J. Miltat

Micromagnetic understanding of current-driven domain wall motion in patterned nanowires

In order to explain recent experiments reporting a motion of magnetic domain walls (DW) in nanowires carrying a current, we propose a modification of the spin transfer torque term in the Landau-Lifchitz-Gilbert equation. We show that it explains, with reasonable parameters, the measured DW velocities as well as the variation of DW propagation field under current. We also introduce coercivity by considering rough wires. This leads to a finite DW propagation field and finite threshold current for DW propagation, hence we conclude that threshold currents are extrinsic. Some possible models that support this new term are discussed.

Quasiballistic magnetization reversal.

We demonstrate a quasiballistic switching of the magnetization in a microscopic magnetoresistive memory cell. By means of time resolved magnetotransport, we follow the large angle precession of the free layer magnetization of a spin valve cell upon application of transverse magnetic field pulses. Stopping the field pulse after a 180 degrees precession rotation leads to magnetization reversal with reversal times as short as 165 ps. This switching mode represents the fundamental ultrafast limit of field induced magnetization reversal.

Phase coherent precessional magnetization reversal in microscopic spin valve elements

We evidence multiple coherent precessional magnetization reversal in microscopic spin valves. Stable, reversible, and highly efficient magnetization switching is triggered by transverse field pulses as short as 140 ps with energies down to 15 pJ. At high fields a phase coherent reversal is found revealing periodic transitions from switching to nonswitching under variation of pulse parameters. At the low field limit the existence of a relaxation dominated regime is established allowing switching by pulse amplitudes below the quasistatic switching threshold.

Domain wall dynamics in nanowires

We study, by numerical calculations, the static domain wall structures in nanowires with axial magnetization. Then, applying an axial field, the wall dynamics is computed and compared to analytical models. We show that the one-dimensional Bloch wall dynamics, as first described by Walker, is fully realized in such samples.

Spin transfer into an inhomogeneous magnetization distribution

Based on specific examples, we examine the consequence of spin-polarized current injection into confined model micromagnetic configurations, namely a high remanence state known as the S state and a low, though nonzero, remanence state called the Leaf state. Magnetization dynamics is solved in the space and time domain owing to the Landau–Lifshitz–Gilbert equation. It is shown that, within model bounds, the S state is not propitious to fast switching under the sole influence of a polarized current, even if disregarding the current induced field, whereas Leaf state switching characteristics become extremely complex as soon as due account is made for the latter.

An Introduction to Micromagnetics in the Dynamic Regime

This first review introduces the equations of magnetization dynamics, starting from the basic equations of quantum mechanics. The macrospin model is then considered, i.e. a model in which no spatial variation of magnetization is allowed for. General expressions for the frequencies and decrement rates of small magnetization oscillations are established. Within the macrospin model, the different behaviors of magnetization submitted to pulsed fields are investigated for a set of typical parameters. The existence of ballistic trajectories (the “no-ringing case”) is established. The first part ends with a description of magnetization dynamics based on a Lagrangian formalism. The second part deals with nonuniform magnetization distributions. After recalling the relevant equations for the dynamics of these structures, several results of numerical simulations in the case of submicron size rectangular Permalloy platelets are shown and discussed. The relevance of precessional motion for fast switching characteristics is emphasized. The concept of an apparent damping constant, derived from the temporal evolution of average quantities, is introduced. This apparent damping constant is always larger than the microscopic one. A procedure that checks the accuracy of the time integration of the Landau-Lifshitz-Gilbert equation is described, in which the microscopic damping constant is systematically recalculated. Following an initial quasi-coherent rotation of the magnetization, simulation results reveal the development of large amplitude magnetization waves, which bear some analogy to the spin waves that exist in such confined structures.

Quantitative interpretation of magnetic force microscopy images from soft patterned elements

By combining a finite element tip model and numerical simulations of the tip–sample interaction, it is shown that magnetic force microscopy images of patterned soft elements may be quantitatively compared to experiments, even when performed at low lift heights, while preserving physically realistic tip characteristics. The analysis framework relies on variational principles. Assuming magnetically hard tips, the model is both exact and numerically more accurate than hitherto achieved.

Magnetic properties and domain structure of epitaxial (001) Fe/Pd superlattices

Abstract The magnetic properties of well-characterized (001)Fe/Pd superlattices prepared by molecular beam epitaxy are presented. The saturation magnetization is enhanced due to the polarization of the Pd interface, and analysis of hysteresis loops indicate low coercive fields, abrupt magnetic reversals, and ferromagnetic coupling between the Fe layers for all Pd thickness investigated (10–50 A). It is also found that deposition on a stationary substrate can create a weak uniaxial in-plane anisotropy, which, for one of the two easy directions of Fe, causes a spontaneous rotation of the magnetization by 90° at low fields. This effect is clearly demonstrated by optical Kerr-effect imaging of the magnetic domain structure, and can be mistaken for antiferromagnetic coupling with very weak coupling fields. The strength of this uniaxial anisotropy is found to oscillate rapidly with Pd thickness, suggesting that it is very sensitive to the microstructure.

Wall structures in ferro/antiferromagnetic exchange-coupled bilayers: A numerical micromagnetic approach

Two-dimensional wall structures in ferro/antiferromagnetically exchange-coupled bilayers are calculated by means of numerical micromagnetic simulations. Extended wall tails occurring in non-compensated structures are duly accounted for via the introduction of a variable meshing along one space direction. Constant charge finite volumes and the application of general boundary conditions in the presence of interlayer exchange coupling and/or surface anisotropy further characterize the computation scheme. Simulated wall structures in zero field are compared with various approximate analytical models and the ranges of validity of the latter are made explicit. Hard-axis field intrinsic magnetization and hysteresis properties are outlined, and a sketch of the wall structure phase diagram is proposed for given material parameters and a specific geometry.

A magnetic force microscopy analysis of soft thin film elements

Several different soft magnetic materials have been investigated by means of Magnetic Force Microscopy (MFM) using commercial Atomic Force Microscope (Nanoscope III) with slope detection. The observed contrast of basically solenoidal magnetization distributions in nanocrystalline Fe-Permalloy multilayer and single-layer Permalloy thin film elements not only reveals wall locations, and thus the subdivision of the magnetic volume into domains, but also a decomposition into subdomains hitherto undocumented. A strong influence of roughness on the magnetic fine structure is also demonstrated. Additional observations of the classical lancets of Goss texture in bulk Iron confirm the fact that the symmetries of the observed contrast are consistent with those of pendicular component of magnetization within a volume close to the sample surface. >