Olivier Redon

Analytical investigation of spin-transfer dynamics using a perpendicular-to-plane polarizer

An analytical investigation of the dynamic properties of spin-transfer torque (STT) devices comprising a perpendicular-to-plane polarizer (PERP) is reported. Several analytical solutions for PERP are derived from Landau-Lifshitz-Gilbert (LLG) equation including the spin-torque term, and proved its validity by comparing with numerical simulations using the single domain model. The critical spin-torque amplitude a/sub JC/ is linearly proportional to the in-plane anisotropy field of the sample. It shows a good agreement with simulation assuming single domain except in the case of large positive current where the polarizer magnetization is also excited. Several analytical solutions for precession frequency are derived which showed a good agreement with single domain simulations. The main results reveal that, considering the thermal stability criterion, the switching current density in PERP based structures is larger than that in their longitudinal polarizer counterparts when the cell area is smaller than a certain value. However, the switching is much faster in the PERP. Also, it is found that the precession frequency in PERP based structures can be tuned from about 1 GHz to 20 GHz by only changing the current without any external field.

Thermally assisted switching in exchange-biased storage layer magnetic tunnel junctions

A thermally assisted writing procedure is proposed in a tunnel junction based magnetic random access memory cell. The magnetic layers of the tunnel junction are both exchange-biased with antiferromagnetic layers, the reference layer having a much higher blocking temperature than the storage layer. In the operating mode, a current pulse sent through the junction generates enough heat to raise the temperature of the storage layer above its blocking temperature, without affecting the pinning of the reference layer. The concept is demonstrated here for an isolated junction using an homogeneous external magnetic field.

Structural, magnetic and transport properties of NiFeχAg(1−χ) heterogeneous alloys

Abstract We carried out a comprehensive study of structural, magnetic and electrotransport properties of as-deposited and annealed (Ni 80 Fe 20 ) χ Ag( 1−χ ) heterogenous alloys prepared by sputtering. The NiFe atomic concentration was varied between 15% and 40%. These alloys consist of small magnetic particles (Ni 80 Fe 20 ) embedded in a nonmagnetic matrix (Ag). The structures of these alloys were investigated by X-ray diffraction, scanning electron microscopy and high-resolution cross-section transmission electron microscopy. The magnetic measurements were made using SQUID magnetometry and ferromagnetic resonance. Magnetoresistance was measured with a conventional four-point probe between 1.5 K and room temperature in field range 0–6T. Three contributions to the magnetoresistance of these granular alloys have been clearly identified: the spin-valve (or giant) magnetoresistance as in multilayers, scattering on magnetic fluctuations (as in any ferromagnetic metal around its magnetic ordering temperature), and anisotropic magnetoresistance. These three contributions have their own dependences on the size of the magnetic particles, on the degree of intermixing between Ni 80 Fe 20 and Ag, and on temperature. We discuss the different shapes and amplitudes of magnetoresistance versus Ni 80 Fe 20 concentration or temperature and their evolution upon annealing in terms of the relative roles of these three contributions. The magnetoresistance in multilayers (current in-plane or perpendicular to the plane) and granular alloys are also compared.

Giant magnetoresistance in sputtered and melt-spun alloys

Abstract We present an overview of our recent results on giant magnetoresistance in as-prepared and annealed heterogeneous alloys obtained by sputtering or melt-spinning. Three series of alloys have been investigated: melt-spun CoxCu(1-x), sputtered NiFexAg(1-x) and sputtered (Co70Fe30)xAg(1-x) alloys, with x between 5 and 40%.

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.

Magnetoresistance of Ag/Co70Fe30 layered and granular structures

Abstract Magnetoresistance and magnetization measurements were performed on sputtered Ag/CoFe multilayers, with various CoFe and Ag thicknesses. For thicknesses larger than 6 A, CoFe layers are continuous. The amplitude Δ ϱ/ϱ 0 of the giant magnetoresistance (GMR) is maximum around 7 A of CoFe and 12 A of Ag, reaching 30% in 700 Oe at room temperature and 100% at 4.2 K. For lower CoFe thicknesses, the magnetic layers break into oblate particles. These particles exhibit a superparamagnetic behaviour and the system a ‘granular-like’ magnetoresistance. Magnetization and magnetoresistance behaviours have been modelized in each case, introducing bilinear and biquadratic exchange couplings in multilayer systems, or the size distribution and interactions of the particles in granular multilayers.

Crossover in heating regimes of thermally assisted magnetic memories

This work investigates the tunnel junction heating process for micrometer and submicrometer size junctions to be used in a thermally assisted magnetic random access memories write scheme. The time evolution of the heating process was obtained from experimental measurements and numerical thermal simulations. Simulation results show an initial temperature regime at very short pulse widths associated with the intrinsic heating of the junction (adiabatic regime). In this regime, for the same power density, the temperature increase is independent of the junction area. In the studied geometry, for pulse widths around 1ns and higher an additional heating occurs in the electrical leads (diffusion regime). The write power density is in this case lower for large junction areas. The use of thermal barriers is an effective mean to decrease the power density required for writing and to eliminate its junction area dependence.

Magnetic properties and hyperfine interactions in sputtered (Co70Fe30)36Ag64 heterogeneous alloy

Abstract Granular thin films of (Co 70 Fe 30 ) 36 Ag 64 alloy prepared by sputtering and annealed at different temperatures were investigated by X-ray, Mossbauer and magnetization measurements. The composition of the different particles, their size distribution, the direction of magnetization, their magnetic interaction and the anisotropy energy were studied as a function of the annealing process. A comparison of the magnetic and magnetotransport properties has been carried out. Interfacial scattering is found to play a dominant role in the magnetoresistance of this series of alloys.

Analytical investigation of spin transfer dynamics using a perpendicular-to-plane polarizer

An analytical investigation of the dynamic properties of spin-transfer torque (STT) devices comprising a perpendicular-to-plane polarizer (PERP) is reported. Several analytical solutions for PERP are derived from Landau-Lifshitz-Gilbert (LLG) equation including the spin-torque term, and proved its validity by comparing with numerical simulations using the single domain model. The critical spin-torque amplitude a/sub JC/ is linearly proportional to the in-plane anisotropy field of the sample. It shows a good agreement with simulation assuming single domain except in the case of large positive current where the polarizer magnetization is also excited. Several analytical solutions for precession frequency are derived which showed a good agreement with single domain simulations. The main results reveal that, considering the thermal stability criterion, the switching current density in PERP based structures is larger than that in their longitudinal polarizer counterparts when the cell area is smaller than a certain value. However, the switching is much faster in the PERP. Also, it is found that the precession frequency in PERP based structures can be tuned from about 1 GHz to 20 GHz by only changing the current without any external field.

Thermo-magnetic random access memory: a new route for low power applications

The concept of thermo-magnetic writing was demonstrated and important features such as selectivity and the protection against stray fields were confirmed. Moreover, submicron size magnetic tunnel junctions can be written using 5 ns heating pulses proving that the thermo-magnetic approach is competitive with the conventional schemes in terms of write speed. In this approach, both magnetic electrodes of the tunnel junction are exchange-biased with antiferromagnetic layers, the reference layer having a much higher blocking temperature. During the write operation, the selection transistor is opened and a current pulse is sent through the stack. The heat generated in the vicinity of the tunnel barrier allows reaching the blocking temperature of the storage layer without affecting the reference layer. The system is then cooled under a magnetic field, larger than the coercive field of the storage layer.