Oscillatory reduction of domain wall depinning field by transverse magnetic field in ferromagnetic permalloy nanowires

Reported herein is a possible way of controlling the depinning field of magnetic domain walls (DWs) by using a magnetic field H(T) transverse to the nanowire. A typical notch structure-in the form of triangles on both edges of ferromagnetic Permalloy nanowires-is employed to pin the DWs. The depinning field of the DW initially pinned at the notch is then measured with respect to H(T). Interestingly, it is experimentally found that the depinning field is drastically decreased to almost 0 with increasing H(T), due to the internal shift of the DW position at the notch. Moreover, it is experimentally observed that an oscillatory behavior of the depinning field occurs with respect to H(T), Micromagnetic calculation is performed to model the depinning behavior of the DW pinned at the notch structure with respect to H(T). It is ascribed to the natural edge roughness of the nanowire, which means the edge roughness plays an important role in determination of the depinning field.     

Control of domain wall pinning in ferromagnetic nanowires by magnetic stray fields

We have found that the depinning field of domain walls (DWs) in permalloy (Ni(81)Fe(19)) nanowires can be experimentally controlled by interactions between magnetic stray fields and artificial constrictions. A pinning geometry that consists of a notch and a nanobar is considered, where a DW traveling in the nanowire is pinned by the notch with a nanobar vertical to it. We have found that the direction of magnetization of the nanobar affects the shape and local energy minimum of the potential landscape experienced by the DW; therefore, the pinning strength strongly depends on the interaction of the magnetic stray field from the nanobar with the external pinning force of the notch. The mechanism of this pinning behavior is applied for the instant and flexible control of the pinning strength with respect to various DW motions in DW-mediated magnetic memory devices.     

Translational positioning of a magnetic domain wall in ferromagnetic nanowires using a stray field filter

Understanding the interaction between the magnetic domain wall and the various artificial defects in ferromagnetic nanowires has been of utmost importance for the future realization of the spintronic devices based on the magnetic domain wall motion in nanowires. In this work, the chirality filter effect of the magnetic domain wall in T-shaped ferromagnetic nanowires with a stray field filter was investigated via micromagnetic simulation. A tapered wire was attached to the flat nanowires to form a potential barrier or well for the domain wall propagating along them. For the domain wall passing through the potential barrier or the potential well, the spin structure of the domain wall and the interaction between the domain wall and the potential barrier/well were investigated in detail. The chirality-dependent translational positioning of the domain wall was intensively examined for the potential barrier and potential well cases. The domain wall chirality transmission on relatively long length scales using a series of potential wells was explored.     

Dynamics of field-driven domain-wall propagation in ferromagnetic nanowires

Ferromagnetic nanowires are likely to play an important role in future spintronic devices. Magnetic domain walls, which separate regions of opposing magnetization in a nanowire, can be manipulated and used to encode information for storage or to perform logic operations. Owing to their reduced size and dimensionality, the characterization of domain-wall motion is an important problem. To compete with other technologies, high-speed operation, and hence fast wall propagation, is essential. However, the domain-wall dynamics in nanowires has only been investigated in the last five years and some results indicate a drastic slowing down of wall motion in higher magnetic fields. Here we show that the velocity-field characteristic of a domain wall in a nanowire shows two linear regimes, with the wall mobility at high fields reduced tenfold from that at low fields. The transition is marked by a region of negative differential mobility and highly irregular wall motion. These results are in accord with theoretical predictions that, above a threshold field, uniform wall movement gives way to turbulent wall motion, leading to a substantial drop in wall mobility. Our results help resolve contradictory reports of wall propagation velocities in laterally confined geometries, and underscore the importance of understanding and enhancing the breakdown field for practical applications.     

Field-dependent spin waves in high-aspect-ratio single-crystal ferromagnetic nanowires

We investigate the spin wave (SW) modes in high-aspect-ratio single-crystal ferromagnetic nanowires (FMNWs) using an all-optical time-resolved magnetooptical Kerr effect (TR-MOKE) microscope. The precessional magnetization dynamics in such FMNWs unveil the presence of uniform and quantized SW modes that can be tuned by varying the bias magnetic field (H). The frequencies of the modes are observed to decrease systematically with a decreasing magnetic field, and the number of modes in the spectrum is reduced from four to three for H < 0.7 kOe. To understand these results, we perform micromagnetic simulations that reveal the presence of edge, standing wave, and uniform SW modes in the nanowires (NWs). Our simulations clearly show how the standing wave and uniform SW modes coalesce to form a single mode with uniform precession over the entire NW for H < 0.7 kOe, reproducing the experimentally observed reduction in modes. Our study elucidates the possibility of manipulating the SW modes in magnetic nanostructures, which is useful for applications in magnonic and spintronic devices.

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Curvature of domain walls

An experiment was performed whereby domain walls were forced to bulge under the influence of an applied easy-axis fieldH. The region where the domain wall is bent appears to be approximately circular. To relate the radius of such a curved domain wall to theory, a model was chosen that takes into account magnetostatic, anisotropy, and exchange energy. The assumption was made that anisotropy and exchange energy, as well as the magnetostatic contribution due to the local magnetization distribution of a domain wall, may be summarized in a wall energy term. The energy of the entire system has been calculated under the constraint that the domain wall should be circular and that it is inscribed tangentially into a triangle. Minimizing the total energy with respect to the radius of curvatureA, the result can be expressed approximately byA propto (H - H_{c})^{-1}where Hcis the coercive force. This result fits the model of a membrane under pressure.     

Enhanced ferromagnetic stability in Cu doped passivated GaN nanowires

Density functional calculations are performed to investigate the room temperature ferromagnetism in GaN:Cu nanowires (NWs). Our results indicate that two Cu dopants are most stable when they are near each other. Compared with bulk GaN:Cu, we find that magnetization and ferromagnetism in Cu doped NWs are strongly enhanced because the bandwidth of the Cu t d band is reduced because of the one-dimensional nature of the NW. The surface passivation is shown to be crucial to sustain the ferromagnetism in GaN:Cu NWs. These findings are in good agreement with experimental observations and indicate that ferromagnetism in this type of systems can be tuned by controlling the size or shape of the host materials.     

Self-assembled single-crystal ferromagnetic iron nanowires formed by decomposition

Arrays of perpendicular ferromagnetic nanowires have recently attracted considerable interest for their potential use in many areas of advanced nanotechnology. We report a simple approach to create self-assembled nanowires of alpha-Fe through the decomposition of a suitably chosen perovskite. We illustrate the principle behind this approach using the reaction 2La(0.5)Sr(0.5)FeO(3) --> LaSrFeO(4) + Fe + O(2) that occurs during the deposition of La(0.5)Sr(0.5)FeO(3) under reducing conditions. This leads to the spontaneous formation of an array of single-crystalline alpha-Fe nanowires embedded in LaSrFeO(4) matrix, which grow perpendicular to the substrate and span the entire film thickness. The diameter and spacing of the nanowires are controlled directly by deposition temperature. The nanowires show uniaxial anisotropy normal to the film plane and magnetization close to that of bulk alpha-Fe. The high magnetization and sizable coercivity of the nanowires make them desirable for high-density data storage and other magnetic-device applications.     

Domain walls and transverse shock waves in shape-memory alloys

The martensitic phase transition in shape-memory alloys which is responsible for their ferroelastic and pseudoelastic deformation behaviour is described by a one-dimensional Landau theory. Domain walls between Martensite variants and between Austenite and Martensite are modelled as transverse shock waves. From the balance of momentum and of energy the jump of temperature across and the speed of the domain walls are calculated. The direction of motion follows from the entropy principle. Under certain conditions a moving domain wall acts like a heat engine converting free energy into mechanical work.     

High ferromagnetic transition temperature in PbS and PbS:Mn nanowires

Spontaneous magnetization measured in the temperature range 5-300 K with high ferromagnetic transition temperature (T(c)) has been observed in both undoped and Mn doped (2-8 mol %) PbS nanowires (diameter 30 nm) in polymer. For undoped sample, we find T(c) ~ 290 K while for doped samples T(c) varies between 310-340 K depending on Mn concentrations. Both T(c) and coercive fields are critically dependent on Mn concentrations. Coercive fields show a T(0.5) dependence with temperature for a moderate concentration of Mn (4 mol %) in PbS while it deviates from T(0.5) behavior for higher Mn concentrations. Anionic defects arising out of nonstoichiometric growth is solely responsible for the observed magnetism in undoped PbS nanowires. The role of intrinsic strain along with reduced dimensionality in determining such high T(c) and overall magnetizations has been discussed.     

Magnetostatic interactions and coercivities of ferromagnetic soft nanowires in uniform length arrays

First-order reversal curve diagrams have been used to investigate magnetostatic interactions and average coercivity of individual wires in soft ferromagnetic uniform length nanowire arrays. We present a method for identifying these physical parameters on the out-of-plane first-order reversal curve diagrams: the position of the irreversible part on the critical axis is a good approximation to the average value of the nanowire coercivity and the maximum interaction field is equal to the interaction field at saturation. Their dependence upon material (CoFeB and Ni) and nanowire length are presented. The magnetostatic interactions increase linearly with length, in agreement with a model developed previously. The global array coercivity, obtained from magnetization curves, is generally lower than the apparent average coercivity for individual nanowires. This coercivity reduction increases linearly with the magnetostatic interactions. The general shape of the out-of-plane first-order reversal curve diagrams is compared with those obtained from a theoretical moving Preisach model.     

Optical index profile at an antiparallel ferroelectric domain wall in lithium niobate

Unexpected optical contrast at antiparallel domain walls is observed in non-stoichiometric lithium niobate. This is imaged using near-field scanning optical microscopy. A detailed modeling of the imaging process is performed, and a comparison of the experimental and simulation images is used to extract the index profile across a single antiparallel domain wall.     

Pressures on cylindrical magnetic domain walls

The pressures on cylindrical magnetic domain walls are discussed with particular emphasis on bubbles, that is, near-circular domains. In the case of anisotropic wall energy, bubbles will be slightly elliptical, and a formula is derived relating the ellipticity to the anisotropic wall energy. Various formulations for the average magnetic field at a domain wall are presented. The problem of cutting bubbles off from a strip wall is investigated. It is seen that the field required to cut off a bubble can be up to 40 percent less than the 4πMsrequired to push two parallel walls together for certain geometries     

Atomic spin structure of antiferromagnetic domain walls

The search for uncompensated magnetic moments on antiferromagnetic surfaces is of great technological importance as they are responsible for the exchange-bias effect that is widely used in state-of-the-art magnetic storage devices. We have studied the atomic spin structure of phase domain walls in the antiferromagnetic Fe monolayer on W(001) by means of spin-polarized scanning tunnelling microscopy and Monte Carlo simulations. The domain wall width only amounts to 6-8 atomic rows. Although walls oriented along <100> directions are found to be fully compensated, detailed analysis of <110>-oriented walls reveals an uncompensated perpendicular magnetic moment. Our result represents a major advance in the field of antiferromagnetism, and may lead to a better understanding of the magnetic interaction between ferromagnetic and antiferromagnetic materials.     

Stability of magnetic recording on domain walls

The effect of a surface anisotropy of the easy-plane and easy-axis types, as well as an auxiliary layer of magnetically hard particles, on the stability of magnetic data recording on domain walls with respect to the action of external magnetic fields has been studied. It is established that the surface anisotropy does not significantly influence the stability of magnetic recording, while the auxiliary layer even considerably increases this stability and allows the information bit size to be fixed.     

Wave propagation along domain walls in magnetic films

It is shown by theoretical analysis that domain walls can support two different types of waves denoted as "magnetostatic interface waves" and "wall displacement waves". The magnetostatic interface waves are similar in character to magnetostatic surface waves. The wall displacement waves are analogous to displacement waves on strings and to capillary waves on the surface of liquids, Dispersion relations are derived for both types of waves. The displacement waves are approximately dispersion free at high wavenumbers. At low wavenumbers their dispersion diagram reflects the incipient instability of the straight domain wall against sinusoidal displacement, which occurs as the field gradient is reduced.     

Interferences in light deflection by ferroelastic domain walls

Deflection of light is studied in a crystal of glycine phosphite containing two twin walls. When the crystal is rotated in the incident laser beam, interferences are observed in both the direct beam and in the main deflected beam (A or B) for both polarizations of the incident light. The contrast is especially high, because the mutual tilt angle of the principal axes is close to 45 degrees in this twinned crystal. On this principle, fundamental-harmonic beam splitters could be built from as-grown twin crystals. Furthermore, the electrical modulation of the light deflected by ferroelectric-ferroelastic crystals can be now explained in terms of interference effects.     

Anisotropic conductance at improper ferroelectric domain walls

Transition metal oxides hold great potential for the development of new device paradigms because of the field-tunable functionalities driven by their strong electronic correlations, combined with their earth abundance and environmental friendliness. Recently, the interfaces between transition-metal oxides have revealed striking phenomena, such as insulator-metal transitions, magnetism, magnetoresistance and superconductivity. Such oxide interfaces are usually produced by sophisticated layer-by-layer growth techniques, which can yield high-quality, epitaxial interfaces with almost monolayer control of atomic positions. The resulting interfaces, however, are fixed in space by the arrangement of the atoms. Here we demonstrate a route to overcoming this geometric limitation. We show that the electrical conductance at the interfacial ferroelectric domain walls in hexagonal ErMnO(3) is a continuous function of the domain wall orientation, with a range of an order of magnitude. We explain the observed behaviour using first-principles density functional and phenomenological theories, and relate it to the unexpected stability of head-to-head and tail-to-tail domain walls in ErMnO(3) and related hexagonal manganites. As the domain wall orientation in ferroelectrics is tunable using modest external electric fields, our finding opens a degree of freedom that is not accessible to spatially fixed interfaces.