Permalloy Nanowires Research Articles

Reciprocal Damon-Eshbach-type spin wave excitation in a magnonic crystal due to tunable magnetic symmetry

We report spin-wave (SW) propagation in a one-dimensional magnonic crystal (MC) explored by all electrical spectroscopy. The MC consists of a periodic array of 255 nm wide permalloy nanowires with a small edge-to-edge separation of 45 nm. Provoking antiparallel alignment of the magnetization of neighboring nanowires, we unexpectedly find reciprocal excitation of Damon-Eshbach type SWs. The characteristics are in contrast to ferromagnetic thin films and controlled via, both, the external magnetic field and magnetic states. The observed reciprocal excitation is a metamaterial property for SWs and attributed to the peculiar magnetic symmetry of the artificially tailored magnetic material. The findings offer great perspectives for nanoscale SW interference devices.

Thermal effects in spin-torque assisted domain wall depinning

We have investigated the magnetization reversal in V-shaped permalloy (Py) nanowires under high dc currents via anisotropic magnetoresistance (AMR) measurements. Utilizing a diamond substrate as heat sink, current densities up to $2\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.16em}{0ex}}{\text{A}/\text{m}}^{2}$ can be applied. These high current densities enabled us to observe spin-torque assisted switching in thermal equilibrium, without the influence of transient effects. At high currents, the field driven magnetization reversal process is influenced by Joule heating, Oersted fields, and spin-torque effects. These contributions can be identified in our experiment due to their different symmetry properties under magnetization and current reversal. We obtain two important results when evaluating the spin-torque efficiency $\ensuremath{\epsilon}$, which is proportional to the nonadiabaticity parameter $\ensuremath{\beta}$. First, we find different values for $\ensuremath{\epsilon}$ within the same sample, obtained with just a slight variation of the experimental parameters. Second, within the temperature range of 77 to 327 K, $\ensuremath{\epsilon}$ is found to be constant.

Shape Effect of Magnetic Nanoelements as Biomolecular Labels for Magnetic Biosensors

Four magnetic nanoelements of different shapes were investigated at varying distances from a permalloy nanowire to see how the shape of the nanoelements affected the resultant demagnetizing field from the nanoelements onto the nanowire. All results were simulated using micromagnetic simulations by Object Oriented Micromagnetic Framework (OOMMF). It is shown that the square nanoelements performed the best and that the shape anisotropy has an important role in determining the external demagnetizing field for nanoelements.

Microwave assisted resonant domain wall nucleation in permalloy nanowires

We have designed a system to study microwave assisted domain wall nucleation in permalloy nanowires. We find a substantial decrease in the nucleation field when microwave fields are applied, in comparison to pulse fields. A clear resonance peak is observed in the frequency dependence of the nucleation field, which coincides with the uniform mode ferromagnetic resonance frequency. Owing to the well-defined nucleation process, the switching field distribution is small in contrast to previous reports. Our results show that localized microwave field provides an efficient tool for injecting domain walls into magnetic nanowires.

Longitudinal and transverse spin current absorptions in a lateral spin-valve structure

Spin absorption effects for the longitudinal and transverse spin currents in a Permalloy nanowire have been examined by using a lateral spin valve consisting of a V-shaped ferromagnetic injector and detector. The reduction of the nonlocal spin signal due to spin absorption for the transverse spin current was found to be stronger than that for the longitudinal spin current. This result is quantitatively explained by considering the angular dependence of the effective spin polarization, and the fact that the transverse spin relaxation length is less than half that of the longitudinal spin relaxation.

Current-induced domain wall motion in magnetic nanowires with different dimensions

The current-driven domain wall motion was investigated on permalloy nanowires with different dimensions by micromagnetic simulations. The critical current density increased with the reduction in both the width and thickness of nanowires because of the enhanced hard-axis anisotropy. At a thickness of 5 nm, the critical current density decreased with the reduction of the nanowire width because of the reduced domain wall width.

Vortex Domain Wall Formation in Permalloy Nanowires with Geometric Pinning Sites

An optimal geometric pinning site on Permalloy nanowires of varying widths has been investigated and applied in a magnetic memory scenario using micromagnetic simulations. Minimal limits on two key factors; the applied field length and the domain wall formation length are established such that vortex domain walls are reliably formed in the structures to facilitate lower powered domain wall movement using spin-polarised current. The symmetric wires with the nanoconstrictions at both sides have been found to favour the formation of the vortex domain wall compared with the asymmetric wires with the nanoconstrictions at only one side of the wires. The detailed micromagnetic simulations show that the domain wall formation length and the applied field length are optimal to form the vortex domain walls when they are equal to the nanowire width.

Vortex Domain Wall Formation in Nanowires With Twin Pinning Sites

A set of symmetric twin pinning sites of varying distances on Permalloy nanowires of different widths have been investigated and applied in a racetrack memory scenario using micromagnetic simulations. The nanowire width as well as pinning sites distance were found to affect the vortex domain wall formation, while the writing performance is subject to the writing head dimensions and the external field strength. Nanowires with a width of 600 nm and 800 nm with twin pinning sites at a distance of 1 m have been found to favor the formation of the vortex domain wall compared with 400 nm and 1 m nanowires. The detailed micromagnetic simulations show that a pinning site distance of 1 m and a nanowire width of 800 nm are optimal for an information storage device. The results of the write scenario simulations carried out for this device indicate that, for a successful writing process, an applied field length (AFL) of value 1 m and an applied field magnitude (AFM) of -0.05 are most suitable for information writing processes.

Generation and annihilation of domain walls in nanowires by localized fields

We study the creation and annihilation of domain walls in a permalloy nanowire by local Oersted fields of current pulses passing through a perpendicularly aligned stripline. The occurrence of both processes is investigated for current pulses of different polarities and for various external magnetic fields. Reliable creation and annihilation are achieved for small and zero external fields, while higher externally applied fields lead to the suppression of both processes as well as to the creation of multiple domain walls in the wire.

Stability of spinmotive force in perpendicularly magnetized nanowires under high magnetic fields

Spinmotive force induced by domain wall motion in perpendicularly magnetized nanowires is numerically demonstrated. We show that using nanowires with large magnetic anisotropy can lead to a high stability of spinmotive force under strong magnetic fields. We observe spinmotive force in the order of tens of μV in a multilayered Co/Ni nanowire and several hundreds of μV in a L10-ordered FePt nanowire; the latter is two orders of magnitude greater than that in permalloy nanowires reported previously. The narrow structure and low mobility of a domain wall under magnetic fields in perpendicularly magnetized nanowires permits downsizing of spinmotive force devices.

Time-Domain Observation of the Spinmotive Force in Permalloy Nanowires

The spinmotive force associated with a moving domain wall is observed directly in Permalloy nanowires using real time voltage measurements with proper subtraction of the electromotive force. Whereas the wall velocity exhibits nonlinear dependence on magnetic field, the generated voltage increases linearly with the field. We show that the sign of the voltage reverses when the wall propagation direction is altered. Numerical simulations explain quantitatively these features of spinmotive force and indicate that it scales with the field even in a field range where the wall motion is no longer associated with periodic angular rotation of the wall magnetization.

Clocking magnetic field-coupled devices by domain walls

We study the interaction between propagating domain walls in permalloy nanowires and nanomagnetic dots made of Co/Pt multilayers. The magnetic behavior of dots placed above/below the domain wall conductor is strongly influenced by the field of the wall. Using micromagnetic simulations we show that the field generated by the domain wall can switch the coupled dots to their magnetically ordered ground state. This effect can be exploited for clocking magnetic field-coupled devices and possibly substitute electrical clocking structures.

Geometric dependence of static and kinetic pinning of domain walls on ferromagnetic nanowires

We investigate two distinct pinning mechanisms—denoted as static and kinetic pinning of magnetic domain wall (DW) in Permalloy nanowires with different widths. Both pinning situations are realized at an artificial notch on U-shaped Permalloy nanowires, depending on the initial DW states, moving or pinned. We find experimentally that the kinetic and static depinning fields simultaneously increase as the width of the nanowire decreases, whereas a difference between static and kinetic depinning fields monotomically decreases. This is ascribed to the shape anisotropy field of the DWs depending on the geometry of nanowires based on one-dimensional collective model.

Magnon magnetoresistance of NiFe nanowires: Size dependence and domain wall detection

The magnetoresistance of permalloy (Ni84Fe16) nanowires of various widths (down to 50 nm) has been measured for fields applied along the wires. The enhancement of the shape anisotropy in the narrowest widths leads to the disappearance of the anisotropic magnetoresistance signal, the remaining contribution to the magnetoresistance being that of the magnons. Using constrictions to pin a domain wall, we show that the magnon magnetoresistance signal can give access to the position of the domain wall along the wire.

Temperature-dependent dynamics of stochastic domain-wall depinning in nanowires

The temperature dependence of domain-wall depinning in permalloy nanowires is investigated by measuring depinning fields and corresponding depinning times as a function of the external magnetic bias field. Domain walls are pinned at triangular notches in the nanowires and detected noninvasively by Hall micromagnetometry. This technique allows one to acquire depinning-field and depinning-time distributions in the temperature range between 5 and 50 K and thus to determine the stochastics of the depinning process. The results are discussed in terms of the Néel–Brown model for thermally activated magnetization reversal, assuming a single energy barrier to overcome. In general, the cases presented deviate from this description and give a clear indication that a more complex term for the energy landscape of domain-wall depinning at constrictions in nanowires is obligatory.

Magnetic soft x-ray microscopy of the domain wall depinning process in permalloy magnetic nanowires

Full-field magnetic transmission x-ray microscopy at high spatial resolution down to 20 nm is used to directly observe field-driven domain wall motion in notch-patterned permalloy nanowires. The depinning process of a domain wall around a notch exhibits a stochastic nature in most nanowires. The stochasticity of the domain wall depinning sensitively depends on the geometry of the nanowire such as the wire thickness, the wire width, and the notch depth. We propose an optimized design of the nanowire for deterministic domain wall depinning field at a notch.

Current-induced domain wall motion in permalloy nanowires with a rectangular cross-section

We performed micromagnetic simulations of the current-induced domain wall motion in permalloy nanowires with rectangular cross-section. In the absence of the nonadiabatic spin-transfer term, a threshold current, Jc is required to drive the domain wall moving continuously. We find that Jc is proportional to the maximum cross product of the demagnetization field and magnetization orientation of the domain wall and the domain wall width. With varying both the wire thickness and width, a minimum threshold current in the order of 106 A/cm2 is obtained when the thickness is equivalent to the wire width. With the nonadiabatic spin-transfer term, the calculated domain wall velocity ν equals to the adiabatic spin transfer velocity u when the current is far above the Walker limit Jw. Below Jw, ν=βαu, where β is the nonadiabatic parameter and α is the damping factor. For different β, we find the Walker limit can be scaled as Jw=α|β-α|Jc. Our simulations agree well with the one dimensional analytical calculation, suggesting the findings are the general behaviors of the systems in this particular geometry.

Direct comparison of domain wall behavior in permalloy nanowires patterned by electron beam lithography and focused ion beam milling

Nominally identical permalloy nanowires, with widths down to 150 nm, were fabricated onto a single-electron transparent Si3N4 membrane using electron beam lithography (EBL) and focused ion beam (FIB) milling. Transmission electron microscopy (TEM) experiments were performed to compare the nanostructures produced by these two techniques in what we believe is the first direct comparison of fabrication techniques for nominally identical nanowires. Both EBL and FIB methods produced high quality structures with edge roughness being of the order of the mean grain size 5–10 nm observed in the continuous films. However, significant grain growth was observed along the edges of the FIB patterned nanowires. Lorentz TEM in situ imaging was carried out to compare the magnetic behavior of the domain walls in the patterned nanowires with anti-notches present to pin domain walls. The overall process of domain wall pinning and depinning at the anti-notches showed consistent behavior between nanowires fabricated by the two methods with the FIB structures having slightly lower characteristic fields compared to the EBL wires. However, a significant difference was observed in the formation of a vortex structure inside the anti-notches of the EBL nanowires after depinning of the domain walls. No vortex structure was seen inside the anti-notches of the FIB patterned nanowires. Results from micromagnetic simulations suggest that the vortex structure inside the anti-notch can be suppressed if the saturation magnetization (Ms) is reduced along the nanowire edges. A reduction of Ms along the wire edges may also be responsible for a decrease in the domain wall depinning fields. Whereas the two fabrication methods show that well-defined structures can be produced for the dimensions considered here, the differences in the magnetic behavior for nominally identical structures may be an issue if such structures are to be used as conduits for domain walls in potential memory and logic applications.

Magnetisation reversal in permalloy nanowires controlled by near-field charge interactions

This study investigates the interaction between transverse head-to-head domain walls (THHDWs) in permalloy nanowires and magnetostatic charges located within a tripod structure in close proximity. This interaction enables remote pinning of the THHDW. Eight different interactions are measurable, depending on the initialising field sequence. The pinning strengths of an attractive potential well and a repulsive barrier are compared in this paper. Based on experimental measurements, micromagnetic simulations, and a monopolar analytical model, we demonstrate that pinning strength depends on the inhomogeneous charge distribution within the tripod and the relative orientation of the THHDW which faces the tripod.

Detection of Local Oersted Field Generated at the Junction Between Ferromagnetic Nanowire and Electrode

We report local concentration of the Oersted field at the junction of electric contact geometry. The local Oersted field is numerically calculated with a simplified analytical model. This local field is then verified experimentally by observing magnetization reversal due to the current injection into Permalloy nanowires under magnetic field bias. The threshold current of magnetization reversal is found to be linearly dependent on the strength of the magnetic-field bias with a proportional coefficient (6.9 ± 0.2) × 10-11 Oe·m2/A, which provides evidence of the local concentration of the Oersted field at the junction.