Domain Wall Position Research Articles

Effect of Fiber Laser Treating on Magnetic Domains in the Grain-Oriented Silicon Steel: Imaging Domains by Bitter, MFM and Kerr Microscopy

Abstract A magnetic domain laser scribing technique of grain-oriented 3.2% silicon steel has been investigated for the direct influencing on the magnetic domain wall. The magneto-optical Kerr effect was employed to obtain a visible contrast between antiparallel domains. The effects of laser treating on domain wall were observed. The Bitter, MFM and Kerr microscope pictures showed that domain-wall positions did not repeat precisely from cycle to cycle, particularly at high inductions, and that the average domain-wall spacing decreased with increasing density of laser scribing lines. Two phenomena have been discovered that are difficult to explain (1) that the hardness decreases with increasing laser energy and (2) that the coercivity decreases with decreasing laser energy. A semi-quantitative relationship has been found between the domain patterns and used fiber laser treating method. The behavior of patterns in an applied magnetic field is shown, and based on that a two-dimensional laser lines configuration is proposed for one of the less complicated surface patterns.

Domain wall engineering through exchange bias

The control of the structure and position of magnetic domain walls is at the basis of the development of different magnetic devices and architectures. Several nanofabrication techniques have been proposed to geometrically confine and shape domain wall structures; however, a fine tuning of the position and micromagnetic configuration is hardly achieved, especially in continuous films. This work shows that, by controlling the unidirectional anisotropy of a continuous ferromagnetic film through exchange bias, domain walls whose spin arrangement is generally not favored by dipolar and exchange interactions can be created. Micromagnetic simulations reveal that the domain wall width, position and profile can be tuned by establishing an abrupt change in the direction and magnitude of the exchange bias field set in the system.

Precise control of magnetic domain wall displacement by a nanosecond current pulse in Co/Ni nanowires

Magnetic domain wall (DW) motion induced by nanosecond current pulses is investigated in Co/Ni nanowires where the DW motion is driven by either adiabatic spin transfer torque (STT) or spin Hall torque (SHT). The DW displacement, including transient displacement and steady-state displacement, is found to be linearly related to the current pulse duration for both current-induced torques. The transient displacement is found to be less than 10% of the DW displacement. This result implies that the DW position can be controlled by tuning the duration of the current pulse, which enables a robust operation of racetrack memory.

Generation and manipulation of domain walls using a thermal gradient in a ferrimagnetic TbCo wire

We demonstrate the ability to create, control the propagation, and annihilate domain walls in 25-nm thick Tb22Co78 ferrimagnetic alloy wires using a temperature gradient under a constant applied field. The temperature gradient is generated by passing a current through the wire, and the domain wall properties are imaged using Kerr microscopy. The manipulation of the domain wall is made possible by creating a temperature gradient such that the temperature at one end of the wire is above the compensation temperature for the TbCo alloy, while the other end remains below the compensation temperature. By tuning the intensity of the applied magnetic field and the current flowing inside the wire, it is possible to carefully control the domain wall position that can then be stabilized under zero applied field and current.

Fabrication of L-shaped Fe4N ferromagnetic narrow wires and position control of magnetic domain wall with magnetic field

We grow a 15-nm-thick ferromagnetic Fe4N epitaxial film on a SrTiO3(001) substrate by molecular beam epitaxy, and process it into approximately 0.5-µm-wide and 24-µm-long L-shaped ferromagnetic narrow wires by electron-beam lithography and Cl2 reactive ion etching. Their longitudinal directions are set in parallel to the magnetic easy axes, Fe4N[100] and [010]. With applying external magnetic field in the direction parallel to Fe4N[100] or [010], the position of domain wall is controlled either on the upper side or lower side of the corner. This experiment is the preliminary step toward current-driven domain wall motion in Fe4N having a negative spin polarization.

Successive Logic-in-Memory Operation in Spin Wave-Based Devices With Domain Wall Data Coding Scheme

Data coding by current-driven domain wall (DW) depinning/pinning assisted by Oersted field was proposed and selective control of DW position was achieved in a perpendicularly magnetized wire in micromagnetic simulations. Sequential operations of logic operation following data coding have also been numerically demonstrated for spin wave and DW-based device. Inductive detection of interferrometric spin wave packets whose phase has been shifted by a DW pair showed successful logic operation in the optimized current pulse sequence applied for controlling the DW positions.

Engineering 180° ferroelectric domains in epitaxial PbTiO3 thin films by varying the thickness of the underlying (La,Sr)MnO3 layer

Epitaxial ferroelectric thin films of PbTiO3 (PTO) grown on top of nominally La0.7Sr0.3MnO3 (LSMO) submicron hillocks on Nb-doped SrTiO3 (100) substrate were investigated by means of scanning transmission electron microscopy. 180° ferroelectric domains were observed in the c-axis oriented PTO films. The formation and configuration of ferroelectric domains and domain walls were found to exhibit strong correlation with the thickness of the underlying LSMO hillocks. The domain walls start at the locations of the hillocks where the LSMO layer has a thickness of about 3 nm. Our results demonstrate that controlling the thickness variation (shape) of the LSMO hillocks can manipulate the position and density of the ferroelectric domain walls, which are considered to be the active elements for future nanoelectronics.

Position of the transverse domain wall controlled by magnetic impurities in rectangular magnetic nanowires

We have performed numerical simulations to demonstrate that the domain wall movement can be controlled introducing a distribution of magnetic impurities in a nanowire. In particular, we have considered two identical impurities equidistant from the nanowire width axis. Pinning and scattering sites for the domain wall can be defined by magnetic impurities, consisting of a local variation of the exchange constant. The domain wall motion was induced by application of a magnetic field pulse and our results indicate that it is possible to control the domain wall position.

Persistent conductive footprints of 109° domain walls in bismuth ferrite films

Using conductive and piezoforce microscopy, we reveal a complex picture of electronic transport at weakly conductive 109° domain walls in bismuth ferrite films. Even once initial ferroelectric stripe domains are changed/erased, persistent conductive paths signal the original domain wall position. The conduction at such domain wall “footprints” is activated by domain movement and decays rapidly with time, but can be re-activated by opposite polarity voltage. The observed phenomena represent true leakage conduction rather than merely displacement currents. We propose a scenario of hopping transport in combination with thermionic injection over interfacial barriers controlled by the ferroelectric polarization.

Pure spin current-induced domain wall motion probed by localized spin signal detection

We demonstrate the displacement of domain walls via pure diffusive spin currents in a nonlocal spin valve geometry, without any externally applied fields. We implement a localized detection of the domain wall position by simultaneous nonlocal spin signal measurements using contacts on both sides of the spin current conduit, which allows us to determine the domain wall position even underneath the spin current conduit. Using this detection method, we probe the domain wall position as it moves across the spin current conduit when sweeping a field or on current application. Injecting pure spin currents in the nonlocal architecture, we find that in our optimized geometry we can displace a transverse head-to-head or tail-to-tail domain wall without any externally applied fields at effective spin currents $<$${10}^{10}$ A/m${}^{2}$, showing that this method can be a viable avenue to low-power domain wall manipulation.

Multi-level domain wall memory in constricted magnetic nanowires

We report a new type of multibit per cell (MBPC) magnetic memory wherein the movement and position of domain wall (DW) can be controlled precisely using spin polarized current. Out of two investigated configurations, the one with in-plane magnetization offers faster DW motion, and hence is suitable for high-speed applications, although stability may be an issue. In contrast, stable DWs were observed in the perpendicular configuration. Furthermore, the DW position can be controlled through a sequence of pulses with different magnitudes. Controlling the DW position offers a novel MBPC magnetic memory with high performance compared to other solid state memories.

Voltage-gated pinning in a magnetic domain-wall conduit

In spintronic devices relying on magnetic domain-wall (DW) motion, robust control over the DW position is required. We use electric-field control of perpendicular magnetic anisotropy to create a voltage-gated pinning site in a microstructured Pt/Co/AlOx DW conduit. A DW pins at the edge of a gate electrode, and the strength of pinning can be tuned linearly and reversibly with an efficiency of 0.22(1) mT/V. This result is supported by a micromagnetic model, taking full account of the anisotropy step at the gate edge, which is directly caused by a change in the electron density due to the choice of material.

On the YFe11Mo intermetallic characterization

Rare-earth intermetallic compounds adopting the tetragonal ThMn12–type structure and containing high Fe concentrations have attracted considerable attention in the field of permanent magnets. Among them, the Y-Fe-Mo series has been extensively investigated, especially by X-ray diffraction (XRD), but the microstructural characterization was very limited.In the present work, Y:11Fe:Mo has been prepared by melting Y, Fe and Mo in an arc furnace followed by splat-quenching and/or annealing treatments. The structure, chemistry and magnetic domain configurations of the resulting polycrystalline aggregates have been investigated by XRD, scanning and transmission electron microscopies (SEM and TEM, respectively), energy dispersive X-ray spectroscopy (EDS) and Lorentz microscopy.A special emphasis was given to Lorentz microscopy, where contrast is based on the Lorentz deflection imposed on electrons by the passage through a magnetic specimen. The magnetic domains have been imaged by the intermediate lens in Fresnel mode, with the objective lens switched off. In these conditions the intermediate lens is defocused so that out-of-focus images of the specimen are formed: the magnetic domain walls are imaged as alternate bright (convergent) and dark (divergent) lines. For the overfocused image, bright lines occur at the position of domain walls for which the magnetisation on either side is deflecting the electrons towards the wall, whereas dark lines are observed at the walls for which the magnetisation on either side is deflecting the electrons away from the wall. The opposite contrast is observed at the underfocused image.The results have shown that the tetragonal YFe11-xMoy phase is predominant, with a cellular dendritic morphology (Figure 1), but a considerable presence of -Fe(Mo) could be inferred, forming a coarse intercellular eutectic mixture. Significant Fe segregation occurred during annealing. However, this composition variation corresponded to an extremely limited evolution of the lattice parameters, the Rietveld analysis pointing to Fe vacancies at the 8i sites on the annealed material. XRD and EDS results indicate that the fraction of point defects in the ThMn12-type structure adapts to the processing route and that the stable configuration depends on the temperature. The current study also showed that grain boundaries are usually associated with domain walls and that YFe11Mo grains present internal domain walls forming stripe/maze patterns characteristic of high anisotropy materials (Figure 2), while residual -Fe(Mo) grains exhibit vortex configurations (see arrows in Figures 2 (a-c)).The work was supported by the Portuguese Science Foundation through the CTM/48617/2002 and PEst-OE/CTM-UI0084/2011 grants.

Domain walls in ring-shaped nanowires under rotating applied fields

We present a study of the motion of domain walls confined to 1D propagating energy minima in ferromagnetic nanowires. The energy minima are defined by the combination of the geometry of a ring-shaped planar nanowire and the influence of an external magnetic field, and may be controllably propagated via rotation of this field. Focused magneto-optic Kerr effect measurements are used to characterize the walls' behavior at a range of field amplitudes and frequencies. Combining these measurements with simple models allows us to demonstrate that the domain walls propagate by thermally assisted “hopping” between defect sites and that the relative smoothness of their motion can be controlled by variation of the applied field strength. Frequency-domain analysis indicates that the nanowires retain domain wall structure, rather than form quasi-saturated states, over a large range of applied magnetic fields and including fields that result in smooth wall motion. Our results are important to applications where tight control of domain wall position takes precedent over speed, for example, where domain walls are used to trap and transport magnetized particles.

ChemInform Abstract: Magnetic Domain Wall Motion by Spin Transfer

AbstractReview: 67 refs.

Domain wall manipulation in magnetic nanotubes induced by electric current pulses

We propose that the injection of electric currents can be used to independently manipulate the position and chirality of vortex-like domain walls in metallic ferromagnetic nanotubes. We support this proposal upon theoretical and numerical assessment of the magnetization dynamics driven by such currents. We show that proper interplay between the tube geometry, magnitude of the electric current and the duration of a current pulse, can be used to manipulate the position, velocity and chirality of a vortex domain wall. Our calculations suggest that domain wall velocities greater than 1 km s−1 can be achieved for tube diameters of the order of 30 nm and increasing with it. We also find that the transition from steady to precessional domain wall motion occurs for very high electric current densities, of the order of 1013 A m−2. Furthermore, the great stability displayed by such chiral magnetic configurations, and the reduced Ohmic loses provided by the current pulses, lead to highly reproducible and efficient domain wall reversal mechanisms.

Current-induced magnetic domain wall motion below intrinsic threshold triggered by Walker breakdown

Controlling the position of a magnetic domain wall with electric current may allow for new types of non-volatile memory and logic devices. To be practical, however, the threshold current density necessary for domain wall motion must be reduced below present values. Intrinsic pinning due to magnetic anisotropy, as recently observed in perpendicularly magnetized Co/Ni nanowires, has been shown to give rise to an intrinsic current threshold J(th)(0). Here, we show that domain wall motion can be induced at current densities 40% below J(th)(0) when an external magnetic field of the order of the domain wall pinning field is applied. We observe that the velocity of the domain wall motion is the vector sum of current- and field-induced velocities, and that the domain wall can be driven against the direction of a magnetic field as large as 2,000Oe, even at currents below J(th)(0). We show that this counterintuitive phenomenon is triggered by Walker breakdown, and that the additive velocities provide a unique way of simultaneously determining the spin polarization of current and the Gilbert damping constant.

Restoration the domain structure from magnetic force microscopy image

This contribution gives an approximation method to calculate the stray field of the scanning plane from the magnetic force microscopy (MFM) force gradient image. Before calculation, a Butterworth low-pass filter has been used to remove a part of the noise of the image. The discrete Fourier transform (DFT) method has been used to calculate the magnetic potential of the film surface. It shows that the potential is not correct because the low-frequency noise has been enlarged. The approximation method gives a better result of the potential and proves that the MFM force gradient of the perpendicular component image also gives the perpendicular component of the stray field. Supposing that the distance between the tip and the sample is as small as near zero, the force gradient image also gives the magnetic charge distribution of the film surface. So if the orientation of the film from hysteresis loop is known, then the domain structure of the film can be determined. For perpendicular orientation, the absolution value of the perpendicular component of stray field gives the domain and domain wall position. For in-plane orientation, the absolution value of in-plane component of stray field gives the domain and domain wall position.

Low Energy Magnetic Domain Wall Logic in Short, Narrow, Ferromagnetic Wires

We present circuit simulation results of an implementation of universal logic that operates at low switching energy. Information is stored in the position of a single domain wall in a thin, short ferromagnetic wire. The gate is switched by current-driven domain wall motion, and information is read out using a magnetic tunnel junction. The inputs and outputs of the device are currents controlled by voltage clocks, making it compatible with CMOS. Using devices that operate at 100-1 mV, we simulate a shift register circuit and a full-adder circuit. The simulations show that the magnetic logic gates can operate at lower switching energy than CMOS electronics.

Novel Method for Reducing Operation Current in Magnetic Domain-Wall Motion by the Use of Curved Wedge Nanowire

We propose a method to reduce the operation current of domain-wall (DW) motion in ferromagnetic nanowires with perpendicular magnetic anisotropy. It is based on the phenomena that a DW placed in wedged nanowire geometry moves spontaneously without any external magnetic field by consumption of the DW energy. Based on energy consideration, we found that there exists a geometry-induced effective field, which depends solely on the radius of circular arc intersecting normal to both the nanowire edges. Geometry for a constant effective field is analytically given with a curved wedge shape, which keeps the radius unchanged irrespective of the DW position. The validity of the analytic prediction is confirmed by a micromagnetic simulation.