Magnetization Reversal Mode Research Articles

The angular dependence of the exchange bias under the planar domain wall model

Based on the principle of minimal energy, the angular dependence of exchange bias (ADEB) under planar domain wall model has been investigated in detail. Three different types of the initial states, monostable, bistable and tristable states, will be formed in exchange-biased systems by adjusting the relative strength of the uniaxial anisotropy, unidirectional anisotropy and the planar domain wall energy. These initial states determine the ADEB directly. The jump phenomena were still existent in the ADEB. Additionally, it can be found that the magnetization reversal mode is sensitive to the orientation of the magnetic field. Two distinct modes of the magnetization rotation, half-plane rotation and whole-plane rotation, can be observed when the applied field points along different orientations. These modes of the magnetization rotation can be identified by different features of the transverse hysteresis loops. Our investigations about the ADEB and the magnetization reversal modes are helpful to interpret the magnetic behaviors of some exchange-biased systems, which can be described by planar domain wall model.

Measuring and Actuating Devices Based on Shape Memory Ferromagnets

The possibility of application of shape memory ferromagnetic materials (SMFM) in measuring and actuating devices of systems for automation and control of production processes is considered. Adevice implementing a bicyclic configuration with distributed magnetizing coil and a mode of pulse magnetization reversal of the active components from SMFM is suggested. Mathematical models are designed for analyzing and synthesizing the device. Results of an experimental study are presented.

A new reversal mode in exchange coupled antiferromagnetic/ferromagnetic disks: distorted viscous vortex.

Magnetic vortices have generated intense interest in recent years due to their unique reversal mechanisms, fascinating topological properties, and exciting potential applications. In addition, the exchange coupling of magnetic vortices to antiferromagnets has also been shown to lead to a range of novel phenomena and functionalities. Here we report a new magnetization reversal mode of magnetic vortices in exchange coupled Ir20Mn80/Fe20Ni80 microdots: distorted viscous vortex reversal. In contrast to the previously known or proposed reversal modes, the vortex is distorted close to the interface and viscously dragged due to the uncompensated spins of a thin antiferromagnet, which leads to unexpected asymmetries in the annihilation and nucleation fields. These results provide a deeper understanding of the physics of exchange coupled vortices and may also have important implications for applications involving exchange coupled nanostructures.

Angular Dependence of the Coercivity in Electrodeposited Co–Pt Nanostructures With a Tube–Wire Morphology

We have fabricated Co-Pt cylindrical nanostructures comprised of a pair of nanotube and nanowire segments via direct electroplating into anodic alumina (AAO) membranes. The fabrication of such nanostructures is possible due to the penetration of sputtered gold (Au) nanoparticles inside the template, which serve as nucleation spots. The current transient monitored during the deposition process allowed us to distinguish between the nanotube and nanowire formation regime during the electrodeposition. In this paper, we report on the systematic changes in the angular dependence of the coercivity behavior due to the change in the length of the nanowire segment of these nanostructures, while keeping the nanotube segment constant. Considering the range of parameters studied, we found that the tube segment always reverses its magnetization through a vortex domain wall, whereas the nanowire segment reverses its magnetization through a transverse domain wall. Our results are in good agreement with literature. The possibility to alter the magnetization reversal mode in such nanostructures provides an attractive way to control the motion of the magnetic domain walls.

Magnetization Dynamics and Reversal Mechanisms in Ni Nanowire and Nanotube Arrays

Highly ordered arrays of Ni nanowires (NWs) and nanotubes (NTs) were electrodeposited into porous anodic alumina templates with 200 nm pore diameter. The geometrical parameters of the NW/NT arrays were tuned by the deposition conditions. The fabricated NWs and NTs had various lengths depending upon deposition time and NTs had a wall thickness of ~40 nm. Morphological characterizations were performed using a scanning electron microscope and transmission electron microscope yields the topology of NWs and NTs, structural properties were determined using X-ray diffraction, and magnetic characterization was done using SQUID. Dynamic properties have been studied by ferromagnetic resonance technique in frequency sweep mode. A comparative study of the magnetization reversal processes was also performed by analyzing the angular variation of resonance frequency of NWs/NTs. The resonance frequency increases linearly with magnetic field for studied arrays. In NWs the magnetization reversal mode is curling mode, whereas for NTs it is coherent rotation mode.

Effect of misaligned unidirectional and uniaxial anisotropies on angular dependence of exchange bias

We report a numerical study of the angular dependences of low-temperature exchange bias field (ADEB) and coercivity in the ferromagnetic/antiferromagnetic bilayers with misaligned unidirectional and uniaxial anisotropies. Through choosing a proper antiferromagnet the conventional symmetry in the ADEB may be broken, while the novel behaviors are also dependent on the angle between induced unidirectional and intrinsic uniaxial anisotropies. Finally, we draw conclusions that the two anisotropies with a small misalignment together determine the asymmetric ADEB properties around the easy axis. In contrast, after the magnetically hysteretic measurement rotating through the hard axis, a large misalignment between the anisotropies may change the magnetization reversal mode at the decreasing branch of loop, besides weakening the positive loop shift. Thus the strength of exchange bias field is suppressed while the coercivity is enhanced. • Role of the unidirectional anisotropy induced by cooling field direction is studied. • The ADEB behaviors are used to reveal the influence of anisotropies. • Coercive fields with unidirectional anisotropy are related to the measuring directions. • Novel behaviors of magnetization reversal modes with measuring directions are found.

Magnetization reversal in Co zigzag nanocolumns grown by glancing angle deposition

A Co zigzag nanocolumn sample was grown by glancing angle deposition using molecular beam epitaxy. A structural study was carried out using scanning electron microscopy and the magnetic properties and the magnetoresistance were investigated at room temperature. Each zigzag consisted of six arms tilted with respect to each other by approx. 60°, had a club-like shape, diameter in the range of 20–100nm, a single arm length of about 150nm and a total height of approx. 370nm. The magnetization hard axis was observed for the magnetic field oriented parallel to the zigzag long axis and the easy axis for the field oriented perpendicular to the long axis. The system exhibited very high maximum coercivity values, which could find potential applications. The angular dependences of the coercive field and the remanent to saturation magnetization ratio, as well as magnetoresistance curves, indicated the presence of curling magnetization reversal mode.

Kinetics of magnetization reversal in a heterophase nanomagnet with spatially modulated anisotropy

Magnetization reversal modes in a thin-film NiFeCuMo ferromagnet (FM) with periodically varying in-plane anisotropy are studied by the magneto-optical indicator film (MOIF) technique. The uni-directional anisotropy in FM regions exchange-coupled to a FeMn antiferromagnet (AFM) film in the form of square mesh stripes is alternated by the uniaxial anisotropy in the FM regions inside this mesh. It is shown that the boundaries formed along the edges of these stripes, which separate FM regions with different anisotropy, crucially influence the kinetics of domain-structure transformation in both types of FM regions. It is established that the lateral exchange anisotropy in the ferromagnet, which is determined by the stabilization of the spin distribution in the FM layer along the FM-(FM/AFM) interface, leads to the asymmetry of the magnetization reversal in FM regions bordered with an FM/AFM structure. Anisotropy of the mobility of 180-degree “charged” and “uncharged” domain walls situated, respectively, perpendicular and parallel to the unidirectional anisotropy axis is revealed. The difference observed between the mobilities of charged and uncharged domain walls is attributed to the difference in the spin distribution in these walls with respect to the unidirectional anisotropy axis and is a key factor for the difference between the magnetization reversal kinetics in horizontal and vertical exchange-biased FM stripes. Drastic differences are revealed in the asymmetry of magnetization reversal processes in mutually perpendicular narrow stripes of FM/AFM structures. Possible mechanisms of magnetization reversal in low-dimensional FM-(FM/AFM) heterostructures are discussed with regard to the effect of domain walls localized on the edges of AFM layers.

Angular dependence of the magnetic properties of cylindrical nanostructures with wire–tube morphology

We have investigated the angular dependence of the magnetic properties of cylindrical nanostructures comprised of a pair of nanotube and nanowire segments. We compared the magnetic behavior of these structures with the properties observed in an isolated nanowire and nanotube. Thus, we observed that the hysteresis curves of the cylindrical nanostructure with wire–tube morphology exhibits a step or plateau corresponding to a partial pinning of the domain wall at the interface between wire and tube sections. However, this step gradually disappears as we change the angle at which the external magnetic field is applied, thus enabling control of nucleation and release of a domain wall. Moreover, for small angles the nucleation and propagation of domain walls is observed, while for large angles is observed a quasi-coherent rotation. Finally, the possibility to alter the magnetization reversal mode in such nanostructures provides an attractive way to control the motion of magnetic domain walls.

Complex magnetic reversal modes in low-symmetry nanoparticles

A detailed numerical analysis of the magnetization reversal processes in T-shaped nanoparticles has been carried out. Attention has been focused on the influence of the symmetry of the particle on the formation, propagation, and interaction of internal magnetic structures such as domain walls, vortices, and antivortices. Results show that the lower the degree of symmetry of the particle, the more complex the reversal process is. Thus, symmetry represents an additional ingredient to control the magnetic properties of ferromagnetic nanoparticles.

Angular first-order reversal curves: an advanced method to extract magnetization reversal mechanisms and quantify magnetostatic interactions

The magnetic properties of ordered hexagonal arrays of Co nanowires (NWs) and nanotubes (NTs) with diameters of 50 nm and interwire/tube distances of 105 nm were studied using first-order reversal curves (FORCs). We report an advanced analysis of angle dependent first-order reversal curves (AFORCs), measured by changing the angle of the applied magnetic field from θ = 0° (parallel to the wire/tube axis) to 90° (perpendicular). This method allowed us to determine the magnetization reversal mode and to retrieve quantitative information on the magnetostatic interactions between NWs and between NTs. In particular, we found a sharp increase in the coercivity distribution of the NT arrays for θ > 70°, which is attributed to a transition between vortex and transverse reversal modes. Local magnetic interactions are found to prevail in the Co NT arrays, steadily increasing from θ = 0° to 90°. However, in the Co NW arrays the mean magnetic interactions decrease as θ increases, going from ones similar to local interactions to ones smaller than them.

Magnetization switching detection of a single permalloy nanomagnet using magneto-transport measurements

The magnetoresistance of Permalloy (NiFe) nanomagnets is studied at room temperature. For low aspect ratios, the main contribution is the anisotropic magnetoresistance, whereas for higher aspect ratio this contribution vanishes. We show that magnon magnetoresistance measurements need to be taken into account to detect precisely the magnetization switching of a single NiFe nanomagnet of dimensions 200 × 32 × 30 nm3. Experimental results are found to be in agreement with micromagnetic simulations and we give the analytical expressions to compute this effect. The measurement of the angular dependence on the magnetization switching allows studying the magnetization reversal mode of a single nanomagnet.

Magnetic Anisotropy Evolution in CoFe/Au Barcode Nanowire Arrays

Owing to their multifunctional properties, multisegmented metallic nanowires have generated interest for potential applications in magnetics and biology. In this paper, we present the magnetic properties of Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> /Au barcode nanowire arrays with CoFe and Au segments of various diameters and thicknesses. The distinct anisotropic behavior and magnetization reversal process depending upon these geometric factors were investigated. As the diameter increased (50-200 nm), the easy-axis direction changed from parallel (//) to perpendicular (⊥) to the nanowire axis. As the intersegment spacing increased (t <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Au</sub> = 50-200 nm), the easy-axis direction tended to lie in the plane of the nanowires. In addition, we examined the changes in the magnetization reversal modes.

Magnetic reversal modes in cylindrical nanowires

We perform a systematic study based on micromagnetic simulations of the demagnetization process for cylindrical nanowires with different crystalline structure. These simulations correspond to the most commonly reported electrodeposited nanowires, based on permalloy, nickel, iron and cobalt, with crystal structure tailored by electrodeposition parameters. The dependence of coercivity and remanence on the nanowire diameter, the angular dependence of coercivity and the corresponding reversal modes are calculated and discussed. Extensive comparison of the obtained coercive field value with available experimental data is presented. Depending on the crystallographic structure, the nanowires reverse magnetization by transverse or vortex domain walls and can exhibit the vortex structure along the whole nanowire length. The state diagrams for reversal modes in different materials as a function of the exchange correlation length, nanowire diameter and the anisotropy direction are presented. Magnetic force microscopy images corresponding to different structures are also evaluated by the micromagnetic simulations.

Reversal mode instability and magnetoresistance in perpendicular (Co/Pd)/Cu/(Co/Ni) pseudo-spin-valves

We have observed distinct temperature-dependent magnetization reversal modes in a perpendicular (Co/Pd)4/Co/Cu/(Co/Ni)4/Co pseudo-spin-valve, which are correlated with spin-transport properties. At 300 K, magnetization reversal occurs by vertically correlated domains. Below 200 K the hysteresis loop becomes bifurcated due to laterally correlated reversal of the individual stacks. The magnetic configuration change also leads to higher spin disorders and a significant increase in the giant magnetoresistance effect. First order reversal curve measurements reveal that the coupled state can be re-established through field cycling and allow direct determination of the interlayer coupling strength as a function of temperature.

Frequency dependence of magnetization reversal in thin Fe(100) films

We have studied the magnetization reversal of epitaxial Fe(100) films in an ac magnetic field as a function of frequency between 0.1 and 1.2 MHz by polarized neutron reflectivity. Different modes of magnetization reversal are excited depending on the ac field frequency, those with domain nucleation and propagation, and those with a coherent magnetization rotation. At low frequencies the magnetization follows the external field adiabatically via nucleation and propagation of antiparallel 180${}^{\ensuremath{\circ}}$ domains. At higher frequencies additional 90${}^{\ensuremath{\circ}}$ domain walls are being nucleated and propagate. Beyond 0.5 MHz, the magnetization reversal becomes progressively suppressed and finally comes to a stop at about 1 MHz.

Magnetoresistive behavior and magnetization reversal of NiFe/Cu/CoFe/IrMn spin valve GMRs in nanoscale

The magnetoresistance behavior and the magnetization reversal mode of NiFe/Cu/CoFe/IrMn spin valve giant magnetoresistance (SV-GMR) in nanoscale were investigated experimentally and theoretically by nanosized magnetic simulation methods. Based on the Landau-Lifshitz-Gilbert equation, a model with a special gridding was proposed to calculate the giant magnetoresistance ratio (MR) and investigate the magnetization reversal mode. The relationship between MR and the external magnetic field was obtained and analyzed. Studies into the variation of the magnetization distribution reveal that the magnetization reversal mode, that is, the jump variation mode for NiFe/Cu/CoFe/IrMn, depends greatly on the antiferromagnetic coupling behavior between the pinned layer and the antiferromagnetic layer. It is also found that the switching field is almost linear with the exchange coefficient.

Modeling and analysis of nano-sized GMRs based on Co, NiFe and Ni materials

Magnetic simulation method is introduced to analyze giant magnetoresistances (GMRs) in nanoscale for nano-sized biosensors. A spin valve model with special gridding corresponding to the exchange interaction length is proposed to study the influence of easy axes, exchange coefficients, pinning fields and feature widths on magnetization reversals and hysteresis characteristics of nano-sized GMRs with different pinned layer and free layer materials of Co, NiFe and Ni. The switching field is found to be almost linear with the pinning field and decrease with the absolute exchange coefficients and the feature widths for the nano-sized GMRs. The increase rate of each depends on the spin valve stacks. Further investigations into variations of the magnetization distribution reveal that the initial magnetization distribution and the magnetization reversal mode depend greatly on easy axes and materials The dependence on easy axes based mainly on the magnetocrystalline anisotropy is illustrated in detail.

5 Tdots/in2 bit patterned media fabricated by a directed self-assembly mask

FePt bit patterned media (BPM) was fabricated with a self-assembled polymer mask with a feature size of 12 nm pitch (equivalent to 5 Tdots/in2 ). A 3.5 nm FePt film with high c-axis crystal orientation was prepared for the magnetic recording layer. A solvent vapor annealing process was applied for obtaining uniform directed self-assembling of polystyrene (PS)-polydimethylsiloxane (PDMS) diblock copolymer pattern. Pattern transfer from a polymer mask to FePt layer was achieved by employing a carbon hard mask. In spite of excellent magnetic characteristics of FePt layer, the fabricated FePt BPM showed small coercivity (Hc) of 6 kOe and large switching field distribution (SFD) of 21%. These results are due to the etching damage of FePt dots. Disordering of FePt L10 phase by the etching damage reduced magnetic anisotropy energy (Ku). The damaged portion became a nucleus of the magnetization reversal and reduced Hc. Distribution of the damaged volume and the extent of the Ku reduction contributed to large SFD. This model is supported by the experimental data of magnetic field angle dependence of switching field. The result suggests the domain wall motion type of magnetization reversal mode, where the domain wall is created at the interface between the damaged portion and the internal high-Ku region.

Magnetization Reversal in Chemically Synthesized Hexagonal Cobalt Microplatelets

We report the magnetization reversal modes in hexagonal cobalt microplatelets prepared via the solvothermal chemical synthesis method. Experimental results do not reflect any effect of anisotropy due to the shape of an individual platelet and the shape of the overall geometry of the ensemble of platelets. Micromagnetic simulations on a single hexagonal cobalt platelet demonstrate different kinds of reversal modes depending upon the direction of the applied magnetic field, whereas for a group of three platelets the reversal modes of a single platelet are manifested in a slightly modified form due to the weak magnetostatic interactions among themselves. In this case, a poor qualitative agreement is observed with the experimental hysteresis loops. A much better qualitative agreement with the experimental loops is observed for a random ensemble of seven platelets, and a strongly collective magnetization reversal is observed in this case.