A theory of magnetization reversal in nanowires

Applied Physics Letters, 2008

We microscopically demonstrate that the magnetic domain is controllably nucleated and erased in the uniformly magnetized wire using a current pulse in small magnetic fields. Lorentz microscopy is performed in Permalloy nanowires with in-plane anisotropy. The stochastic nature of the magnetization reversal due to spin wave and thermal excitations in the absence of magnetic field completely disappears and turns into deterministic in the presence of small magnetic field, which enables the magnetization reversal control. We interpret that the phenomena are associated with Zeeman energy stabilization.

Europhysics Letters (epl), 2001

It is shown that a pulsed current driven through Ni nanowires provokes an irreversible magnetization reversal at a field distant from the spontaneous switching field Hsw by ∆H of as much as 40 % of Hsw. The state of the magnetization is assessed by magnetoresistive measurements carried out on single, isolated nanowires. The reversible part of the magnetization follows that of a uniform rotation. The switching occurs between the two states accessible otherwise by normal field ramping. ∆H is studied as a function of the angle between the applied field and the wire, and also of the direction of the pulsed current. The results are interpreted in terms of spin-flip transfer from the spin-polarized current to the magnetization, while the switching is approximated by a curling reversal mode.

Journal of Applied Physics, 2008

Magnetic nanowires have been shaped in the form of spirals and arranged in different patterns. A two-dimensional periodic array of Fe spiral structures was fabricated by electron-beam lithography. The spirals had a radius of 2.8 m, a linewidth of 100 nm, and a thickness of 20 nm. The magnetization reversal was studied by longitudinal vector magneto-optic Kerr effect ͑MOKE͒ in specular geometry as well as in Bragg MOKE geometry, using the diffraction spots from the grating for hysteresis measurements. The measurements are compared with the results of micromagnetic simulation, which allows a detailed interpretation of the experimental data. The magnetization reversal is characterized by an onion state in remanence and a coercivity which is different for the inner and outer parts of the spiral structures. In general the inner parts of the spiral are more stable and switch later than the outer ones. The switching of the outer parts depends on the boundary condition.

IEEE Transactions on Magnetics, 2009

Arrays of Ni ferromagnetic nanowires of three different diameters (20, 40, and 170 nm) are obtained by electrodeposition into nanoporous alumina templates. Hysteresis curves parallel and perpendicular to the applied field are studied by angle dependent vector vibrating sample magnetometry. Hysteresis curves, from high remanence and coercivity for the smallest diameter, to nearly anhysteretic curves for the largest diameter, can be tuned by varying the diameter of the nanowires. The results show that the magnetic response of these arrays is a combination of coherent and incoherent rotation of magnetization of the nanowires.

Phase Transitions, 2002

Magnetization reversal in Co nanostructures is simulated over a wide range of time-scales, from fast switching processes on a ps time-scale to thermally activated reversal on a µs time-scale. Langevin dynamics is used as well as time quantified Monte Carlo methods for the simulation of a classical spin system modeling elongated Co nano-particles. We study the behavior of the magnetization during the reversal, the energy barriers which are relevant for the thermally activated long-time behavior and the corresponding characteristic times.

Journal of Materials Science, 2018

Ordered arrays of NiFe/Cu multisegmented nanowires (NWs) are fabricated by ac pulse electrodeposition method into the 25-lm thick anodic aluminum oxide templates with a pore diameter of about 40-100 nm inter-pore distance. The behavior of magnetostatic interactions between neighboring NiFe/Cu NWs as well between magnetic segments of the same wire related to the NW length and the magnetic segment thickness is presented. The first-order reversal curves (FORCs) results for two given magnetic shape anisotropies, a nearly diskshaped and a rod-shaped one, reveal a single domain magnetic state along with a constant peak value of FORC coercivity distribution (H c FORC). However, the Major Hysteresis Loop coercivity (H c MHL) shows a significant reduction with an increase in length. In addition, the magnetostatic interaction distribution along the H u axis of FORC diagrams shows a weakly decreasing behavior, in disagreement with existing phenomenological model. In order to resolve this contradiction, the reversible and irreversible components of magnetization were measured. For arrays of multisegmented NWs, the contribution of the reversible components of magnetization rises up to about 70% as NW's length increases which is in contrast for arrays of uniform NWs where a nearly zero reversibility is reported.

FeCoNi nanowire arrays (175 nm in diameter and lengths ranging from 5 to 40 μm) were fabricated into nanopores of hard-anodized aluminum oxide templates using pulsed ac electrodeposition technique. Increasing the length had no considerable effect on the composition and crystalline characteristics of Fe 47 Co 38 Ni 15 nanowires (NWs). By eliminating the dendrites formed at the bottom of the pores, we report a careful investigation on the effect of magnetostatic interactions on magnetic properties and the effect of nanowire length on reversal modes. Hysteresis loop measurements indicated that increasing the length decreases coercivity and squareness values. On the other hand, first-order reversal curve measurements show a linear correlation between the magnetostatic interactions and length of NWs. Comparing reversal modes of the NWs both experimentally and theoretically using angular dependence of coercivity, we find that when L r22 μm, a vortex domain wall mode is only occurred. When L 422 μm, a non-monotonic behavior indicates a transition from the vortex to transverse domain wall propagation. As a result, a critical length was found above which the transition between the reversal modes is occurred due the enhanced interactions. The transition angle also shifts toward a lower angle as the length increases. Moreover, with increasing length from 22 to 31 μm, the single domain structure of NWs changes to a pseudo single domain state. A multidomain-like behavior is also found for the longest NWs length.

Journal of Applied Physics, 2005

We have performed experiments on current-induced domain-wall motion ͑CIDWM͒ in the case of the domain walls ͑DW͒ trapped within the nanoscale constrictions in patterned NiFe structures. Direct observation of current-induced magnetization reversal was achieved and critical current densities j c were measured in the presence of easy-axis magnetic fields. The direction of CIDWM was found to be along the direction of the electron motion in absence of an applied magnetic field and in the direction of the field when in the presence of even relatively weak fields. Data for the field dependence of j c for both uniform and fast rising pulses suggest that the current, regardless of polarity, assists in the depinning of the DW. Only for the dc case does the data strongly reveal the influence of the electron pressure in promoting or hindering DW motion.

Journal of Magnetism and Magnetic Materials, 2007

We have investigated the magnetization reversal mechanism and the effects of magnetostatic interaction in arrays of lithographically defined Permalloy nanowires with a fixed width of 185 nm, spacing of 35 nm and film thicknesses from 10 to 120 nm. The magnetization reversal has been investigated with vectorial Magneto-Optical Kerr Effect magnetometry. The vectorial hysteresis loops (in-plane magnetization components parallel and perpendicular to the applied field) recorded with the external field applied perpendicular to the length of the wires (hard magnetization direction), show a transition from coherent rotation to inhomogeneous reversal mode for wires' thickness above 80 nm. The effects of dipolar interactions are evidenced by the variation of the saturation field in the hard-axis hysteresis loops as a function of wires thickness. In detail, the analysis of the saturation field vs. wires thickness shows that the dipolar interactions start to play a role for wires thickness X20 nm. r

The Journal of Physical Chemistry C

The magnetic properties of radially-oriented Co, Ni, and CoNi alloy nanowires synthesized by pulsed electrodeposition into porous alumina structures are measured and compared with those of similar nanowires grown in a planar geometry. The alloy composition affects the anisotropy axis direction, which is determined by the balance between the magnetocrystalline and shape anisotropies, lying transverse to the nanowires for Co samples and along the nanowire axis for Ni. Monte Carlo simulations were performed to model the magnetic hysteresis of the radiallyoriented and planar geometry nanowires using an approach based on a conical distribution of anisotropies. The model provides an excellent fit compared with experimental hysteresis loops.