Improved coherence of ultrafast spin-transfer-driven precessional switching with synthetic antiferromagnet perpendicular polarizer

International audienceThe coherence of the precessional switching was compared in planar spin-valves comprising either an additional simple perpendicular polarizer or a synthetic antiferromagnet perpendicular polarizer. A significant improvement in the precession coherence was observed experimentally in the second type of samples. Micromagnetic simulations were performed to study the effect of the stray field from the perpendicular polarizer. They provide an explanation for the gradual loss of coherence of the precession in terms of vortex formation, which occurs much faster when a simple perpendicular polarizer is used

First order reversal curves and intrinsic parameter determination for magnetic materials; Limitations of hysteron-based approaches in correlated systems

The generic problem of extracting information on intrinsic particle properties from the whole class of interacting magnetic fine particle systems is a long standing and difficult inverse problem. As an example, the Switching Field Distribution (SFD) is an important quantity in the characterization of magnetic systems, and its determination in many technological applications, such as recording media, is especially challenging. Techniques such as the first order reversal curve (FORC) methods, were developed to extract the SFD from macroscopic measurements. However, all methods rely on separating the contributions to the measurements of the intrinsic SFD and the extrinsic effects of magnetostatic and exchange interactions. We investigate the underlying physics of the FORC method by applying it to the output predictions of a kinetic Monte-Carlo model with known input parameters. We show that the FORC method is valid only in cases of weak spatial correlation of the magnetisation and suggest a more general approach

The particle interaction effects in the field-cooled and zero-field-cooled magnetization processes

The present theories explaining the mechanism of particle interaction within a fine particle system driven by the thermal agitation assign the increase of the interaction strength either to an increase of the particle anisotropy due to the environment reaction to its dipole moment, or to the occurrence of a collective state. The particle interaction effects on the field-cooled (FC) and zero-field-cooled (ZFC) magnetization curves are the anisotropy effect, referring to the increase of the temperature TMAX, corresponding to the ZFC curve maximum, with increasing sample volume concentration, and the mean-field effect, referring to the flattening of both, FC and ZFC, magnetization curves with increasing sample demagnetizing factor, without altering TMAX in the low applied field limit. We demonstrate that the Onsager mean-field model is able to recover an increase of the particle anisotropy with increasing sample volume concentration using a cavity having the shape of an oblate ellipsoid, the eccentricity increasing with increasing sample volume concentration. The proposed explanation is the formation of particle clusters having a uniaxial symmetry in the particle arrangement (chain-of-particles). We show that the anisotropy effect of interactions is due to not only an increase of the particle anisotropy with increasing sample volume concentration, but also to a temperature-dependent interaction field distribution due to the local non-homogeneity of the particle dispersion. The proposed model is able to recover the experimental FC and ZFC initial susceptibility curves for various concentrations of γ-Fe2O3 nanoparticle systems

Precessional spin-transfer switching in a magnetic tunnel junction with a synthetic antiferromagnetic perpendicular polarizer

International audienceThis paper reports sub-nanosecond precessional spin-transfer switching in elliptical magnetic tunnel junction nanopillars. This result is obtained in samples integrating a synthetic antiferromagnetic perpendicular polarizer and a tunnel junction with in-plane magnetized electrodes. The out-of-plane precession of the free layer magnetization results in oscillations of the switching probability as a function of the pulse width. At 9.25 MA/cm2 current density, these oscillations have a period of 1 ns with a high degree of coherence