Current-induced domain wall motion

The present understanding of domain wall motion induced by spin-polarized electric current is assessed by considering a subset of experiments, analytical models, and numerical simulations based on an important model system: soft magnetic nanowires. Examination of this work demonstrates notable progress in characterizing the experimental manifestations of the “spin-torque” interaction, and in describing that interaction at a phenomenological level. At the same time, an experimentally verified microscopic understanding of the basic mechanisms will require substantial future efforts, both experimental and theoretical.     

Controlled domain wall motion by current into patterned-U Ni80Fe20 wires

The current-induced domain wall motion was observed experimentally in the case of the domain wall trapped at the semicircular arc within the U shape Ni₈₀Fe₂₀ wire. The measurement of the current-induced domain wall motion was achieved by adding a biased field before switching field and a critical current density was measured. We found two magnetic domain structures in the U pattern. At zero fields, the vortex domain wall nucleated at the semicircular arc of the U pattern. Continuous magnetic state without wall was investigated in near-switching field.     

Transient domain wall displacement under spin-polarized current pulses

This paper investigates the non steady-state displacement of magnetic domain walls in a nanostrip submitted to a time-dependent spin-polarized current flowing along the nanostrip. First, numerical micromagnetic simulations show that a domain wall can move under application of a current pulse, and that the displacement resulting from a conversion of the domain wall structure is quantized. The numerical findings are subsequently explained in the framework of simplified analytic models, namely the 1D model and the point-core vortex model. We then introduce the concept of an angle linked to the magnetization of a general domain wall, and show that it allows understanding the transient phenomena quite generally. Simple analytic formulas are derived and compared to experiments. For this, charts are given for the key parameters of the domain wall mechanics, as obtained from numerical micromagnetic simulations. We finally discuss the limitations of this work, by looking at the influence of temperature elevation under current, presence of a non-adiabatic term, and of disorder.     

Anomalous magnetization processes and non-symmetrical domain wall displacements in L10 FePt particulate films

Anomalous magnetization processes and non-symmetrical domain wall displacements in the minor loop of L1₀ FePt particulate films were investigated by magnetization measurements and in situ magnetic force microscopy. Magnetization (M) decreases dramatically on increasing the magnetic field to ∼3 kOe after which M becomes small and constant in the range of 5–20 kOe as observed in the successive measurement of minor loops. The domain wall displacement is non-symmetrical with respect to the field direction. The anomalous magnetization behavior was attributed to the non-symmetrical domain wall displacement and large magnetic field required for domain wall nucleation. Energy calculations from modeling suggest that non-symmetrical domain wall displacement is caused by the existence of metastable domains in which the domain edges are stuck to the particle boundaries.     

Cobalt antidot arrays on membranes: Fabrication and investigation with transmission X-ray microscopy

We have developed a method to fabricate ferromagnetic antidot arrays on silicon nitride membrane substrates for electron or soft X-ray microscopy with antidot periods ranging from 2 μm down to 200 nm. Observations of cobalt antidot arrays with magnetic soft X-ray microscopy show that for large periods, flux closure states occur between the antidots in the as-grown state and on application of a magnetic field, domain chains are created which show a spin configuration at the chain ends comprising four 90° walls. Pinning of the domain chain ends plays an important role in the magnetization reversal, determining the length of the chains and resulting in preservation of the domain chain configuration on reducing of the applied magnetic field to zero.     

Micromagnetic simulations for terabit/in head/media systems

Bit-patterned (BP) recording is a candidate for extending magnetic data storage towards 10 Tb/in² bit densities. An analysis of the design tolerances is carried out using dynamic micromagnetic simulations and statistical models. The effects of distributions of the magnetic material properties on phase margin and addressability error-rate during writing are investigated. At 1.3 Tb/in² a rapid increase of the error-rate is observed when the write-synchronization deviates from the optimum phase φ₀. Estimates of the fabrication and write-synchronization tolerances are derived from the phase margins. It is shown that the switching-field distribution (from intra-island variations and inter-island interactions) as well as the fabrication and synchronization tolerances must be tightly controlled for Tb/in² applications. At ultra-high densities, BP media may need to be combined with energy-assisted writing, which is referred to as second-generation BP recording.     

Magnetization reversal of single domain permalloy nanowires

Magnetization reversal process and magnetoresistance (MR) hysteresis of single domain permalloy nanowires are numerically investigated by using OOMMF. It is shown that the abrupt jumps in the magnetoresistance are due to the domain formation and domain wall propagation so that a magnetic domain suddenly switches from one state into another. A nonmonotonic angular dependence of the jump (switching) field is found. Coherent rotation mode is responsible for the smooth variation of MR curves. The nucleation pattern of newly born domains depends on the tilted angle of external field.     

Domain wall motion in ferromagnetic systems with perpendicular magnetization

Although we lack clear experimental evidence, apparently out-of-plane magnetized systems are better suited for spintronic applications than the in-plane magnetized ones, mainly due to the smaller current densities required for achieving domain wall motion. [Co/Pt] multilayers belong to the first category of materials, the out-of-plane magnetization orientation arising from the strong perpendicular magnetocrystalline anisotropy. If the magnetization arranges itself out-of-plane narrow Bloch walls occur. In the present paper, both field and current-driven domain wall motion have been investigated for this system, using micromagnetic simulations. Three types of geometries have been taken into account: bulk, thin film and wire, and for all of them a full comparison is done between the effect of the applied field and injected current. The reduction of the system's dimension induces the decrease of the critical field and the critical current, but it does not influence the domain wall displacement mechanism.     

Influence of Temperature on Equilibrium Separation Between Vertical Bloch Lines in OHBs in Garnet Bubble Films

The diameters of the ordinary hard bubbles （OHBs） and soft bubbles in epitaxial garnet films are measured under the microscope at various temperatures. It is found that the bubble diameters of OHBs increase with temperature, and it is concluded that the equilibrium separation between two neighbouring vertical Bloch lines （VBLs） Seq is widened with increasing temperature. At the same time, the results can be understood simply as that there are more VBLs in the domain walls of the first dumbbell domains （IDs） than those in walls of OHBs at the same temperature.     

Back to the future: Polarised positron beams

Plans for the development of a polarised mono-energetic positron beam at the University of Bath are described. Pilot measurements with the existing beam, modified only by increasing the source-moderator separation, have demonstrated that the helicity of the present beam is sufficient to show sensitivity to ferromagnetism in bulk iron. However, an increase in flux and helicity is required if the beam is to be successfully used to probe magnetic structures of current technological interest, and strategies to achieve both are described. Potential applications in magnetic semiconductor structures and magnetic nanoparticles are discussed.     

Exact solution of eddy current losses produced by a domain wall with nonzero thickness

The influence of the domain wall thickness on eddy current losses has been calculated. It is shown that, for samples with low magnetic anisotropy and small thickness, the reduction of magnetic losses with respect to the zero thickness domain wall must be seriously taken into account.     

Dynamics of two interacting dipoles

We study the deterministic spin dynamic of two interacting magnetic moments with anisotropy and dipolar interaction under the presence of an applied magnetic field, by using the Landau–Lifshitz equation with and without a damping term. Due to different kinds of interactions, different time scales appear: a long time scale associated with the dipolar interaction and a short time scale associated with the Zeeman interaction. We found that the total magnetization is not conserved; furthermore, for the non-dissipative case it is a fluctuating function of time, with a strong dependence on the strength of the dipolar term. In the dissipative case there is a transient time before the total magnetization reaches its constant value. We examine this critical time as a function of the distance between the magnetic moments and the phenomenological damping coefficient, and found that it strongly depends on these control parameters.     

Domain structures of ultrathin magnetic nanobelts

We use a simple magnetization model to determine domain structures of ultrathin magnetic nanobelts. A train of alternate domains are formed along the length direction. Optimal domain length decreases with belt width. Experimental domain length distributions of Fe bilayer nanobelts can be naturally explained. This approach should be applicable to similar nanomagnets.     

Magnetic vortex driven by non-uniform injection of spin-polarized current in nano-scale spin valves

We demonstrate that a current pulse of a non-uniform spin-polarized current density in a nanomagnet can drive, apart from magnetization reversal a static magnetic vortex. This vortex configuration can be achieved in low shape anisotropy spin valves of elliptical cross-sectional area. These non-uniform configurations exist also in presence of either ion mill damages below the nano-aperture or thermal effects at low temperature. We performed a numerical experiment of spin-torque driven ferromagnetic resonance in a magnetic vortex configuration, our results predict a frequency response with a few maxima and minima related to small oscillation of the vortex state around its equilibrium configuration.     

Magnetic domain wall pinning and coercivity in FePt/MgO and FePt/Pt thin films

FePt thin layers have been epitaxied either on Pt(0 0 1) or on MgO(0 0 1) substrates, and magnetically characterized using extraordinary Hall effect magnetometry and magnetic force microscopy. The coercivity originates in both cases from the pinning of domain walls on structural defects. Whereas the coercivity increases with the FePt layer thickness in FePt/Pt samples, it decreases in FePt/MgO samples. This discrepancy is explained on the basis of structural observations, and of atomistic simulations of magnetic domain wall pinning.     

CPP-GMR heads with a current screen layer for high areal density

We investigated the potential for high-areal-density recording in current-perpendicular-to-plane giant magnetoresistive (CPP-GMR) heads with a current screen layer. The current screen layer is a nano-oxide layer with confined current paths. We fabricated the current screen CPP-GMR heads with a narrow sensor width of 40–50 nm, a high MR ratio of 17%, and low-resistance-area (RA) product of 0.2 Ω μm². The fabricated heads showed a sufficiently high signal-to-noise ratio (SNR) of 30–40 dB. No extra noise, such as spin-torque noise, was observed. Linear density of 1360 kFCI from the head with the magnetic read width of 45 nm was obtained. Distribution of sensor resistance due to nano-hole area distribution in the current screen layer can be reduced with low-RA film. Spin-torque noise can be suppressed by reducing the current-induced field and controlling the shape anisotropy. Accordingly, the current screen CPP-GMR head is a promising candidate that has the potential for high-areal-density recording.     

Contribution of magnetostatic interaction to magnetization reversal of Fe3Pt nanowires arrays: A micromagnetic simulation

Using the micromagnetic simulations, we have investigated the magnetization reversal and magnetostatic interaction of Fe₃Pt nanowires arrays with wire diameters lower than 40 nm. By changing the number of interacting nanowires, N, interwire distance, a, and wire diameter, D, the effects of magnetostatic interaction on coercivity and remanence are investigated in detail. According to the simulated results, the contribution to the stray field induced by surface perpendicular magnetization at the end of wires is established.