N. Vernier

Damping of CoxFe80−xB20 ultrathin films with perpendicular magnetic anisotropy

We use vector network analyzer ferromagnetic resonance to study the perpendicularly magnetized CoFeB films. We report the dependence of the anisotropy, the g-factor, and the damping upon the Fe-Co compositional ratio in the amorphous and crystalline states. The damping and the anisotropy increase upon crystallization but vary little with composition on the Fe-rich side. At high cobalt content, the anisotropy lowers while the damping and the sample inhomogeneity increase. The compositional dependences seem to extrapolate from the properties of bulk CoFe alloys, with differences that can be understood from the correlated impacts of spin-orbit interaction on anisotropy, g-factor, and damping.

Low depinning fields in Ta-CoFeB-MgO ultrathin films with perpendicular magnetic anisotropy

We have studied the domain-wall dynamics in Ta-CoFeB-MgO ultra-thin films with perpendicular magnetic anisotropy for various Co and Fe concentrations in both the amorphous and crystalline states. We observe three motion regimes with increasing magnetic field, which are consistent with a low fields creep, transitory depinning, and high fields Walker wall motion. The depinning fields are found to be as low as 2 mT, which is significantly lower than the values typically observed in 3d ferromagnetic metal films with perpendicular magnetic anisotropy. This work highlights a path toward advanced spintronics devices based on weak random pinning in perpendicular CoFeB films.

Domain decoration in dipolar coupled ferromagnetic stacks with perpendicular anisotropy

The dipolar stray field effects between two nonuniformly magnetized ferromagnetic Co∕Pt stacks with perpendicular anisotropy are investigated by polar magneto-optical Kerr effect microscopy. Decoration of a reversed domain in the hard stack by a domain ring in the soft stack is evidenced and interpreted by magnetostatic calculations. Mirrored 360° domain walls are strongly stabilized by these interactions.

Magnetic domain structure and dynamics in interacting ferromagnetic stacks with perpendicular anisotropy

The time and field dependence of the magnetic domain structure at magnetization reversal were investigated by Kerr microscopy in a structure consisting of a hard and a soft ferromagnetic Co∕Pt multilayer stack with perpendicular anisotropy, separated by a thicker nonmagnetic Pt spacer layer. Large local inhomogeneous magnetostatic stray fields appear as soon as a nonuniform magnetic area exists within one of the stacks and induce a correlated domain structure within the other. The long range nature of this magnetostatic interaction gives rise to ultraslow dynamics even in zero applied field, i.e., it affects the long time domain stability. Due to this additional interaction field, the magnetization reversal under short magnetic field pulses differs markedly from the well-known slow dynamic behavior. Namely, in high field, the magnetization of the coupled harder layer has been observed to reverse more rapidly by domain wall motion than the softer layer alone.

Magnetic phase diagrams of He ion-irradiated Pt/Co/Pt ultrathin films

Through the magneto-optical Kerr effect, we determined the magnetic phase diagrams of Pt/Co(t)/Pt ultrathin film structures uniformly irradiated by He ions in the (T, F) and (t, F) planes, where F is the ion fluence, T the temperature and t the Co film thickness. Three ferromagnetic states with out-of-plane, in-plane and oblique spin configurations were identified. For a fixed Co thickness, an out-of-plane spin state is favoured at low fluence, while in-plane and oblique states are stabilized at higher fluence. Thus, He ion irradiation controls the magnetic systems gradual evolution from Ising-type to XY-type. We show that a similar evolution also occurs at room temperature when increasing the Co thickness for a given fluence value.

Measurement of magnetization using domain compressibility in CoFeB films with perpendicular anisotropy

We present a method to map the saturation magnetization of soft ultrathin films with perpendicular anisotropy, and we illustrate it to assess the compositional dependence of the magnetization of CoFeB(1 nm)/MgO films. The method relies on the measurement of the dipolar repulsion of parallel domain walls that define a linear domain. The film magnetization is linked to the field compressibility of the domain. The method also yields the minimal distance between two walls before their merging, which sets a practical limit to the storage density in spintronic devices using domain walls as storage entities.

Tailoring magnetism in CoNi films with perpendicular anisotropy by ion irradiation

This paper reports on the influence of ion irradiation on the magnetic properties of Co∕Ni multilayers with perpendicular magnetic anisotropy (PMA). This material is a very promising candidate for ultrahigh density spintronic applications since it exhibits high polarization and low damping parameters. We show that PMA can be tailored in a controlled way by using uniform He+ ion irradiation or focused Ga+ ion beam.

Universal domain wall dynamics under electric field in Ta/CoFeB/MgO devices with perpendicular anisotropy

Electric field effects in ferromagnetic metal/dielectric structures provide a new route to control domain wall dynamics with low-power dissipation. However, electric field effects on domain wall velocities have only been observed so far in the creep regime where domain wall velocities are low due to strong interactions with pinning sites. Here we show gate voltage modulation of domain wall velocities ranging from the creep to the flow regime in Ta/Co40Fe40B20/MgO/TiO2 structures with perpendicular magnetic anisotropy. We demonstrate a universal description of the role of applied electric fields in the various pinning-dependent regimes by taking into account an effective magnetic field being linear with the electric field. In addition, the electric field effect is found to change sign in the Walker regime. Our results are consistent with voltage-induced modification of magnetic anisotropy. Our work opens new opportunities for the study and optimization of electric field effect at ferromagnetic metal/insulator interfaces.

Magneto-optical microscopy as a favourite tool to probe focused ion beam patterning at low dose

Checking the effect of focused Ga ion beam (FIB) irradiation on a target at very low dose is still a challenge. High-resolution magneto-optical microscopy and scanning electron microscopy (SEM) are used to investigate weakly irradiated tracks patterned by FIB on a Pt/Co(1 nm)/Pt ultrathin ferromagnetic film. The two techniques give complementary results on surface film morphology and magnetism. Magneto-optical microscopy, linked to tiny ion rearrangements at interfaces, is found to be more sensitive than SEM to detect damages after irradiation at very low dose.

Ring-shaped Racetrack memory based on spin orbit torque driven chiral domain wall motions

Racetrack memory (RM) has sparked enormous interest thanks to its outstanding potential for low-power, high-density and high-speed data storage. However, since it requires bi-directional domain wall (DW) shifting process for outputting data, the mainstream stripe-shaped concept certainly suffers from the data overflow issue. This geometrical restriction leads to increasing complexity of peripheral circuits or programming as well as undesirable reliability issue. In this work, we propose and study ring-shaped RM, which is based on an alternative mechanism, spin orbit torque (SOT) driven chiral DW motions. Micromagnetic simulations have been carried out to validate its functionality and exhibit its performance advantages. The current flowing through the heavy metal instead of ferromagnetic layer realizes the “end to end” circulation of storage data, which remains all the data in the device even if they are shifted. It blazes a promising path for application of RM in practical memory and logic.