to take advantage of the spin of electrons in addition to their charge to obtain new phenomena and conceive innovative electronic components. The first application of spinelectronics is in magnetoresistive heads for computer disk drives based on the giant magnetoresistance phenomenon. The discovery of tunnel magnetoresistance in magnetic tunnel junctions has allowed the emergence of a new kind of non-volatile memory called magnetic random access memory (MRAM). It potentially combines the advantages of all existing memories: non-volatility of FLASH, speed of SRAM, density of DRAM, hardness to ionizing radiations and endurance. Many research groups are nowadays investigating the use of these magnetic components for other logic applications. Spintronic phenomenon is the spin transfer effect allows controlling the magnetization of a magnetic nanostructure directly with a spin-polarized current. It attracts a considerable interest since it provides a new write scheme in MRAM and allows conceiving frequency tunable RF components.

Dynamic heating in submicron size magnetic tunnel junctions with exchange biased storage layer

The dynamic heating of submicron size junctions developed for the thermomagnetic write scheme was investigated. The dependency of the heating power density with the voltage pulse width (Δt) was measured and shows a [1−exp(−Δt∕t0)] phenomenological variation in the investigated range of pulse widths (5ns–1s). For submicron junction areas and constant pulse width, the heating power required for writing decreases as the junction area is reduced. The thermomagnetic write scheme was demonstrated through cycling of the cell between the two low and high resistance levels. This scheme was also shown to be insensitive to stray magnetic fields even in absence of any magnetic shielding of the memory.

Exchange bias effects in submicron antiferromagnetic-ferromagnetic dots prepared by nanosphere lithography

Exchange bias effects have been observed in an array of submicron antiferromagnetic-ferromagnetic dots prepared by nanosphere lithography. The angular dependences of the exchange bias field HE, coercivity HC, and squareness ratio MR/MS of the array of dots have been measured and compared with those of a continuous film prepared under the same conditions. An HE enhancement of around 30% is observed along the field-cooling direction in the dots. The possible mechanisms responsible for this enhancement are discussed. Moreover, magnetic imaging reveals that complex magnetization reversal mechanisms occur in the nanostructures, which cause significant differences in the shape of the hysteresis loops, when compared to the exchange biased continuous films.

Correlated vortex chiralities in interacting permalloy dot patterns

Magnetostatic interactions in different polycrystalline permalloy dot patterns for dots with diameter 900nm and thickness 30nm are investigated through magnetic force microscopy imaging, magneto-optic Kerr effect measurements, and micromagnetic simulations. Magnetization reversal occurs through vortex nucleation∕annihilation. Vortex nucleation and annihilation fields for different interacting arrays were observed to vary with pattern type and with the applied field direction and were shown to follow the prediction of a simple point-dipole model. The vortex chirality was established by imaging vortices in a small applied field. The vortex chirality distribution was determined for different arrays and a well-defined ordering phenomenon of the vortex chirality was observed in two-dot and zigzag-dot chain patterns as well as in a honeycomb array of dots. Micromagnetic simulations well account for the type of vortex chirality ordering observed and attribute it to magnetostatic interactions that induce correlated C-state magnetization configurations among nearest neighbor dots prior to vortex nucleation.Magnetostatic interactions in different polycrystalline permalloy dot patterns for dots with diameter 900nm and thickness 30nm are investigated through magnetic force microscopy imaging, magneto-optic Kerr effect measurements, and micromagnetic simulations. Magnetization reversal occurs through vortex nucleation∕annihilation. Vortex nucleation and annihilation fields for different interacting arrays were observed to vary with pattern type and with the applied field direction and were shown to follow the prediction of a simple point-dipole model. The vortex chirality was established by imaging vortices in a small applied field. The vortex chirality distribution was determined for different arrays and a well-defined ordering phenomenon of the vortex chirality was observed in two-dot and zigzag-dot chain patterns as well as in a honeycomb array of dots. Micromagnetic simulations well account for the type of vortex chirality ordering observed and attribute it to magnetostatic interactions that induce correlat...

Observation of asymmetric Bloch walls in epitaxial Co films with strong in-plane uniaxial anisotropy

Combined studies involving magnetic force microscopy and micromagnetic simulations are used to investigate the domain wall structure in epitaxial Co(1010) thin films with strong in-plane uniaxial magneto-crystalline anisotropy. This letter shows experimental evidence that, for such a system, the domain wall structure transforms from an asymmetric Bloch wall into an asymmetric Neel wall upon decreasing the film thickness from 100 to 20 nm. This transition occurs without cross-tie wall formation. Furthermore, it is found that from the four possible energetically equivalent asymmetric Bloch wall configurations, only two are stabilized along a single domain wall. For a given wall, the transition from one configuration to the other involves the simultaneous reversal of the polarity of the Bloch core and the Neel cap.

Magnetization reversal in sub-100 nm magnetic tunnel junctions with ultrathin MgO barrier biased along the hard axis

We report on room temperature magnetoresistance and low frequency noise in sub-100u2009nm elliptic CoFeB/MgO/CoFeB magnetic tunnel junctions with ultrathin (0.9u2009nm) barriers. For magnetic fields applied along the hard axis, we observe current induced magnetization switching between the antiparallel and parallel alignments at dc current densities as low as 4u2009×u2009106u2009A/cm2. We attribute the low value of the critical current to the influence of localized reductions in the tunnel barrier, which affects the current distribution. The analysis of random telegraph noise, which appears in the field interval near a magnetization switch, provides an estimate to the dimension of the pseudo pinholes that trigger the magnetization switching via local spin torque. Micromagnetic simulations qualitatively and quantitatively reproduce the main experimental observations.